ISSN:0069-3030    

DOI:10.1039/OC96764FX001

出版商:RSC    

年代:1967

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC96764BX003

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

2. PHYSICAL METHODS OF STRUCTURE DETERMINATION Part (i) Nuclear Magnetic Resonance Spectroscopy By J. Feeney (Varian Research Laboratory Walton-on- Thames Surrey) NUCLEAR magnetic resonance spectroscopy continues to have widespread use as a powerful means of determining molecular structures. In fact almost all papers concerned with organic structural and stereochemical work contain some reference to the technique. However this report does not attempt to reflect the routine analytical usage of the n.m.r. technique but rather to present the more specialised aspects of its application. Some of the more noteworthy contributions to the field have been con- cerned with critical re-examination of several standard procedures to assess their validity. This will be evident in the sections of the report dealing with kinetic conformational and double-resonance studies.For example there is now firm experimental confirmation of the validity of the method of full line- shape analysis for studying kinetic processes. Special attention has been devoted to two of the more recent developments namely n.m.r. studies in liquid-crystal media and the application of intra- molecular nuclear Overhauser effects in making spectral assignments. Chemical Shifts.-At present one cannot carry out the necessary detailed quantum mechanical calculations to explain chemical shifts found for the various nuclei in different molecular environments. To explain differences in observed chemical shifts one often resorts to specific physical models (such as those based on anisotropic inductive or electric-field effects).Despite the limitations of such an approach to the problem it is often possible to calculate chemical-shift differences which are of useful practical consequence. For example Whalley and his co-workers'. have modified the McConnell equation used for estimating shielding contributions from anisotropic effects to enable it to be used for cases where the distance between the shielding group and the observed proton is small compared with the length of the induced dipole. They found that when estimating the chemical-shift change accom- panying,a change of substituent one must consider the shielding effects of all bonds displaced as well as those of all the bonds introduced.For the case of carbon-carbon double bonds deshielding is predicted for protons in the J. W. Apsimon W. G. Craig P. V. Demarco D. W. Mathieson L. Saunders and W. B. Whalley Tetrahedron 1967,23,2339. J. W. Apsimon. W. G. Craig P. V. Demarco D. W. Mathieson L. Saunders and W. B. Whalley Tetrahedron 1967,23,2357. J. Feeney regions at the ends of the double bonds but a proton outside this region will be shielded whether it is in the plane of the double bond or above it which is not as had been predicted previously. Following on this work Karabatsos (1) and his co-workers3 suggested that anisotropic effects for carbonyl groups are not as predicted previously. Thus a proton HA in (1) in the plane of the carbonyl group might be expected to be shielded rather than deshielded.The observed temperature-dependence of chemical shifts in some substituted ketones and aldehydes can be best interpreted in terms of rotameric population changes and the above shield’ng considerations. Anisotropic shielding effects in puckered cyclobutane rings & and heterocyclic nitrosamine~~ have also been examined. Aromatic ring-current chemical-shift contributions have been calculated and compared favourably with available experimental data for even alternant pentacyclic hydrocarbons6 and cyclic polyene~.~ Discussion of the validity of the ring-current approach has continued.8* Adcock and Dewar” have examined an extensive series of 1- and 2-fluoro- naphthalenes the substituent effects on the chemical shifts are shown to differ from substituent effects on physical and chemical properties of other side-chains.In an examination of hydrocarbons and their ions linear correlations of ‘H and I3C chemical shifts with the corresponding calculated partial charges on the atoms were found.” Chemical shifts of methyl groups in ortho-toluenes cis-vinyl hydrogens in olefinic systems and the H-3 aromatic protons in 1,2-disubstituted benzenes have been found to give linear correlations with an empirical parameter Q = P/Ir3 where P is the C-X bond polarisability I is the first ionisation potential and I is the C-X bond length.” A linear correlation between chemical-shift changes of a proton and its distance from a neighbouring methyl group has been pointed out for derivatives of bicyclo[2,2,l]heptane and bicyclo[2,2,2]octane.l3 G. J. Karabatsos G. C. Sonnichsen N. Hsi and D. J. Fenoglio J. her. Chem. Soc. 1967 89 5067. N. Nakagawa S. Saito A. Suzuki and M. Itoh Tetrahedron Letters 1967 1003. Y. L. Chow Angew. Chem. 1967,79,51. T. B. Cobb and J. D. Memory J. Chem. Phys. 1967,47,2020. F. Baer H. Kuhn and W. Regel 2.Naturforsch. 1967,22a 103. * J. I. Musher J. Chem. Phys. 1967,46,1219. J. M. Gaidis and R. West J. Ctrem. Phys. 1967,46 1218. lo W. Adcock and M. J. S. Dewar J. Amer. Chem. SOC.,1967,89,379. N. C. Ba$d and M. A. Whitehead Theor. Chim. Acta 1966,6 167. l2 W. B. Smith and J. L. Roark J. Amer. Chem. SOC. 1967,89,5018. l3 E. Pretsch H. Immer C. Pascual K. Schaffner and W. Simon Helv.Chim. Acta 1967,50 105. Physical Methods of Structure Determination 7 Gusten and his co-~orkers'~ have studied the influence of ring substituents on the spectra of substituted cis-and truns-stilbenes. Coupling Constants.-Coupling constants especially proton-proton and carbon-proton coupling constants are more amenable to theoretical calcu- lation than are chemical shifts and considerable success in calculating coupling constants has been achieved in the past few years. Using a valence-bond variational calculation for cis and frans H-H coupling constants in ethylene Chandra and Narasimhan lS have produced values in excellent agreement with observed values (calculated JHH trllm 18-73 JHHcis13-35 c./sec.; observed 19.1 and 11.5 respectively).They treated the problem as an eight-electron eight-orbital (six 0-and two n-orbitals) problem. Gunther16 has discussed the effects of delocalisation on H-H spin coupling in unsaturated systems. H-H geminal coupling constants have been calculated using the Dirac-Van Vleck vector model.17 Dixon18 has reported a MO theory for proton spin-spin coupling constants which is simplified such that one requires only two MO parameters one for the n-interactions of carbon hybrid orbitals and one for the interaction of orbitals on the same atom to give a reasonable prediction of observed trends in coupling constants. Attempts to calculate the H-F coupling constant in hydrogen fluoride using a refined perturbational calculation indicate that the calculated values are extremely sensitive to the wave functions used and it was concluded that presently available wave functions do not have the required accuracy.' The Pople and Santry20$21 MO approach has been used successfully to calculate 13C-H and 29Si-H coupling constants.22.23 From a theoretical interpretation of substituent effects on such coupling constants in methanes and silanes it was predicted that the electronegativity of the substituents will be of paramount importance. Dixon has used a valence-bond treatment to rationalise the proportionality between 13C-H coupling constants and the carbon s-~haracter.~~ Several interesting correlations between coupling constants and other molecular parameters have been pointed out. The most interesting of these is an empirical relationship found between vicinal proton-proton coupling constants and reaction rates for elimination and substitution type processes in aliphatic system^.^ Karplus type correlations between some H-F vicinal l4 H.Gusten and M. Salzwedel. Tetrahedron 1967,23 173. 187. Is P. Chandra and P. T. Narasimhan,Mol. Phys. 1967,12,523. l6 H. Gunther Tetrahedron Letters 1967,7%7. H. G. Hecht J. Phys. Chem. 1967,71,1761. W.T. Dixon J. Chem SOC.(A) 1967 1879. l9 Y. Kato and A. Saika J. Chem. Phys. 1967,46 1975. 2o J. A. Pople and D. P. Santry Mol. Phys. 1964,8,1. 21 J. A. Pople and D. P. Santry Mol. Phys. 1965,9 311. 22 R. Ditchfield M. A. Jensen and J. N. Murrell J. Chem. SOC.(A) 1967 1674. 23 J. N. Murrell P. E. Stevenson and G. T.Jones Mol. Phys. 1967 12,265. 24 W. T. Dixon Theor. Chim. Actu 1966,4,359. 25 W. T. Dixon Tetrahedron Letters 1967,2531. J. Feeney coupling constants and dihedral angles have been observed.26 In a series of asymmetrical benzylic compounds the JHH gem values between the non-equivalent protons of the methylene group show a linear correlation with Hammett o-c~nstants.~~* 28 From an examination of an extensive series of perfluorovinyl compounds CF,=CFX Rest2' has reported several linear relationships between JFFgem values and the mean of the "F chemical shifts of the terminal ethylenic CF2 fluorine nuclei a different linear relationship is observed depending on wehther X is organic a main-group metal a metalloid or a transition metal.In a series of substituted fluorobenzenes the 13C-H and 13C-F coupling constants were found to depend on both the substituent and its position of sub~titution.~~ 13C-H coupling at a particular position usually increases with increase in electron-withdrawing power of the sub- stituent. Long-range coupling constants. Calculations of the long-range (5 bond) coupling constants in buta-1,3-dienes indicate that in the s-trans-isomer the n-mechanism dominates the co~pling.~ Long-range couplings have been reported for carboxylic acid estersJ2 (CH3COOCH, JCH3-CH3 0.21 c./sec.) alkyl formatesJ3 (HACOOCHXCHY JAX -0432 to -1.00 JAY+0.42 to +0.63 c./sec.) and orcinol derivativesJ4 (2) (JCH3-H-ortho 0*6&-0.80,ScH3-H-poro 0.57 c./sec.) and such couplings in 1,3-dioxans have been explored further.35 26 K.L. Williamson Yuan-Fang Li F. H. Hall and S. Swager J. Amer. Chem. SOC. 1966,88,5678. 27 R. R. Fraser P. Hanbury and C. Reyes-Zamora Canad. J. Chem. 1967,452481. R. W. Franck and J. Auerback Ccnad. J. Chem. 1967,452489. 29 A. J. Rest J. Chem. Phys. 1967,47 1559. 30 S. Mohanty and P. Venkateswarlu,MoI. Phys. 1967,12 277. 31 A. V. Cunliffeand R. K. Harris MoI. Phys. 1967 13 269. '* K. Hayamizu and 0.Yamamoto J. MoI. Spectroscopy 1967,22 119. '' K. Hayamizu and 0.Yamamoto J. Mol. Spectroscopy 1967,23 121. 34 D. T. Witiak D. B. Patel and Y. Lin J. Amer. Chem. SOC. 1967,89 1908. '' J. E. Anderson J. Chem. SOC. (B) 1967,712. Physical Methods of Structure Determination 9 In 1-methylpyrene (3) the CH absorption appears as a doublet (JHae3 1.3c./sec.) this splitting is much larger than normally observed for interaction between a methyl group and an adjacent aromatic proton and indicates that there is a substantial amount of double-bond character in the intervening bond.36 A similar effect is observed in 9-methylphenanthrene (4).A study of long-range stereospecific H-F coupling in the t-butyl derivative (5) has revealed the presence of hindered rotation.37 The room-temperature spectrum of the methyl groups shows a doublet (JHF2.9 c./sec. 6H) and a singlet (JHF-0,3H) which is evidence for two different types of methyl group existing in the hindered form of the molecule shown in (5). At 200" a single doublet absorption (JHF2.2 c./sec.) is observed.Pascual and Simon3' have examined several compounds containing geminal methyl groups; long-range CH,-CH coupling ( >O-3 c./sec.) is observed only in cases where the carbon atom bonded to the two methyl groups is either substituted by one or two oxygen functions or is sp2-hybridised. Observation of line-broadening resulting from stereospe,cific long-range coupling in epoxidation products has been used to determine the stereochemical course of the reaction.,' Signs of coupling constants. To measure the success of theoretical attempts to predict coupling constants it is necessary that we should know not only the magnitude of a coupling constant but also its absolute sign. It is not easy to measure directly absolute signs of coupling constants (it can be done by studying spectra recorded for molecules dissolved in the nematic phase of a liquid crystal) but fortunately they can often be obtained indirectly with greater ease by measuring relative signs of coupling constants in systems where one of the coupling constants is known to have a particular absolute sign (e.g.J13C-H for directly bonded nuclei is always positive). Relative signs of coupling constants can often be determined from a complete spectral analysis in cases where the nuclei are strongly coupled. However easier and more direct methods are available by using one of a variety of double-resonance techniques (such as selective decoupling;' spin-tickling:' nuclear Overhauser effects,42 transient selective irradiati~n?~ and transient nut at ion^.^^).For example using heteronuclear spin-tickling on the spectrum of dimethylphenylphos- phine (CH,),PhP JlsGSlp(-14 & 1 c./sec.) is shown to be opposite in sign to J13(-~ (+130-3 & @2) which proves that J13~31pis absolutely nega- tive in sign.45 In the protonated cation of this phosphine (CH3),PhPH+Br- the sign of J13C-31~ (+56 1 c./sec.) is shown to be absolutely positive 36 E. Clar B. A. McAndrew and M. Zander Tetrahedron 1967,23,985. 37 J. P. N. Brewer H. Heaney and B. A. Marples Chem. Comm. 1967,27. 38 C. Pascual and W. Simon Helv. Chim. Acta 1967,50 94. 39 R. G. Carlson and N. S. Behn J. Org. Chem. 1967,32 1363. 40 R. Freeman and D. H. Whiffen Mol. Phys. 1961,4,321. 41 R. Freeman and W. A. Anderson J. Chem. Phys. 1962,37,2053.42 R. Kaiser J. Chem. Phys. 1963,39,2435. 43 R. A. Hoffman B. Gestblom. and S. Forsen. J. Chem. Phys. 1964,40,3734. 44 J. A. Ferretti and R. Freeman J. Chem. Phys. 1966,44 2054. 45 W. McFarlane Chem. Comm. 1967,58. 10 J. Feeney by the same method. The absolute signs of several other coupling constants have been measured in this way for example 13C-119Sn (-) 'H-"'Sn (-),46 I3C-l4N (+) between directly bonded nuclei in methyl i~ocyanide,~~ 29Si-H (+) in tetramethyli~lane,~~ P-H and P-F coupling constants P-P (+) for directly bonded phosphorus nuclei,4i and C-H ( +)and P-H (+) for directly bonded nuclei in dimethyl phosphite" (CH,O),P(O)H. Manatt and his co-~orkers~~ have determined the relative signs of coupling constants for the systems F-C-P F-C-P-H P-H F-C-F F-C-P-F P-F F-C-C-P F-C-C-F F-C-C-C-P F-C-C-C-F F-C-P-P P-P H-C-P H-C-P-P and F-C-P-P-C-H in the series of molecules CF,PH2 CF3PF2 (CF3)2PH (CF3)2 PF F[CF2] 2pc12 F[ CF2] ,Pcl2 (CF3)2PP(CH3)2 and CF3P-+ P(CH3)2.Multiple-resonance techniques were used to determine the relative signs of the six proton-proton coupling constants in isopropenylacetylene (6).' By irradiating at the CH frequency in a double-resonance experiment one obtains a simple decoupled spectrum expected for an ABX stin system on which tickling experiments can be carried out to give signs of J,AB JAX and JBx. An interesting alternation in signs is observed for JpH (+25 c./sec.) and JpHe ( T6 c./sec.) in the phospholene (7).52 Double Resonance.-Even the more sophisticated double-resonance ex-perimenrs are now being used routinely not only by the specialist but also by those interested mainly in the molecular structural information available Me HA from n.m.r.studies. This results from the widespread availability of instru- mentation capable of making such measurements with relative ease. However the need for caution in analysing decoupled spectra when large decoupling 46 W. McFarlane J. Chem. SOC.(A),1967,528. 47 W. McFarlane J. Chem. SOC.(A),1967 1660. 48 R. R. Dean and W. McFarlane Mol. Phys. 1967 12,289. 49 S. L. Manatt D. D. Elleman A. H. Cowley and A.,B. Burg J. Amer. Chem. SOC.,1967,89,4544. W. McFarlane J. Chem. SOC.(A) 1967,1148. 51 H. M. Hutton and T. Schaefer Cunad. J. Chem.1967,45 1165. 52 D. Gagnaire J. B. Robert and J. Verrier Chem. Comm. 1967 819. Physical Methods of Structure Determination 11 fields are used has been stressed by Connor and his co-~orkers.~~ When yH2/2x becomes comparable in magnitude with the chemical shifts (vA -vx) between irradiated and observed groups the spectrum can become more complicated rather than simplified. Bloch Siegert type shifts also complicate the analysis of such spectra. Because of these problems it is not possible to carry out clean spin-decoupling experiments in the AMX spin system of vinyl acetate. When one is decoupling to achieve a simplified but still complex spectrum from an even more complex spectrum one is faced with difficulties unless there is some feature of the decoupled spectrum to indicate when optimum de- coupling conditions have been achieved.An excellent example of where such experiments are possible is provided by the application of double resonance to the n.m.r. spectrum of triptycene (8).54When the methine proton frequency is irradiated the resulting decoupled spectrum is that expected from an AA’BB’ spin system which is still a complex spectrum. Because one knows that the decoupled spectrum must show the symmetry one expects for an AA’BB’ type spectrum the optimum decoupling conditions can be achieved with some confidence. Spin-tickling has been used to obtain a complete analysis of AA’BB’ spin system from the ring protons in dimethyl cis-and trans-1,2-dibromo-cyclobutane-l,2-dicarbo~ylate.~~ The effects of relaxation in n.m.r.double- resonance spectra of weakly coupled spin systems have been ~onsidered.’~ Several computer programmes for the calculation of n.m.r. double-resonance spectra have been developed.57 Cyc1obutanes.-It is only recently that the n.m.r. parameters of cyclobutanes have become well documented. Fleming and Williams58 have now provided a comprehensive summary of all the literature proton-proton coupling constants for cyclobutanes cyclobutanones cyclobutenes and cyclobutenones. For cyclobutanes Jujccjs4.6-1 1.5 Juierrans2.@-10.7 JBem-10.9 to -14.4 c./sec. The Juiccisand Juictrans values are often similar in magnitude in the same molecule but Juiccisis usually larger than Juictrons; this was found to be the case in twenty cyclobutanes studied by Weitkamp and K~rte.~~ When the cyclo- ’’ T.M. Connor D. H. Whiffen and K. A. McLauchlan Mol. Phys. 1967,13,221. 54 K. G. Kidd G. Kotowycz and T. Schaefer Canad. J. Chem. 1967,45,2155. 55 E.Lustig E. P. Ragelis N. Duy and J. A. Ferretti J. Amer. Chem. SOC.,1967,89 3953. ” B.D.Nageswara Rao and L. Lessinger Mol. Phys. 1967 12 221. 57 G.Govil and D. H. Whiffen Mol. Phys. 1967,12,449. 58 I. Fleming and D. H. Williams Tetrahedron 1967,23,2747. 59 H.Weitkamp and F. Korte Tetrahedron 1966,Supplement No. 7,75. 12 J. Feeney butane ring is incorporated into a strained bicyclo[2,l,l]hexane system very different coupling constants from the above are observed (Jgemvalues in the range -5.4 to -8-4 c./sec.). In cyclobutenes coupling constants between olefinic protons are in the range 25-40c./sec.while vicinal coupling between protons attached to adjacent sp3-and sp2-carbons is small (0-1 c./sec.). The spectrum of cyclobutanone (9) has recently been reassigned after analysing as an AA’BB’B”B’” spin system :60 the coupling constants are JAA -11-1; JABcis +10.0;JBBp -17.5;JAW +6.4 ;JBWIcis +4.6 ;JBB,**trpns -2*8c./sec. Weitkamp and Korte” found that in the twenty cyclobutanes they studied the shielding values for the ring protons were always in the range z 6-2-8.2 and the chemical-shift contributions were found to be additive. Studies of Nuclei other than Hydrogen.-Carbon-1 3. Considering the enormous potential in this field comparatively few publications on ”C resonance appeared in 1967.Weigert and Roberts6’ have used spectrum- accumulation techniques to record the ’ spectrum of benzene. The molecule constitutes a seven-spin system with all nuclei being magnetically nm-equiva- lent and the spectrum was fully analysed using a computer to give JCCH+1.0 JmCH+7*4 and Jcccm -1.1 c./sec. Grant62 has likewise examined 13C- enriched acetic acid (such that 0.2%of molecules had two carbon-13 nuclei) to observe the spectrum of the species ‘3CH313COOH. By heteronuclear decoupling he showed that Jcc (57.6 c./sec.) has the same sign as JCH(positive). ’ resonance has been used successfully to characterise aromatic petroleum fraction^.^ The various types of aromatic and aliphatic carbons give clearly distinguishable ‘ absorption signals in the accumulated spectra.A theoretical interpretation of ’ shifts in aromatic compounds (biphenyl naphthalene phenanthrene and pyrene) which includes the effects of G-and n-electron densities on a local-charge parameter has been put forward.64 It has been noted that carbon atoms in a spatially crowded environment have chemical shifts at higher fields than e~pected.~’ A comparison66 of “B chemical shifts in some boron-nitrogen compounds (H,BNH3) with the 13C chemical shifts in the analogous alkanes (H,CCH,) indicates a linear relationship given by 613C(p.p.m.) (from benzene) =1-716”B (p.p.m.) (from BF,.Et,O) +81 ‘O L. H. Sutcliffe and S. M. Walker J. Phys. Chem. 1967,71 1555. 61 F. J. Weigert and J. D. Roberts J. Amer. Chem. SOC.1967,89,2967. 62 D. M. Grant J. Amer. Chem. SOC. 1967,89 2228. ’’S. A. Knight Chem. and Ind. 1967 1921. 64 T. D. Alger D. M. Grant and E. G. Paul J. Amer. Chem. SOC.,1966,88 5397. ”D. M. Grant and B. V. Cheney J. Amer. Chem. SOC. 1967,89,5315. 66 B. F. Spielvogel and J. M. Purser J. Amer. Chem. SOC. 1967,119 5294. Physical Methods of Structure Determination 13 13C studies of substituted methyl ben~oates,~~ cyclopropyl ketones,68 and substituted an is ole^^^ have been reported by Stothers and his co-workers. When the 3C spectra of 4-substituted pyridines70 are examined the observed substituent effects are very similar to those observed in benzene (to 3 p.p.m.). 3C chemical shift data from fifteen methylcyclohexanes have been rep~rted.~' Litchman and Grant72 have measured 13C-13C coupling-constant variations with changes in polar substituents X in the series XC(CH,) ;charge polarisa- tion effects due to electronegative groups do not play a major role in controlling the coupling constants.The implication of this is that 13C-13C values provide a better criterion for bond hybridisation than do 13C-H values. Weigert and Roberts7 have characterised the hybridisation in three-membered ring systems by studying such effects in halogenated cyclopropanes. 13CH Satellite spectra. In 'H spectra of organic molecules one can often detect multiplets arising from the molecules containing 13C nuclei present in 1.108 % natural abundance. When the 13C is directly bonded to protons one observes a large doublet splitting from 13C-'H spin-spin interaction with components almost symmetrically disposed about the much more intense resonance band for the analogous protons bonded to "C atoms (for example in methane JcH is 125 c./sec.).In some molecules the introduction of a 13C atom results in nuclei which are magnetically equivalent in the non-13C- containing molecule becoming magnetically non-equivalent. For such mole- cules the 13CH satellite spectra often feature coupling constants which do not appear in the spectrum of the 12C-containing molecules. For example benzene shows a single absorption for the normal spectrum obtained for the "C-containing molecules. The molecules containing one 13C atom have nuclei which are magnetically non-equivalent such that we now have a seven-spin system.74 From a detailed analysis of the 13CH satellite spectra the ring proton-proton coupling constants can be deduced (Jortho7.54 J,,, 1.37 JpWa0.69 JcH 158.34 c./sec.).Hill and Roberts75 have examined the 13CH satellites of cyclobutene and their findings in conjunction with information from the 'H spectrum of deuteriated cyclobutene derivatives provide a more accurate set of coupling constants. It is of paramount importance to include non-bonded C-H coupling constants in the analysis of the 13CH satellite spectra ; a re-examination of the 13CH spectra of p-benzoquinone including such non-bonded coupling constants gave values for the H-H coupling constants very different from those reported previou~ly.~~ Finer and Harris77 67 K.S. Dhami and J. B. Stothers Canad. J. Chem. 1967,45,233. 68 D. H. Marr and J. B. Stothers Canad. J. Chem. 1967,45 225. 69 K. S. Dhami and J. B. Stothers Canad. J. Chem. 1966,44,2855. 70 H. L. Retcofsky and R. A. Friedel J. Phys. Chem. 1967,71,3592. 71 D. K. Dalling and D. M. Grant J. Amer. Chem. SOC.,1967,89,6612. 'I2 W. M. Litchman and D. M. Grant J. Amer. Chem. SOC. 1967,89,6775. 73 F. J. Wiegert and J. D. Roberts J. Amer. Chem. SOC. 1967,89,5962. 74 J. M. Read R. E. Mayo and J. H. Goldstein J. Mol. Spectroscopy 1967,22,419. 75 E. A. Will and J. D. Roberts J. Amer. Chem. SOC.,1967,89 2047. " G. Govil J. Chem. SOC.(A),1967 1416. 77 E. G. Finer and R. K. Harris Mol. Phys. 1967 13 65. 14 J. Feeney have analysed the 13CH satellites in systems which normally have high sym-metry (X,AA'X' systems) where the reduced symmetry allows one to extract coupling constants which are not featured in the normal spectrum.A unique set of spectral parameters has been obtained from analysis of the 13CH spectra of acetaldehyde diethyl acetal The 13C-H coupling con- stants for the two non-equivalent protons (a b) in the methylene group are not equal (JcH 141.01 and 139-64 c./sec.) which provides another criterion of non-equivalence in such structures. 3CH satellites of homodimers of thimine and dimethylthymine have provided coupling constants essential to the stereochemical characterisation of the molecules.79 From the 13CH spectrum of cyclopropane,gOit is possible to obtain Jgem-4.34 & 0.03; JCk8.97 & 0.01 ; J,,,, 5.58 & 0.01 c./sec.Oxygen-17. A review on the chemical applications of 170nuclear resonance has appeared recently.'' By using 170labelling one can study the reversible hydration and dehydration of acetaldehydeg2 and other carbonyl-containing' molecules. Thus in aqueous solution we obtain R'R2C(OH) + R1R2C=0 + H20 and by enriching the aliphatic carbonyls in 170and observing the intensities of the two different 170signals one can obtain the equilibrium constant for the above equilibrium. The carbonyl 170absorption is in the range -520 to -560 p.p.m. from H21 70,while the gem-diol has a similar chemical shift to that of water.83 170-enriched asymmetric fl-diketones give a broad 170band for the keto-tautomer while two bands are observed for the two non-equivalent oxygen nuclei of the enolic f01-m.~~ By measuring the chemical shifts of the enol peaks one can estimate the equilibrium constant for the equilibrium involving the two possible enolic tautomers PhCO-CH :C(OH)Me + PhC(0H):CH-CO-Me Nitrogen.Mathias and his co-workersa5 have used the heteronuclear double-resonance method to measure 14N chemical shifts in some thioamides " L. S. Rattet L. Mandell and J. H. Goldstein J.Amer. Chem. SOC.,1967,89 2253. 79 D. P. Hollis and S. Yi Wang J. Org. Chem. 1967,32 1620. V. S. Watts and J. H. Goldstein J. Chem. Phys. 1967,46,4165. 81 B. L. Silver and Z. Luz. Quart. Rev. 1967 458. P. Greenzaid Z. Luz and D. Samuel J. Amer. Chem. SOC.,1967,89 756. " P. Greenzaid Z.Luz and D. Samuel .I.Amer. Chem. SOC.,1967.89. 749. 84 M. Gorodetsky Z. Luz and Y.Mazur J. Amer. Chem. SOC., 1967,89 1183. 8s P. Hampson and A. Mathias Mol. Phys.. 1967 13 361. Physical Methods of Structure Determination and thia~oles.~~ 2- and 8-hydro~yquinolines,~~ In this method the 14N frequencies are measured by observing the effect on the NH proton spectrum of a second irradiating field operating at the frequency of the I4Nnucleus under investigation. Mathia~~~ showed that 2-amino- and 2-methylamino-benzo- thiazole exist in the amino-form (11) rather than the imino-form (12) and that for the 2-hydroxyquinoline studied the 0x0-form (13) rather than the hydroxy- form (14) exists.86 Witanowski has measured 14N chemical shifts by direct H (1 1) (1 2) H (13) observation ofthe nuclei in aromaticnitro-compounds,88 nit rile^,^' is on it rile^,^^ and nitro alkane^.^' In nitroalkanes the 14Nshifts are observed over a range of 70 p.p.m.;it was found possible to predict chemical shifts to 1 p.p.m. accuracy by using simple additivity rules for substituents CH, RCH, and C1 in the series R'R2R3C*N02.Witanowski has proposed two primary reference standards for I4N studies nitromethane for organic systems and the nitrate ion for aqueous systems.88 The chemical-shift difference between the two standards is zero. An extensive collection of '5N-H coupling constants for directly bonded nuclei and also for three- and four-bond coupling has been rep~rted.~' In the syn-isomers of oximes geminal JISN-Hvalues in the range 2-6-4.2 c./sec.have been observed while in the anti-isomers the range of values is from 14.2to 16.3 and is much larger because of the effects of the nitrogen l~ne-pair.~' 14N-l H coupling constants have been measured in alkylpyridinium halides" and in enammonium saltsg3 [for example in (CH,),N+CH* CH, JNHrrMs +5-6 JNHgem +3.6 JNHck +2*6 c./sec.]. The relative signs and solvent- 86 P. Hampson and A. Mathias Chem. Comm. 1967,371. '' A. Mathias Mol. Phys. 1967 12 381. M. Witanowski L. Stefaniak and G. A. Webb J. Chem. SOC. (B),1967 1065. 89 M. Witanowski Tetrahedron 1967,23,4299. 90 M. Witanowski and L. Stefaniak J. Chem. SOC. (B),1967 1061. 91 A. K. Bose and I. Kugajevsky Tetrahedron 1967,23 1489; J. P.Kintzinger and J. M. Lehn Chem. Cornrn..1967. 660. 92 J. F. Biellmann and H. Callot Bull. SOC. chim. France 1967,2 397. " J. M. Lehn and R. Seher Chem. Comm.. 1966. 847. 16 J. Feeney dependencies of "N-'H spin-coupling constants in ["N]quinoline its ethiodide and ["N]quinoline oxide have been mea~ured.'~ Solvent Effects.-Solvent effects on chemical shifts. Benzene-induced solvent effects have been extensively used in investigations of the 'H spectra of pyri- dines,g5 q~inolines,~~ pyrroles," indoles," ortho-and meta-substituted methoxybenzene~,'~ polymethoxy-substituted aromatic^,'^ three-membered ring carbonyl epoxides," ketonic derivatives of cyclopr~pane,~~ amines,' O0 and maleic anhydride. ' ' Further discussion of the mechanism of the benzene-induced solvent effects has been reported.lo' On the basis of evidence from freezing-point diagrams and the effects of aromatic solvents on chemical shifts of t-butyl and 1-adamantyl halides Fort and Lindstrom'03 have suggested that a specific 1 :1 complex is not formed but rather that there is a slight geometrical ordering of the solvent around the solute molecules with a rapid exchange between ordered and non-ordered solvent molecules; such an arrangement would give the n.m.r. spectrum expected for a long-lived species without being thermodynamic- ally equivalent to such a species. The usefulness of dimethyl sulphoxide as a solvent for hydroxy-compounds has been further exploited;'04 not only are exchange processes suppressed in this solvent but the observed hydroxy group chemical shifts become independ- ent of solvent at low concentrations of the alcohol.Solvent effects on halogenopr~penes'~~ and substituted pyridinesio6 have also been studied. Abraham and Cooper'07 have extended their electrostatic theory of medium effects to account for the effect of the medium on the energy differences between rotational isomers of any solute molecule; in the case of 1,1,2-trichloroethane the theory gives good agreement with the experimental values of the energy difference between rotational isomers in the liquid and vapour states. Homer'" has described an experimental method for determining the contribution to the shielding from the magnetic anisotropy of the solvent. Solvent effects on coupling constants.Because coupling constants are in- sensitive to anisotropic effects it is possible that studying solvent effects of coupling constants (such as JcH)might provide a better way of investigating 94 K. Tori M. Ohtsuru K. Aono Y. Kawazoe and M. Ohnishi J. Amer. Chem. SOC.,1967,89,2765. 95 J. Ronayne and D. H. Williams J. Chem. SOC.(B) 1967 805. 96 J. H. Bowie J. Ronayne and D. H. Williams J. Chem. SOC.(B) 1967,535. 9' H. M. Fales and K. S. Warren J. Org. Chem. 1967,32 501. 98 D. W. Boykin A. B. Turner and R. E. Lutz Tetrahedron Letters 1967,817. 99 J. Seyden-Penne D. Arnaud J. L. Pierre and M. Plat Tetrahedron Letters 1967,3719. loo D. J. Barraclough P. W. Hickmott and 0.Meth-Cohn Tetrahedron Letters 1967,4289. lol C. Ganter L. G.Newman and J. D. Roberts Tetrahedron 1966 Supplement No. 8 507. lot J. Ronayne and D. H. Williams J. Chem. SOC.(B) 1967,540. lo3 R. C. Fort and T. R. Lindstrom Tetrahedron 1967,23,3227. lo4 R. J. Ouellette D. L. Marks and D. Miller J. Amer. Chem. SOC. 1967,89,913. lo' F. Hruska D. W. McBride and T. Schaefer Canad. J. Chem. 1967,45 1081. loci R. J. Chuck and E. W. Randall J. Chem. SOC.(B),1967,261. lo' R. J. Abraham and M. A. Cooper J. Chem. SOC.(B) 1967,202. lo* J. Homer Tetrahedron 1967,23,4065. 17 Physical Methods of Structure Determination solute-solvent interactions than by studying chemical-shift changes with solvent.'0g J in bromoform has been measured in 30 different solvents (204.31 c./sec. in cyclohexane 211.60 in dimethylformamide) and the solvent effects on J values for thirteen other substituted methanes have been also reported.The results can be explained in terms of specific interactions such as hydrogen-bonding.'Og Weak hydrogen-bonding effects (self-association) have also been invoked to explain the changes in JcHin chloroform with changes in temperature [observed changes from 20943-209.0 c/sec. from -61" to +58" (vapour phase)]. '' The solvent dependence of H-F coupling constants in trifluoroethylene and vinyl chloride has been measured :I1' the orientation of the solute dipole strongly affects the solvent-dependencies of the JHFgem values. Electric-field effects from the solvent appear to be the main factor controlling the changes in coupling constants but the strong molecular associations will alsocontribute.Hutton and Schaefer1l2 have found that solvent effects on JHFck and JHHgem in 1-chloro-1-fluoroethylenedepend primarily on the dielectric constant of the medium. JHForrho and (JHFortho + JHFnreta) in substituted fluorobenzenes increase algebraically as the dielectric constant of the medium increases.' ' Small changes of the coupling constants hetween ring protons in nitro- aromatics have been measured on changing solvents (largest change is +0.14 & 0.1 c./sec.).114 Kinetic Studies.-The most notable contributions to this field have been contained in papers dealing with the validity of the various methods of studying kinetic processes using n.m.r. studies. Anet and Bourn"' have made a careful study of the kinetic parameters for changes in the conformation of C2H11]-cyclohexane by two different methods namely full line-shape analysis and the double-resonance method of Hoffman and Forsen.' ' The line-shape analysis was carried out at different temperatures with the deuterium nuclei decoupled ; thus the system is a simple problem involving two equally populated sites undergoing exchange between axial and equatorial positions.Both methods gave values of AF* AH* and AS* for the chair-to-boat process in good agreement with each other. However the AS* result is different from that obtained by a third method involving spin-echo measurements.' ' Sheppard and Harris'I8 have drawn attention to the difficulties encountered in deter- mining meaningful values of entropy changes from n.m.r.studies of ring- inversion in cyclohexane. log V. S. Watts and J. H. Goldstein J. Phys. Chem. 1966,70,2887. 'lo A. W. Douglas and D. Dietz J. Chem. Phys. 1967,445 1214. S. L. Smith and A. M. Ihrig J. Chem. Phys. 1967,445 1181. 'I2 H. M.Hutton and T. Schaefer Canad. J. Chem. 1967,45,1111. 'I3 H. M. Hutton B. Richardson and T. Schaefer Canad. J. Chem. 1967,45 1795. S. L. Smith and A. M. Ihrig J. Mol. Spectroscopy 1967,22 243. 'I5 F. A. L. Anet and A. J. R. Bourn J. Amer. Chem. SOC. 1967,89 760. R. A. Hoffman and S. Forsen J. Chern. Phys. 1963,39,2892. 'I7 A. Allerhand F. Chen and H. S. Gutowsky J. Chem. Phys. 1965,42,3040. N. Sheppard and R. K.Harris J. Mol. Spectroscopy 1967,23,23. J. Feeney Mannschreck and his co-workers' '' have measured the rates of internal rotation around C-N amide bonds in the rotamers (15) and (16) of N-benzyl- N,2,4,6-tetramethylbenzamideby two different methods namely line-shape analysis and by an equilibration method.The equilibration method involves isolating form (15) as a pure rotamer and then observing the formation of form (16) with time from its n.m.r. spectrum. The activation parameters obtained by the two methods are in good agreement. Gutowsky and his co-workers'20 showed concurrently that there was good agreement between the parameters obtained from complete line-shape analysis and an equilibration method for the rate of internal rotation about the C-N amide bond in N-benzyl-N-methyl formamide. Thus we now have experimental confirmation of the validity of the method of line-shape analysis for studying kinetic processes.There has been an interesting use of line-shape analysis on the 19'Hg-H proton satellite spectra of dimethylmercury to study intermolecular exchange of methyl 122 In the presence of aluminium chloride catalyst the exchange is very rapid and the satellite bands disappear completely which indicates the exchange of methyl groups to be an intermolecular process.'21 Inability to detect 33S-'9F satellites in the "F spectrum of sulphur tetra- fluoride has been advanced as evidence for the existence of an intermolecular rather than intramolecular fluorine exchange process. 123 The large number of n.m.r. studies of intramolecular rate processes have included the following.Ring-inversion in 1,1-difluorocyclohexanes,' 24 ~ycloheptane,'~' cyclohep- tene,'25 and heterocyclic analogues of metacyclophane (17).'26 Internal rotation about the following bonds :phenyl-oxygen bond in phenolic esters,127 the ==C-N bond in enamines,'28~'29 the =N-N bond in hydra- A. Mannschreck A. Matthews and G. Rissmann J. Mol. Spectroscopy 1967,23 15. 120 H. S. Gutowsky J. Jonas and T. H. Siddall J. Amer. Chem. Soc. 1967,89,4300. N. S. Ham E. A. Jeffery T. Mole and S. N. Stuart Chem. Comm. 1967,254. D. N. Ford P. R. Wells and P. C. Lauterbur Chem. Comm. 1967,616. 123 E. L. Muetterties and W. D. Phillips J. Chem. Phys. 1967,46 2861. 124 S. L. Spassov D. L. Griftith E. S. Glazier K. Nagarajan and J. D. Roberts J. her. Chem. SOC.1967,89 88. R. Knorr C. Ganter and J. D. Roberts Angew. Chem. Internat. Edn. 1967,6 556. 126 I. Gault B. J. Price and I. 0.Sutherland Chem. Comm. 1967 540. 12' T. H. Siddall W. E. Stewart and M. L. Good Canad. J. Chem. 1967,45 1290. lZ8 A. Mannschreck and U. Koelle Tetrahedron Letters 1967,863. Y.Shvo E. C. Taylor and J. Bartulin Tetrahedron Letters 1967 3259. Physical A4 ethods of Structure Determination 19 zones,12s the N-0 bond in ON-diacylhydroxylamines (18),I3O the N-N bond in NN'-diacylhydrazines (19),' ' the aryl-nitrogen bonds in substituted and the N-N bond in heterocyclic nitro~amines.'~~ amide~,'~~ Studies of hindered rotation have also been carried out on NN-dimethylcarbamates,' 34 amides,' 35 thioamides,' 35 and nitroaromatic amines.' 36 Dimethylnitrosamine (20) has been studied in the vapour state at various temperatures to provide R ,co.,N-0 R"C0 separate estimates of the intra- and inter-molecular contributions to the barrier to internal rotation about the N-N bond.'37 The vapour-state results agree well with those observed in dilute solutions in carbon tetrachloride. Nitrogen-atom inversion in 1,2,6-trimethylpiperidine where the nitrogen inversion is found to be three orders of magnitude slower than that in tertiary acyclic arnine~.'~' An examination of the 'H n.m.r. spectrum of l-alkyl- aziridine (21) at various temperatures confirms that steric factors accelerate the inversion but by a much smaller amount than previously th0~ght.I~' Synchronous inversion at two bonded nitrogen atoms in ring compounds (22) has been further in~estigated.'~' Valency isomerism in 1,2-divinylaziridine (23) can be conveniently studied using n.rn.r.l4' 13' B.J. Price and 1. 0.Sutherland Chrm. Comm.. 1967 1070. 131 G. J Bishop B. J. Price and I. 0.Sutherland. Chrm. Comm.. 1967 672. 13' Y Shvo. E. C. Taylor. K Mislow. and M. Rahan. .I Amer rlicm SOC. 1967.89.4910 I" Y. L. Chow Angew. Chem. internat. Edn. 1967,6,75. 134 E. Lustig W. R. Benson and N. Duy J. Org. Chem. 1967,32 851. 13' R. C. Neuman D. N. Roark and V. Jonas J. Amer. Chem. SOC.,1967,89,3412. 136 J. A. Weil A. Blum A. H. Heiss and J. K. Kinnaird J. Chem. Phys. 1967,46 3132. 13' R. K. Harris and R. A. Spragg Chem. Comm. 1967,362. J. J. Delpuech and M. N. Deschamps Chem.Comm. 1967 1188. S. J. Brois J. Amer. Chem. SOC. 1967,89,4242. J. E. Anderson and J. M. Lehn 1.Amer. Chem. SOC. 1967,89,81. E. L. Stogryn and S. J. Brois J. Amer. Chem. SOC.,1967,89 605. J. Feeney Conformational Studies.-Conformational studies using n.m.r. have often relied on the fact that t-butylcyclohexyl systems when used as model com- pounds are assumed to give chemical shifts for the remote ring-protons which are the same as those in a related system without the t-butyl group. However this assumption has been shown to be in~0rrect.l~~ Thus attempts to obtain conformational parameters by relating averaged chemical shifts of a proton in a mobile cyclohexane system to those observed in the related fixed t-butyl- cyclohexyl derivatives may not always be justified.To study interconverting systems under conformational equilibrium one needs to estimate the parameters in the individual conformers. Some of the problems involved in this process can be circumvented by a method which combines dipole-moment data with coupling-constant data to obtain informa- tion about mobile ring systems.'43 This is based on the existence of a linear correlation between the squares of the dipole moments and the sum of the vicinal coupling constants JAx+ J, in a series of conformationally inverting ring compounds with similar geometry and similar polar substituents [for example a series of trans-1,2-dihalogenocyclohexanes(24)l. These problems do not arise when one can measure the parameters in the spectra of the indi- vidual conformers by freezing out the separate conformers.For example the conformational equilibria found in trans- 1,4-dibromo- trans-1,4-dichloro- and trans-1 -bromo-4-chloro-cyclohexane can be determined with confidence by integrating the separate axial and equatorial proton signals observed in the low-temperature spectra of the molecules. 144 The conformational preference of the trifluoromethyl group in cyclohexanes has been measured. 14' At -73" cis-4-methyl-1-trifluoromethylcyclohexane shows two CF fluorine absorption bands corresponding to CF,-axial ($cp3 66.63 p.p.m. from CFCl,) and CF,-equatorial (+cF3 74.5 p.p.m.) positions in the two frozen-out conformers. It is suggested that the difference between the two CF groups could be exploited in assigning the configuration of carboxy- substituents at centres of unknown stereochemistry in rigid molecules ; the suggested method consists of converting the carboxy-group into a trifluoro- methyl group using SF, which is an easy and selective process.The CF group thus formed would reveal its configuration through its "F chemical shift.',' In six-membered rings of type (25) the ratio of the average vicinal coupling S. Wolfe and J. R. Campbell Chem. Comm.,1967,872. 143 C. Altona H. R. Buys H. J. Hageman and E. Havinga Tetrahedron 1967,23,2265. 144 G. Wood and E. P. Woo Canad. J. Chem. 1967,45,2477. 14' E. W. Della J. Amer. Chem. Soc. 1967,89 5221. Physical Methods of Structure Determination J,,,, to the average value of J, is independent of the electronegativity of X and Y.146In molecules which adopt a perfect chair conformation this ratio has a value of ca.2.0. Deviations from this value have been used to indicate devia- tions to non-chair and distorted-chair conformations in a qualitative fashion. By examining cyclohexanols and pyranose derivatives in dimethyl sulphoxide solution stereospecific long-range coupling constants involving hydroxy- protons can be observed and used in conformational analysis.'47* 148 For trans-and cis-4-methylcyclohexanol,the respective values for JCHoHare 4.64 and 3.60 c./s~c.'~~ The small differences in the values render this method more difficult to use for conformational studies than some of the other methods. Conformational studies have been carried out on 1,1,4,4-tetramethyl-cyclohexanes,' 49 protonated methylcyclohexanones,' 50 tetrahydropyran,' 1,3-dioxan,'52 1,3-0xathiolans,'~~ cyclobutanes,' 54 cycl~butanols,'~~ and cyclo but ylamines ' ' and on hexahydro-1,3,5- trimethyl- 1,3,5-triazine-S- tri -oxan.Carbonium Ions.-A review article on n.m.r. studies of carbonium ions has ap~eared.'~' Olah and his co-~orkers'~~-'~~ have published a most extensive systematic study of protonated organic molecules all examined in the same protonating media. By using the strong acid system FSO,H-SbF dissolved in sulphur dioxide solution at -60° OlahlS8 and Wein~tein'~' with their co-workers have protonated ethers,' aliphatic alcohols 160 aldehydes,lS9 ketones,16' carboxylic acids,' 58 thiols and sulphides,162 aldimines and keti- mines,'63 and other organic molecules.When methyl alcohol is treated in 146 J. B. Lambert J. Amer. Chem. SOC. 1967,89 1836. 147 J. J. Uebel and H. W. Goodwin J. Org. Chem. 1966,31,2040. 14' J. C. Jochims G. Taigel A. Seeliger P. Lutz and H. E. Driesen Tetrahedron Letters 1967,4363. 149 R. W. Murray and M. L. Kaplan Tetrahedron 1967,23 1575. 150 T. D. J. D'SiIva and H. J. Ringold Tetrahedron Letters 1967 1505. G. Gatti A. L. Segre and C. Morandi J. Chem. SOC. (B),1967 1203. 15' J. E. Anderson F. G. Riddell and M. J. T. Robinson Tetrahedron Letters 1967,2017. 153 D. J. Pasto F. M. Klein and T. W. Doyle J. Amer. Chem. SOC.,1967,89 4368. 15' G. M. Whitesides J. P. Sevenair and R. W. Goetz J. Amer. Chem.SOC. 1967,89 1135. Is' I. Lillien and R. A. Doughty J. Amer. Chem. SOC. 1967,89 156. 15' H. S. Gutowsky and P. A. Temussi J. Amer. Chem. SOC. 1967,89,4358. 157 H. Cheradame and G. Mavel Ann. Chim. France 1966,1,449. Is' G. A. Olah and J. M. Bollinger J. Amer. Chem. SOC. 1967 89 2993; G. A. Olah and D. H. O'Brien ibid. p. 1725. M. Brookhart G. C. Levy and S. Winstein J. Amer. Chem. SOC. 1967,89 1735; G. A. Olah D. H. O'Brien and M. Calin ibid. p. 3582. 160 G. A. Olah J. Sommer and E. Namanworth J. Amer. Chem. SOC. 1967,89 3576. 16' G. A. Olah M. Calin and D. H. O'Brien J. Amer. Chem. SOC.,1967,89,3586. 162 G. A. Olah D. H. O'Brien and C. U. Pittman J. Amer. Chem. SOC. 1967,89,2996. 163 G. A. Olah and A. M. White J. Amer. Chem. SOC. 1967,89 3591 ;G. A.Olah and P.Kreien Buhl ibid. p. 4756. 22 J. Feeney this way the species CH,OH,+ (FOH2' -9.4 p.p.m.) is obtained because exchange rates are negligible at -60" in these systems one can measure the coupling JHH 3.6 c./sec. At temperatures above +60° the protonated alcohols break down to form carbonium ions (R++ H30+),and the kinetics of the cleavage have been studied by n.m.r. for several alcohols.160 For aliphatic aldehydes tiOH+ values for the protonated species are in the range -15 to -17.p.pm. In protonated acetaldehyde at -60° one can detect two forms of the species which differ in that the added proton can be either cis or trans to the hydrogen on the carbonyl carbon atom.'" For protonated ketones16' tiOH+ values in the range -13 to -15 p.p.m.are observed while for protonated acetone the values are tiOH+ -14.93 p.p.m. and J 1.0 c./sec. The parameters for protonated thiols'62 and alkoxy carbonium ions'58 are CH3SH2+,ti,, 6.45 p.p.m. JHH 8.0 c./sec.; CH30CH2+,tiCHz+ 9.94 p.p.m. J 1.0 c./sec. The long-range coupling J 1.0 c./sec. observed for CH30CH2+is much larger than the values found normally in ethers which suggests the formation of an sp2-centre as indicated in structure (26). a+,Me 8 Liquid-crystal Studies.-A liquid-crystal solvent in the nematic phase has many of its molecules aligned with respect to themselves in domains; in a magnetic field these domains are homogeneously ordered to give an even more ordered system. In 1963 Saupe and Englert'64 demonstrated that molecules dissolved in the nematic phase of a liquid-crystal solvent are themselves aligned to a high degree in the magnetic field.The n.m.r. spectra of molecules examined in this way are substantially different from normal n.m.r. spectra and it is possible to obtain information which is not available in normal solvents where we do not see any anisotropic magnetic interactions (those dependent on orientation of the molecules with the magnetic field). The observed absorption bands are well resolved because although the molecules are ordered they do not lose their translational motion which averages out intermolecular dipole- dipole interactions. Absorption bands extending over a range of several thousand cycles per second are observed and these can be explained in terms of direct dipole-dipole intramolecular interactions which we do not see normally in a high-resolution n.m.r.spectrum. Molecules such as cyclopropane which have magnetically equivalent nuclei and show only a single absorption when examined in an isotropic solvent give rise to complex spectra when examined in the nematic phase of a solvent such as 4,4'-di-n-hexyloxyazoxy-benzene.'65 From a detailed analysis of the spectrum'65 one can extract the absolute signs and magnitudes of JHHck +9.5 & 1 and JHHtrans +55 & 1.0 164 A. Saupe and G. Englert Phys. Rev. Letters 1963 11 462. 165 L. C. Snyder and S. Meiboom J. Chem. Phys. 1967,47 1480. Physical Methods of Structure Determination c./sec. The 13CHsatellite spectra in the nematic-phase spectrum have also been analysed.By assuming the symmetry of cyclopropane to be D, and that the C-C bond length is 1.510A one can calculate the C-H bond length 1.123 and the HCH angle 114.4".In addition to obtaining information concerning relative bond lengths bond angles absolute signs and magnitudes of indirect spin-spin coupling constants dipole-dipole interactions and chemical-shift anisotropies one can also use the technique for studying the nature of the nematic phase of liquid-crystal solvents. Excellent review articles dealing with the theory and application of liquid-crystal solvents in n.m.r. spectroscopy have been written.'66-'68 Here we will be content to examine the requirements for obtaining such spectra and the extent of the new information available therein.A typical liquid-crystal material which has been used is p-azoxyanisole which has a temperature range for the nematic phase of 118-135"c. To obtain high-quality spectra it is necessary to eliminate temperature gradients in the sample and to maintain the temperature constant to +0.1" if possible. It is also desirable to ensure that the sample is homogeneous with regard to its composition. A significant advance in the experimental technique was provided by the discovery that eutetic mixtures of various liquid-crystals can be chosen such that the mixture is in the nematic phase at the normal probe operating temperat~re.'~'~ For example a 4:1 mixture of (27) and (28) has a melting point of 29"and a clearing point of 78".16' (28) It is usual to examine the samples in the non-spinning condition because rapid rotation would destroy the molecular alignment with the magnetic field.However it has been shown recently"' that slow rotation (5-10 c./sec.) appears to improve resolution without interfering with the other properties of the spectrum. A new kind of nematic phase has been described recently. This is formed from a mixture of c8 or Cl0 alkyl sulphates the corresponding alcohol sodium sulphate and water in the proportions 40 :5 :5 :50 respec-ti~e1y.l~~ The nematic-phase temperature range is from 10 to 75"c and it is 166 A. D. Buckingham and K. A. McLauchlan Progr. N.M.R. Spectroscopy 1967,2 16' G.R. Luckhurst Quart. Rev. 1968 in the press. 16* G. R. Luckhurst &err.Chem.-Ztg 1967,68 113. 169 H. Spiesecke and J. Bellion-Jourdan Angew. Chem. Internat. Edn. 1967,6,450. 170 D. Demus 2.Naturforsch. 1967,22a 285. E. E. Burnell and C. A. de Lange personal communication. K. D. Lawson and T. J. Flautt J. Amer. Chem. SOC.,1967,89 5489. J. Feeney possible to spin the samples to improve resolution. Some of the systems which have been examined recently by the liquid-crystal technique are cyclopro- pane,165 cy~lobutane,'~~ (JHHet + 10.4,JHHtrnns +4.9c./sec.) methyl fluoride'74 (J13cHis shown to be absolutely positive J13cFnegative JHFyem positive) and ethyl iodide' " (the spectrum was analysed using a modified composite- particle technique). Saupe and his co-workers have examined 3C-labelled a~etonitrile,'~~ methyl iodide,176 methan01,'~~ acetylene,177 and several acetylenic compounds1 77 by this method.Buckingham and Burnell' 78 have studied chemical-shift anisotropies in oriented molecules. They recommend using molecules of high symmetry (CF, CH, SF,) as internal reference materials and advocate caution in interpreting small shifts observed on passing from nematic to isotropic phase of liquid-crystal solution. Bernheim and Krugh' 79 found that they could determine "F chemical-shift anisotropies with reasonable accuracy but it was more difficult in the case of protons. Spectral Assignments from Nuclear Overhauser Effects.-In 1965 Anet and Bourn'" illustrated how nuclear Overhauser experiments can be used to determine which nuclei in a molecule are in close spatial proximity to each other by virtue of their being connected through an intramolecular spin-lattice relaxation mechanism.For this experiment to be successful it is necessary that the intramolecular effects should dominate the relaxation ;thus intermolecular relaxation effects must be minimised by efficient degassing of the system and by using solvents which do not contain high-magnetic-moment nuclei. For two protons A and B in close spatial proximity (the intramolecular relaxation is inversely dependent on the sixth power of the internuclear distance) if this relaxation mechanism is the dominant one then when one saturates protons A in a double-resonance experiment there is an increase of as much as 50 % in the integrated intensity of the band for the B nuclei.Thus if the assignment of absorption A is certain then it is possible to infer the assignment for the B nuclei. This technique provides an additional powerful aid in spectral assignment and is becoming used more widely. For example Nouls and his co-workers18 ' have examined the n.m.r. proton spectrum of a mixture of the two isomers (29) and (30)by this method. Irradiation of (29)at the CH frequency causes a 30 % increase in intensity of HA which identifies the absorptions for both CH and HA protons in (29) where the nuclei are in close spatial proximity. This experi- ment was successful even though trifluoroacetic acid was used as the solvent. 173 S. Meiboom and L. C. Snyder J. Amer. Chem. SOC.,1967,89 1038. 17* R.A. Bernheim and B. J. Lavery J. Amer. Chem. SOC.,1967,89 1279. C. M. Woodman Mol. Phys. 1967,13,365. 17' A. Saupe G. Englert and A. Povh 'Advances in Chemistry Series' 1967 vol. 63 p. 51. G. Englert A. Saupe and T. P. Weber 2.Naturforsch. 1968,22a 152. A. D. Buckingham and E. E. Burnell J. Amer. Chem. SOC.,1967,89,3341. 179 R. A. Bernheim and T. R. Krugh J. Amer. Chem. SOC., 1967,89,6785. 180 F. A. L. Anet and A. J. R. Bourn J. Amer. Chem. SOC., 1965,87,5250. J. C. Nouls G. Van Binst and R. H. Martin Tetrahedron Letters 1967,4065. Physical Methods of Structure Determination Similar experiments have been used to distinguish between not only stereo- isomers but also individual conformers of a single compound. For example 7-methoxy-7,12-dihydropleiadeneexists as a 2:1 mixture of axial and equa- torial conformers at -2W (31)and (32).Ig2When the C-12hydrogen (axial) in (32) is irradiated the C-7axial proton signal increases in intensity by 27%.Similar experiments on (31)give no increase in intensity. H OMe Hx: \ Woods and his co-worker~'~~ have also reported work on the ginkgolides (33)where irradiation at the t-butyl proton frequency causes the absorptions of the I J E and F protons in close spatial proximity to increase their intensity and thus confirms the assignments. This experiment was successful even though the solvent was trifluoroacetic acid; it is suggested that the bulky nature of the t-butyl group isolates the protons I J E and F from close approach of the solvent molecules.Novel Techniques.-The problem of resolution enhancement has received considerable attention during the last year. Four methods of measuring a small long-range coupling constant in 3-bromothiophen-2-aldehyde have been c~nsidered.'~~~ 18' (i) Trial and error fitting of the observed multiplet profile by a set of component lines each having the line-shape taken from a known single in the spectrum. (ii) A linear transformation (referred to as convolution) of the observed curve using a function which has such a form that it enhances resolution at the expense of sensitivity. (iii) Examination of Fourier transform lS2 J. G. Colson P. T. Lansbury and F. D. Saeva J. Amer. Chem. SOC.,1967,89,4987. "' M. C. Woods I. Miura Y. Nakadaira A. Terahara M.Maruyama and K. Nakanishi Tetra-hedron Letters 1967,321. R. R. Ernst personal communication. R. R. Ernst R. Freeman B. Gestblom and T. R. Lusebrink Mol. Phys. 1967,13,283. J. Feeney spectra. (iv) An analogue transformation taking place within the spectrometer subtracting a suitable portion of the second derivative from the slow-passage absorption signal. Freeman and Gestblom'86 have discussed anothu method of measuring very small coupling constants using a double-resonance technique ; this is based on the fact that two nuclear sites within the same molecule will experience almost exactly correlated local fields due to field G-ihomogeneity. Double- resonance effects which result from interactions on the molecular scale can be used to take advantage of this fact.Freeman and Gestblom' 87 have shown that fine-structure can be transferred from one resonance absorption to another using double-resonance techniques. If one of the X-lines in an AX-type spectrum is irradiated with a weak field (yxH2/27c 4I JAxI) then each of the A-lines is split into a doublet (spin- tickling); if there is any fine-structure on the X resonance due to coupling to a third group of nuclei Z and if yxH,/2x + I Jxz(,the same fine-structure is observed on the two components of the A-doublet except that the splitting is reduced by a factor of 2. This method has been used to detect very small long-range coupling constants ( +0.05 c./sec.). Reports have appeared concerning further extensions to the method of Hoffman and Forsen' l6 for measuring kinetic processes using double-resonance techniques.When this was used to study the intramolecular reversible slow process in p-nitrosodimethylaniline (34) on irradiation at the H-3 Me M,Me absorption frequency not only the signal from the H-5 proton collapses but also those from H-2 and H-6. This is because H-5 and H-6 have almost identical chemical shifts and being strongly coupled lose their identity as separate nuclei; H-2 and H-6 are interchanged by rotation about the N-aryl bond which explains why H-2 also disappears.'88 Comparatively few papers dealing with high-field (220 Mc./sec.) proton spectra have appeared. '89-1 ' The conformational differences between native and denatured forms of ribonuclease lysozyme and cytochrome C have been investigated.' 89 The authors detected high-field resonances (6 0.7 p.p.m.) in the folded native forms which are absent in the denatured forms because of absence of ring-current effects on the shielding of a certain CH proton in the la6 R.Freeman and B. Gestblom J. Chem. Phys. 1967,47,2744. la' R. Freeman and B. Gestblom J. Chem. Phys. 1967,47 1472. I. C. Calder P. J. Garratt and F. Sondheimer Chem. Comm. 1967,41. lag C. C. McDonald and W. D. Phillips J. Amer. Chem. SOC., 1967,89 1967. C. C. McDonald W. D. Phillips and J. Lazar. J. Anzer. Chem. SOC.,1967,89 4166. 191 N. S. Bhacca and D. Horton Chem. Comrn. 1967 867. Physical Methods of Structure Determination denatured form where the nucleus is no longer in close proximity to an aro- matic ring.In the 220 Mc./sec. 'H spectrum of single-stranded deoxyribo- nucleic acid (DNA),there are two separate thymine methyl absorptions (one for those near to a purine and the other for those near to a pyrimidine neighbour in the 5'-po~ition).'~' By examining enantiometers such as alkylarylcarbinols in optically active solvents the absolute configurations of several enantiomers have been assigned.Ig2 Deuterium resonance has been used to study the mechanism of nucleophilic substitution reactions. 193 35Cl ions have been used as chemical probes in molecules of biological interest by observing 35Cl line-width vari-ations under different condition^."^ This latter method should prove to be a useful technique for investigating biological systems.Burgen and his co-workers have studied line-width variations in 'H spectra to investigate hapten- antibody complex formation. lg5 Miscellaneous Studies.-The limitations of the sub-spectral method of analysis have been discussed in some detail.'96 '97 Hydroxychromenes (39-437) have been characterised by observing the effects of acetylation on the chemical shifts of neighbouring proton^."^ Thus only in structures (35) and (37) does acetylation cause deshielding of the peri-Ha by 0-3-0-4 p.p.m. From examination of an extensive series of 3-aminoacrylic esters (38) J,, proton-proton coupling constants ranging from 7-5 to 15.0 c./sec. have been observed.' 99 Some other compounds examined by n.m.r. were cyclopropanes200-202 192 W.H. Pirkle and S. D. Beare J. Amer. Chem. SOC.,1967,89 5486. 193 L. K. Montgomery A. 0.Clouse A. M. Crelier and L. E. Applegate J. Amer. Chem. SOC. 1967,89,3453. 19' R. L. Ward and J. A. Happe Biochem. Biophys. Res. Comm. 1967,28,785. 195 A. S. U. Burgen 0.Jardetzky J. C. Metcalfe and N. Wade-Jardetzky Proc. Nar. Acad. Sci. U.S.A. 1967,58,447. 196 P. Diehl and D. Trautmann MoZ. Phys. 1966,11,531. 19' P. Diehl and P. Weissenhorn Helv. Chim. Acta 1967,50 143. 198 A. Arnone G. Cardillo L. Merlini and R. Mondelli Tetrahedron Letters 1967,4201. W. Bottomley J. N. Phillips and J. G. Wilson Tetrahedron Letters 1967 2957. 'O0 D. T. Longone and A. H. Miller Chem. Comm. 1967,447. '01 K. L. Williamson and B. A. Braman J. Amer. Chem. Soc. 1967,89,6183.'O' J. Lee C. Parkinson P. J. Robinson and J. G. Speight J. Chem. SOC. (B) 1967,1125. 28 J. F eeney RHN ,R' H/C=C;R" (38) (the shielding effects of alkyl substituents have been summarised) dia~oles,~'~ tetrazole~,''~ pyra~oles,~'~ triaz~les,~'~*''~ methyl silane derivatives,206 nucleo~ides,~~~ substituted styrenes,2 polyurethanes,208 quin~xalines,~'~ helix-coil transformations in polypeptides,' and histidine residues in proteim2l2 203 G. B. Barlin and T. J. Batterham J. Chem. SOC.(B) 1967 516. '04 W. Freiberg C. F. Kroger and R. Radeglia Tetrahedron Letters 1967 2109. '05 C. L. Habraken H. J. Munter and J. C. P. Westgeest Rec.Trao.chim. 1967,86 56. E. A. V.Ebsworth and S. G. Frankiss Trans. Faraday SOC. 1967,63 1574. 207 R.J. Cushley. K. A. Watanabe and J. J. Fox J. Amer. Chem. SOC. 1967,89,394. '08 E. G. Brame R. C. Ferguson and G. J. Thomas Analyt. Chem. 1967,39,517. 209 P. J. Brignell A. R. Katritzky R. E. Reavill G. W. H. Cheeseman and A. A. Sarsfield J. Chem. SOC.(B) 1967 1241. 0.Gurudata J. B. Stothers and T. D. Talman. Canad. J. Chem. 1967,45 731. 2'1 J. A. Ferretti Chem. Comm. 1967 1030. J. H. Bradbury and P. Wilairat Biochem. Biophys. Res. Comm. 1967,29 84.

ISSN:0069-3030    

DOI:10.1039/OC9676400005

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

2. Part (ii) ELECTRON SPIN RESONANCE By A. Horsfield (Varian Research Laboratory Walton-on- Thames Surrey) OF the books and reviews on electron spin resonance (e.s.r.) appearing in 1967 the book by Carrington and McLachlan' is especially noted. It gives a clear account of the principles and theory of magnetic resonance and it is illustrated by a wide variety of applications in chemistry. Aromatic radical- ions in solution have been treated by Gerson' and Bowers,3 and both authors give extensive compilations of data with splitting constants and unpaired spin densities. Geske4 has summarised important work in studies of conformation and structure determination by e.s.r. There is a review of applications of the technique to free-radical studies in solid polymers,' and other general reviews have Experimental techniques in e.s.r.are explained in books by Assenheim' and Poole." Finally an atlas of e.s.r. spectra' ' from free radicals in solution and in solids in which preparative details hyperfine splitting constants and references are listed should prove to be useful in many laboratories. Free Radicals in Solution.-The greatest effort in e.s.r. work of interest to the organic chemist continues to be devoted to investigations of free radicals in the liquid phase. Many new radicals in solution have been reported and their hyperfine structures analysed. In most cases there is little difficulty in reconciling the spin density distribution of the unpaired electron orbital as determined from the measured hyperfine splitting constants using the McConnell relationship1 (for protons) and the Karplus-Fraenkel expression' (for 3C I4N etc.) with the unpaired spin density distribution calculated from molecular orbital (MO) theory using the Hiickel and M~Lachlan'~ approxi-mations.Non-alternant hydrocarbon radical-ions remain a problem however ' A. Carrington and A. D. McLachlan 'Introduction to Magnetic Resonance,' Harper and Row New York 1967. F. Gerson 'Hochauflosende ESR-Spektroskopie,' Verlag Chemie Weinheim 1967. K. W. Bowers Ado. Magn. Resonance 1965,l. D. H. Geske Progr. Phys. Org. Chem. 1967,4 125. ' S. E. Bresler and E. N. Kazbekov Russ. Chem. Rev. 1967,36 298. A. H. Maki Ann. Rev. Phys. Chem. 1967,18,9. ' N. M. Atherton A. J. Parker and H. Steiner Ann.Reports 1966,63 62. A. Horsfield Ann. Reports 1966,63 257. H. M. Assenheim :Introduction to Electron Spin Resonance,' Hilger London 1967. lo C. P. Poole jun. 'Experimental Techniques in Electron Spin Resonance,' Wiley London 1966. l1 B. H. J. Bielski and J. M. Gebicki 'Atlas of Electron Spin Resonance Spectra,' Academic Press New York 1967. l2 H. M. McConnell J. Chem. Phys. 1956,24 764. M. Karplus and G. K. Fraenkel J. Chem. Phys. 1961,35 1312. l4 A D. McLachlan Mol. Phys. 1960,3,233. 30 A. Horsfzeld and Mobius and Plato" conclude that the full self-consistent-field MO methodI6 is necessary to calculate satisfactory welectron spin distributions in these cases. For the dibiphenylene ethylene cation consideration of steric hindrance improves the agreement with e~periment.'~ The normal seven-line hyperfine spectrum of the benzene anion made by sodium reduction in solution changes with increase in temperature to a 15-line spectrum with a triplet splitting of 6.5 G and a quintet splitting of 1.7 G.Since this change may be reversed by further contact with sodium at low temperature it is suggested" that the unpaired electron becomes stabilised in the symmetric antibonding 7c-orbital (for which triplet and quintet splittings of 7.2~ respectively are and 1.9~ predictedI8 by Huckel MO theory) in a permanent Jahn-Teller distortion. The reason why the degeneracy between the symmetric and antisymmetric anti- bonding orbitals should be lifted in this case is not clear. Reinterpretation of the spectrum of the pentacene cation confirmed that the pairing theorem for cation and anion radicals of alternant hydrocarbons is not violated by penta- cene as previously suggested.20 Valence bond calculations for aromatic radical-ions give slightly different spin densities at the same carbon in corres- ponding anions and cations suggesting a partial explanation for the small differences in corresponding proton hyperfine splittings in the oppositely charged radical pairs.21 The observed spectra of simple aromatic cyanide and isocyanide 23 correlate well with predictions from Hiickel MO theory assuming a linear R-N-C bond arrangement.The nitrogen hyperfine splitting constants are greater for isocyanides than for cyanides in corres- ponding compounds.22 Triarylmethyl radicals have been recorded.The decrease of 0.5G in the p-proton hyperfine splittings in 2,2',2",6,6',6"-hexamet hoxytriphenylmethyl compared with triphenylmethyl is consistent with a pronounced angle of twist ( -50') for the aryl rings.24 The triphenylaminium radical was prepared by oxidation of the parent amine and the proton splittings are in good agree- ment with MO the~ry.~ s Radical-anions containing cumulated unsaturated chains such as diphenylacetylene26 and diphenyldia~etylene~' have been examined and their spectra satisfactorily explained. Heterocyclic radical-ions have been widely ~tudied.~ A summary of *-' K. MSbius and M. Plato 2.Naturforsch. 1967,22a 929. l6 A. T. Amos and L. C. Snyder J. Chem. Phys. 1964,41,1773; ibid.1965,42,3670. W. Kbhnlein K. W. BSddeker and U. Schindewolf Angew. Chem. 1967,79 318. " A. Carrington Quart. Rev. 1963,17,67. l9 J. R. Bolton J. Chem. Phys. 1967,46,408. 2o K. W. Bowers and F. J. Weigert J. Chem. Phys. 1966,44,416,3645. T. H. Brown and M. Karplus J. Chem. Phys. 1967,46,870. 2Z G. F. Longster J. Myatt and P. F. Todd J. Chem. SOC.(B) 1967,612. 23 E. Brunner R. Miicke and F. Dorr Z. Phys. Chem. (Frankfurt),1966,50,30. 24 M. J. Sabacky C. S. Johnson jun. R. G. Smith H. S. Gutowsky and J. C. Martin J. Amer. Chem. SOC.,1967,89,2054. " H. van Willigen J. Amer. Chern. SOC.,1967 $9,2229. R.E. Soda D. 0.Cowan and W. S. Koski J. Amer. Chem. SOC. 1967,89,230. '' J. C. Chippendale P. S. Gill and E. Warhurst Trans. Faraday SOC.,1967,63 1088.J. C. M. Henning. J. Chem. Phys. 1966.44.2139. Part (ii) Electron Spin Resonance 31 work on nitrogen heterocyclics is given by HenningZ8 who confirmed that the isotropic nitrogen splitting constants are dependent on x-spin density of nearest neighbour atoms as well as on the nitrogen in a 0-71 expression analogous to that for 13C splittings.I3 The nitrogen splittings in the acridine mononegative ion fit Henning’s results.29 Slightly different o-x parameters for nitrogen hyperfine splittings have been suggested by Talcott and Myers.30 Detailed calculations of 71-electron spin densities in polyazine anions3 suggest that the McConnell relationship” may not be satisfactory for obtaining carbon x-spin densities from proton splittings for this class of compound as generally assumed and that this may be resonsible for the variety32 of 0-71 parameters for nitrogen reported in the literature.Other heterocyclic radical- ions33-38 containing nitrogen oxygen sulphur and selenium have been examined particularly anhydrides and substituted irnide~.~~-~~ Heterocyclic analogues of diphenylpicrylhydrazyl and a series of thio-phosphonium hydrazyls have been synthesised and measured by Ryzhmanov and his co-worker~.~~ The radical obtained by electrochemical reduction of 4,4’-dinitrodiphenyl-methane was found to possess an unexpected hyperfine structure consisting of 5 x 5 x 5 lines. It wasshown that the anion loses a proton from the methylene bridge forming radical (1). Calculations confirm that the unpaired spin density is high at the nitrogens and negligible at the central carbon in agreement with the observed ~pectrum.~’ (1) A protonated form of di-(p-methoxyphenyl) nitroxide has been found with a hyperfine splitting of 9.5~for the odd proton.41 The stable nitroxide di(trichloromethy1) nitroxide gives a well resolved spectrum and the chlorine splitting is found to be 1.25 G.~~ Phenyl t-butyl nitroxides which spontaneously dissociate during isolation can be stabilised by steric hindrance due to 29 H.G. Hoeve and W. A. Yeranos Mol. Phys. 1967,12,597. 30 C. L. Talcott and R. J. Myers Mol. Phys. 1967,12,549. 31 P. J. Black and C. A. McDowell Mol. Phys. 1967,12,233. 3f P. T. Cottrell and P. H. Rieger Mol. Phys. 1967,12 149. 33 N. M. Atherton J.N. Ockwell and R. Dietz J. Chem. SOC.(A) 1967 771. 34 S. F. Nelsen J. Amer. Chem. SOC.,1967,89 5256 5925. 35 R. E. Sioda and W. S. Koski J. Amer. Chem. SOC.,1967,89,475. 36 M. Hirayama Bull. Chem. SOC.Japan 1967,40 1557. 37 M. M. Urberg and E. T. Kaiser J. Amer. Chem. SOC.,1967,89 5931. ’* J. P. Keller and R. G. Hayes J. Chem. Phys. 1967,46,816. 39 Yu. M. Ryzhmanov B. M. Kozyrev Yu. V. Yablokov D. P. Elchinov and R. 0.Matevosyan Doklady. Akad. Nauk S.S.S.R.,1966 171 1120; F. G. Valitova and Yu. M. Ryzhmanov ibid. 1966 170,1124. 40 B. I. Shapiro V. M. Kazakova and G. M. Kipkind Zhur. strukt. Khim. 1966,7,612. 41 H. Hogeveen H. R. Gersmann and A. P. Praat Rev. Trav. chim. 1967,86 1063. 42 H. Sutcliffe and H. W. Wardale J. Amer. Chem. SOC. 1967,89 5487.A. Horsfield substitution at the para-position and the e.s.r. data of a number of such compounds have been tabulated.43 Conformational studies. Examples of conformational isomerism in free radicals observed by e.s.r. have been reported particularly for semidiones. In the case of 3,4,5-trimethoxy-phenylglyoxalsemidione anions different g-factors are found for the two isomers. This is attributed to ion-pair formation which stabilises the cis-isomer (2) and modifies the n-n* excitation energy of the molecule and hence the q-fa~tor.~~ Unusual stability is found for semidiones substituted with cyclopropyl groups. This group has pronounced conforma- tional preference (3) with the methine H atom about 8"from the nodal plane.4s Both syn- and anti-6-ethylbicyclo[3,l,O]hexan-2-one are oxidised in basic dimethyl sulphoxide solution to the same semidione having hyperfine structure consistent with the more stable anti-structure (4); the syn-isomer initially gives a mixture of signals which decay to those of the anti-structure provided that oxygen is not excluded.46 n Changes in the e.s.r.hyperfine structure demonstrate that stereospecific exchange occurs at carbon-4 in bicyclo[3,1,0]hexane semidiones (5) in per- deuteriodimethyl s~lphoxide.~~ The duroquinol cation spectrum is analysed at low temperature in terms of cis-and trans-is~mers.~~ A trans-relationship is assigned to the halogen with respect to the C=N bond in 1-bromo- and 1-iodo-fluorenones for which large halogen splittings are found because of favourable geometry for overlap between orbitals on the iminoxy-function 43 A.Calder and A. R. Forrester Chem. Comm. 1967,682. 44 C. Corvaja P. L. Nordio and G. Giacometti J. Amer. Chem. Soc. 1967,89 1751. 45 G. A. Russell and H. Malkus J. Amer. Chem. SOC.,1967,89 160. 46 G. A. Russell J. McDonnell and P. R. Whittle J. her. Chem. Soc. 1967,89 5515 5516. 47 P. D. Sullivan J. her. Chem. Soc. 1967,89,4294. Part (ii) Electron Spin Resonance 33 and the halogen.48 cis-trans-Isomerism is also found in p-dialkoxybenzene cations with the percentage of the trans-form increasing with the size of the alkyl Ion-pairs in solution. Ion-pairing between metal cations and radical-anions in solution is well established by e.s.r.and inter- and intra-molecular exchange of cations has been demonstrated. With sodium anthracene and sodium 2,6-di-t-butylnaphthalene,the sodium hyperfine splitting is dependent on solvent and temperature.” Equilibrium between an ion-pair and free ions does not explain the observed temperature-dependence but a model involving equilibrium between a tight ion-pair (with a large sodium splitting) and a loose solvated ion-pair (with a smaller sodium splitting) accounts satis- factorily for the results and equilibrium constants and enthalpy changes may be derived. Rate constants for interconversion between the pairs are obtained from hyperfine line-width changes.” The same concept of tight and loose ion-pairs has been invoked for the a~enaphthylene~’ and phthalonitrile anions.52 Molecular orbital calculations show that the most favourable configuration for the acenaphthylene pair has the gegen-ion above the five- membered ring.” Ion-pairing has been found for the pyracyclene ~emiquinone.’~ The cyclo- hoptatrienide dianion-radical shows a large double sodium splitting of 1.76 G in ethereal solvents suggesting the formation of contact triplets with two sodium cations.54 The alternating line-width effect observed with the 5,5,10,10-tetramethyl-5,lO-dihydrosilanthreneanion is due to cation exchange between the two silicon atoms which carry high spin density in their 3d-0rbitals.~’ rn-Dinitrobenzene anions have been examined under varied conditions and the associated dynamic frequency line-shifts measured.These are small ( hl 20 mG). They arise because of modulation of isotropic hyperfine splitting from intramolecular motions or fluctuating complex formation with solvent molecules or cations in the surroundings. In this example the data are repre- sented by a two-state model involving two nitrogen hyperfine splitting constants with out-of-phase correlations due to complexing of the nitro- groups.’‘Similar measurements have also been made with dinitromesitylene and dinitrodurene anions.57 Electron and proton transfer reactions. In optically active hexahelicene where large asymmetry exists in the n-electron system a factor of 4 difference in the electron exchange rate constants kDD and kD between the molecule and its 48 B. C. Gilbert and R. 0.C.Norman J. Chem. SOC.(B),1967,981. 49 W. F. Forbes P. D. Sullivan and H. M. Wang J. Amer. Chem. SOC.,1967,89,2705. A. Crowley N. Hirota and R. Kreilick J. Chem. Phys. 1967,46,4815. 51 M. Iwaizumi M. Suzuki T. Isabe and H. Azumi Bull. Chem. SOC.Japan 1967 40 1325; A. M. Hermann A. Rembaum and W. R. Carper J. Phys. Chem. 1967,71 2661. ” K. Nakamura and Y. Deguchi Bull. Chem. SOC.Japan 1967,40,705. 53 S. F. Nelsen B. M. Trost and D. H. Evans J. Amer. Chem. Soc. 1967,89,3034. 54 N. L. Bauld and M. S. Brown J. Amer. Chem. SOC.,1967,89,5417. 55 E. G. Janzen and J. B. Pickett J. Amer. Chem. SOC. 1967,89,3649. 56 R. J. Faber and G. I(.Fraenkel,J. Chem. Phys. 1967,47,2462. 57 R. D. Allendorfer and P. H. Rieger J. Chem. Phys. 1967,46,3410. 34 A. Horsjield radical-anion is found for like optical configurations and their enantiomer~.~~ The rate constant for electron exchange between the stilbene anion and trans-stilbene is lo9 1.mole-'sec.-' and the activation energy is about 3 kcal.mole-'. These are typical values for such organic redox reactions which are probably diffusi~n-controlled.~~ Electron exchange between the mono- and di-anions of cyclo-octatetraene has also been observed.60 Reduction of aliphatic and aromatic nitro-compounds can be effected by electron transfer from organic donors such as RCHOH (R = H or alkyl) generated in a flow system. Non-equivalencg of the nitrogen splittings in dinitro-compounds is evidence for ion-pair formation between the anion and the donor.61 Proton transfer equilibria involving semiquinones from hydroquinone and catechol were studied in an aqueous flow system by Carrington and Smith.62 Alternating line-width effects and spectral changes were attributed to protona- tion at rates dependent on the pH of the solution.At low pH the hydroxy- proton splitting can be observed.62 Proton exchange accounts for the acid- catalysed tautomerism of the monoprotonated trans-biacetyl semidione radical which exhibits a pH-dependent alternating line-width effect. The process is first-order with respect to hydrogen ions and the rate constant is 4.5 x lo9 1. mole-'sec.-' at room temperat~re.~~ Transient radical and kinetic studies. OH radicals generated from TiC13 and H202 in a flow system react with oximes producing P-hydroxy-nitroxides by addition at the oxime double bond since the observed nitrogen splittings (-13 G) are too small for iminoxy-radicals ( -30 G).64 In the case of formamide and acetamide two different modes of attack by OH are found since the radicals HCONH and CH2CONH2 are unambiguously identified by their e.s.r.spectra. HCONH which has a large nitrogen splitting (21-6G) is probably formed by rearrangement of CONH2? According to pH two radical types are detected in reactions between transient OH radicals and benzenoid compounds in flow systems. Addition of OH to the ring giving cyclohexadienyl radicals occurs or alternatively groups (H C02H or CH20H) are lost from side-chains by acid-catalysed heterolytic bond ruptures induced by OH- elimination from the ring.66 Transient radicals from alkaline ferricyanide oxidation of hydroxamic acids have been identified as RC(O)N6 -anions which react on further oxidation through a molecular rearrangement to alkylhydroxylamine carbamate OCONRO -,radi~al-anions.~~ 58 R.Chang and S. I. Weissman J. Amer. Chem. SOC.,1967,89 5968. s9 R. Chang and C. S. Johnson jun.. J. Chem. Phys. 1967,46,2314. 6o F. J. Smentowski and G. R. Stevenson,J. Amer. Chem. SOC.,1967,89,5120. 61 W. E. Griffths G. F. Longster J. Myatt and P. F. Todd J. Chem. SOC.(B) 1967,533 I. C. P. Smith and A. Carrington Mol. Phys. 1967,12,439. 63 R. J. Pritchett Mol. Phys. 1967. 12 481. 64 J. Q. Adams J. Amer. Chem. SOC.,1967,89,6022. 65 P. Smith and P. B. Wood Canad. J. Chem. 1966,44,3085. 66 R.0.C. Norman and R. J. Pritchett J. Chem. SOC.(B) 1967,926. 67 D. F. Minor W. A. Waters and J. V. Ramsbottom,J. Chem. SOC.(4, 1967 180. Part (ii) Electron Spin Resonance Radical intermediates detected during decomposition of nitrosoacetanilides in benzene have been identified as diazotate radicals p-RC,H,N=N-o and this was checked by 15N substitution.68 A series of consecutive radical reactions initiated by electron transfer with 4,4'-dinitrobenzophenone in alkaline solution has been investigated by e.s.r. and a reaction mechanism propo~ed.~' EndorStudiesof FreeRadicals.-Endor or electron nuclear double resonance can be applied to free radicals which have e.s.r. spectra exhibiting hyperfine structure. The technique consists of applying variable-frequency r.f.power to a sample in the cavity of a spectrometer which is maintained continuously on an electron resonance line under conditions of partial saturation. As the r.f. frequency is swept a change in the electron resonance absorption signal is observed whenever this frequency corresponds to resonance between two of the nuclear sub-levels in the scheme of hyperfine energy levels of the radical. Each class of equivalent nuclei gives rise to two Endor lines corresponding to the two situations where the coupled electron is parallel or antiparallel to the magnetic field. The method is of advantage in analysing complex spectra where the n + 1lines from a group of n equivalent protons in the e.s.r. spectrum reduce to two lines in the Endor spectrum and when several groups of equi- valent protons are present the enormous simplification of the spectrum is obvious.Although the method has been used with solids for some years it was not successfully applied to radicals in solution until it was realised that intense r.f. fields are req~ired.~' A review on the application of Endor to free radicals in liquid and solid phases has been recently published by H~de.~' The power of Endor in analysing radical spectra containing small proton splittings that cannot be resolved in the e.s.r. spectra has been demonstrated with substituted triphenylmethyl derivatives containing -CH,SCH3 side-chains (6) where the methyl proton splitting was detected. Rapid exchange was found at -20" between conformers that were detectable at -80°.72The G.Binsch E. Merz and C. Riickhardt Chem. Ber. 1967,100,247. 69 B. I. Shapiro V. M. Razakova,and Ya. K. Syrkin Doklady. Akad. Nauk. S.S.S.R. 1966,171,156. 'O J. S. Hyde and A. H. Maki J. Chem. Phys. 1964,40,3117. 71 J. S. Hyde 'Magnetic Resonance in Biological Systems,' ed. A. Ehrenberg B. G. Malmstrom and T. ViinngArd Pergamon Oxford 1967. 72 J. S. Hyde R. Breslow and C. &Boer J. Amer. Chem. SOC. 1966,88,4763. B* 36 A. Horsfield 2,4,6-triphenylphenoxy-radical(7) spectrum has been analysed by e.s.r. using selective deuteriation and computer spectrum synthesis.73 Although deuteria- tion proved necessary to analyse the same spectrum by End~r,’~ because of near degeneracy of certain splittings the latter method has the advantage that deuteriated sites are eliminated from the Endor spectrum while deuterium splittings remain in the e.s.r.spectrum. A theoretical study of saturation effects in free-radical solutions showed that strong Endor r.f. fields can split e.s.r. lines associated with two or more equivalent protons and this has been confirmed e~perimentally.~ The effect is similar to spin-tickling in n.m.r. double resonance. The advantage of using Endor for free radicals in solids is a dramatic improvement in resolution. Electron resonance lines that are typically several gauss wide owing to dipolar broadening by neighbouring magnetic nuclei have line-widths of the order of 100 KHz in the Endor spectrum. So far Endor has not been applied to polycrystalline samples or glasses but this should be possible.71 The improved resolution with Endor has made it very successful for investigating radiation damage in single crystals where the e.s.r.analysis is complicated by overlapping spectra from a single radical species trapped in different crystallographic orientations or where several different radicals are involved. In irradiated a-aminoisobutyric acid the radical (CH,),e C02- which is observed in one conformation at room temperature is distinguished in three conformations by Endor when the crystal is cooled to The method was found essential for the identification of the radical in irradiated histidine hydrochloride since there are four crystallographic sites for radiation damage in the orthorhombic unit cell of the crystal.77 The application of Endor to triplet-state molecules is reported by Hutchison and Pearson who studied the ground-state triplet fluorenylidene (8) trapped in single-crystals of diazofluorene.All proton hyperfine splittings were detected and their hyperfine splitting tensors evaluated allowing the distribution of unpaired spin density in the molecule to be mapped7* at C-1 C-2 (2-3 and C-4 and at C-9 by 13C labelling. N.m.r. Studies of Free Radicals.-Although n.m.r. is strictly not relevant to this section of the Report information about electron resonance hyperfine splitting constants can be obtained from the n.m.r. spectra of free radicals in solution. The Fermi contact shift (6,)for a proton in a radical is given by 6 = -ay,y,hH/161~~kT where a is the hyperfine splitting and ye and 7 are the magnetogyric ratios of the electron and proton.Thus the magnitude of an e.s.r. splitting constant 73 K. Dimroth A. Berndt F. Bar R. Voiland and A. Schweig Angew. Chem. 1967,79,69. 74 J. S. Hyde J. Phys. Chem. 1967,71 68. ’’ J. H. Freed J. Phys. Chem. 1967,71 38; J. H. Freed D. S. Leniart and J. S. Hyde J. Chem. Phys. 1967,47 2762. 76 J. W. Wells and H. C. Box J. Chem. Phys. 1967,47,2935. ” H. C. Box H. G. Freund and K. T. Lilga J. Chem. Phys. 1967,46; 2130. C. A. Hutchison and G. A. Pearson J. Chem. Phys. 1967,47 520. Part (ii)Electron Spin Resonance can be obtained from the n.m.r. contact shift and the sign of the splitting constant is given by the direction of the shift.A review on n.m.r. of paramagnetic systems has been published.79 Hausser and his co-workers used n.m.r. to determine small hyperfine splitting constants which are below the limit of resolution of a normal e.s.r. spectrometer." Because broadening of the n.m.r. lines occurs in addition to the contact shift it was first thought that the method was limited to measuring small hyperfine splittings (-100 rnG)." However. it has been demonstrated that splittings as large as 5 G can be measured if a 'wide-line' n.m.r. spectro- meter is employed." d The n.m.r. results for hyperfine splittings have been compared with direct em. measurements for the biphenyl anion' and for several nitroxide radicals,82 and good agreement was found. In the case of the particular nitroxides studied (9) and (lo) which should exist as chair-shaped molecules the occurrence of single lines from inequivalent axial and equatorial groups indicates rapid interconversion of conformers.The n.m.r. method promises to be useful in unravelling complex e.s.r. solution spectra particularly where resolution is marred by small unresolved splittings. Since n.m.r. transitions are measured however the sensitivity is inferior to that obtained in e.s.r. measurements and concentrated solutions of the free radical must be employed. Spin-labelling Techniques.-Since unpaired spins are essential for electron spin resonance the spin-labelling technique was developed for investigating large diamagnetic biomolecules. It consists of grafting stable free radicals usually nitroxides on to the molecule to make it paramagnetic and specific sites can be labelled by choosing appropriate radicals and reaction conditions.The detailed hyperfine structure of the spectrum of the radical-label gives information about the structure motion and reactions of the biomolecule and nucleic acids proteins and enzymes have been studied in s~lution.'~ By using single crystals of labelled proteins and enzymes McConnell has shown that molecular symmetry axes which are non-coincident with crystal sym- metry axes can be detected through the e.s.r. spectra and evidence of allosteric conformational changes can be obtained.71*83 l9 E. de Boer and H. van Willigen Progr. N.M.R. Spectroscopy 1967,2. K. H. Hausser H.Brunner and J. C. Jochims Mol. Phys. 1966,10,253. G. W. Canters and E. de Boer Mol. Phys. 1967,13,395. 82 R. W. Kreilick J. Chem. Phys. 1967,46,4260. H. M. McCannell and J. C. A. Boeyens J. Phys. Chem. 1967,71.12. A. Horsfield The spin-labelling method has been extended to small organic molecules. A general method for converting ketones into stable nitroxide radicals or oxazolidines (1l) has been de~ised.'~ The maleic anhydride anion grouping (1 1) or semifuraquinone (12) is also a useful label especially for bicyclic systems.' 0-0- b* (13) (14) Hyperfine splittings from several semifuraquinone adducts (butadiene cyclo- hexadiene etc.) have been compared with values obtained with semiquinone (13) and semidione (14) spin labels; they are found to change with radical geometry.' The technique appears promising for investigating molecular structure and conformation in polycyclic derivatives since long-range hyper- fine splittings observed in a number of bicyclic and tricyclic semidiones and semiquinones have allowed structural assignments.86 The origin of these splittings is thought to be through x-bond-x-bond interactions.Similar long- range splittings are found in iminoxy-radicals produced from bicyclic ketones such as bicycle[3,2,2]nonan-6-one.' Free Radicals in Solids.-Electron resonance continues to be widely em- ployed in identifying free radicals produced by irradiation processes. By using single crystals the anisotropy or variation of hyperfine splittings with orientation of the radicals in the field can be determined and radicals can often be identified unambiguously.For example in X-irradiated crystals of hydroxyurea it is concluded that the radical is HzNC0AH formed by loss of OH. From the principal values of the nitrogen hypertine splitting tensor it is found that the unpaired electron is localised to the extent of 41 % in the nitrogen 2p-orbital normal to the -fi-H plane in the radical.88 Hydroxy-group 84 J. F. W. Keana S. B. Keana and D. Beetham J. Amer. Chem. SOC.,1967,89,3055. S. F. Nelsen and E. D. Seppanen J. Amer. Chem. SOC. 1967,89,5740. 86 D. Kosman and L. M. Stock Tetrahedron Letters 1967 1511; S. F. Nelsen and B. M. Frost ibid. 1966 5737. 87 A. Caragheorgheopol M. Hartmann K. Kiihmstedt and V.E. Sahini Tetrahedron Letters 1967,4161. 88 H. W. Shields P. J. Hamrick jun. and W. Redwine J. Chem. Phys. 1967,46 2510. Part (ii) Electron Spin Resonance 39 abstraction is also notcd in alloxan monohydrate to give the radical rNHCO~(OH)NHC07.89 The principaI hyperfine splitting values of the hydroxy-hydrogen are in good agreement with those for HOtHC0,- in irradiated lithium glyc~llate,~~ showing that the OH group must lie in the plane normal to the 2p-orbital of the free-radical carbon. Characteristic hyperfine splitting anisotropy for one a-proton and two P-protons (attached to the sp2-and adjacent sp3-carbon atoms respectively) confirms that the radical formed by radiolysis of succinamide is (H2NOCKHCH2(CONH2).91 The analogous radical CH(CONH,) is formed in malonamide together with a o-radical formed by loss of a hydrogen atom from the amine group (H2NOC)CH2COfiH;the -fiH proton has a large nearly isotropic splitting of about ~OG,indicating a contribution of about 16% from the hydrogen 1s-orbital to the unpaired electron orbital.92 In trifluoroacetamide the cF3 radical is found at 77"~.The observed 19F and 13C hyperfine splittings show that the radical has the same structure as found in solution with an FCF angle near the tetrahedral value. cF2CONH2 is also found as well as a new radical attributed to fiH2C0.93 Irradiation of methyltriphenylphosphonium chloride gives the radical- cation (C6H5)3P+c&. Rapid rotation of the methylene resulting in magnetic equivalence of two protons is observed at room temperature while at 100"~ the radical is essentially stationary with non-equivalent methylene protons.94 Endor was used to identify CH3tH(COzH) as a minor product in irradiated succinic acid despite serious overlap of lines from the main radical (C02H)~HCH,(C02H).gs The methyl group was found to be rotating at 77"~,in contrast to the case of irradiated alanine where the same radical is found,96 but where the methyl group rotation is frozen at 77"~, showing that methyl rotation is a function of the matrix in which the radical is trapped.The radical found in irradiated 3,3'-dithiodiproprionic acid is (C02H)CH,CH2S.97 From g-factor anisotropy it is concluded that the un-paired spin is Iocalised in a p-x orbital on sulphur.The isotropic splittings of the adjacent methylene protons fit a B cos% expression similar to that found for P-protons in aliphatic radicals,96 where B is 12.8~ and 8 is the dihedral angle between the S-C-H plane and the C-S x-plane. Other sulphur-containing radicals of the type RtHSR' were found in urea clathrates containing sulphides like di-n-hexyl sulphide and diethyl sulphide in which the unpaired spin a9 M. Kashiwagi J. Chem. SOC.Japan 1966,87,1294. 90 D. Pooley and D. H. Whiffen Trans. Faraday SOC. 1961,57 1445. 91 M. Kashiwagi J. Chem. SOC.Japan 1966,87,1298. 92 N. Cyr and W. C. Lin Chem. Comm. 1967 192. '' M. T. Rogers and L. D. Kispert J. Chem. Phys. 1967,47,3193. 94 E. A. C. Lucken and C. Mazeline J. Chem. SOC.(A),1967,439. 95 S. F.J. Read and D. H. Whiffen MoZ. Phys. 1967,12 159. 96 I. Miyagawa and K. Itoh J. Chem. Phys. 1962 36 2157; A. Horsfield J. R Morton and D. H. Whiffen MoZ. Phys. 1962,5 115. 97 Y. Kurita Bull. Chem. SOC.Japan 1967,40,94. A. Horsfield density is located mainly in the p-n orbital on the sp2-carbon rather than on sulphur.98 Free radicals in irradiated polymers have been studied by e.s.r. In the photodegradation of poly(methy1 methacrylate) it is concluded that the residual monomer molecules are the centres converted into radicals.99 Wedum and Griffith have examined various monomer radicals by irradiation of urea inclusion compounds containing monomer esters and decane. 'O0 The decane functioned as a spacer between ester molecules in the urea cavities and pre- vented polymerisation.The anisotropy of the proton hyperfine splittings of the monomer radicals could therefore be examined. A new radical in y-irradiated tetrafluoroethylene-hexafluoropropene copolymer has been identified as -CF&(CF,)CF,-. This radical can be converted into its isomer -CF,CF(CF2)CF2CF2-under U.V. illumination and the process reversed by thermal annealing. An analogous reversible photoinduced isomerisation between CH3C(CH3)CH3and CH3CH(cH2)CH3was discovered during U.V. exposure of y-irradiated isobutyl bromide.' O2 The peroxide radicals -CF2CF2O2 formed by irradiation of polytetrafluoroethylene in air are converted into the propagating radical -CF,cF2 by U.V. irradiation in a vacuum.' O3 Information about the course of radiation reactions in solids can be obtained by variable-temperature studies in e.s.r.A number of precursor radicals have been detected by irradiation and e.s.r. measurement at low temperature. The radical-ions CH3C022-in sodium a~etate,"~ CH,BrCO,H-in bromo- acetic acid,'" and CF3C02H-in ammonium trifluoroacetate'06 suggest electron capture as the primary process; on raising the temperature re-arrangements to more stable radicals occur such as to cF2C02-in the latter case. '06 The radicals CH2S04-and CH3CHS04- initially observed in irradiated potassium alkyl su1phates,lo7 decompose above 200°K leaving SO3-. The radical-pair CH2CONH2 * * -CHFCONH is found at 77°K in monofluoroacetamide but on warming CH2CONH2 is quantitatively con- verted into cHFCONH2 which is the product of room-temperature irradia- tion.This suggests reaction between specific neighbouring molecules as follows:'08 CH2FCONH2 cH2CONH2 + F F + CH,FCONH2 -CHFCONH + HF CHZCONH + CH,FCONH thermal CHFCONH + CH,CONH 98 0.H. Griffin and M. H. Mallon J. Chem. Phys. 1967,47,837. 99 R. E. Michel F. W. Chapman and T. J. Mao J. Chem. Phys. 1966,45,4604. loo E. D. Wedum and 0.H. Griffith Trans. Faraday SOC. 1967,63,819. lo' M. Iwasaki K. Toriyama,T. Sawaki and M. Inone J. Chem. Phys. 1967,47,554. M. Iwasaki and K. Toriyama,J. Chem. Phys. 1967,46,2852. S. Siege1 and H. Hedgpeth J. Chem. Phys. 1967,46,3904. lo4 D. G. Cadena V. Mendez and J. R. Rowlands Mol. Phys. 1967,13 157. J. R. Suttle and R. J. Lontz J. Chem. Phys. 1967,46 1539.lo6 F. D. Srygley and W. Gordy J. Chem. Phys. 1967,46,2245. lo' P. B. Ayscough and A. K. Roy Trans. Faraday Soc. 1967,63,1106. lo* M. Iwasaki and K. Toriyama,J. Chem. Phys. 1967,46,4693. Part (ii) Electron Spin Resonance E.s.r. spectra from radicals in rigid glassy solutions and polycrystalline samples are less easily interpreted than in single-crystal studies since aniso- tropic hyperfine splittings and g-factors cause broadening of the e.s.r. lines because the radicals are randomly oriented. Nevertheless many new results have appeared. The benzene positive radical has been prepared by Carter and Vincow by photoionisation in a rigid H,SO matrix. The proton hyperfine splitting is 4.44~ at -150",'09 and this agrees quite well with the value calculated from the Colpa-Bolton relationship.' lo The positive ions of hexamethyl- and hexaethyl-benzene have also been observed.' ' ' High-energy irradiated pyridine has attracted attention.' 12-' l4 It is suggested that the spectrum previously attributed to the pyridine cation is in fact due to the 2-pyridyl radical.'"* 'l3 The radicals in electron-irradiated cyclohexane-1,2-diol exhibit e.s.r. spectra consistent with the trapping of two conformationally isomeric radicals cis-and trans-cyclohexane-l,2-diol-l(15) at 77"~.These undergo intramolecular conversion into cyclohexanone-2 or 2-hydroxycyclohexanone-6 on warning to room temperature. ' 0' 0-- e- cis (1 5) trans In cyclohexanecarboxylic acid three radical species are observed at 153"~ of which CsHloC CO,H is stable up to the melting point with reversible changes in spectrum due to chair-chair conversion.The other cyclohexyl radicals decay at 188"~."~ Two types of radical are found in the solid phase radiolysis of nitroalkanes. RCHNO radicals are given by the methyl and ethyl compounds; for homologues higher than C2,and for all nitroalkanes in vitreous ethanol solution radicals of the type R&(O)OH or R&(O)OR' are formed by addition to the nitro-group. These types may be distinguished by their respective nitrogen hyperfine splitting constants. '' Free radicals formed by radiation damage in a number of amino-acids and compounds of biological interest have been identified.76. 77 Using Io9 M. K. Carter and G. Vincow J.Chem. Phys. 1967,47,292. 'Io J. P. Colpa and J. R. Bolton MoZ. Phys. 1963,6 273. '11 M. K. Carter and G. Vincow J. Chem. Phys. 1967,47,302. H. J. Bower J. A. McRae and M. C. R. Symons Chem. Comm. 1967 542. C. David G. Geuskens A. Verhasselt P. Jung and J. F. M. Orth MoZ. Phys. 1966 11 599. 114 K. Tsuji H. Yoshida and K. Hayashi J. Chem. Phys. 1967,46,2808. T. Ohmae H. Sakurai S. I. Ohnishi K. Kuwata and I. Nitta J. Chem. Phys. 1967,46 1865. P. M. K. Leung and J. W. Hunt J. Phys. Chem. 1967,71,3177. 11' C. Chachaty and C. Rosilio J. Chim. phys. 1967,64,777. R. S. Mangioavacina Radiation Res. 1967,32 27. H. Shields P. Hamrick and D. DeLaigle J. Chem. Phys. 1967,46 3649. ''O H. C. Box H. G. Freund and E. E. Budzinski J. Chem. Phys. 1967,46,4470.J. W. Wells and H. C. Box J. Chem. Phys. 1967,47,2935. J. B. Cook J. P. Elliott and S. J. Wyard Mol. Phys. 1967 13,49. A. Hors3eld the Endor technique it was found that the radical in histidine hydrochloride is formed by addition of a hydrogen atom at the 2-position (16) in the imidazole ring.77 Hydrogen-atom addition at a double bond also occurs in cytosine.’22 Multiradicals and the Triplet State.-Further work on the theoretical interpretation of the e.s.r. spectra of diradicals has continued. The effect of temperature-variation of the electron-electron exchange parameter J on the positions intensities and widths of the spectral lines has been e~amined.”~’ 124 The sign of J for di-(2,2,6,6-tetramethyIpiperidin-4-yl1-0xide)carbonate has been determined from the variation of line-width differences within the spectrum with temperature.124 A symmetrical nitroxide triradical has been observed by Hudson and Lu~khurst.’~’ The full theoretical spectrum for a triradical is calculated including the doublet and quartet transitions and compared with the ex- perimental one. The observed alternating line-widths in the experimental spectrum are explained on the basis of modulation of the electron exchange parameter J. An aromatic even-alternant hydrocarbon with a quintet ground-state has been prepared and observed by e.s.r. rn-Phenylenebisphenylmethylene(17) was formed by photolysis of 1,3-di-(a-diazobenzyl)benzenein a host single- crystal of benzophenone.’ 26 The magnitudes of the zero-field splitting para- meters D and E (which are respectively a measure of the electron-electron dipolar interaction and its deviation from cylindrical symmetry) found from the electron resonance spectrum are in reasonable agreement with values predicted theoretically by Hig~chi,’~~ and they are consistent with a planar configuration for the molecule.The most convenient method for studying triplet states by e.s.r. is to examine rigid glassy solutions of the required solute at 77°K. From an analysis of the line- shape the zero-field splitting parameters D and E can be obtained although lZ3 S.H. Glarum and J. H. Marshall J. Chem. Phys. 1967,47 1374. 124 H. Lemaire J. Chim.phys. 1967,64 559. A. Hudson and G. R.Luckhurst Mol. Phys. 1967,13,409. K. Itoh Chem.Phys. Lerters 1967 1 235. IZ7 J. Higuchi J. Chem. Phys. 1963,38 1237 and 39 1847. Part (ii) Electron Spin Resonance 43 the information about their correlation with the symmetry axes of the molecule is lost.128 Three new methods for generating substituted methylenes which have triplet ground-states have been reported. Methylenes have been obtained in low-temperature glasses by photolysis of alkaline salts of toluene-p- sulphonylhydrazones which decompose through diazo-intermediates. This is a useful general method where the diazo-precursors of the methylene com- pounds are non-isolable or dangerously reacti~e."~ Direct evidence from the known e.s.r. spectrum of diphenylmethane has proved that oxirans decompose under photolysis to give carbene intermediates (18) as previously s~ggested.'~' Oxirans thus form another set of precursors to methylenes in addition to diazo-compounds and bisazides :l Ph\c-c ,Ph hv * c + RCO-Ph 77°K phO'\ph R' 'o/ 'Ph R = H,Ph (1 8) Photolysis of geminal azides at 77"~, sensitised by benzophenone (19)yields the corresponding methylene.The e.s.r. results show an initial triplet signal attributed to an a-azido-nitrene which decays as the diphenylmethylene signals build up.'32 R = Ph R' = H Ph (19) A number of cyclopentadienyl cations have been prepared and their triplet states detected. For pentachlorocyclopentadienylthis is the ground-state as predicted by molecular orbital theory; for the pentaphenyl compound the triplet is a low-lying excited state which becomes progressively higher for larger less symmetrical substituted cyclopentadienyls because Jahn-Teller distor-tions lead to unsymmetrical singlets of lower energy.'28 Other triplet states of interest have been observed.Carbenes with n-benzene systems next to the divalent carbon possess triplet ground-states and several examples such as dibenzo[a,dlcycloheptenylidene (20) have been made and compared with diphenylmethylene all have zero-field splitting parameters in the region of D = 0.4 cm.-' and E = 0.01cm.-' requiring one unpaired electron to be largely localised on the bivalent carbon and the other to be R.Breslow H. W. Chang R. Hill and E. Wasserman J. Amer. Chem. Soc. 1967,89 1112. lZ9 R.E.Moser J. M. Fritsch and C. N. Matthews Chem.Comm. 1967,770. 130 H. Kristinnsson and G. W. Griffin Angew Chem. 1965,77 859;J. Amer. Chem. SOC. 1966 88,1579. 13' A. M. Trozzolo W. A. Yager G. W. Griffin H. Kristinnsson and I. Sarkar J. Amer. Chem. SOC.,1967,89 3357. L. Barash-E. Wasserman and W. A. Yager J. Amer. Chem. SOC.,1967.89 3931. 133 I. Moritani S-I. Murahashi M. Nishino Y. Yamamoto K.Itohard and N.Mataga J. Amer. Chem. SOC.,1967,89,1259. A. Hors3eld delocalised into the ~t-orbital.'~~ The dianion of triphenylene has a triplet ground-state and in highly solvating solvents the e.s.r. spectrum shows trigonal spin distribution. In methyl tetrahydrofuran and mixtures with more polar solvents it is found that two triplet species exist without trigonal symmetry owing to perturbation of the spin distribution by the gegen-ions in two geometrically different i~n-pairs.'~~ From computer simulation of the e.s.r.line-shape it has been shown that there are two dimerisation products with triplet character in the rigid matrix spectra of alkali-metal reduction products of p-diketones and their structures (21) and (22) have been assigned for dibenzamide and dibenzoylmethane.' 35 An excited triplet state was found for solid solutions of tetramethylpyrazine in durene. The zero-field splitting para- meters indicate that this state is of n-n* ~haracter,'~~ in contrast with the n-z* character for the parent pyrazene compound where a much closer approach for the unpaired electrons is possible. Changes occurring in the zero-field splitting parameters with substitution in unsaturated ring compounds have been noted and related to charge distrib~tion.'~~~ 138 In naphthalene the influence of substitution at the a-position is greater than at the p.'38 The decrease in D compared with naph- thalene for the excited triplet state of 1,l'-binaphthyl is due to increased delocalisation of the unpaired e1e~trons.l~~ Examples of radical-pairs with exchange-coupled electrons have been reported.Exchange aligns the spins so that an effective triplet state is obtained with dipolar interactions between the unpaired electrons and hence AMs = +2 transitions which occur at half-field and which are characteristic of the triplet state are observed. Radical-pairs between similar radicals are already known.'40 There are two recent examples of pairs between dissimilar radicals; a pair formed by a hydrogen atom and a methyl radical separated by one 134 H.van Willigen J. A. M. van Brockhoven and E. de Boer Mol. Phys. 1967,12,533. lJS F. W. Pijpers H. van Willigen and J. J. Th. Gerding Rec. Trau. chim. 1967,86 511. lJ6 J. S. Vincent J. Chem. Phys. 1967,47 1830. 13' B. Smaller E. C. Avery and J. R. Remko J. Chem. Phys. 1967,46 3976. 138 W. S. Veeman and W. J. van den Hoek Mol. Phys. 1967,13 197. 139 S. P. Solodovnikov and Yu. B. Saks Zhur. strukt. Khim. 1966,7 802. I4O Y. Kurita and M. Kashiwagi J. Phys. SOC.Japan 1966,21,558; J. Chem Phys. 1966,44,1727. Part (ii) Electron Spin Resonance undamaged molecule of methane in a matrix of y-irradiated methaneI4' at ~OK,and the radical pair cH2CONH * .-cHFCONH2 in y-irradiated mono- fluoroacetamide'42 at 77°K. Such pairs are confirmed by measurement of their hyperfine splittings which are just half the values found in the separate radicals. Other radical-pairs detected by their half-field (AMs = +2) tran-sitions have been found in y-irradiated polymers (polyethylene poly- p~opene)'~~ The triplet species in frozen solutions containing bipyridyl at 77"~. radical anions (R-)and metal cations (M') has been to have the structure R -M+R -14' W. Gordy and R. Morehouse Phys. Rev. 1966,151,207. 142 M. Iwasaki and K. Toriyama J. Chem. Phys. 1967,46,4693. 143 M. Iwasaki and T. Ichikawa J. Chem. Phys. 1967,46,2851. 144 J. I).W. van Voorst W.G. Zijlstra and R. Sitters Chem. Phys. Letters 1967,1,321.

ISSN:0069-3030    

DOI:10.1039/OC9676400029

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

2. Part (iii). OPTICAL ROTATORY DISPERSION AND CIRCULAR DICHROISM By P.M. Scopes (Westfield College London N. W.3) DURING 1967 there has been a big increase (more than 40%) in the number of papers on the optical rotatory dispersion (0.r.d.) and circular dichroism (cad.) of organic molecules (excluding macromolecules) and even more dramatic increases in the number of publications discussing c.d. of inorganic complex ions and magnetic optical rotatory dispersion (m.0.r.d.). Reviews have been published on aromatic compounds,’ and on the general relationship between optical activity and the structure of organic compounds.2 -’ This report deals primarily with the o.r.d./c.d. of the smaller organic mole- cules but in addition these techniques are widely applied to the study of natural macromolecules particularly proteins and nucleic acids.The pro- ceedings of the International Conference on the Conformation of Biopolymers held at Madras in January 1967 have been published and include a wealth of material on o.r.d./c.d.6 An important paper has appeared describing a simple spectropolarimeter attachment for recording fully compensated m.0.r.d. ~pectra,~ and in another instrumental advance flash photolysis and 0.r.d. have been combined’ in order to investigate possible structural changes occuring on electron-excitation and also the absolute configuration of excited states. During 1967 several leading workers have emphasised that our present knowledge of the relationship between the structure of organic molecules and their o.r.d./c.d.usually permits us to determine only one unknown factor at a time from an experimental measurement. Thus an 0.r.dJc.d. curve may be used either to determine the configuration of a molecule (directly or by empirical comparison with other chemically and conformationally analogous molecules of known configuration) or to study the conformation of molecules whose configuration is already known. It is rarely possible to determine both configuration and conformation at the same time.when both are unknown. Mason has exposed the problem clearly in work on the c.d. and absolute P. Crab& and W. Klyne Tetrahedron 1967,23,3449. P. Crabbt in ‘Topics in Stereochemistry’ ed. E. L. Eliel and N. L. Allinger Interscience New York 1967 vol.1 p. 93. ’ A. D. Liehr Transition Metal Chem. 1966,2 165. K. Kuriyama Japan Analyst. 1966 Ann. Rev. 27R ’ M. Suzuki J. Synthetic Org. Chem. Japan 1966,24,885. ‘Conformation of Biopolymers,’ vols. I and 11 ed. G. N. Ramachandran Academic Press London 1967. ’J. T. Clerc H. K. Wipf and W. Simon Helv. Chim. Acta 1967,50 1794. * P. A. Carapellucci H. H. Richtol and R. L. Strong J. Amer. Chem. Soc. 1967,89 1742. P.M.Scopes configuration of Troger's base (l).9 The molecule is conformationally labile and each enantiomer (1R,3R)or (1S,3S) can adopt either a 'folded' conforma- tion (central rings twist) or an 'open' conformation (central rings half-boat). Calculations showed that c.d. maxima of the same sign would be expected either for the (1R,3R) configuration in the folded conformation or for the (1S,3S) configuration in the open conformation and therefore it was im- possible to deduce the absolute configuration of Troger's base from c.d.measurements alone. In fact other conformational studies suggest that the molecule exists predominantly (>90%) as the folded conformer at room temperature and from c.d. measurements ( +)-Troger's base has been allotted the (1R,3R) configuration (2). Me0--N Me&> ' (2) Snatzke has commented on similar problems in connection with the c.d. of a-ketols in the curcurbitacin series;" the known configurations of some steroid 17-carboxylic acids have been used'' to discuss their preferred con- formations. Another source of difficulty arises from unsuspected solvent interactions and Djerassi Kjaer and their colleagues'2 have drawn attention to the need for caution in assigning configuration to chiral molecules solely on the basis of the sign of the Cotton effect in systems where the solute-solvent interaction is unknown.Even in a given solvent cases are recorded of homologous com- pounds of identical chirality exhibiting Cotton effects of opposite sign. Carboy1 Chromophore.-The 0.r.d.lc.d. of the carbonyl group has been more extensively studied than that of any other chromophore and this is reflected in the number of papers which apply 0.r.d. as a routine tool in stereo- chemical problems. Of these applications more than 60 "/ are to ketones and many make use of the Octant Rule,13 which has recently been applied14 to S.F. Mason G. W. Vane K. Schofield R. J. Wells and J. S. Whitehurst J. Chem. SOC.(B) 1967,553. lo G. Snatzke P. R. Enslin C. W. Holzapfel and K. B. Norton J. Chem. Soc.(C),1967,972. G. Gottarelli W. Klyne and P. M. Scopes J. Chem. Soc.(C) 1967 1366. l2 E. Bach A. Kjaer R. Dahlbom T. Walle B. Sjoberg E. Bunnenberg C. Djerassi and R.Records Acta Chem. Scand. 1966,20,2781. l3 W. Mofitt R. B. Woodward A. Moscowitz W. Klyne and C. Djerassi J. Amer. Chem. SOC. 1961,83,4013. l4 J. Go& C. Djerassi and J. M.Conia Bull. SOC.chim. France 1967,950. Part (iii) Optical Rotary Dispersion and Circular Dichroism 49 cyclo butanones. Studies of these compounds indicated an increase in con- formational rigidity at -192” (cf.Velluz and Legrand14”). For much o.r.d./c.d. work it is useful to eliminate the complications of flexible conformationally labile compounds and to work with rigid molecules. Such monoterpene ketones of the camphor type have been used by Coulombeau and Rassat15 to study solvent effects. The authors showed that bicyclo[2,2,1]- heptanones have a c.d. band at 300 mp which is always greater in magnitude for a polar than for a non-polar solvent. There is no possibility of confor- mational change in these ketones and the authors were able to exclude the possibility of compound formation ; they therefore attribute the observed difference to solvation of the ketones in the polar hydroxylic solvents and suggest a model for the ketone-solvent complex. Cookson and his co-workers have used py-unsaturated ketones and their aryl derivatives in the dehydrocamphor and dehydroepicamphor series to study the degree of mixing of the n -,7t* and 7c -+ IC*transitions of the chromo- phore.I6 As expected the transitions show A& of opposite sign and the effects of mixing are greatest in the aryl derivatives.0.r.d. or c.d. curves have also been used to study the conformation of diastereoisomeric 4-hydroxymenth- ones’ and of diastereoisomeric keto-acids derived from menthofuran photo- peroxide. In many cases the 0.r.d. of ketones has been used to determine the stereo- chemistry of natural products. The Cotton effect of a complex keto-lactone was used to assign the absolute stereochemistry to ginkg~lide’~ (independently cross-checked by Bijvoet X-ray2’) and the c.d.of py-unsaturated ketones has been used to establish’’ stereochemistry in the pelenolides (a new group of macrocyclic sesquiterpene lactones). The 0.r.d. of the alkaloid ( -)-pelletierine confirms22 the configuration R previously allotted to the single asymmetric centre by chemical transformations. Ketones derived from the tricothecane skeleton23 and from ~edrane~~ have been studied by c.d. and cyclopropyl and epoxy-ketones related to umbellulone by 0.r.d.” 0.r.d. curves have been used by Djerassi26 to investigate the base-catalysed equilibration at C-14 of 15-0x0-steroids. The proportions of different con- 14’ L. Veiluz and M. Legrand Compt. rend. 1967 C 265 663. Is C. Coulombeau and A. Rassat Bull.SOC. chim. France 1966 3752. l6 D. E. Bays R. C. Cookson and S. MacKenzie,J. Chem. Soc.(B) 1967,215; D. E. Bays and R. C. Cookson ibid. p. 226. ‘I T. Suga T. Shishibori and T. Matsuura J. Org. Chem. 1967,32,965. C S. Foote M.T. Wuesthoff and I. G. Burstain Tetrahedron 1967,23 2601. l9 M. Maruyama A. Terahara Y. Nakadaira M. C. Woods Y. Takagi and K. Nakanishi Tetra-hedron Letters 1967 315. 2o N Sakabe S. Takada and K. Okabe Chem. Comm. 1967,259. ‘’ M. Suchy,Z. Samek V. Herout R. B. Bates G. Snatzke and F. Sorm,Coll. Czech. Chem. Comm. 1967,32 391 7. 22 H. C. Beyerman L. Maat and J. P. Visser Rec. Trau. chim. 1967,86,80. 23 G:Snatzke and Ch. Tamm Helv. Chim. Acta 1967 50 1618. 24 W Wojnarowski and G. Ourisson Bull SOC. chim. France 1967 219.25 R. T. Gray and H. E. Smith Tetrahedron 1967,23,4229. 26 A R Van Horn and C. Djerassi J. Amer. Chern. SOC.,1967,89,651. P. M.Scopes formers present in (+)-6/3-bromo-3,8dimethyldecahydroazulen-5-one in various solvents have been studied by ~.d.~' Two groups of workers28* 29 have independently synthesised a number of interesting heterocyclic ketones e.g. S( + )-1-oxoquinolizidine (3). The results of Yamada and Kunieda2* appear to show that tertiary nitrogen has an 'anti-Octant' effect (i.e.,makes an Octant contribution of sign opposite to that of an alkyl group) although the results are subject to some slight con- formational uncertainty. The Cotton effects of many halogen-substituted @-unsaturated ketones have been re~orded,~' but the substituent effect could not be correlated with an octant rule.Interesting carbohydrate ketones have been investigated by Paulsen and his colleagues3' (cf. other carbohydrate work32). OMe OMe (4) OMe Aromatic Chromopbores.-An extensive general survey of the 0.r.d. and c.d. of aromatic compounds has been published.' Much of the work discussed in this review is only partly complete and theoretical interpretation is possible only in a few limited cases; its value lies in the breadth of the field covered and in the suggested areas for future research. The absolute configurations of ( -)argemonine (4)33 and (+)-Troger's base (1)9 have been determined by Mason by the coupled oscillator method as (1S,5S) and (1R,3R) respectively. These results are in accord with those obtained for ( -)-argemonine by rigorous chemical degradation (Battersby and his co-~orkers~~).Empirical comparisons of 0.r.d. curves with those of reference compounds of known configuration have been used to determine '' K. Kuriyama T. Iwata M. Moriyama M. Ishikawa H. Minato and K. Takeda,J. Chem. Soc.(C) 1967,420. S. Yamada and T. Kunieda Chem. and Pharrn. Bull. (Japan) 1967,15,490. 29 S. F. Mason K. Schofield and R. J. Wells J. Chem. Soc.(C) 1967 626. 'O J. C. Bloch and S. R. Wallis J. Chem. Soc.(B),1966 1177. K. Heyns J. Weyer and H. Paulsen Chem. Ber. 1967 100,2317 32 H. Paulsen Chem. Ber. 1967,100 515 806. 33 S. F. Mason K. Schofield R. J. Wells J. S. Whitehurst and G. W. Vane Tetrahedron Letters 1967 137; S.F. Mason G. W. Vane and J. S. Whitehurst Tetrahedron 1967,23,4087. 34 A. C. Barker and A. R. Battersby Tetrahedron Letters 1967 135; J. Chem. Soc.(C) 1967 1317. Part (iii) Optical Rotatory Dispersion and Circular Dichroism 51 the absolute configurations of plicatic acid3’ (by reference to other lignans) and of latif~lin~~ (by reference to quinol diacetates). Aromatic amines aromatic alcohols and their derivatives have been studied by four groups. The 0.r.d. of phenyl- and diphenyl-propylamines and related imides has been compared with that of corresponding compounds in the cyclohexylpropylamine series.37 The o.r.d./c.d. of other a-and P-aryl- amine~~~ and the 0.r.d. of some aromatic alcohols have also been rep~rted.~’ Papers have also appeared on P-arylcarboxylic acids4’ and on compounds containing the thiophen or furan chrom~phores.~’ Indole alkaloids of the yohimban and corynanthean groups have been extensively studied,42 as have some oxindole alkaloids.43 From detailed work on lycorine and.related compounds Takeda and his co-workers have proposed an ‘octant rule’ for the Cotton effect associated with the 280-290 mp aromatic transition44 in this group. Other work has appeared on cinchona alkaloids,45 lupin alkaloids,46 and compounds of the rhoeadine groups.47 Nitrogen Chromophores.-Multiply- bonded nitrogen chromophores have been studied by several gro~ps.~~-’’ Snatzke and Himmelrei~h~~ have reported c.d. data for many 20-0x0-steroids with heterocyclic rings (pyrazoline oxazoline and isoxazoline) fused at positions 16 and 17.The observed c.d. maxima can be correlated with the stereochemistry of the steroid side-chain. A paper by Chilton and Krahn4’ describes a survey of arabinose derivatives with a heterocyclic ring (benzimidazole quinoxaline or phenyltriazole) at C-2 (sugar numbering). Preliminary results suggest that the benzimidazole and quinoxaline derivatives with S-configuration at C-2 have positive Cotton effects at 245 and 315 mp respectively. 35 R. J. Swan W. Klyne and H. MacLean Canad. J. Chem. 1967,45 319. 36 D. M. X. Donnelly B. J. Nangle P. B. Hulbert W. Klyne and R. J. Swan J. Chem. Soc.(C) 1967,2450. 3’ A. La Manna V. Ghislandi P. B. Hulbert and P. M. Scopes 11. Farmaco(Ed. Sci.) 1967 22 1037. 38 L.Verbit J. Amer. Chem. SOC.,1966,88,5340; J. C. Craig R. P. K. Chan and S. K. Roy Tetra-hedron 1967,23 3573. “ 0.Cervinka and 0.Btlovsk9 Coll. Czech. Chem. Comm. 1967,32 4149. 40 L. Verbit and Y. Inouye J. Amer. Chem. SOC. 1967 89 5717; L. Verbit and P. J. Heffron Tetrahedron 1967,23 3865. 41 L. Verbit E. Pfeil and W. Becker Tetrahedron Letters 1967 2169. 4’ W. Klyne R. J. Swan N. J. Dastoor A. A. Gorman and H. Schmid Helv. Chim. Act& 1967 50,115; C. M. Lee W. F. Trager and A. H. Beckett Tetrahedron 1967,23,375; W. F. Trager C. M. Lee J. D. Phillipson and A. H. Beckett Tetrahedron 1967,23,1043;M. Von Strandtmann R.Eilertson and J. Shave] J. Org. Chem. 1966,31,4202. 43 J. L. Pousset J. Poisson R. J. Shine and M. Shamma Bull. SOC.chim. France 1967 2766; A.F. Beecham N. K. Hart S. R. Johns and J. A. Lamberton Tetrahedron Letters 1967,991. 44 K. Kuriyama T. Iwata M. Moriyama K.Kotera Y. Hamada R. Mitsui and K. Takeda J. Chem. Soc.(B) 1967 46. 45 G. G. Lyle and W. Gaffield Tetrahedron 1967,23 51. 46 S. I. Goldberg and R. F. Moates J. Org. Chem. 1967,32 1832. 47 F. Santavy J. Hrbek,and K. Blaha Coll. Czech. Chem. Comm. 1967,32,4452. 48 G. Snatzke and J. Himmelreich Tetrahedron 1967,23,4337. 49 W. S. Chilton and R. C. Krahn J. Amer. Chem. SOC.,1967,89,4129. 52 P. M. Scopes The linear azide chromophore has also been studied in a series of steroidal azides. Sulphur Chromophores.-The 0.r.d. of the sulphoxide chromophore has been discussed for compounds in which the sulphur atom is part of an acyclic chain,” and for oxides of cyclic sulphur compounds.52 In either case diastereo- isomeric sulphoxides differing only in the configuration at sulphur give Cotton effects of opposite sign.0.r.d. data have also been reported for some rigid cyclic ~ulphides,’~ and c.d. data for a series of substituted dithiolans (dithi~acetals).’~ RippergerS5 has studied the c.d. of a number of dithiourethanes containing the chromophore R S C(:S) N. The experimental results can be interpreted in terms of a quadrant rule in which the regional sign distribution is the same as that previously founds6 for carboxy- and related chromophores. Phosphorus Chromopbore.-Mislow and his co-workers have reporteds7 0.r.d. data for a pair of diastereoisomeric menthylmethylphenylphosphinates (5).The chirality of the arylphosphoryl chromophore dominates the o.r.d. and positive and negative Cotton effects were observed for the diastereoisomers having R-and S-configuration respectively at the phosphorus atom. Ph 3 H,C*NYP=O I &(5) Chromophoric Derivatives.-The 0.r.d. and c.d. of N-thiobenzoyl and N-phenylthioacetyl derivatives of or-amino-acids have been studied in different s0lvents.’~958 Cotton effects have also been reported for N-(2-pyridyl-N- oxide)amin~-acids,~~ for thioamides of cycloalkanedicarboxylic acids,60 and for selenophenyl and selenonaphthyl esters of amino-acids.6 ’ Mono-and Di-enes.-The c.d. of dethiogliotoxin6’ shows a negative band at 265 mp as would be expected from the known chirality of the diene system.This may help to explain the apparent anomaly between the 0.r.d. of gliotoxin C. Djerassi A. Moscowitz K. Ponsold and G. Steiner J. Amer. Chem. SOC., 1967,89 347. ” D. N. Jones and M. J. Green J. Chem. Soc.(C) 1967,532; D. N. Jones M. J. Green M. A. Saaed and R. D. Whitehurst Chem. Comm. 1967 1003. ’’ R. Nagarajan B. H. Chollar and R. M. Dodson Chem. Comm. 1967 550; P. B. Sollman R. Nagarajan and R. M. Dodson ibid. p. 552. 53 P. Laur H. Hauser J. E. Gurst and K. Mislow J. Org. Chem. 1967 32 498. 54 R. C. Cookson G. H. Cooper and J. Hudec J. Chem. Soc(B) 1967,1004. 55 H. Ripperger Angew. Chem. Internat. Edn. 1967,6,704. ’tj J. P. Jennings W. Klyne and P. M. Scopes J. Chem. SOC.,1965 7211 7229. ’’ R. A. Lewis 0.Korpium and K.Mislow J. Amer. Chem. SOC.,1967,89,4786. ’’ G. C. Barrett J. Chem. Soc.(C) 1967 1. ’’ V. Tortorella and G. Bettoni Chem. Comm. 1967 321. 6o Y. Inouye S. Sawada M. Ohno and H. M. Walborsky Tetrahedron 1967,23 3237. “ K. Blaha I. Fric and H. D. Jakubke Coll. Czech. Chem. Comm. 1967,32 558. 62 H. Ziffer U. Weiss and E. Charney Tetrahedron 1967,23 3881. Part (iii) Optical Rotatory Dispersion and Circular Dichroism 53 f I C,H,wCdO,C.CH I ;I (6) itself and its absolute configuration established by Bijvoet X-ray measure- ments (cf. last year's Ann. Reports). The chirality of isolated double bonds has been studied further.63 Carboxy-chromophores.-A detailed survey64 has been made of the 0.r.d. of alp-unsaturated lactones in the steroid cardenolides which show the first extremum of a Cotton effect at 260 mp; the sign can be correlated with the configuration at C-17 and C-14.Some confusion exists in the literature over the three quite separate attempts which have been made56*65*66 to correlate the o.r.d./c.d. of lactones with their molecular geometry. Legrand and Bu~ourt~~ have now considered the sign of lactone Cotton effects in terms of the conformation of the lactone ring. The carboxy-group Sector Rule (originally developed for lactones' 6 has now been used to study the 0.r.d. of the -CO*O- chromophore in esters (steroid acetates)68 and in steroid 17P-carboxylic acids' ' and dinorcholanic acids,69 all of known configuration. The signs of the carboxy-group Cotton effects for these acids and esters can all be rationalised in terms of the Sector Rule and the preferred conformations suggested in the literature for these compounds.0.r.d. has also been used to investigate the absolute configuration of some long-chain hydro~y-acids,~' and to study the effect of alkali on the rotation of a-hydroxy-carboxylic acids.7 A positive Cotton effect has been observed7' for R( +)-[1-2Hl]butyl acetate (6) an ester whose optical activity arises solely from isotopic sub- stitution. The c.d. of malimide and tartarimide has been reported.73 The 0.r.d. of small peptides is receiving growing attention. Detailed papers have appeared74 on glycine peptides having one leucine residue in different positions in the glycine chain on protected peptides of alanine and ~erine,~~ A.Yogev D. Amar and Y. Mazur Chem. Comm. 1967 339. 64 F. Burkhardt W. Meier A. Fiirst and T. Reichstein Helu. Chim. Acta 1967,50,607. 65 G. Snatzke H. Ripperger Chr. Horstmann and K. Schreiber Tetrahedron 1966 22 3103. 66 H. Wolf Tetrahedron Letters 1966 5151. " M. Legrand and R. Bucourt Bull. SOC.chim. France 1967,2241. '' J. P. Jennings W. P. Mose and P. M. Scopes,J. Chem. Soc.(C),1967 1102. 69 G. Gottarelli and P. M. Scopes J. Chem. Soc.(C),1967 1370. 'O T. H. Applewhite R. G. Binder and W. Gafield J. Org.Chem. 1967,32 1173. 7? K. Droll and V. Klingmiiller Tetrahedron Letters 1967 2795. 72 L. Verbit J. Amer. Chem. SOC. 1967,89 167. 73 H. R. Dave and M. K. Hargreaves Chem. Comm. 1967 743. 7* A. F. Beecham Tetrahedron Letters 1967 21 1 Tetrahedron 1967 23 4481 ; Austral.J. Chem. 1967,20 1983. 75 P. M. Scopes D. R. Sparrow J. Beacham and V. T. Ivanov J. Chem. Soc.(C) 1967 221. P. M.Scopes and on some di- and tri-peptides containing aromatic amino-acid residue^.'^ 0.r.d. curves have been interpreted by Shields and M~Dowell’~ as evidence of secondary structure in a protected tetrapeptide. MagneticCircularDichroism.-Important papers on7 m.c.d. which appeared at the end of 1967 will be considered fully in next year’s report. ’tj E. W.Gill Biochim. Biophys. Acta 1967 133,381. ’’ J. E. Shields and S. T. McDowell J. Amer. Chem. SOC.,1967,89 2499. B. Briat D. A. Schooley R. Records E. Bunnenberg,and C. Djerassi J. Amer. Chem. SOC.1967 89,6170,7062 cf. also D. A. Schooley E. Bunnenberg and C. Djerassi Proc. Nut. Acad. Sci. U.S.A. 1966,61377.

ISSN:0069-3030    

DOI:10.1039/OC9676400047

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

2. Part (iv). MASS SPECTROSCOPY By John M.Wilson (Department of Chemistry University of Manchester Manchester 13) General Methods of Interpretation.-The most interesting feature of publica- tions in mass spectrometry in the past year has been the tendency among many of the groups in the field to concentrate on much more detailed work on a narrower range of systems. Substituent effects as described in last year's Report have been widely used and give interesting results even if linear Hammett plots cannot always be obtained. In the case of ions which bear the substituent a linear relationship can be found between intensity ratios and o-values only at low electron energies,' i.e. when th; intensity ratio is a measure of kl and not of k,. The hypothesis of charge localisation in the molecular ion is supported by two types of correlation RC,H4COCH3+' kl,RC6H4+' + -COCH k4Decomposition products firstly between ionisation potentials and fragmentations of bifunctional mole- cules.2 The dominant fragmentations of nitrogen- and sulphur-containing carboxylic acids are those typical of compounds bearing the functional group with the lowest ionisation potential.In molecules of the type (1) there is a (M -C2H4)'. ion of appreciable abundance except when R = NR,. In the latter compound the nitrogen-bearing ring carries the positive charge and the typical ketone fragmentation is s~ppressed.~ R Metastable ions. Several authors have reported decompositions of meta- stable ions that cannot be single proce~ses,~ for example the elimination of two molecules of water from 3,17-dihydro~y-steroids.~" McLafferty has developed his method of identifying the structures of ions by examining the peak shapes and intensities of their metastable ions.Ions of the same structure M.M. Bursey and F. W. McLafferty J. Aiiicv-. Cheril. Soc.. 1967 89 1. H. J. S ec and G. Junk J. Amer. Chem. SOL..,1967,89 790. 'T. J. Wachs and F. W. McLafferty J. Amer. Chem. SOC.,1967,8!3 5044. (a) E. Caspi J. Wicha and A. Mandelbaum Chem. Comm. 1967 1162; (b)J. Seibl Helu. Chim. Acrri 1967 50 263; H. Budzikiewicz F. v. d. Haar and H. H. Inhoffen. Annalen 1967,701 23. John M.Wilson from different molecules will decompose through metastable ions which will have the same shape and the same intensity relative to each other although not relative to the precursor ion.The intensity of metastable ions relative to their precursor ion is found to be a function of the number of vibrational degrees of freedom in the original molec~le.~ This is in agreement with the quasi- equilibrium theory. Measurements of appearance potentials and of metastable ions in benzonitrile show a very slow rise in k with electron energy.6 Measure- ments of half-lives for the process C,HSN+. -+ C6H4+.+ HCN suggest that the maximum value of k is 5 x lo-*. ’ Theoretical calculations have been published of peak shapes for metastable ions decomposing in all regions of a double-focussing mass spectrometer.8 Values of kinetic-energy release for processes in a large number of aromatic hydrocarbons have been obtained.’ AIthough there are a few exceptions,’* triply charged ions are generally of very low abundance but examination of the metastable ions of 9,lO-diphenylanthracene suggests that a considerable proportion of the ions in the spectrum may have a triply charged ion as their origin.l1 Conventional mass spectra of isomeric alkanes and alkenes are often very similar.In many cases where they are indistinguishable or barely dis- tinguishable the metastable spectra are quite different.12 This does not apply to cases such as the isomeric xylenes where decomposition takes place through a common intermediate. Fragmentation Mechanisms.-General fragmentation processes found in common types of functional group are well covered by two books published this year.13 New work of interest has been mainly concerned with rearrange- ments.Hydrogen rearrangements. Further work has been done on the elimination of small molecules from molecular ions of saturated compounds. H,S loss from C5H1 ,SH is 1,3 and 1,4.14 Although HF elimination from C4H9F is also 1,3 and 1,4 in the case of C6HI3F it is 1,5.” Ions of the type (2) undergo a McLafferty rearrangement to product (3) but this does not rearrange further to (4) and (5). This would suggest that keto+nol tautomerism is not generally a favoured process in the mass spectrometer.16 The case of phenol which is thought to rearrange to the keto-form prior to the extrusion of carbon mon- F. W. McLafferty and W. T. Pike J. Amer.Chem. SOC. 1967,89 5951. I. Herter and C. Ottinger Z. Naturforsch. 1967,22a 40. ’ I. Herter and C. Ottinger Z. Naturforsch. 1967,22a 1141. J. H. Beynon and A. E. Fontaine Z. Naturforsch. 1967,22a 334. M. Barber K. R. Jennings and R. Rhodes 2.Naturforsch. 1967,22a 15. lo M. I. Bruce Chem. Comm.,1967 593. l1 K. R. Jennings and A. F. Whiting Chem. Comm. 1967 820. l2 F. W. McLafferty and T. A. Bryce Chem. Comm.,1967,1215. l3 G. Spiteller. ‘Massenspektrometrische Structuranalyse organischer Verbindungen,’ Veriag Chemie Weinheim/Bergstr. 1966; H. Budzikiewicz C. Djerassi and D. H. Williams ‘The Mass Spectra of Organic Compounds,’ Holden-Day SanFrancisco 1967. l4 A. M. Dufield W. Carpenter and C. Djerassi Chem. Comm. 1967 109. l5 W. Carpenter. A. M. Dufield and C.Djerassi Chem. Comm. 1967 1022. l6 J. K. McLeod J. B. Thomson and C. Djerassi Tetrahedron 1967,23,2095. Part (iv).Mass Spectroscopy +. +. 4.. +. (3) oxide is hardly relevant since this process requires an electron energy of 15 ev. Similar conclusions were reached from an examination of the mass spectra of + P-keto-esters. C3H60 * ions are formed by ionisation of acetone McLafferty rearrangement of alkan-2-ones and double rearrangement of alkan-4- and -5-ones. The metastable ions for the decompositions of these ions all have different intensities,18 which would suggest that all have different structures the latter two being (6) and (7). Ideas about the electronic requirements of the McLafferty rearrangement are contradictory.Djerassi suggested'' that since this process was suppressed in some even-electron systems radical character was a necessity at the hydrogen-accepting site. A study of the spectra of a number of substituted butyrophenones suggests that there are two different substituent effects operating. Rearrangement is facilitated by substituents which increase the positive charge at the reactive centre and also by substitu- ents which increase the radical character at the carbonyl group invoking larger participation from canonical forms such as @)?'The operation of the pro- cesses (9) +(10)and (1 1) +(12) shows that such rearrangements may take place when there is positive charge but no radical character present at the carbonyl group.21 Compounds which show behaviour contrary to the above generalisa- tions are ketones of the type (13) which undergo a McLafferty rearrangement with formation of PhCH=CH2+ .at electron energies lower than that necessary to ionise the carbonyl group.22 Other published work on this rearrangement + + l7 R.I. Reed and V. V. Takhistov Tetrahedron 1967,23,4425. 18 F. W. McLafferty and W. T. Pike. J. Amer. Chem. Soc.. 1967.89 5953. 19 C. Djerassi M. Fischer and J. B. Thomson Chem. Comm. 1966 12. 2o F. W. McLafferty and T. J. Wacks J. Amer. Chem. Sac. 1967,89 5043. " M. Kraft and G. Spiteller Chem. Comm. 1967 943. ''J. L. Occolowitz Austral. J. Chem. 1967,20 2387. John M.Wilson ~cH‘Ii3 ’ H’ includes a full discussion of isotope effects. These are nearly all in the range 0.75-1 with the exception of compound (14; X = NH or 0).In the latter compounds the values are in the region of 0.5 presumably because the non- bonding electrons of the X atom contribute to the transition state to give a more unsymmetrical C-H-C system.23 Where there are two unsaturated systems in the molecule which can both undergo this type of rearrangement a ketonic group will decompose more rapidly than an ester or an aromatic ring.Other types of hydrogen rearrangement have been investigated including the double rearrangement in ethers e.g. C2HS+’OC6Hl3+ C2H,0H2+ + C6H1,. In a large proportion of the product ions both rearranged hydrogen atoms come from C-5,and the labelling results can only be explained in terms of a partial randomisation process involving steps such as those shown in Scheme A.25 There has been a general revival of interest in processes involving randomisa- tion in molecular ions.Double-bond shifts in olefin ions are generally associ- ated with random hydrogen migrations,26 but in cyclohexenes they appear to be specific proce~ses.~’ In the n-propane ion all the hydrogen atoms retain their identity even in long-lived states which give metastable decompositions.28 23 J. K. McLeod and C. Djerassi J. Amer. Chem. SOC. 1967,89 5182. 24 J. K. McLeod and C. Djerassi J. Org. Chem. 1967,32 3485. ’’ W. Carpenter A. M. Duffield and C. Djerassi J. Amer. Chem. SOC. 1967,89 6164. 26 B. J. Millard and D. F. Shaw J. Chem. SOC.(B) 1966,664. ’’ T.-H.Kinstle and R. E. Stark J.Org. Chem. 1967,32 1318. ’* C. Ottinger J. Chem. Phys. 1967,47 1452. Part (iv).Mass Spectroscopy All decompositions of the benzene molecular ion are preceded by complete randomisation of the hydrogen atoms. It has been suggested that this may not be a hydrogen migration but a skeletal reorganisation through prismane (15) and benzvalene (16) intermediate^,^^ but '3C-labelling work necessary to h settle this point has not yet been done. The decomposition of pyridine by loss of HCN is likewise preceded by H-rand~misation,~' but in a similar elimination from [1-2H]quinazoline there is 90% loss of HCN.31 The loss of HCN from the benzonitrile ion appears to take place by two different routes one involving complete hydrogen randomisation the other being a specific 1,2-elimination ;32 it is possible that these are decompositions of different electronic states of the molecular ion.Skeletal rearrangements. This type of process has been found to occur in a large variety of organic compounds and its occurrence has been compre- hensively covered in a review,33 so this report will cover only a few selected examples. There has been a lively controversy about the structure of the ion formed by elimination of carbon monoxide from a-pyrone. This was thought to be cyclised to a f~ran,~~ but there has been evidence to the contrary. If the hypothesis is correct [3-2H]-a-pyrone and [6-2H]-a-pyrone should both yield the same product [2-2H]furan. In the further decomposition C4H40+ . -+ C3H3+ the retention of deuterium in C3H3+ is different in these two cases suggesting that the product cannot be f~ran.~' It has been pointed out however that C3H3+ can be formed by an alternative route in which C-3 and C-6 do not become equivalent (1 7) 4(19),36so the furan theory is not ruled out.Intensities of metastable ions for the decomposition of the C4H40' . ion from pyrone are quite different from those in the spectrum of f~ran,~~ but this is a comparison 29 K. R. Jennings Z. Naturforsch. 1967,22a 455. 30 D. H. Williams and J. Ronayne Chem. Comm. 1967 1129. 31 T. J. Batterham A. C. K.Triffett and J. A. Wunderlich J. Chem. SOC.(B),1967,892. 32 R. G. Cooks R. S. Ward and D. H. Williams Chem. Comm. 1967 850. 33 P. Brown and C. Djerassi Angew. Chem. 1967,79,481.34 C. S. Barnes and J. L. Occolowitz Austral. J. Chem. 1964 17 975. 35 W. H. Pirkle J. Amer. Chem. SOC. 1965,87 3022. P. Brown and M. M. Green J. Org. Chem. 1967,32 1682. 37 F. W. McLaNerty and W. T. Pike J. Amer. Chem. SOC. 1967,89 5955. John M.Wilson of a molecular ion and a fragment ion so a difference in internal energy may be responsible for this effect. Another approach has been to examine the kinetic energy release in the metastable ion for Mf. -+ C,H,O+’. This energy does not vary within the group of compounds (20; R = p-H p-OMe p-Ph) where the charge or radical-stabilising ability varies considerably. The authors consider that this evidence is in favour of an unsymmetrical structure (21) in which the aryl-group n-electrons are orthogonal to the electron-deficient orbitals.38 This reporter feels however that we do not know enough yet about the conditions under which internal energy may be converted into kinetic energy to allow us to make decisions based on this evidence.t Ph Ph(37H4R The rearrangement which results in loss of carbon dioxide from organic carbonates has been studied in detail3’ In the case of aryl methyl carbonates the oxygen of the methoxy-group is lost and the process is promoted by electron- withdrawing substituents. The product ion behaves in its further decomposi- tions like ArOMe’ rather than o-MeC,H,OH+ so a mechanism can be written (22)-423). In the corresponding process in the spectra of diary1 carbonates aryl groups with electron-releasing substituents migrate more readily.A number of migrations of groups containing oxygen or nitrogen can be explained by a general mechanism as shown in Scheme B.,Oa Such a mechanism can rationalise the formation of PhCH OMe+ in the spectra of P-bromo-p-phenylpropionic acid and dimethyl phenylsuccinate and other ions in the spectra of dicarPoxylic Another example is the methoxy-migration found with 4-methoxy- cyclohexanone discussed in last year’s Report this process is found in 38 M. M. Bursey and L. R. Dusold Chem. Comm. 1967 712. 39 P. Brown and C. Djerassi J. Amer. Chem. SOC.,1967,89 2711. ’* (a) R. G. Cooks and D. H. Williams Chem. Comm. 1967 51 ; (b) I. Howe and D. H. Williams Chem. Comm. 1967,733. 41 (a)M. M. Green D. S. Weinberg and C.Djerassi J Amer. Chem. Soc. 1966,88 3883; (b) ibid. 1 967,89 5 190. Part (iv).Mass Spectroscopy Y-x Y-x +Y-x I-I-II Ph-CH-CHZ Ph-CH -CH 2 Ph-CH-CH, + R I +I Ph-CH=Y + CH,=X SCHEME B cyclohexanones and cycloheptanones with hydroxy- or alkoxy-groups at the 3- or 4-po~itions.~’~ In the formation of the ion RCH=CHCO+ from 4,4-dialkylcyclohexenones (24) substituent effects suggest that an R group and an R’ group migrate.42 The same authors report similar findings for benzylidene- cyclohe~anones.~~ The structural requirements for the elimination of keten from the molecular ions of UP-unsaturated ketones are the same as those for the photochemical conversion into bicyclohex[3,l,0]an-2-ones and the elimina- tion should go as in (25).44 In the mass spectra of substituted benzophenones there is a rapid loss of carbon monoxide from the doubly charged molecular ion which is not observed with the singly charged species.This may be due to the coupling of a diradical species such as (26).45 Labelling work demonstrates that the methyl group lost from a-ionone is not one of the gem-dimethyls but the vinylic group. This can be explained by invoking a cyclisation (27)-+(28).46 The decomposition C,H2C1,+ . -+ C2C12+. in the spectrum of 3,S-dichloropyridazine suggests that the former ion may be a cy~lobutadiene.~’ Ions C,H3 from various dienes have a cyclic structure.48 Several examples + 42 R. L. N. Harris F. Komitsky jun. and C. Djerassi J. Arner. Chem. SOC.,1967,89 4765.43 R. L. N. Harris F. Komitsky jun. and C. Djerassi J. Amer. Chem. SOC. 1967,89 4775. 44 A. L. Burlingame C. Fenselau W. J. Richter W. G. Dauben G. W. Shaffer and N. D. Vietmeyer J. Amer. Chem. SOC.,1967,89 3346. 45 F. W. McLafferty and M. M. Bursey Chem. Comm. 1967 533. 46 A. F. Thomas and B. Willhalm Tetrahedron Letters 1967 5129. 47 S. J. Weininger and E. R. Thornton J. Amer. Chem. SOC. 1967,89 2050. 48 J. L. Franklin and A. Mogenis J. Chem. Phys. 1967,71 2820. 62 John M. Wilson have been found of elimination of methylene from large ions. 49 Other systems in which skeletal rearrangements have been found include unsaturated esters," azobenzenes," azoxy-compounds,52 a~ylthiophens,~~ aromatic N-~xides,~~ dimet hylaminopyrimidines N-methyltriflu~roacetanilide,~ thioesters 57 and thi~nylaniline.~~ Miscellaneous.-Both 1,4-59 and 1,2-quinones(jo dehydrogenate in the heated inlets of mass spectrometers.These are presumably surface reactions. Work still continues on methods of determining the positions of double and triple bonds in aliphatic molecules. The field-ionisation mass spectra of alkynes show intense peaks corresponding to alkyl ions formed by field- dissociation at the propargylic bond but the spectra are complicated by ion- molecule reactions taking place in the multilayer at the emitter tip.61 An alternative method involves the conversion of the acetylene in a single reaction into a mixture of two ethylene ketals which will decompose very predictably.62 Two methods have been suggested for olefins.The simplest involves direct mass-spectrometric examination of the epoxide which will give intense peaks due to ~t-cleavage.~~ The alternative method is to hydroxylate the double bond and then methylate the hydroxy-gr~ups.~~ This method has been shown to be useful for compounds with more than one double bond. Long-chain dialkyl ethers have much simpler spectra when the molecules are weakly excited; with a source temperature of 90" and 12 ev electrons the only frag- ments observed from R,O are (RH)'. R+,and ROH,+.(j5 The mass spectra of catenanes are of interest in that a molecular ion cor- responding in mass to the sum of both rings is found. In the spectrum of (29)(j6 the elimination of keten gives rise to ions containing both rings but processes which involve fission of bonds in either of the rings produce ions of the same mass as found in the isolated ring compound.There is one ion which requires a hydrogen-transfer from one ring to the other; this could be called an intra- molecular ion-molecule reaction. 49 B. D. Tilak K. G. Das and H. M. El-Narnaky Experientia 1967,23,609. 50 D. H. Williams R. G. Cooks J. H. Bowie P. Madsen G. Schroll and S. 0.Lawesson Tetra-hedron 1967,23 51 J. H. Bowie G. E. Lewis and R. G. Cooks J. Chem. SOC.(B) 1967 621. 52 J. H. Bowie G. E. Lewis and R. G. Cooks Chem. Comm. 1967 284; Austral. J. Chem. 1967 20 1601. 53 J. H. Bowie R. G. Cooks S. 0.Lawesson and C. Nolde J. Chem. SOC.(B) 1967,616. 54 J. H. Bowie R. G. Cooks N. C. Jamieson and G.E. Lewis Austral. J. Chem. 1967,20 1601. 55 Y.Rahamin J. Sharvit A. Mandelbaum and M. Sprecher J. Org. Chem. 1967 32 3856. 56 R. A. W. Johnstone D. W. Payling and A Prox Chem. Comm. 1967 826. 57 J. H. Bowie S. 0.Lawesson F. Duss P. Madsen and R. G. Cooks Chem. Comm. 1967,346. 58 B. E. Job Chem. Comm. 1967 44. 59 R. T. Aplin and W. T. Pike Chem. and Znd. 1967 2009. 6o K. Ukai K. Hirose A. Tatematsu and T. Goto Tetrahedron Letters 1967 4999. 61 B. C. Patterssn and M. Seakins Trans Faraday SOC.,1967,63 1863. 62 H. E. Audier J. P. Beguk P. Cadiot and M. Fetizon Chem. Comm. 1967 200. 63 R. T. Aplin and L. Coles Chem. Comm. 1967 858. 64 W. Niehaus jun. and R. Ryhage Tetrahedron Letters 1967 4999. 65 M. Spiteller-Friedmann and G. Spiteller Chem. Ber.1967 100 79 66 W. Vetter and G. Schill Tetrahedron 1967,23 3079. Part (iu). Mass Spectroscopy A detailed study has been made of the relative ease of loss of alkyl radicals from ketals. As would be expected the most pronounced difference is between primary secondary and tertiary radicals but the increase in fragmentation probability with size within a homologous series is still ~onsiderable.~ Ionisation potentials of ally1 radicals are almost identical with those of isomeric non-allylic radicals,68 i.e. allylic stabilisation is the same for radicals as for carbonium ions. Stereochemical effects. Epimeric steroid pairs give distinguishable spectra.69 The stereochemistry of the A/B ring junction in 4,4-dimethylandrostan-6-one affects some of the fragment intensities particularly where there is a difference in the distance between sites of exchange of a rearranging hydr~gen.~' trans-Methyl 2,3,3-trimethylcyclopentylacetatehas a much more intense peak for the McLafferty rearrangement than its ~is-isomer.~' Presumably this process is favoured by the cis-relationship of the carbonyl group and the tertiary hydrogen atom in the trans-isomer.Similarly in the case of the 2-acyl norbornanes the endo-isomer rearranges more rapidly.' In the 2,3-dimethylcyclobutanone series the cis-isomer always decomposes more rapidly than the trans-isomer but the difference shows up in all fragmentation processes not only in those involving the bond between the methyl-substituted carbon atoms.73 In p-lactams such as (30) the effect is more specific.The products of fission a are more abundant from the cis-isomer than from the trans-isomer and the reverse applies to fission b.74 In the spectra of some 2-aryl-3-acylazetidines there is reported the first case of stereospecific process. The compounds in which there are aryl and acyl groups trans to each other show an (M -OH)' peak in their spectra which is completely absent from those of the cis-isomers. The molecular arrangement necessary is thought to be as in (31).75 67 J. T. B. Marshall and D. H. Williams Tetrahedron 1967 23 321. 68 S. Pignataro A. Cassuto and F. P. Losing J. Amer. Chem. SOC. 1967,89 3693. 69 N. S. Wulfson V. I. Zaretskii V. L. Sadovskaya A. V. Zakharychev S. N. Ananchenko and I. V. Torgov Tetrahedron 1967 23 3667; V.I. Zaretskii N. S. Wulfson and V. G. Zaikin ibid. p. 3683. 'O H. E. Audier M. FCtizon and P. Foy Bull. SOC. chim. France 1967 1271. J. Cason and A. I. A. Khodair J. Org. Chem. 1967,32 575. 72 A. F. Thomas and B. WiIIhaIm Helv. Chim. Acta 1967 50 826. 73 H. E. Audier J. M. Conia M. Fktizon and J. Gore Bull. SOC. chim. France 1967 787. 74 H. E. Audier M. FCtizon H. B. Kagan and J. L. Luche Bull. SOC. chim. France. 1967 2297. 75 J. L. Imbach E. Dornes N. H. Cromwell H. E. Baumgarten and R. G. Parker J. Org. Chem. 1967.32. 31 23 John M. Wilson Chemical ionisation. The reactions of s-butyl ions (from n-butane) and t-butyl ions (from isobutane) with olefins have different reaction rates.76 The study of such gas-phase reactions may be useful for the identification of the ions used.In benzene at high pressure C6H6+ is unreactive towards benzene except possibly in a symmetrical charge-transfer process. All fragment ions from benzene react by electron-transfer with C6H6 to form C6H6+.77 Chemical ionisation mass spectra of cycloalkanes show (M -H)+ ions whose abundance can be correlated with the number of hydrogen atoms available for abstrac- ti~n.~~ The reaction leading to the formation of cyclo-olefin ions by elimination of side-chains appears to be unique in this sort of system in being endothermic. In the case of aromatic hydrocarbons the most important processes are proton- ation and alkylation of the aromatic ring.79 Some hydride-abstraction does take place but only from saturated C-H groups in side-chains.In toluene and cycloheptatriene the hydrogen atoms retain their identity.80 76 M. S. B. Munson J. Amer. Chem. SOC. 1967,89 1772. 77 F. H. Field P. Hamlet and W. F. Libby J. Amer Chem. SOC.,1967,89 6035. '13 F. H. Field and M. S. B. Munson J. Amer. Chem. SOC. 1967,89,4272. 79 M. S. B. Munson and F. H. Field J. Amer. Chem. SOC. 1967,89 1047. *' F. H. Field J. Amer. Chem. SOC.,1967,89 5328.

ISSN:0069-3030    

DOI:10.1039/OC9676400055

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

2. Part (v). X-RAY CRYSTALLOGRAPHY By George Ferguson (Department of Chemistry The Universi&y of Glasgow Glasgow W.2 Scotland) CRYSTALLOGRAPHY has been included in the ‘Physical methods’ section of annual reports for the first time and the literature coverage is slightly different from that of the previous organic crystallography section. The aim of this report is to give details of the more important organic structures which have been determined by single-crystal diffraction methods during the year ;papers which only give barest details of constitution of e.g. natural products will be reported in other chapters and have been omitted here. More than 250 relevant papers have appeared but space requirements have necessitated omitting some of these from this review.The advent of computer-controlled diffractometers makes accurate data collection more rapid and also makes the determination of the absolute configuration’ of organic molecules more certain. More accurate data can also allow details of bonding electron distribution and hydrogen-atom positions to be determined but for precise location of hydrogen atoms the increasing availability of neutron diffraction facilities is invaluable. The crystallographic phase problem remains the major difficulty in crystal structure analysis and was overcome in 53% of the structures reported here by the heavy-atom method; ‘direct’ methods’ were described in 15% of the papers and the structures described in the remaining 32% mainly those of small molecules were solved from a knowledge of molecular geometry vector or trial-and-error met hods.It is probably timely to insert a word of warning here to non-crystallo- graphers about the significance of ‘estimated standard deviations’ of bond lengths and angles which should always be quoted in crystallographic papers. It is generally recognised that it is not safe to claim that e.g. a bond length is significantly different from a comparable value unless the difference is at least three times the estimated standard deviation. This appears to have been overlooked on occasion and the situation is further complicated because in the majority of cases the estimated standard deviations are themselves under- estimated owing to the computational approximations used in deriving them.Carboxylic Acids and Related Compounds.-Crystals of deuteriated oxalic acid dihydrate (C02D),,2D20 are not isomorphous with those of (C02H), J. M. Bijvoet Proc. k. ned. Akad. Wetenschap. 1949,52 313. See also S. Ramaseshan in ‘Ad- vanced Methods of Crystallography,” Academic Press London 1964 p. 67. For a brief review see M. Gerloch and R. Mason Ann. Reports 1966,689. George Ferguson 2H20. A three dimensional X-ray analysis3 has determined the C-€ distance as 1-539 & 0.005 8 and shown the oxalic acid and water molecules to be linked by hydrogen bonds forming a three-dimensional network. The hydrogen-bond distances O-D-.-O are 2.540 2.854 and 2.822 A. The deuterium atom positions have been determined by neutron diffraction ;4 the bond lengths in the D20 molecule are 0.946 and 0.960 A and D-0-D is 110.5".The lack of optical activity in meso-tartaric acid was usually ascribed to intramolecular compension; the two halves of the molecule were supposed to be mutually related either by a mirror plane or a centre of symmetry.X-Ray analyses of a number of crystalline modifications of the acid,5 however show that in the solid a staggered conformation (1) is adopted as is found in meso- tartrates. The lack of optical activity is thus caused by intermolecular com- pensation of the conformational antipodes. meso-PP'-Dimethyladipic acid has a crystallographic centre of symmetry6 and hence adopts a fully staggered conformation in the crystal. HO CO H Me HO@:02H C02H An X-ray arbitration7 has unequivocally established the absolute configura- tion of (+)-2-isopropyl-7-methylglutaric acid as (2) in agreement with the work of Norin,' but at variance with the conclusion of Djerassi and co-worker~.~ It follows that the configurations assigned' to ( +)-sabinene (-)-umbellulone the thujanes and their congeners are confirmed but that the absolute configuration assigned to ( -)-methylisop~legone~must now be represented as (3).A further consequence of these findings is that the abso- lute configurations of numerous derivatives of ( -)-methylisopulegone and (-)-menthone must be revised. The C-0 bond lengths in aminomalonic acid show that the molecule is + present in the crystal as the zwitterion C02H*CHNK,*C0,-.'0Molecules of F.F. Iwasaki H. Iwasaki and Y. Saito Acta Cryst. 1967,23,64. F. F. Iwasaki and Y. Saito Acta Cryst. 1967,23 56. G. A. Bootsma and J. S. Schoone Acta Cryst. 1967,22 522. E. Martuscelli Ricerca sci. 1967 37 53; P. Ganis E. Martuscelli and G. Avitabile Ricerca sci. 1966,36,689. M. R.Cox H. P. Koch W. B. Whalley M. B. Hurtshouse and D. Rogers Chem. Comrn 1967 212. T. Norim Actu Chem Scand. 1962 17 640. E. J. Eisenbraun F. Burian J. Osiecki and C. Djerassi J. Arner. Chem. SOC.,1960,82 3476. lo J. A. Kanters J. Kroon P. T. Beurskens and J. A. Vliegenthart Actu Cryst. 1966 21 990. Part (u) X-Ray Crystollagraphy eaminocaproic acid NH .[CH,] -CO,H which undergo polycondensation in the solid state to Nylon 6 are also present in the crystal as zwitterions with three hydrogen atoms 0.9 8 from each nitrogen atom." Each zwitterion forms three N-H*.*O hydrogen bonds (2.728 2.753 and 2.814 A) with adjacent molecules to form a double-layered structure.The analyses of a number of butadienes of photochemical interest have appeared. The structure12 of monomethyl-trans-trans-muconate MeO,C* CH CH * CH :CH C02H con-sists of stacks of hydrogen-bonded molecule pairs (O-H***O 2.62 A); double bonds of nearest neighbour molecules make contact along the short c axis of the crystal and are presumably responsible for the photochemical formation of cyclobutanes. Interstack contacts of 3.6 A between terminal atoms of the butadiene chain may account for initiation of polymerisation. In the dimethyl- m~conate,'~ analysis of the bond angles at the methoxycarbonyl group suggests that the axis of the methyl group makes an angle of 5" with the 0-C vector interpreted as evidence for a bent bond.Accurate analyses have also yielded precise measurements for trans-trans-~orbamide'~ and trans-trans-muconodi- nitrile.15 Two polymorphic modifications of azelaic acid HO2C*[CH2],*CO2H,have been studied.I6 The two carboxyl groups of the a-form have different dimensions [C(l)-O(1) = 1.253 C(l)-0(2) = 1.266 8 while C(9) O(3) = 1.283 and C(9)-0(4) = 1.192A). The &form has crystallo- graphic two-fold (C,) symmetry with the C-0 distances 1.231 and 1.303 A. Both compounds crystallize to give infinite hydrogen-bonded chains. Deca- trans-3-trans-7-dienedioicacid has a crystallographic centre of symmetry and the torsional angle at C(2)-C(3) (double bond eclipsed to hydrogen) is -125-5" while at C(4)-C(5) (double bond eclipsed to methylene) the torsional angle is -1-6".This is thought to be the first observation of a cis-conformation in the crystalline state for this kind of bond.17 The occurrence and authenticity of some 'very short' hydrogen bonds in which the hydrogen atom is symmetrically positioned between two oxygen atoms have been reviewed.l8 Very short hydrogen bonds are found in the isomorphous rubidiumlg and potassium hydrogen diaspirinates.,' The metal ion lies on a two-fold axis and makes contact with an oxygen atom from each of six different aspirinate residues and each aspirinate makes contact with three different metal ions.The acidic hydrogen atom is involved in a very short hydrogeh bond with 0 0 2.455 0.005 A which connects two aspirinate residues across a centre of symmetry. A neutron diffraction study of G. Bodor A. L. Bednowitz and B. Post Acta Cryst. 1967,23 482. D. Rabinovich and G. M. J. Schmidt J. Chem. SOC.(B) 1967 286. l3 S. E. Filippakis L. Leiserowitz and G. M. J. Schmidt J. Chem. Soc. (B) 1967 290. l4 S. E. Filippakis L. Leiserowitz and G. M. J. Schmidt J. Chem. SOC.(B),1967 297. l5 S. E. Filippakis L. Leiserowitz and G. M. J. Schmidt J. Chem. Soc. (B),1967 305. J. Housty and M. Hospital Acra Cryst. 1967 22 288. E. Martuscelli Acta Cryst. 1967 23 1086. J. C. Speakman Chem Comm. 1967 32. l9 S. Grimvall and R. F. Wengelin J. Chem.Soc. (A),1967 968. L. Manojlovic and J. C. Speakman J. Chem. SOC.(A) 1967,971. C* 68 George Ferguson ammonium tetroxalate has located the hydrogen atoms accurately.2 The structure involves seven distinct hydrogen bonds ranging in length (0 -* 0) from 2.899-2.472 A and the corresponding 0-H distances tend to increase inversely from 0.945 to 1-102A. The four hydrogen atoms of the ammonium ion form four nearly linear N-H.**O bonds with N-.*O lengths 2.927 2.938 2.950 and 2.974 (each +0004)8 and N-H 1.004,1.022 1.015 and 0.995 (each +0.010) A. The oxalate residues have their carbon and oxygen atoms coplanar and C-C 1.544 1549 (each +0.005),and 1549 (&0-003)A. The ionization in the di-ionized salt lithium ammonium citrate monohydrate occurs in the central and one of the terminal carboxyl groups.22 The backbone of the citrate ion is fully extended in a plane roughly perpendicular to that of the central carboxy- and hydroxy-groups.The lithium ion is tetrahedrally co- ordinated by oxygen atoms from four different citrate ions. Considerable overcrowding exists in the anion of the monorubidium salt of furantetra-carboxylic acid and the resulting strain is accommodated in the usual ways by (a)in-plane splaying-out and out-of-plane bending of the exocyclic carbon atoms and (b)rotation of the carboxyl-group planes with respect to the furan ring plane.23 There is a close approach (2.386 f0.013A) between two oxygen atoms of adjacent carboxy-groups but no hydrogen atoms were located. exo-N-Methylacetanilide has crystallographic rn-symmetry with the N- methylacetamido-group in the mirror plane and the benzene ring normal to it.24 The N-C /\distance is short (1.344A)revealing appreciable double-bond character and the C=O distance (1.263 8,) is somewhat longer than the corresponding distances in related compounds.Ethyl carbamate crystals (urethane EtO CO NH2) consist of chains of centrosymmetric hydrogen- bonded dimers (N-H-**O 2.96 A) which are interconnected by further N-H-**O hydrogen bonds. The C=O and C-NH distances are 1.221 and 1.345A. The N-C-0 angle is 123.6°.25Acrylamide crystallises in a similar fashion.’ L-Ornithine hydrochloride exists as a zwitterion NH3 + [CH,] CH(NHi) CO,-.Cl-in the solid state each nitrogen atom making three N-H***O hydrogen bond^.'^ The molecule is characterised by two planar groups the carboxy-group and the aliphatic chain.The absolute configuration of (+)-2-benzylglutamic acid hydrobromide hydrate has been determined as D. The carbon skeleton of the glutamic acid is different from other glutamic acids (which are reviewed) due to internal rotations of the C-C bonds.28 The molecular and crystal structures of the hydrochloride and hydrobromide of 21 M. Currie J. C. Speakman and N. A. Curry J. Chem. SOC.(A) 1967 1862. 22 E. J. Gabe J. P. Glusker J. A. Minkin and A. L. Patterson Acta Cryst. 1967,22 366. 23 L. L. Martin and 1. C. Paul Acta Cryst. 1967,22 559. 24 B. F. Pedersen Acta Chem. Scand. 1967,21 1415. 25 €3. H. Bracher and R. W.H. Small Acta Cryst. 1967,23 410. 26 I. V. Isakov Zhur. strukt. Khim. 1966,7 898. 27 A. Chiba T. Ueki T. Ashida Y. Sasada and M. Kakudo Acta Cryst. 1967,22,863. T. Ashida Y. Sasada and M. Kakudo Bull. Chem. SOC. Japan 1967,40 476. Part (v) X-Ray Crystallography 69 L-tryptophan have also been determined.29 The iodine-iodine distance in di-iodo-L-tyrosine dihydrate (4) (6.05 A) is shorter than was expected. The molecules are held in the crystal by an extensive hydrogen-bond net involving the carboxy- amino- and phenolic groups of the molecule and the water molecules.30 The stereochemistry of L-leucine hydrobromide has been deter- mined3' and that of L-valine hydrochloride revised.32 The crystal structures of pentane and octane have been refined.33 In pentane the average C-C bond length is 1.533 0.006 the C-C repeat distance is 2.539 f0.006 A and the average C-C-C angle is 112.1 & 0.3".The shortest intermolecular C*-C distances are 3.90 3-95 and 3.96 A. In octane the average C-C bond length is 1525 f0.007 A the C-C repeat distance is 2.545 + 0.007 8 and the average C-C-C angle is 113.3 f0.6". The shortest intermolecular C...C contact is 3-65 A. OH CH2*CH(NH,) .CO,H I0 9 Me CH Aromatic Molecules and Related Systems.-The structure3' of 1,6-epoxy[lO]annulene (5) is very similar to the carboxylic acid derivative of 1,6-methano[ 10Jann~lene.~~ The eight carbon atoms not directly associated with the bridge i.e. C(2&(5) and C(7t(lO) are essentially coplanar while the re-entrant carbon atoms C(1) and C(6) each lie 0.35 A above the plane with the oxygen atom 1.25 A from it.The C-0-C- angle is 102" compared with 99.6 % for the related angle in the carbocycle. The tendency towards bond alternation noted in the carb~cycle~~ is not seen in the epoxy[lO]annulene where all bond lengths are within two standard deviations of 1.39 A. The 29 T. Takigawa T. Ashida Y. Sasada and M. Kakudo Bull. Chem. SOC.Japan 1966,39,2369. 30 J. A. Hamilton and L. K. Steinrauf Acta Cryst. 1967,23 817. 31 E. Subramanian Acta Cryst. 1967,22 910. 32 0.Ando T. Ashida Y.Sasada and M. Makudo Acta Cryst. 1967 23 172. 33 H. Mathisen N. Norman and B. F. Pedersen Acta Chem. Scand. 1967 21,127. 34 N. A. Bailey and R.Mason Chem. Comm. 1967 1039. 35 M. Dobler and J.D. Dunitz Chim. Acta. 1965 48 1429. 70 George Ferguson bond-length distribution in the peripheral 14-membered ring of trans-15,16-diethylhydropyrene (6) is characteristically aromatic36 (the lengths are in the range 1-393-1.401 A). A measure of the non-planarity of the ring is the displacement of C(3) (0.232 A) from the plane containing the four carbon atoms bonded to the internal substituent. In the corresponding methyl derivative37 the distance is only 0.117 A. The C(6)-C(9) bond is abnormally long (1-574 A) and the angle C(6)-C(9)-C(10) (115.7") is much greater than tetrahedral. The effect of these distortions is to increase the separation of the two methyl hydrogens at C(10) from the carbon skeleton of (and presumably from the 71-electron cloud of) the 14-membered ring.The transannular bond length in azulene-1,3-dipropionic acid is 1.490 A 0.008 A strongly suggestive of a Csp2-Csp2 single bond.38 The average value of the peripheral ring bonds is 1.393 A. The internal C-C-C bond angles at C(4) C(6) and C(8) are in good agreement with each other and their average of 130.1" is greater than that 127.43 of the remaining internal angles in this ring. l-Methylamino-7- methylimino-1,3,5-cycloheptatriene (7) a typically disubstituted aminotropone- imine has essentially mm2 (C?,,)symmetry including the methyl groups which are staggered with respect to the nearest ring hydrogen at0ms.j' The molecular structure can be described in terms of a 10-electron x-system encompassing the seven-membered ring and the two nitrogen atoms.The single amino- hydrogen atom appeared in a final difference Fourier synthesis as two half- weight peaks symmetrically situated with respect to the two nitrogen atoms a finding consistent with the n.m.r. i.r. and chemical evidence that the nitrogen atoms are equivalent. The i.r. absorption spectra of a number of o-halogeno-aroyl compounds both in solid state and solution have been correlated with the crystal structures as revealed by three-dimensional X-ray analy~es.~' For ortho-substituted benzoic acids the preferred conformation in the solid state is usually that with the carbonyl group adjacent to and the hydroxy-group remote from the ortho-substituent. An analysis of the o-toluic acid structure4' shows sur- prisingly that the displacements of the carboxy- and methyl groups from the benzene plane are not large.For o-nitrobenzoic acid42 the plane of the carboxyl group and that of the nitro-group make angles of 23.4 and 54.7" respectively with the benzene-ring plane. The effect of intramolecular over- crowding causes the exocyclic carbon atom and the nitrogen atom to be dis- placed out of the aromatic plane in opposite directions by 0.209 and 0.160 respectively; the C-C and C-N bonds are also displaced sideways. The angle of tilt of the carboxy-group plane to the benzene plane in o-fluorobenzoic 36 A W. Hanson. Acta Cryst.. 1967. 23. 476. 37 A. W. Hanson Acta Cryst. 1965 18 599. 38 H. L. Ammon and M. Sundaralingam,J. Amer. Chem. SOC.,1966,88,4794.39 P. Goldstein and K. N. Trueblood Acta Cryst. 1967 23 148. 40 G. Eglinton G. Ferguson K. M. S. Islam and J. S. Glasby J. Chem. SOC.(B),1967 1141. 41 C. Katayama A. Furusaki and I. Nitta Bull. Chem. SOC.Japan 1967,40 1293. 42 T. D. Sakore S. S. Tavaie and L. M. Pant Acta Cryst. 1967,22 720. M. Kurashi M. Fukuto and A. Shimada. Bull. Chem. SOC. Japan 1967,40,1296. Part (u) X-Ray Crystallography acid is 6.7".43Overcrowding in 2,6-dimethylbenzoic acid causes the carboxy- group plane to be rotated 53"out of the benzene-ring plane and there are also small displacements (0.01 and 0-03A) of the methyl carbon atoms from the aromatic plane.44 Both polymorphs of terephthalic acid have been investigated. In one polymorph which was thoroughly refined:' the bond lengths indicate a small amount of quinonoid character in the benzene ring.The carboxy-group planes are rotated about the exocyclic C-C bond 5" out of the benzene plane. The structure of ammonium acid o-carboxybenzene sulphonate NH4+ (C6H4*C02H*S0,) consists of ammonium ions and infinite chains of o-carboxybenzene sulphonate iyns with intermolecular 0-H 0 bonds 0.. (264 A) between carboxy- and sulphonate groups.46 The carboxy-group plane makes an angle of 50.7" with the benzene plane; other effects of steric interaction include shifts of the sulphur (-0.06 A) and carboxy-carbon (+0.13 .$) from the benzene plane and deviations of ring bond-angles from normal values. Crystals of orthanilic acid NH,f *C,H4*S0 have an N--H-*O bond system which contains a 'bifurcated' bond of intra- and inter-molecular character47 (the hydrogen-atom positions were confirmed by neutron diffraction analysis of the Okl projection).The residual electron density map contains features attributable to bonding electrons; for the majority of C-C bonds these consist of dumbell shaped peaks which extend approximately 1 .$ above and below the plane of the benzene ring. P-Sulphanil- amide has been studied4* by X-ray and neutron diffraction. The bond lengths in the molecule suggest that there is a small but significant contribution of quinonoid resonance form to the structure. The distribution of residual electron density within the benzene ring is explained in terms of effects resulting from electron redistribution at bonding.The hydrogen-bond system closely resembles that found in a-sulphanilamide and the N-H**.O bonds are about 0.25 A longer than in related zwitterion compounds. The dimensions of the two structurally distinct molecules of p-aminobenzoic acid in the crystal asymmetric unit also suggest a small amount of quinoid character.49 Both amino- and carboxy-groups are displaced slightly from the benzene-ring planes. Me 1 43 G. Ferguson and K. M. S. Islam Acta Cryst. 1966,21 1000. 44 R. Anca S. Martinez-Carrera and S. Garcia-Blanco Acta Cryst. 1967 23 1010. 45 M. Bailey and C. J. Brown Acta Cryst. 1967 22 387. 46 Y. Okaya Acta Cryst. 1967 22 104. 47 S. R. Hall and E. N. Maslen Acta Cryst. 1967 22 216. 48 A. M. O'Connell and E. N. Maslen Acta Cryst.; 1967 22 134.49 T. F. Lai and R. E. Marsh Acta Cryst. 1967 22 885. 72 George Ferguson The tricarbonylchromium o-toluidine molecule adopts the eclipsed con- figuration (8) in which the carbonyl-chromium vectors point closely towards the benzene carbon atoms which are ortho and para to the amino-~ubstituent.~~ The configuration (9) of tricarbonylchromium methylbenzoate is that in which the carbonyl-chromium vectors point towards the 1-,3-,and 5-positions of the benzene ring.51 These results support the proposition that the orientation of the tricarbonylchromium moiety in a tricarbonylchromium substituted benzene reflects the electron-withdrawing or -releasing character of the benzene substi tuen t. The o-nitro-groups in 2,4,6-trinitrophenetole are rotated out of the phenyl ring one by 62" and the other by 33".52The p-nitro-group is almost coplanar with the phenyl ring (rotation angle 3") and the plane of the OEt group is almost perpendicular (88") to the phenyl ring plane.The structure of the Meisenheimer complex of 1,l-dimethoxy-2,4,6-trinitrobenzene K+ [C6H2(N02),(OMe)2]-,2H,0 has been determined.53 Although the angle C(2)-C(l)-C(6) in the benzene ring is 110",the ring is nearly planar (largest deviation 0.03 A). The nitro-groups at the 2-,4- and 6-positions are twisted 6",6" and 11" out of the benzene plane. None of the substituent groups of N-methyl-N-2,4,6-tetranitroaniline (tetryl) is coplanar with the benzene ring.54 The angles made by the nitro-group planes with the aromatic ring are 25" 23" and 44"; the nitramine-group plane is inclined at 65" to the ring.The non-planarity of the 4-nitro-group may be caused in part by the formation of an intermolecular hydrogen bond between a benzene-ring hydrogen and an oxygen atom of a neighbouring nitramine group. 1,3-Dichloro-2,4,6-trinitrobenzenehas crystallographic two-fold symmetry; the nitro-group in the 2-position is rotated 75" from the plane of the benzene ring and the 4-nitro-group through 37°.55In contrast the nitro-groups of 1,3-diamino-2,4,6-trinitrobenzene are not rotated out of the aromatic plane ;56 there are apparent intramolecular hydrogen bonds between adjacent amino- f and nitro-groups (N-*.O 2.52-260 A). The benzene ring is distorted so as to relieve the overcrowding of the planar conformation.The structure analysis of N,3-dimethyl-4-bromo-2,6-dinitroaniline shows that the N-methyl group is in contact with the 2-nitro-group thus confirming earlier n.m.r. and i.r. res~lts.~ ' The bonding to the exocyclic carbon atom in potassium p-nitrophenyl- dicyanomethide (10) is ~yrarnidal.~' The cyano-groups and the nitro-group are significantly displaced from the phenyl-ring plane in the same direction. " 0.L. Carter A. T. McPhail and G. A. Sim J. Chem. SOC.(A),1967 228. 51 0.L. Carter A. T. McPhail and G. A. Sim J. Chem. Soc. (A),1967 1619. 52 C. M. Gramaccioli,R. Destro and M. Simonetta Chem. Comm. 1967,331. 53 H. Ueda N. Sakabe J. Tanaka and A. Furusaki Nature 1967,215,956. 54 H. H. Cady Acta Cryst. 1967,23 601.55 J. R. Holden and C. Dickinson J. Phys. Chem. 1967,71 1129. 56 J. R. Holden Acta Cryst. 1967,22,545. " S. Abrahamsson M. Innes and B. Lamm. Acta Chem. Scand. 1967,21,224. 58 R. L. Sass and C. Bugg Acta Cryst. 1967,23 282. Part (u) X-Ray Crystallography K+ 9-Dicyano-2,4,7-trinitrofluorene(11) is over~rowded.~' A slight twist of the fluorene framework into a propeller shape leaves outer rings out of parallelism by 3.2". The 4-nitro-group is rotated out of the molecular plane by 19.3". The force tending to maximise the resonance energy of this group is believed to be responsible for the intramolecular overcrowding of the observed conformation the slight deviation from planarity of the fluorene framework and angular distortion in C-C-N bond angles.10-Dicyanomethylene anthrone (12) is forced by considerable steric strain into a non-planar configuration which apparently accounts for its inability to form complexes.60 The molecular conformation which closely resembles that of bianthronylidene involves a bend of 28" in the anthracene skeleton a dihedral angle of 144" between the planar dicyanomethylene group and the anthracene skeleton and a twist or non-parallelism of 5" out-of-plane between the direction defined by the 9,lO- carbon atoms and the corresponding directions in the outer rings. A clearance of 2.848 between overcrowded carbon atoms is thus achieved. In crystals of propargyl2-bromo-3-nitrobenzoate there is an intermolecular H(ethynyl)*.- O(carbony1) distance of 2.398 and the C-H -**O angle is 156".61 When the ester is in dilute solution in carbontetrachloride the C-H stretching frequency of the ethynyl group is at 3313 cm.- '.In the solid state the acetylenic C-H stretch appears at 3266 cm.-'. Both the carboxy- and the nitro-groups are rotated considerably out of the plane of the benzene ring the former by 53" and the latter by 60". A C-H***O hydrogen bond is also found in the crystal structure of ~-oxo-bis[chlorobis(pentane-2,4-dionato)titanium(1v)] chloroform solvate.62 The chloroform molecule apparently takes part in a 59 J. Silverman A. P. Krukonis and N. F. Yannoni Acta Cryst. 1967,23 1057. 6o J. Silverman and N. F. Yannoni J. Chem. SOC.(8, 1967,194. 61 J. C. Calabrese A. T. McPhail and G. A. Sim J. Chem.SOC.(B) 1966 1235. 62 K. Watenpaugh and C. N. Caughlan. Inorg. Chem. 1967.6,963 George Ferguson bifurcated C-H *-*Ohydrogen bond to two oxygens of acetylacetonate groups. The C(chloroform)***O distances are 3.21 and 3-35 A. PhCOSCPh II N-OH (13) NO The accepted structures for benzil a-(13) and p-monoxime have been con- firmed by an X-ray analysis of a p-bromobenzoate derivative of the a-mon- ~xime.~~ The cation side-chain of noradrenaline hydrochloride is maximally extended and the catechol moiety is planar.64 In crystals of 2-bromo-1,l-di- p-tolylethylene the atoms are distributed in three planes; the ethylene plane and the two tolyl rings which are rotated by 24.4" (trans to Br) and 67.9" (cis to Br).65 A variety of reducing agents convert o-benzoylbenzoic acids and their acid chlorides to 3,3'-diarylbiphthalidyls(14).Two isomers the racemic (DL) (m.p. 226") and the meso (m.p. 262") forms are produced in unequal amounts. The more abundant (meso)form has been shown to exist in the solid state in the fully staggered conformation by an analysis of the dibromo-derivative (14,R = Br).66 The red compound obtained by coupling diazotised 2-nitro-4- toluidine and acetoacetanilide at higher temperatures has been shown to be a-(4-methyl-2-nitrophenylazo)acetanilide.The molecule is planar and the configuration of the carbon atom to which coupling occurs is trig~nal.~~ The molecular arrangement in crystals of the iodide of NN'-diphenyl-p-phenylene-diamine and in the perchlorate of 4,4'-bis(dimethy1amino)diphenylamine radicals have been reported.68 The molecular configuration around N(2) in the 2,2-diphenyl-l-picrylhydrazyl radical (15) is approximately trigonal planar with the phenyl groups twisted at angles of 49 and 22".69 The bond at the digonal nitrogen is bent with a C-N-N angle of 118.5" and the attached picryl carbon is twisted 28.5" out of the plane of the trigonal hydrazine nitrogen ; this non-planarity of the hydrazyl backbone was not predicted from e.p.r.measurements. The picryl-ring plane is further inclined at an angle of 33" to the plane of the digonal nitrogen. The o-nitro-groups are K. A. Kerr J. M. Robertson G. A. Sim and M. S. Newman Chem. Comm. 1967 170. 64 D. Carlstrom and R. Bergin Acta Cryst. 1967,23 313." G. L. Casalone C. Mariani A. Magnoli and M. Simonetta Acta Cryst. 1967,22 228. H. Manohar V. Kalyani M. V. Bhatt and K. M. Kamath Tetrahedron Letters 1966 5413 V. Kalyani H. Manohar and N. V. Mani. Acta Cryst. 1967,23 272. '' C. J. Brown J. Chem. SOC.(A),1967,405. 68 K. Toman D. Ocenaskova and K. Huml. Acta. Cryst. 1967,22,29 32 69 D. E. Williams J. Amer. Chem. SOC. 1967,89,4280. Part (u) X-Ray Crystallography twisted out of the picryl ring plane by 25" and 55" while the p-nitro-group is twisted by 13". The hydrazyl N-N bond distance is 1.334 A intermediate between expected values for single and double N-N bonds. /o Me -C< ? Me-ol@) I k0 Fe I (16) In 5-acetoxy-6-methoxy-8-nitroquinoline(16) the exocyclic C-0-C angles average 117" instead of tetrahedral owing to steric repulsions between ring atoms and those of the methoxy and acetoxy-gro~ps.~~ Packing and intramolecular steric requirements force the nitro- and acetoxy-groups to rotate out of the ring plane by 59" and 79" respectively.The resonance energy of the acetoxy-group is sufficient to maintain its planarity in the overcrowded environment but the quinoline deviates slightly from planarity. The cyclo- pentadiene rings in 2-biphenylylferrocene (17) are eclipsed ; the first phenyl ring of the biphenylyl group is rotated 43" out of the cyclopentadienyl plane and the outer phenyl ring is rotated 58" out of the plane of the first phenyl ring.71 These rotations relieve the strain which would exist in a planar model for the C,H4*C6H4*C6H,group.2-Chloro-l,8-phthaloylnaphthalene(18) is folded about a line joining the carbonyl carbon atoms and is distorted by the chlorine atom. The naphthalene plane makes an angle of 42" with that of the phthaloyl ring. The strain imposed by the seven-membered ring closing the peri-positions of the naphthalene moiety is accommodated largely by an increase of the valency angle C(l j-C(S)-C(S) from 121.8" in naphthalene to 126-1"and by extension of the C(l)-C(9) and C(SkC(9) bonds from the established value (1.421A) for naphthalene to 1.443and 1-456A re~pectively.'~ 'O M. Sax and R. Desiderato Acta Cryst. 1967,23 319. '' J. Trotter and C. S. Williston J. Chem. SOC.(A) 1967 1379. 72 K. M. S. Islam and G. Ferguson J. Chem.SOC.(B),1967 1134. 76 George Ferguson The crystal structure of p-benzoquinone 2,3-,73 2,5- and 2,6-dimethyl- benz~quinone,~~ (dur~quinone),~~ 2,3,5,6-tetramethyl-1,4-benzoquinone and thymoquinone and their solid-state photochemistry have been discussed76 in terms of their packing arrangements. The structures of the dimers (cyclo- butanes and oxetans) of the 2,5- and 2,6-dimethyl-p-benzoquinonescan be related to the packing geometry in the monomer crystals of parallel double bonds (C==C C==O) with centre-to-centre distances up to 4.3 A. This cor- relation is unsatisfactory for the 2,3-dimethyl derivative. The absence of short parallel contacts in p-benzoquinone and duroquinone may account for the inability of these two quinones to yield dimers.It is postulated that the formation of polymeric material is related to the presence of short contacts between carbon atoms of nearest neighbour non-parallel >C=C < groups whose interaction leads to open-chain dimer diradicals ;these cannot terminate by ring closure but may instead initiate polymerisation. A number of centro- symmetric hydroquinones and their salts have been examined. Chloranilic acid77 (19) and its dih~drate~~ have ring geometry similar to that in p-benzo- quinone but the C-C single bonds are different in length in the two cases 1.455 and 1.501 A and 1.446 and 1.512 I$,respectively and there are two kinds of C-0 distances corresponding to C=O and C-OH bonds. By contrast the carbon ring-systems in ammonium chloranilate m~nohydrate,~' ammonium nitranilate,*' and nitranilic acid hexahydrate (hydronium nitranilate)* are not (20) in quinoidal form.In each case four C-C bonds are of equal length and two are considerably longer (mean values are 1.404 and 1.535 A 1-435 and 1-551A and 1.419 and 1.559A respectively) and the C-0 distances are ofequal length (1.248 1.220 and 1.228 8 respectively). In ammonium nitranilate the nitro-group plane is rotated 6"around the C-N bond out of the carbon plane; in nitranilic acid hexahydrate the corresponding angle is 22". -R HO c1@ 0 J' (19) (20) 53 D. Rabinovich J. Chem. SOC.(I?) 1967 140. 74 D.Rabinovich and G. M. J. Schmidt J. Chem. SOC. (4,1967,127. '' D.Rabinovich G. M. J. Schmidt and E. Ubell J. Chem. SOC.(4, 1967,131.76 D.Rabinovich and G. M. J. Schmidt J. Chem. SOC.(B) 1967,144. 77 E.R.Andersen Acta Cryst. 1967,22 188. 78 E.K.Andersen Acta Cryst. 1967,22 191. 79 E.K.Andersen Acta Cryst. 1967,22 196. E. K. Andersen Acta Cryst. 1967,22,201. E.K.Andersen Acta Cryst. !967,22,204. Part (u) X-Ray Crystallography 77 A refinements2 of the anthraquinone structure rules out the possibility of a formal transfer of one electron from carbon to oxygen in the C=O bond as proposed earlier.83 There is a strong asymmetric O-H***O hydrogen bond in 1,5-dihydroxanthraquinone (21) and electron delocalisation across the quinonoid ring as gauged by the structural parameters appears minimal.84 The anthraquinone nucleus in NN'-diphenyl-1,5-diaminoanthraquinone is planar while the substituent benzene rings are inclined to the central ring system by 62.4"." There is the possibility of intramolecular hydrogen bonding between nitrogen and oxygen atoms which are 2602 8 apart.NN'-Diphenyl- 1,8-diaminoanthraquinonehas two-fold crystallographic symmetry and the phenyl rings are inclined by 61.8"to the anthraquinone nucleus which itself is not quite planar.86 Here too there is the possibility of intramolecular hydrogen bonding between the nitrogen atoms and the adjacent carbonyl oxygen (N-H -02.578A). The anthraquinone nucleus of 1,5-dinitro-4,8-dihydroxy-anthraquinone is approximately planar but the nitro-groups are inclined to it at 88". An intramolecular O-H-*O hydrogen bond is possible (O.*-O 2549 A) but the hydrogen atom was not located.87 Other anthraquinone structures reported include the 1,2- and 1,5-dihydro~y-9,10-anthraquinones~~ and 2,3-di bromo-l,4-an thraquinone.89 The dimer l0,lO-dian thronyl formed by oxidation of anthrone has a two-fold symmetry axis perpendicular to the C(l0)-C((l0) bond. The planes of the two half molecules make an angle of 145" with each other and each half molecule has a bend of 10". The dihedral angle C(9) C(10 jC(lO')C(9') is 74". All bond lengths and angles are normal except the C(LO)-C(lO)' bond (1.60 & 0.006 A).'' Other Cyclic Molecules.-Two derivatives of cyclopropenylidene-cyclo-pentadiene (calicene) (22) have been examinedg '9 92 and yield comparable results for the molecular geometry confirming the calculation^^^ of Dewar and Gleicher which predict strong bond fixation and lack of aromatic character in the calicene system.The bond lengths in the ring of the p-bromo- phenacyl ester of 7,7-dimethylcycloheptatriene-3-carboxylic acid (thujic acid) alternate and it adopts a boat conformation. Thus a norcaradiene arrangement and the planar pseudo-aromatic structure previously proposed for the ring are precluded.94 The molecular geometry implies a small twist in 1,2- and 82 A. Prakash Acta Cryst. 1967,22,439. 83 B. V. R. Murty Z. Krist. 1960 113,445. 84 D. Hall and C. L. Nobbs Acta Cryst. 1966,21,927. 85 M. Bailey and C. J. Brown Acta Cryst. 1967,22,488. 86 M. Bailey and C. J. Brown Acta Cryst. 1967,22,493. M. Bailey and C. J. Brown Acta Cryst. 1967 22 392.J. Guilhem Bull. SOC.chim. France 1967 1656. 89 J. Gaulthier,S. Geoffre and C. Hauw Compt. rend. 1967,264 C,697. 90 M. Ehrenberg Acta Cryst. 1967,22,482. 91 H. Shimanouchi T. Ashida Y. Sasada and M. Kakudo Tetrahedron Letters 1967 61. 92 0.Kennard D. G.Watson J K Fawcett K. A. Kerr C Romers Tetrahedron Letters 1967 3885. 93 M. J. S. Dewar and G. J. Gleicher Tetrahedron 1965 21 3423. 94 R. E. Davis and A. Tulinsky J. Amer. Chent. SOC.,1966,88 4583. George Ferguson 5,6-double bonds. Considerable bond alternation has also been observedg5 in the conjugated seven-membered ring of 8,8-dicyanoheptafulvene (23). A planar (centrosymmetric) cyclobutane ring is found in trans-cyclobutane- 1,3-dicarboxylic acid96 but in the ~is-isomer~~ the ring is puckered with a dihedral angle of 149".The crystal structures of both compounds consist of chains of hydrogen-bonded molecules. In the trans isomer the C-C ring bonds are 1-56? and 1.552 A; the mean C-C ring bond in the cis-isomer is 1.554 A. In crystals of 1,4-truns-cyclohexane-dicarboxylicacid the molecule has a centre of symmetry and hence adopts the chair conformation. The mean C-C4 angle in the cyclohexane ring is 112".98 The erythro-configuration has been determined for 2-(cl-bromophenyl-~-nitroethyl)cyclohexanone obtained from the hydrolysis of the reaction between cyclohexanone enamine and j3-nitro-p-bromostyrene. The cyclohexanone ring has a chair conformation with the carbonyl oxygen nearly eclipsed to the adjacent exocyclic carbon atom.99 The main reaction product of the hydrogenation of 4,6-dinitro- pyrogallol on a rhodium-platinum catalyst is a 1,3-diamino-4,5,6-trihydroxy-cyclohexane.Its dihydrochloride has the all-cis configuration and is a 2-deoxy-cis-inosa- 1,3-diamine. loo (24) (25) (26) cis-cis-cis-1,4,7-Cyclononatetraeneis known to adopt a crown conformation and when complexed with silver nitrate to give a n-complex.of composition C9H1,(AgN0,)3 it retains this conformation having crysta1logr:phic 3/m (C3J symmetry.lo' The conformation of all-cis-1,6-dichlorocyclodeca-1,3,6,8-tetraene (24) is such that each half of the centrosymmetric molecule may be defined by two planes one through C(lO),C(l),C(2) and C(3)Band or& through C(2),C(3),C(4) and C(5) inclined at an angle of 57" to each other.'02 This 95 H.Shimanouchi T. Ashida Y. Sasada M. Kakudo I. Murata and Y. Kitahara Bull. Chem. SOC.Japan 1966,39,2322. 96 T. N. Margulis and M. S. Fischer J. Amer. Chem. SOC.,1967,89,223. '' E. Adman and T. N. Margulis Chem. Comm. 1967,641. 98 J. D. Dunitz and P. Strickler Helv. Chim. Acta 1966,49,2505. 99 M. Calligaris F. Giordano and L.Randaccio Ricerca xi. 1966,36 1333. loo J. H. Palm Acta Cryst. 1967,22,209. R. B. Jackson and W. E. Streib J. Amer. Chem. SOC.,1967,89,2539. lo2 0.Kennard D. G. Watson J. K. Fawcett and K. A. Kerr. Tetrahedron Lettcrs. 1967. 3129. Part (u) X-Ray Crystallography 79 arrangement allows for a separation of 2.5 A between the two inner hydrogens at C(5)and C(l0). The tensions exerted by the silver ions on the double bonds of the germacratriene-AgNO complex (25) and some non-planar deforma- tions have combined to distort the ring much past what can be achieved by a Dreiding model.lo3 The planes of the double bonds are approximately per- pendicular to the plane of the macrocycle (26).The results of the X-ray analysis of 1,1,5,5-tetramethylcyclodecane-8-carboxylicacid do not correspond to a reasonable molecular structure.lo4 When used as a starting point for a strain- energy minimization calculation based on semi-empirical potential functions they lead to two new conformations for the cyclodecane ring.It is shown that the X-ray results can be explained by assuming that the crystal consists of a random mixture of these two conformations.lo' 4 Br (27) (28) (2 9) Bridgehead substituted polycyclics are known to exhibit widely differing solvolysis rates. The explanation of this variation based on changes in methy- lene bridgehead angle is not acceptab1e,lo6 and is further denied by crystal structure analysis of 3-exo-(N-benzyl-N-methylaminomethyl)-2-endo-norborn-anol (27). The angles in the bicycloheptane system are considerably less than tetrahedral reflecting the strain in the system.'07 As closely similar results were obtained from a neutron diffraction study of 3-endo-phenyl-2-endo- bornanol'08 and a gas-phase electron diffraction study of the parent nor-bornane'Og it appears that the geometry of the norbornane skeleton is especially insensitive to the nature and the position of substituents.Molecular geometry rn is demanded of pseudotropine (28) by its space group.'" The N-methyl group is equatorially attached with respect to the piperidine ring which is in deformed chair form such that the separation of the ethylene bridge atoms from C(3) and the N-methyl group is 3 A. Atoms C(1) and C(1') are fairly close; the angle C(l)-N-C(l)' is only 102.5". Most of the bond angles at the carbon atoms forming the tricyclic system of 2-anilino-3-bromo- lo3 F. H. Allen and D. Rogers Chem. Comm. 1967,588. lo4 J. D. Dunitz and H. Eser Helu. Chim. Ada 1967,50 1565. lo' J. D. Dunitz H. Eser M. Bixon,and S. Lifson Helv. Chim. Acta 1967 50 1572; M. Bixon H. Dekker J. D. Dunitz H. Eser S. Lifson C. Mosselman J. Sicher and M. Svoboda Chem.Comm. 1967 360. lo' A. C. Macdonald and J. Trotter Acta Cryst. 18 243 1965; ibid. 19 456 1965. lo' A. V. Fratini K. Britts and I. L. Karle J. Phys. Chem. 1967 71 2482. lo' C. K. Johnson Abstracts Amer. Chem. SOC. Meeting Atlanta Georgia 1967. log W. C. Hamilton Ph.D. Thesis California Institute of Technology 1954. 'lo H. Schenk C. H. MacGillavry S. S. Kolnik and J. Laan Acta Cryst. 1967 23 423. 80 George Ferguson tetrahydro-exo-dicyclopentadiene(29) are somewhat smaller (mean value 104") than tetrahedral the angle at the bridge carbon atom being 94O.l" Br.C,H,COO The formulation of the tricycl0[2,1,0,0,~* 'Ipentane system has been con- firmed by an. X-ray analysis of 1,5-diphenyltricyclo[2,1,0,0,2~ 5]pent-3-yl p bromobenzoate (30).The ring system is under considerable strain there being six C-(2-42 angles of about 60°,three of about 80" and four of about 90". The C(2)-*C(4) non-bonded distance is only 1.99 A.112 Six of the C-C distances are in the range 1.50-1.54 .$; the bond common to the two three- membered ring is 1-44A but the difference may not be significant. The tricyclo- dodecyl system of 12-hydroxy-6-methyltricyclo[5,3,1,12~ 6]dodec-3-yl p-iodo- benzoate (31) adopts a distorted double-chair conformation because of mutual repulsion between methylene groups at C(4) and C(11) and between the 12- hydroxyl and C(9) methylene groups.' ' The low temperature phase of adaman- tane (32) has been reported as departing significantly from 43m symmetry.'14 However another refinement' l5 of the structure starting with molecules having 33m symmetry and tilted 9" about the c axis refines to a structure not significantly different from the new starting point.On the other hand if the refinement is started at the published structure the parameters of which correspond to rather distorted molecules a structure not significantly different from this starting point results although the final R is higher 8.7 as compared with 3.1 % for the symmetrical structure. Donohue and Goodman point out that the occurrence of a false minimum which is quite close to a true minimum is rather disturbing!'I5 c1 Cl (3 3) (35) N. Tanaka T. Aishda T. Sasada M. Kakudo and N. Kasai Bull. Chem. SOC.Japan 1967 40,1574. J. Trotter,C. S. Gibbons N.Nakatsuka and S. Masamune J. Amer. Chem. SOC. 1967,89,2792. '" G. Ferguson; W. D. K. Macrosson J. Martin and W. Parker Chem. Comm. 1967 102. C. E. Nordman and D. L. Schmitkons Acta Cryst. 1965,18 764. J. Donohue and S. H. Goodman Acta Cryst. 1967,22,352. Part (0)X-Ray Crystallography 81 An unusual cage structure (33) results when hexachlorocyclopentadiene dimerizes. A chlorosulphonate group was introduced at one of the apex carbon atoms to overcome disorder of the parent perchloro-derivative. The basic cage structure is trans consisting of four cyclopentane and two cyclo- butane rings all of which are 'in the cyclobutane rings the average bond angle is 87". The bond angles of the cyclopentane rings fall into three groups with 96" at the apex and 108" and 102" at the middle and base res- pectively.Two of the isomeric compounds of formula C12Cl12 obtained by pyrolytic reaction of perchloro-3,4-dimethylenecyclobutanehave been ex-amined. '177 ' ' Perchloro-4,8-dimethylenetricyclo-[3,3,2,0'~ ']deca-2,6-diene (34)' l7 has a crystallographic two-fold axis of symmetry. The four-membered ring is slightly distorted and consequently the conformation of the two adjoining dichloromethylene groups is about 8" deviated from the eclipsed form. The five-membered rings are also puckered the dihedral angle between them being 123". Perchloro-3,4,7,8-tetramethylenetricyclo[4,2,O,O2~ ']octane (35)' '' has a rather simple chair-like form with an approximate symmetry 2/m (C,,,) although only a centre of symmetry is demanded by the space group.Each conjugated system in the molecule is nearly planar in spite of a very close contact between the two adjacent dichloromethylene groups. The nearest approach occurs between the chlorine atoms (3.28 A) which is achieved by increasing the overcrowded C-C-Cl valency angles to 126". Systems Containing Hetero-atoms.-The 2-nitronitrosoethane dimer has the two monomer units bound in the trans-configuration through a centre of symmetry at the mid-point of the N-N bond.'lg Bond lengths are N-N 1.315 f 0-010 N-C 1-462& 0.008 and N-0 1.255 & 0-006 A. Molecular orbital calculations for the nitroso-part of the system indicate that resonance structure (36)makes a substantial contribution to bonding in C-nitroso-dimers.The S-N distances (1.58 and 1-63 .$) in 5,5-dimethyl-N-methylsulphonyl-sulphilimine (Me,S -N. S0,Me) indicate a delocalized S-N-S d bond-system.'20 The S-N-S bond angle is 116.2" the average s-0 distance 1.45 A. The CSpr-S IV bond distances (1.74 A) are rather short which can be explained with the supposition of strong hyperconjugation. A refinement of the thiourea structure with accurate intensity data has led to molecular dimensions S-C 1.720 0.009 N-C 1.340 & 0.006A S-C-N 120-5& 0.5 N-C-N 119.0 & 05°.121The hydrogen atoms are approximately coplanar with the heavy atoms. Thioglycollic acid S*[CH,*C02H12 has mirror symmetry with only the sulphur atom on the mirror plane the C-S-C angle being 96".'22 cis-2-Butene-episulphone(37) has approximately M Y.Okaya and A. Bednowitz Acta Cryst. 1967,22,11 I. 117 A. Furusaki and I. Nitta Tetrahedron Letters 1966 6027. A.Furusaki Bull. Chem. SOC. Japan 1967,40,758. 'I9 F. P. Boer and J. W. Turley J. Amer. Chem. Soc. 1967,89,1034. I2O A.Kalman Acta Cryst. 1967,22,501. 12' M.R. Truter Acta Cryst. 1967,22 556. S. Paul Acta Cryst. 1967,23 491. 82 George Ferguson symmetry. The terminal C-C bond lengths are normal (1.51 A) but the central C-C distance of 1.60 _+ 0.02 A is rather longer and the S-C distances of 1-74 and 1.73 A are a little shorter than might have been expected.'23 The other dimensions are S-0 1.41 1.44 8 and 0-S-0 120". In butadiene sulphone (2,5-dihydrothiophen-l,l-dioxide)(38) the sulphur and carbon atoms lie on crystallographic mirror planes and the molecule has mm2 (C2") symmetry.'24 Average bond lengths are S-0 1.44 S-C 1.79 C-C 1.48 C=C 1.308,;the angles are C-S-C 97",0-S-0 llo" and S-C-C 104".A roughly U-shaped cation is found in 1-p-chlorophenyl-5-isopropylbiguanide hydrochloride (paludrine) (39). The biguanide part of the cation consists of two planes of atoms each containing a CN group and intersecting at 58.9". The mean C-N distance is 1.328 8 and as there is no distinction between formal single and double bonds some x-bonding is present ;this was somewhat unexpected in view of the non-planarity of the biguanide moiety.'25 (37) s -s (41) The dihedral angle at the peroxide bond in dibenzoyl peroxide is 91"; a value of 94" was predicted on quantum chemical grounds.The 0-0 bond length is 1.46 8 and the C-0-0 angle is 110" implying that the bonding orbitals in the oxygen atom are sp3 hybrids.'26 1,2-di-(p-chlorophenoxy)ethane crystallises in a cell containing two crystallographically independent molecules which are of slightly different conformations (the OCH2-CH,O torsional angles are 66 and 81") and are located on two-fold axes in the ~rysta1.l~~ The parent compound 1,2-diphenoxyethane also has two-fold symmetry and the R. Desiderato and R. L. Lass Acta Cryst. 1967,23,430. D. E. Sands and V. W. Day Z. Krist. 1967,124,220. C.J. Brown J. Chem. SOC.(A),1967,60. 126 M. Sax and R. K. McMullan Acta Cryst. 1967,22,281. N. Yasuoka T. Ando and S. Kuribayashi Bull. Chem. SOC.,Japan 1967,40,265.Part (u) X-Ray Crystallography 83 OCH,-CH,O torsional angle is 67°.128 The torsional angle CS-SC in 2,2'-diaminodiphenyl disulphide is 87".12' Tetraethylthiuran disulphide (disulphiram Antabuse) (40) is an antioxidant that interferes with the normal metabolic degradation of alcohol in the body and a crystal structure analysis i /" has established that the molecule contains two planar -S-C-N 'cgroups which have a dihedral angle of 96". The configuration about each nitrogen atom is planar rather than pyramidal and the two C-N bond lengths adjacent to the C=S bonds are both very short 1.33 and 1.36 A. Thus there is strong evidence for the influence of the C=S bond on the C-N bond length.130 A literature survey'31 indicates that in a disulphide group the normal dihedral angle of about 90" between the valencies of the two sulphur atoms corresponds to S-S bond lengths of about 2.03 A while smaller dihedral angles give longer bond lengths.The length difference is assumed to be due partly to lone-pair repulsion which is most pronounced when the dihedral angle is 0" and partly to n-bonding which is most pronounced when the dihedral angle is 90". A graph showing this trend indicates that 2.lOA is the length of a single bond between two divalent sulphur atoms when the dihedral angle is 0". A linear bond-length-bond-order curve for sulphur-sulphur bonds in cis-planar disulphide groups is proposed based on the double-bond length of 1.89 8 and the single-bond length of 2.10 A. The S-S distances found in 4-phenyl-l,2-dithiolium iodide (2.028 It 0.010 A),132thiuret hydrochloride (2.071 f 0.004 A),1333,5-diacetamide-l,2-dithioliumbromide (2.080 f 0.005 A),134 and sodium 3,5-diacetylimino-1,2-dithioltrihydrate (2.05 0.015 A)13s show that the rings are stabilised through n-orbital delocalisation extending over the sulphur-sulphur bonds.No four atoms of the five membered ring in ~~-6-thiocetic acid (41) are coplanar. The torsional angle CS-SC is near 96" and the S-S bond length is normal (2.01 A).136 The discovery of the thio- thiophen 'no bond resonance' system through an X-ray analysis'37 of the dirnethyl derivative (42) has prompted X-ray analyses of the unsymmetrical derivative (43).13* Whereas in the symmetrical molecule the S-S distances are identical (2.35 A) in the unsymmetrical molecule S(lFS(2) 2.51 and S(3) 2.22 A.Similar results [S(l)-S(2) 2-52 and S(2)-S(3) 2.18 A] have been found for the unsymmetrical derivative (44).'39 12* N. Yasuoka T. Ando and S. Kuriyabashi Bull. Chem. SOC. Japan 1967,40,270. A. H. Gornes de Mesquita Acta Cryst. 1967,23 671. I3O I. L. Karie J. A. Estlin and K. Britts Acta Cryst. 1967 22. 273. 13' A. Hordvik Acta. Chem. Scand. 1966,20 1885. A. Hordvik and E. Sletten Acta Chem. Scand. 1966 20 1874. 133 A. Hordvik and J. Sletten Acta Chem. Scand. 1966,20 1907. 134 A. Hordvik and H. M. Kjoge Acta Chem. Scand. 1966,20,1923. 135 A. Hordvik and E. Sletten Acta Chem. Scand. 1966,20 2043. 136 I. L. Karle J. A. Estlin and K. Britts Acta Cryst. 1967 22 567.13' S. Bezzi M. Mamrni and C. Garbuglio Nature 1958,192 247. A. Hordvik E. Sletten and J. Sletten Acta Chem. Scand. 1966,20 2001. 139 S. M. Johnson M. G. Newton I. C. Paul R. J. S. Beer and D. Cartwright Chem. Comm. 1967 1 170. George Ferguson Ph PhCO OH (&;N H (45) The conformation of the bond system 0-C-C=N-in anti-fufuraldoxime is trans (45).The side chain is turned away less from the plane of the furan ring than expected which leads to a rather short intramolecular distance 2.86 A between the oxime oxygen atom and a ring carbon atom. Hydrogen bonds -N-0-H *N-0-H -link the molecules in infinite chains along the screw axis. This type of molecular association appears to be characteristic for aromatic anti-aldoximes.The nearly planar molecules of trans-P-2-furyl- acrylic acid (46) are pairwise hydrogen-bonded at the carboxyl groups (O-H***O 2.67 A); these pairs are arranged in stacks in which parallel ,C=C \/\groups make contacts of 3.84 8 and is probably responsible for the photochemical formation of the dimer of symmetry rn (p-truxinic acid analogue). U.v.-induced polymerisation may occur along a reaction path involving the exocyclic ,C=C \/groups and double bonds of the furan \ ring.14' According to their cell dimensions as well as the geometry of contact of such exocyclic groups the two modifications of trans-P-2-thienylacrylic acid belong to the p-and y-type of the cinnamic acid series and the photochemistry of the two forms is discussed in terms of their packing arrangement^.'^^ a-Chloro-6-valerolactam adopts a half-chair conformation in the solid state with the chlorine atom equatorial; the peptide linkage NH-CO has the cis-c~nfiguration.'~~ The six-membered ring of trimethylene sulphite (47) has a chair conformation with an axial S=O bond.The mean single-bonded S-0 distance is 1.60 A and the S=O distance is 1.45 A. The 0-S=O angles are 107" and the 0-S-0 angle is 100". Within experimental error the molecule has symmetry rn.144 A chair conformation is also adopted by trans-2,3-dichloro-1,4-thioxan (48)with the chlorine atom axial. Its overall geometry is midway between the conformations of the corresponding 2,3-dichloro- derivatives of 1,4-dioxane and 1,4-dithiane.'45 Thioformaldehyde trimer B.Jensen and B. Jerslev Acta Chem. Scand. 1967,21,730. 14' S. E. Filippakis and G. M. J. Schmidt J. Chem. SOC.(B) 1967,229. 142 S. Block S. E. Filippakis and G. M. J. Schmidt J. Chem. SOC.(B) 1967,233. 143 C. Romers E. W. M. Rutten C. A. A. van Driel and W. W. Sanders Acta Cryst. 1967,22,893. 144 C. Altona H. J. Geise and C. Romers Rec. 'Rav. chim. 1966,85 1197. 145 N. de Wolf C. Romers and C. Altona Acta Cryst. 1967,22 715. Part (u) X-Ray Crystallography [CH,SI3 has a chair conformation with the mean S-C distance 1.818 A.146 Chair conformations are also found in 3,3,6,6-tetrabromomethyl-1,2,4,5-tetr~xane’~’ 14* and 1-thia-4-selenocyclohexane-4,4-dibromide. II c1 S c1 (47) (48) N-0 H ,,VH (53) (54) The 1,3-dithiete ring of the desaurin (49) from acetophenone must be planar (the molecule is centrosymmetric) and the ap-unsaturated carbonyl system is in an s-cis conformation.The ten central atoms of the molecule are closely planar and associated with this is a short S***0 distance of 2.64 1$ suggesting a sulphur-oxygen attraction. 149 5-Aminotetrazole is planar and exhibits a large degree of conjugation not implied by the molecular formula (50). The bond lengths are N(l)-N(2) 1.381 N(2)-N(3) 1.255 N(3)-N(4) 1-373 N(4)-C 1,321 N(1)-C 1-329 and N(6)-C 1.577 A.150The anions of 3-substituted 5-phenylrhodamines are S-alkylated by alkyl halides and the formulation of the resulting compounds as the ‘mesionic’ anhydro-2-alkylthio- 3-R-4-hydroxy-5-phenylthiozolium hydroxides has been confirmed by X-ray analysis.The compounds are best represented as betaines with the positive charge on the nitrogen atom and the negative charge on the oxygen (51).’5’ Molecules of NN‘-bisuccinimidyl have a crystallographic two-fold symmetry axis parallel to the N-N bond and non-crystallographic 222 symmetry; the 146 J. E. Fleming and H. Lynton Canad. J. Chem. 1967,45 353. 14’ M. Schulz K.Kirsche and Hohne Chem. Ber. 1967 100,2242. 14’ L. Battelle C. Knobler and J. D. McCullough Inorg. Chem. 1967,6 958. 149 T. R. Lynch I P. Mellor. S. C. Nyburg and P. Yates Tetrahedron Letters 1967 373. K. Britts and I. L. Karle Acta Cryst. 1967 22 308. S. Abrahamsson A. Westerdahl G. Isaksson and J. Sandstrom Acta Chem. Scand. 1967,21 442 86 George Ferguson ring planes make a dihedral angle of 65°.152 In crystals of 3,3'-bisisoxazolyl (52) the molecules lie on centres of symmetry and hence have the trans-conformation.Within experimental error the molecule is planar.' 53 An analysis of 5-nitrouracil monohydrate reveals that the 2,4-dioxo-tautomeric (53) form is adopted. The dihedral angle between the plane of the pyrimidine ring and the nitro-group is 4.7". A hydrogen bond C-H*-.O to a nitro- oxygen (H-a.0 2.30 A) appears to be an important structure-determining factor.154 Thiouracil contains highly polarized thionamide groups the polarization of these groups being position-dependent. The C-S bond lengths are 1.645 and 1.685 ( f 0.006) A,suggesting an important contribution of form (54) to the structure.The packing of the molecules in the crystal lattice is dominated by intermolecular S*-*H-N bonds whose lengths (H--S2.39 and 2.78 A) appear to be a function of the degree of polarization about the sulphur atoms.' s5 R' Et (55) The tetrazene chain in the centrosymmetric structure 1,4-bis-(N-ethyl-1,2- dihydrobenzthiazol-2-y1idene)tetrazene(55) is in the trans-N-trans-trans-N form with bond lengths C=N 1-302,N-N 1-400 and N=N 1.257 A. The plane of the tetrazene chain is inclined at 4.8" to the benzthiazole ~1anes.l~~ Bond angles suggest that there is a considerable amount of strain in the five- membered sultone o-hydroxy-a-toluenesulphonicacid as compared to the six-membered soltone 2-o-hydroxyphenylethanesulphonicacid (56; R1= R2 = H).The C-S-0(3) angle 96.1" is 53" smaller than the corresponding value 101.4" in the six-membered sultone and differences are also found in the S-O(3)-C(1) angles of 108-9 and 116.9". The strain introduced into the five- membered ring may be the basis for an explanation of the difference in hydrolysis rates of the two compounds.'57 A detailed analysis has been made of another six-membered sultone 2-o-hydroxyphenyl-1-phenylpropanesul-phonic acid (56; R' = Me R2 = Ph). The heterocyclic ring is in the half-chair conformation with the phenyl group axial and cis to the equatorial methyl group. The C-S-0(3) angle is here 100.2" and the S-O(3)-C(l) angle is 117.1"; some bond lengths are S-O(periphera1) 1.431 S-O(C) 1.595 S-C 1,797 and C\p2-0 1.426A.158 G.S. D. King J. Chem. SOC.(B) 1966,1224. M. Cannas and G. Marongiu Z. Krist. 1967 124 143. lS4 B. M. Craven Acta Cryst. 1967,23,376. 15' E. Shefter and H. G. Mautner J. Amer. Chem. SOC.,1967,89 1249. lS6 R. Allmann Acta Cryst. 1967 22 246. 157 E. B. Fleischer E. T. Kaiser P. Langford,S. Hawkinson A. Stone and R. Dewar Chem. Comm. 1967 197. *" K. Bjamer and G. Ferguson Acta Cryst. 1967,23,654. Part (u) X-Ray Crystallography ~-Thioxanthen-9-ol-lO-oxide (m.p. 206") has the trans-configuration. In the solid state the S-0 bond occupies a pseudo-equatorial position while the HO-CH dihedral angle is 34O.lS9 During the preparation of thiothixene (57) a drug effective against schizophrenia two isomers cis and trans are produced but of these only one exhibits therapeutic activity and is established as the cis-isomer by an X-ray analysis The two aromatic rings of the thioxanthene moiety are inclined at 141.5".160 3-Allyltropolone reacts with bromine in carbon tetrachloride to form a colourless compound CloH,O,Br whose structure has been determined as 2-bromomethyl-2,3-dihydrofuro[2,3-6]tro-pone with double bond alternation as implied in (58).The longest and shortest bonds in the tropone ring are 1.48 and 1.36 8 respectively; the carbon atoms of the tropone ring are planar but the carbonyl oxygen is slightly off (0.07 A) this plane.161 The irradiation of N-chloroacetyl-p-0-methyl-L-tyrosine results in the production of an unusual rearrangement product whose methyl ester has been identified by X-ray methods as (59).Constraints imposed on the molecule by the five- and seven-membered rings twist the molecule from a planar conformation.162 The two nitrogen atoms of the seven-membered ring in 1,4-benzodiazepins are of different basicity. The X-ray analysis of a derivative (60)shows that one of the nitrogens is pyramidal and the other planar. The *C(CO)(NMe)*C-group is nearly planar but is rotated 50" from the adjacent benzo-group. 63 The product C6HloS3 formed when an alcoholic solution of chloroacetone saturated with hydrogen chloride is treated with hydrogen sulphide has been shown to be 2,5-dimethyl-2,5-endo-thio-l,4-dithiane (61). The angle at the 0 Q Me 159 A. L. Ternay Jun. D. W. Chasar and M. Sax J. Org.Chem. 1967,32,2465. 160 J. P. Schaefer Chem. Comm. 1967. 743 16' H. Shimanouchi,T. Ashida Y. Sasada M. Kakudo L. Murata and Y. Kitahara Bull. Chem. SOC.Japan 1967,40,779. 16' I. L. Karle J. Karle and J. A. Estlin Acta Cryst. 1967,23 494. 163 J. Karle and 1. L. Karle J. Amer Chem SOC.1967 89 804. 88 George Ferguson CI I Me (60) Me (61) bridge sulphur atom is 86.3"while at the other two the values are 95.4and 94.7".The S-C bond lengths range from 1.810-1.850A. The molecule con- forms closely to one with a two-fold symmetry axis through the bridge sulphur atom. The methyl hydrogen atoms conform to this symmetry even though they have different intermolecular environments. 64 The molecular structure of the cyclo-adduct from 4-bromo-N-methoxycarbonylazepineand tetracyano- ethylene has been confirmed (62).A refinement of the structure produces anomalous results because of the unexpected presence of a co-crystallized isomer (in approximately 15% of the molecular sites) apparently originating from cyclo-addition involving 3-bromo-N-methoxycarbonylazepineand tetra- ~yanoethy1ene.l~'More details of the molecular geometry of (63)have been reported.166 The compound of empirical formula SNCCl obtained from ammonia and trichloromethanesulphonyl chloride has been shown to be the centrosymmetric tricyclic compound (SNCCl) (64).167 The central [SNC] ring is in a chair conformation but the outer rings are in half-chair conform- ation atoms except the sulphur atom common to the central ring being coplanar.The chlorine atoms are considerably twisted out of this plane to relieve steric strain arising from their close contact (3.17A). One of the so called 'acid products' of 1-methyl-1,4-dihydronicotinamidehas twice the molecular weight of the parent compound and has been shown to be (65).168 The structure contains nine different six-membered rings and very closely A. M. O'Connell Acta Cryst. 1967 23 623. R. A. Smith J. E. Baldwin and I. C. Paul J. Chem SOC.(B),1967 112. *'' M. G. Newton J. A. Kapacki J. E. Baldwin and I. C. Paul J. Chem. SOC.(4,1967 189. A. C. Hazell Acta Chem. Scund. 1967,21,415. H. L. Ammon and L. H. Jensen Acta Cryst. 1967 23 805. Part (u) X-Ray Crystallography resembles the geometry assumed by two twistane16' molecules with one ring [shown by solid circles in (65)] in common.The skeleton bears the same relationship to twistane as congre~sanel~~ does to adamantane.171 An in- vestigation of a dibromo-derivative of eriostoic acid (66) has confirmed the chemically deduced structure and determined the molecular geometry. Small differences which exist between chemically equivalent portions of the molecule are consistent with distortion of the molecule by packing forces.'72 Triclinic tetraphenylporphyrin (67) is centrosymmetric with one pair of pyrrole groups essentially coplanar with the plane of the porphyrin ring and the other pair carrying the central hydrogen atoms inclined at k6.6"to this plane. This has the effect of increasing the separation of the central hydrogen atoms by 0.2 to 2.36 A.173The structure corresponds closely with the hybrid of the two classical resonance forms of the porphyrin molecule.The diacid species of aay6-tetraphenylporphine and apy6-tetra-4-pyridylporphinehave 3 and pseudo-3 symmetry respectively and exhibit the greatest deviation from planarity yet found in any porphyrin This distortion is almost certainly due to mutual repulsion of the four hydrogen atoms on the inner nitrogen atoms. The displacements of the porphyrin atoms from their best plane range from -0.93 to +0.87 A. The geometry and absolute configuration of ethyl-1 -thio-a-D-glucofurano- side (68) have been determined.175 The furanose ring is puckered with C(3) 0.59 8 from the plane of 0(1) C(1),C(2)and C(4),and C(2) -0.60 8 from the OH OH (68) H.W. Whitlock J. Amer. Chem. SOC.,1962,84 3412. 170 C. Cupas P. Schleyer and D. J. Trecker J. Amer. Chem. SOC. 1965,87,917. 17' S. Landa Acta Chem. Acad. Sci. Hung. 1962,31 123. M. G. Paton E. N. Maslen and K. J. Watson Acta Cryst. 1967 22 120. 173 S. J. Silvers and A. Tulinsky J. Amer. Chem. SOC. 1967,89 3331. 174 E. B. Fieischer and A. L. Stone Chem. Comm. 1967 332. 175 R. Parthasarathy and R. E. Davis Acta Cryst.,1967 23 1049. George Ferguson OH OH (73) 0 II plane of 0(1),C(1) C(3) and C(4). The lactone function C-C-0-C of D-galactono-y-lactone (69) is planar; the fifth atom of the lactone ring is 0.64 8 out of this plane forming the puckered furan-type conformation very similar to that found in furanose sugar.The C-0 bond adjacent to the carbonyl group is 0.10 8 shorter than the other C-0 bonds which do not differ significantly from a mean of 1.421 The C(ltO(1)H bond (1.3928,) in P-m-arabinose (70) is significantly shorter than the mean value 1.423 8 for the other C-OH bonds in the molecule. There is no significant difference between the two ring C-0 distances 1.44 These observations are consistent with those from other recent determinations of pyranose sugars. In J3-D-glucurono-y-lactone (71) the rings neither of which is planar are inclined to each other so that the best planes containing four atoms in each make a dihedral angle of 111*3°.178Two of the substituents are endo contrary to the rule that stable derivatives of two fused five-membered rings compounds are those with the minimum number of endo-sub~tituents.'~~ The lactone group in the molecule is not planar having a carbon atom 0.26 8 out of the plane of the C.CO.0 group which is planar.The C-0 bond adjacent to the carbonyl group is again 0.10 8 shorter than other formal single C-0 bonds in the molecule. The a-l,d-linked glucopyranose residues of methyl P-maltopyranoside (72) have the C(1) chair conformation with interatomic distances which are normal for single bonds with the exception of the C-0 G. A. Jeffrey R. D. Rosenstein and M. Vlasse Acta Cryst. 1967,22 725. "'S. H. Kim and G. A. Jeffrey Acta Cryst. 1967 22 537. S. H. Kim G. A. Jeffrey R. D. Rosenstein and P. W. R. Corfield Acta Cryst. 1967,22 733.R. D. Guthrie and J. Honeyman 'Introduction to the Chemistry of the Carbohydrate,' Oxford University Press London 1964. Part (u) X-Ray Crystallography 91 bonds. The methyl f3-glycoside bond C(l’)-O(l’)Me is 1.375A. It is significantly shorter than the mean value of 1.427 A in contrast to the a-glycosidic link joining the two glucopyranoside residues which is 1.416 A. This difference in glucosidic bond lengths appears to be correlated with relative lengths of the C-0 ring bonds which are observed equal in one ring and unequal in the other.’” The primary alcohol group in the a-anomer of the pyranose form of L-sorbose is disordered which leads to an apparent shortening of corres- ponding C-OH bonds. With the exception of those bonds the C-C and C-0 distances do not differ significantly from the mean values of 1.516 and 1.424 8 respectively.lS’ P-Lyxose (73) occurs in the conversion form leq2ax- 3eq4eq with C-C bond lengths in the range 1.509-1.538 C(1)-O(1) 1.364 & 0.006 and the other C-0 bond lengths varying from 1.399-1.435 & 0.006 I$.Nine of the fourteen bond angles in which only carbon and oxygen are involved deviate significantly from the tetrahedral value. Comparison with other nucleosides and nucleotides shows that the glycosidic bond length (1.40 A) in 5-bromo-5-deoxyuridine (74) is shorter than normal and that the conformation of the C(5’)-0(5’) bond is not that most commonly found. In 5-bromouridine the glycosidic bond length (1-49 A) and the C(5’)-O(5’) bond conformation are normaLiS3 In both compounds the uracil base is essentially planar but atom C(1’) is significantly out of the uracil plane; the sugar ring is puckered with C(2’) lying 0.59 and 0.52 I$ out of the plane of the other four ring-atoms (for the deoxyribose and ribose sugar respectively).Complexes.-The molecular adducts LiBr,2NH2 [CH2] NH2 and LiC1,2NH2-[CH2I2-NH2 are isomorphous. The structure consists of infinite molecular chains held together by NH.-X- bonds. There are no Li+-X- bonds. Each lithium atom is tetrahedrally surrounded by four amino-groups from three ethylenediamine molec~les.~ LiCl also forms a complex with 84 pyridine (py) of composition LiC1,2py,2H20. The ability of deoxycholic acid (DCA) to form well-defined molecular compounds with many organic substances is well known.With p-di-iodo benzene (PDIB) crystals of com- position 2DCA PDIB are obtained and the ‘guest’ molecule is held in channels which run parallel to the crystal c axis in the ‘host’ system. With naphthalene benzanthracene and phenanthrene the structures must be disordered. 86 The equatorial isomer of perhydrotriphenylene (PHTP) gives rise to a wide variety of inclusion compounds with different kinds of molecules ranging from those with a nearly spherical (e.g. CCI,) or planar shape (benzene) to linear molecules and macromolecules. The adducts have a channel-like structure with the PHTP molecules arranged in infinite stacks whose axes are parallel to the three-fold axis of the molecules. The structures of three different adducts S.S. C. Chu and G. A. Jeffrey Acta. Cryst. 1967,23 1038. S. H. Kim and R. D. Rosenstein Acta Cryst. 1967,22 648. A. Hordvik Acta Chem. Scand. 1966,20 1943. J. Iball C. H. Morgan and H. R. Wilson Proc. Roy. SOC.,1966 A 295,320. F. Durant P. Piret and M. van Meerssche Acta Cryst. 1967,23 780. F. Durant P. Piret and M. van Meerssche Acta Cryst. 1967,22 52. A. Damiani E. Giglio N. Morosoff R. Puliti and I. Rosen Ricerca xi.,1967,37,42. D 92 George Ferguson have been described:'87 (a) one with a n-hydrocarbon n-heptane (which is isomorphous with those containing n-ethers n-carboxylic acids n-esters and also iso-octane and CCI,) (b)that with chloroform (c) that with cyclo- hexane. In case (a) no coherence is observed between the rows of included molecules and the host structure but in (b)and (c) the presence of simple molecular ratios between the host and guest compounds leads to formation of more complex structures.In the (b) adduct two kinds of non-equivalent included molecules are present; the (c) structure results from the packing of parallel PHTP molecules arranged along helices with a small radius (0.40 A) and nine residues in two pitches with rows of regularly spaced cyclohexane molecules. Other different crystal structures have been observed with dioxan with benzene toluene or bromoform; and with some substituted polybuta- 1,4-dienes. The structural results on two different modifications of pure PHTP showing different stability at room temperature are also reported.In the 2 1 complex of 1,3,7,9-tetramethyluric acid (TMU) and coronene the hydrocarbon molecule is sandwiched between two TMU molecules. The coronene to TMU distance is 3.45 A.188 A similar separation (3.48 A) is found between the molecular planes in the 2:l complex of TMU and 3.4-benz- pyrene. This structure is characterised by a plane-to-plane alternate stacking of two TMU and one benzpyrene molecule arranged in infinite columns parallel to the h axis.'89 Molecular geometry virtually identical with that found in free hexamethylene tetramine"' is found in the hexamethylenetetramine(hex) complex CaBr hex 10H20.191p-Chloro- and p-bromophenol form 2 1192 and 1 :1Ig3 complexes with p-benzoquinone. The component molecules of the isomorphous 2 :1 complexes are stacked plane-to-plane discontinuously in groups of three each group consisting of a quinone molecule between two phenol molecules and held together by charge-transfer forces.The isomorphous structures of the 1:l complexes contain the phenol and quinone molecules stacked alternately plane-to-plane in infinite columns. The structure of the 1:1 tetracyanoethylene :naphthalene complex consists of infinite columns of alternate tetracyanoethylene and naphthalene molecules with molecular overlap such that the molecules are positioned as for a Diels- Alder reaction. The mean perpendicular separation of the molecules is 3-30 A.194. 1,2,4,5-Tetracyanobenzeneforms a 1 :1 complex with naphthalene but the structure is disordered with naphthalene molecules adopting one of the alternative orientations with equal probability.The average interplanar G. Allegra M. Favina A. Immitzi A. Colombo U. Rossi R. Broggi and G. Natta J. Chem. SOC.(B),1967 1020. 18' A. Damiani E. Giglio A. M. Liquori R. Puliti and A. Ripamonti J. Mol. Biol.,1967 23 113. 189 A. Damiani E. Giglio A. M. Liquori and A. Ripamonti Acta Cryst. 1967,23 675. L. N. Becka and D. W. J. Cruickshank Proc. Roy. SOC.,1963 A 273,435. 19' L. Mazzarella A. L. Kovacs P. De Santis and A. M. Liquori Acta Cryst. 1967,22,65. lg2 G.G. Shipley and S. C. Wallwork Acta Cryst. 1967,22 585. 193 G. G. Shipley and S. C. Wallwork Acta Cryst. 1967,22 593. 194 R. M Williams and S C. Wallwork Acta Cryst. 1967,22 899. Part (u) X-Ray Crystallography spacing in the complex is 3.43A.195 The structure of the 1:1 complex NNN’N’-tetramethyl-p-phenylenediamine and 1,2,4,5tetracyanobenzenedoes not seem to show the usual n-m interaction between the aromatic rings but indicates n+n interactions localised between the nitrogen atoms of the donor and the cyano-groups of the acceptor.The magnitude of the charge-transfer from the donor to the acceptor was estimated to be 0.24in electron units.‘96 The complex between 7,7,8,8-tetracyanoquinodimethane and bis-(8-hydroxyquinolato)-copper-(11) crystallises with the component molecules plane-to-plane so that the double bond adjacent to one dicyanoethylene group of the tetracyanoquino- dimethane molecules lies over the 5:8 positions of one donor molecule whilst the other double bond is similarly oriented with respect to the benzenoid ring of the centrosymmetrically related donor molecule.The perpendicular separa- tion of the molecules in the region of overlap is about 3.2A.l” The structure of the complex formed from dimeric 8-hydroxyquinoline and chloranil molecules differs from that predicted by the overlap and orientation principle and is remarkably similar to the bis-8-hydroxyquinolinatopalladium(11)-chloranil molecular complex. The 8-hydroxyquinoline dimer is held together by a bifurcated hydrogen-bond system and the occurence of bifurcated hydrogen bonds is di~cussed.’~~ Natural Product Structures.-That there is no change in configuration at C( 11) during the formation of desmotroposantonin from santonin has been established by an X-ray investigation of 2-bromo-( -)-P-desmotroposantonin (75).lg9.Ring B approximates to an envelope rather than the expected half- chair conformation. The five-membered lactone ring is in the usual envelope conformation with C(7)displaced by 0.52 A from the plane through the other atoms of the lactone ring. The fbepoxide configuration in palmarin derivative (76) has been established and the results of the analysis reveal a degree of con- formational distortion that might appear unreasonable from an examination of Dreiding models. The distortion in ring A is best characterised by comparing the C(14)*-*C(2) distance 2-61A).Interaction distance 3.57 with the C(9).*-C(l) between the C(14)methyl and epoxide oxygen is responsible for this and th f separation of these two centres is 2.92A.2ooThe structure and relative stereo- chemistry of bromomexicanin-E are shown in (77).The two five-membered rings are non-planar and are cis-fused to the seven-membered ring which is in a boat conformation.All bond distances and angles have values close to those expected.”l The constitution and absolute stereochemistry of E-caesalpin (78)have been determined. Rings A,B and c are fused trans-anti-trans with A and B in chair and c in half-chair conformations. The C(5)axial hydroxy- 19’ S. Kumakura F. Iwasaki Y. Saito Bull. Chem. SOC.Japan 1967,40 1826. Y. Ohashi H. Iwasaki and Y. Saito Bull. Chem. SOC.Japan 1967,40 1789. 19’ R. M. Williams and S. C. Wallwork Acta Cryst. 1967,23,448. 19’ C.K. Prout and A. G. Wheeler J. Chem. SOC.,(A) 1967,469. 199 A. T. McPhail B. Rimmer J. M. Robertson and G. A. Sim J. Chem. SOC.(B),1967 101. K. M. S. Islam G. Ferguson K. H. Overton and D. W. Melville Chem. Comm. 1967 167. 201 Mazhar-UI-Haque and C. N. Caughlan J. Chem. SOC.(B) 1967,355. George Ferguson group is involved in an intramolecular hydrogen bond (2.65 A) with the C(l) axial hydroxy-group.f02 Structure determinations of testosterone and 8P-methyltestosterone deriva- tives have shown that the conformation of the latter is clearly bent in contrast Me R (75) (76) (79) '02 A. Balmain K. Bjamer J. D. Connolly and G. Ferguson Tetrahedron Letters 1967 5027. Part (u) X-Ray CrystuZZography 95 with the ‘planar’ overall shape of the ring system of testosterone.Ring D of testosterone has the P-envelope conformation while in the P-methyl derivative a half-chair conformation is adopted.203 17~-Bromoacetoxy-9~-lOa-androst-4-en-3-one has contrary to normal 9a- 10P-steroids a zig-zag skeleton [see (79) which also shows the absolute configuration]. Ring A is a distorted half- chair; rings B and c are chair forms and ring D is in the envelope conforma- ti~n.~’~ The stereochemical course of an unusually facile D-homo-rearrange- ment of 20P-p-bromo benzensulphonyloxy- 19-nor-9p 1Oa-pregn-4-ene-3-one to 17a~-p-bromobenzenesulphonyloxy-17a-methyl- 19-nor-9P l0a-D-homo- androst-4-en-3-one (80) has been established by an X-ray The product molecule is free from major conformational distortion; ring A is in the half-chair form and rings B,C and D exhibit chair forms with average valency angles slightly greater than tetrahedral.The structure of diosgenin has been confirmed206 with the stereochemistry implied by (81). The double bond at C(5)C(6) confers a degree of planarity on the A-B ring system. The triterpene 2a-bromoarborinone has structure (82) with a 13P 14~-trans configuration of the methyl groups at the C-D ring junction. Four of the five carbon atoms of ring E are coplanar to within 005 A and C(17) is displaced 0.74 A from this plane.207 In a molecule containing fused five- and six- membered rings four atoms of the 5-membered ring are frequently coplanar while one of the two atoms common to the two rings is out of plane by approxi- mately 0.7 A.The geometry of the perhydrophenanthrene skeleton in eight steroids has been discussed in considerable detail in terms of valency and torsional angles in a paper by Geise et aL208 The geometrical details of the molecules from X-ray structure determinations are compared with those obtained from theoretical considerations on appropriately substituted cyclo- hexane and cyclohexene rings. It is shown that the use of such building material leads to a qualitative agreement. A number of interactions present in a steroid but not in a cyclohexane unit prevents a quantitative agreement. The steroid skeleton (all-trans) has a somewhat bent overall shape. The occurrence of conformational transmission effects is discussed and a number of rules con- cerning the torsional angles around junctions are given.’03 H. Koyama M. Shiro R. Sato Y. Tsukuda,H. Itazaki and W. Nagata Chem. Comm. 1967,812. 204 W. E. Oberhansli and J. M. Robertson,Heh. Chim. Acta 1967,50 53. ’05 R. T. Puckett G. Sim A. D. Cross and J. B. Siddall J. Chem. SOC.(B) 1967 783. *06 E. A. O’Donnell and M. F. C. Ladd Acta Cryst. 1967,23,460. 207 0.Kennard and L. Riva Di Sanseverino Tetrahedron 1967,23 131. 208 H. J. Geise C. Altona and C. Romers Tetrahedron 1967,23,439. George Ferguson Me (86) (87) All six-membered rings in neothiobinupharidine dibromide tetrahydrate (83) are in chair conformations and all substituents are equatorial. There is some molecular disorder in the crystals associated with an approximate non- crystallographic two-fold axis.209 The hydroxy-group in the perchlorate salt of hydroxy-P-isosparteine (84) is attached to C(7)and not the bridged atom C(8)as previously thought.The molecule is the trans-trans-isomer and all four six-membered rings are in the chair conformation. Neglecting the hydroxy- group the molecule has a two-fold axis of molecular symmetry through the bridge carbon atom.210 The stereochemical features of the steroid alkaloid tomatidine (85) have been determined from an X-ray analysis of a hydro- bromide derivative.21 ' The same stereochemistry has also been reported from an X-ray analysis of the isomorphous hydroiodide derivative.2 l2 The cyclo- hexane rings A,B and c are in the chair conformation with average angles close to 110".Four atoms of ring D are coplanar while the fifth C(14) is 0.7 A from this plane. Four atoms of ring E are also coplanar while the spiro-carbon atom C(22)is out of plane by 0.5 8 resulting in the D-E ring system being symmetrically distorted about the line of fusion. The steroidal framework is bent slightly so that bonds C(lOkC(9) and C(13bC(18) are not parallel but inclined.2 The structure of the Crotalaria alkaloid retusamine has been determined as its monohydrated bromocamphorsulphonate salt. The absolute configuration of cation and anions are shown in (86) and (87).The water '09 G. I. Birnbaum Acta Cryst. 1967,23,526. 210 J. M. H. Pinkerton and L. K. Steinrauf J. Org. Chern. 1967,32 1828. 211 0.Kennard L. Riva Di Sanseverino and J.S. Rollett J. Chem. SOC.,(C),1967,956 212 E. Hohne H. Ripperger and K. Schreiber Tetrahedron 1967,23,3705. Part (u) X-Ray CrystulEography 97 molecule forms a hydrogen-bond network linking the OH group of the retusamine cation to two of the sulphonate oxygen atoms. Of particular interest is the very long ring fusion C-N bond (1-64A) since it is readily cleaved on regeneration of the free base to give an eight-membered cyclic amino-ketone exhibiting transannular interaction between the trigonal nitrogen atom and the carbonyl group. Also of general stereochemical interest are the observations that in the anion the bromine atom is in the endo-position and the sulphonate group is attached to C(9) trans-n to the keto-gro~p.'~~ Me -0 (90) CH,OH 0 The structure and absolute stereochemistry of the hydrobromide salt of a Diels-Alder adduct of thebaine has been determined as (88).The molecular shape which is partly defined by a complex cage structure is severely distorted when compared with an idealised Dreiding model. The hydroxy-group forms an intramolecular hydrogen bond with the adjacent methoxyl oxygen. The positian of the hydroxy-hydrogen atom implies that the methoxy-oxygen is sp2 hybridised. The C-O-CH3 valency angle is 120".The other methoxy- group angle is 117".214The structure and absolute stereochemistry of a p-bromobenzoate of a dihydroanhydroacetonide derivative of taxadiene tetraol have been determined as (89).*' The cyclohexane ring trans-fused with the cycloheptane ring is in a slightly distorted chair conformation.The cyclopentene ring is envelope-shaped and cis-fused with the cycloheptane ring which adopts a twisted boat conformation. A study of the quinidine salt of ( -)-1,1'-dimethylferrocene-3-carboxylic acid has defined the absolute *13 J. A. Wunderlich Acta Cryst. 1967 23 846. 214 J. H. van den Hende and N. R. Nelson J. Amer. Chem. SOC.,1967,89,2901. 215 W. R. Chan T. G. Halsall G. M. Hornby A. W. Oxford W. Sabel K. Bjamer G. Ferguson and J. M. Robertson Chem. Comm. 1966,923. George Ferguson stereochemistry of the metalocene and confirmed the accepted absolute stereochemistry of the alkaloid quinidine.2 l6 The absolute stereochemistry and precise details of molecular geometry of gliotoxin (90) have been re-ported.217 The fused rings form a rigid molecular framework with the piper- azinedione constrained to a boat conformation by the disulphide bridge.The chirality of the CSSC group is left handed and the contribution of the di- sulphide-bridged piperazinedione system can be associated with a negative peak (at 233 mk) in the circular dichroism curves of gliotoxin sporidesmin and their derivatives containing the same system. It is therefore concluded that all these compounds have the same absolute stereochemistry. The cyclo- hexadiene ring has a skew chair conformation. An N-brosyl derivative of mitomycin A an anticancer antibiotic has been shown to have structure and absolute stereochemistry (91).218The bond lengths in the aziridine ring are normal (C-N 1.48 and 1.49 A).The aziridine nitrogen is non-basic atypical for an aziridine nitrogen atom even when part of a [3,l,O]bicyclic system and is attributable to steric interference of its fourth valence pair with the free p orbital of N(2). N(1) may thus be considered as an optically active site pro- duced by steric interference. *16 0.L. Carter A. T. McPhail and G. A. Sim J. Chem. SOC.(A),1967 365. *I7 J. Fredrichsons and A. McL. Mathieson Acta Cryst. 1967,23,439. 218 A. Tulinsky and J. H. van den Hende J. Amer. Chem. SOC.,1967,89 2905.

ISSN:0069-3030    

DOI:10.1039/OC9676400065

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

3. (Part i) REACTION MECHANISMS By B.C. Challis (Department of Organic Chemistry Imperial College of Science London S. W.7) Deuterium Isotope Effects.-These are conveniently discussed under three sub-headings primary effects arising from reactions where the isotopically substituted bond undergoes fission ;secondary effects associated with isotopic substitution of bonds not undergoing rupture in the transition state; and solvent effects arising from differences between ordinary water and deuterium oxide as reaction media. Attention is drawn to significant developments in the interpretation of variable isotope effects and these are discussed under both primary and solvent isotope sub-headings in relation to the transition state configuration of proton-transfer reactions.Primary isotope effects. These manifest themselves as a lower rate of deuterium transfer compared to hydrogen (k,lk > 1) and arise from differ- ences between the zero-point vibrational energies of H and D in the ground and transition states. The differences can be calculated for the ground state but not for the transition state where the speculative nature of the vibrations involved evokes considerable enquiry from both theoretical and experimental standpoints. It may be recalled that Westheimer' was the first to suggest from con- sideration of a simple three-centre model that mass-dependent zero-point energy may arise from the unsymmetrical stretching mode (1) in the transition state This led to the prediction in accord with recent experimental findings t+ 4-+ 3 A -H + B + [A---H ---B] (1) that k,/k will pass through a maximum value when the transition state is symmetrical.Westheimer's calculations have been criticised by Bell,2 in particular for failing to take account of bending vibrations and proton tunnelling and this criticism can be justified in at least two recent examples where the experimental isotope effects exceed the calculated energy difference for the loss of stretching vibrations only.3* Thus suggestions have been made that k,/k may in fact be virtually independent of the transition-state or even be at a minimum for the symmetrical configuration.6 F. H. Westheimer Chem. Rev. 1961,61,265. R. P. Bell Discuss. Faraday SOC.,1965,39 16. R.P. Bell and D.P. Onwood J. Chem. Soc.(B),1967,150. 'J. R. Jones R. E. Marks and S. C. Subba Rao Trans. Faraday SOC. 1967,63 111. ' A. V.Willi and M. Wolfsberg Chem. and Ind. 1964,2097. R. F. Bader Canad. J. Chem. 1964,42. 1822. D* 100 B. C.Challis Alber~,~ however has now considered the three-centre transition-state model for proton transfer in more detail and calculated kH/kDfor all possible values of the force constants; it is evident that this model can account for the experimental observations provided no preconceptions are held about the shape of the potential barrier. Also some of these criticisms have been answered by the interesting calculations of More O’Ferrall and Kouba’ employing four- and five-centre transition-state models “(2)and (3) respectively] to account for A-B-H + C + [A-B.***H*...CI # (2) A-B-H + C-D -+ [A-B ..-.H.....C-D]# (3) contributions from bending vibrations and from proton tunnelling. Three important predictions emerge from these calculations Firstly. for proton transferfrom carbon or an ammonium ion to a polvatomic base (MeO- MeS- CN- etc.) kH/kD values pass through a maximum for the symmetrical tran- sition state and are not therefore fundamentally different from those based on stretching vibrations alone. This is illustrated for MeO- in Table 1 where x is the degree of proton transfer. Secondly for proton transferfrom carbon to a monatomic base (e.g. Br- in Table 1 the inclusion of bending vibrations broadens the k,/kD maximum with large isotope effects predicted for product- like transition states.Thirdly the maximum value of kH/kD for proton transfer to carbon is considerably reduced when allowance is made for bending vibrations as illustrated in Table 2 The differences in the predicted isotope effects for the two models are large enough to be detected experimentally and these results must be awaited with interest. TABLE1 Calculated isotope effects from ref. 8 for proton transfer to MeO-and Br- X MeO- Br- 1 1.0 kH/kD 1.0 kH/kD 0-875 3-3 2.8 0.75 5.2 3.8 0.625 7.1 5.5 0-5 7.2 7.3 0-375 5.6 8.3 0.25 4-1 7.8 0.125 3.3 7.2 0 1.1 3-2 W.J. Albery Trans. Farads!) Soc. 1967,63,200. R. A. More O’Ferrall and J Kouba. J. Chm. Soc. (B) 1967,985. Reaction Mechanisms TABLE 2 Maximum isotope effects from ref.8 for proton transfer to curbon Proton Bending and Stretching donator stretching vibrations vibrations only (R,H)C-H 7-0 kH/kD 7.2 kdkD R,Nf-H 6.6 6.6 NEC-H 7.0 10.2 RO-H 7.0 10.2 RS-H 3.0 5.8 F-H 6.8 14.9 CI-H 3.0 7.2 Br-H 2.7 5.9 I-H 2.4 5.8 Until recently the only firm evidence for the incidence of variable primary isotope effects came from studies of pseudo-acid ionization (reported last year)' and from the compilation of Kresge" for aromatic hydrogen exchange reactions. Now Long and his co-workers" report that k,/k ratios for hydrogen exchange in substituted azulenes range from 6 to 9.6. These results which may require slight modification to account for secondary isotope effects,' were obtained by varying the base strength of either the catalysing acid or the substrate.They clearly indicate a maximum isotope effect for the symmetrical transition state and virtually parallel the earlier datag for pseudo-acid ionization Less definitive support comes from other reports of substrate-dependent primary isotope effects in the bromination of acetophenones,4*' for aromatic hydrogen exchange in NN-dimethylanilines l4 and from Kresge and Chiang's l2 detailed kinetic analysis of aromatic hydrogen exchange in trimethoxybenzene. Variable isotope effects for proton transfer from trimethyl- ammonium ions15 and in the mass spectral McLafferty rearrangement l6 have also been rationalised in terms of changes in transition-state symmetry. By and large however the limited data available at present gives no clear indication as to the most satisfactory theoretical representation In fact the choice of an experimental system to test the various theories may be limited to those with uncongested transition states.This is evident from kinetic studies of the iodination of azulene for which proton-loss from the transition state is rate-controlling. The experimental k,/kD ratios are almost independent of the strength of the catalysing base It 15 believed that steric B. C. Challis Ann. Reports 1966,63 278. Soc. 1967 63 200. lo A. J. Kresge Pure Appl. Chem. 1964,8 517. '' J. L. Longridge and F. A. Long J. Amer. Chem. SOC. 1967 89 1287; L. C. Gruen and F. A. Long ibid.,p. 1292. See A. J. Kresge and Y.Chiang J. Amer. Chem. SOC.,1967,89,4411. l3 J. R. Jones R. E. Marks and S. C. Subba Rao Trans. Faraday SOC. 1967,63,933. l4 A. C. Ling and F. H. Kendall. J. Chem. SOC.(B),1967.445. R. J. Day and C. N. Reilley J. Phys Chem. 1967,71 1588 '' J. K. McCleod and C. Djerassi J. Amer. Chem. SOC. 1967,89,5182. 102 B. C.Challis effects arising from solvation of the catalyst may work in opposition to the effect of basicity on the transition-state symmetry and therefore smooth out any variations in the isotope effect.” The manner in which steric interactions enhance k,/kD ratios has been discussed further by Lewis and Funderburk,18 in connection with additional measurements of proton abstraction from 2-nitropropane by pyridine bases. They argue that in addition to proton tunnelling (the isotope effect on Arrhenius A values evident from AdAD= 0.15) steric congestion of the transition state may also cause an abnormal loss of bending zero-point energy.A similar explanation could apply to the iodination of azulene. Less well authenticated claims have also been made that k,/k ratios effect the degree of proton transfer in the transition state (4)of base-catalysed Base .% H. L-c--$ / B a ’*x (4) p-elimination reactions.’ These appear to be substantiated by several recent findings. For instance there is nice agreement in the effect of substrate structure on the magnitudes of both the solvent isotope effect (ie. base = OH- and OD-in H,O and D,O respectively) and the primary isotope effect (i.e.k,,/kD for PhCH2CH2X us. PhCD2CH2X in solvent H,O) for E2 elimination from P-phenethyl derivatives consistent with the notion that both ratios depend on transition state symmetry. *’ More compelling evidence however is that primary k,/k ratios for OH-catalysed (3-elimination from PhCH,CH,SMei and PhCD,CH,SMel depend on the dimethyl sulph- oxide concentration in mixed-aqueous solvents passing through a maximum in about 50% dimethyl sulp%xi&.21 This is the first reported case of a maximum isotope effect for a single reaction. Related measurements of the leaving-group isotope effect (k32S/k34S)22 and other kinetic data” show the variation in k,/kn is consistent with the formation of a more reactant-like transition state (both C,-H and C,-S bonds only slightly stretched) in the dimethyl sulphoxide-rich solvents.Why the C,-H bond stretching should vary with solvent composition is less clear. Cockerill and Saunders” propose that addition of dimethyl sulphoxide desolvates and therefore raises the basicity of the OH- catalyst mainly on the grounds that a comparable solvent effect is not observed in either the Bu‘O-catalysed elimination from 2-aryl- ethyl bromide23 or the OH -catalysed elimination from fluorenyl nitrate.24 E. Grovenstein and F. C. Schmalsteig J. Amer. C~PIII. Soc. 1967.89. 5084. ** E. S. Lewis and L. H. Funderburk. J. Artier. Cheni. So(..,1967,89 2322. l9 J. F. Runnett. Angew. Chem. Inrernrit. Edri.. 1962. 1. 225. lo L. J. Steffa and E. R. Thornton J. Amer. Chem. SOC. 1967,89,6149.21 A. F. Cockerill J. Chem. SOC.(B),1967,964. 22 A. F. Cockerill and W. H. Saunders J. Amer. Chem. SOC. 1967,89,4985. 23 A. F. Cockerill S. Rottschaefer and W. H. Saunders J.Amer. Chem. SOC. 1967,89 901. 24 P. J. Smith and A. N. Bourns Canad. J. Chem. 1966,44,2553. Reaction Mechanisms I03 Also there is plenty of other evidence showing that dimethyl sulphoxide alters substrate basicities’’ and causes remarkable changes in the rates26 and isotope effects2’ of proton-transfer reactions. These observations therefore suggest that suitable studies in aqueous dimethyl sulphoxide solvents may be the easiest way of testing the current theories of primary-isotope effects. Solvent isotope efSects. The underlying theory to kinetic and equilibrium measurements in H,O-D,O mixtures has been substantially modified in recent years and the relevance of these changes is discussed by Albery28 in an authoritive review.However slight changes will be necessary in the theory and numerical calculations on account of experimental confirmation that the equilibrium constant for the fractionation of hydrogen isotopes in H20 and D,O [equation (l)] does not accord with the rule of the geometric mean (i.e. K = 4.0). H20 + D,O + 2HD0 The new experimental value (K = 3~75)~~ is in good agreement with that measured last year.30 The equilibrium constant (L)for equation (2) has been redetermined from electrochemical data and the new value of L = 9.0 & 0.3 also stands in excellent agreement with previous estimates.31 2D30+ + 3H20 + 2H30+ + 3D20 (2) It is generally recognised that measurements in H,O-D,O mixtures are not a reliable criterion of reaction mechanism but useful information as to the extent of proton transfer from the hydronium ion can be obtained for reactions in which this process is rate-controlling. [equation (Z)]. The degree of proton transfer (defined as aiand with values 0 < ai < 1) is obtained from the experimental measurements via equations which describe the resemblance between isotopic hydrogens (I.,) in the transition state with those in the solvent. Despite the complexity of these equations their applica- tion is straightforward as is shown in ref. 28. The magnitude of aishould by definition be similar to the Bronsted para- meter (a,) obtained from kinetic studies of general acid catalysis.Nevertheless their exact correspondence has been questioned because the energy differences upon which the two parameters depend are fundamentally different (a comes 2s (a) C. D. Ritchie and R. E. Uschold J. Amer. Chem. SOC. 1967 89 1721; (b) ibid. p. 2752; (c)E. C. Steiner and J. D. Starkey ibid. p. 2751. z6 C. D. Ritchie and R. E. Uschold J. Amer. Chem. SOC. 1967,89 1730. ’’ J. R. Jones Chem. Comm. 1967,710. 28 W. J. Albery Progr. Reaction Kinetics 1967,4 353. 29 J. W. Pyper R. S. Newbury and G. W. Barton J. Chem. Phys. 1967,46,2253. 30 L. Friedman and V. J. Shiner J. Chem. Phys. 1966,44,4639. 31 P. Salomaa and V. Aalto Acta Chem. Scand. 1966,20,2035. 104 B. C. Challis from differences of two potential energy surfaces whereas airefers to zero- point energy differences for a single potential energy surface).Recent ex-perimental studies however show substantial agreement between aiand aB for several reactions such as the hydrolysis of vinyl ethers32 including the related cyanoketen dimethylacetal [NC-CH=C(OMe)2],33 and the hydra- tion of a-methyl~tyrene.~~ Also aB for the acid-catalysed cleavage of allyl- mercuric iodide agrees with a obtained from earlier measurement^.^' Thus solvent isotope effects in H,O-D,O mixtures appear to be a useful alternative to the Bronsted technique of estimating transition state configurations. In connection with this conclusion claims have also been made that entropies of activation (AS#) also reflect the extent of proton transfer in the transition 36 It is suggested that ASfjdecreases with increasing proton transfer.The general utility of this criterion is doubtful as an extensive compilation of A-&2 reactions shows a rough correlation of AS# with sub- strate reactivity but in the opposite sense to that predicted above.37 Also polar substituents are known to interact with the solvent to cause drastic changes in AS#values.38 Solvent isotope effects in solvolysis reactions have been reviewed again this time by a proponent of the importance of initial-state s~lvation.~’ Studies of triphenylcarbinol ionization in H2S04 and D2S04show the HRfunction has the same numerical value in both solvents :40 this unexpected result is similar to that for the Ho function (amine equilibria) several years Secondary isotope effects.The origin of secondary isotope effects has been a controversial matter for some time. Generally speaking however they have been discussed in just two different ways either as non-bonded (steric) inter- actions (on the basis that C-D bonds are shorter than C-H bonds) or as electronic interactions of inductive or hyperconjugative types depending on the site of isotopic substitution. Key references for these two interpretations were given in last year’s report in which recent arguments in favour of the steric model were also summari~ed.~~ But the issue is far from settled parti- cularly in regard to P-deuteriation. Thus recent findings of ring deuteriation at any site increasing the stability of triphenylcarbonium ions,43 but decreasing the rate of proton-abstraction from toluene ~ide-chains,~~ both show that 32 (a)A.J. Kresge and Y. Chiang J. Chem. SOC.(B),1967,53,58;jb) P. Salomaa A. Kankaanpera and M. Lajunen Acta Chem. Scad. 1966,u) 1790. 33 V. Gold and D. C. A. Waterman Chem. Comm. 1967,40. 34 J-C. Simandoux B. Torck M. Hellin and F. Coussemant Tetrahedron Letters 1967 2971. 35 M. M. Kreevoy T. S. Straub W. V. Kayser and J. L. Melquist J. Amer. Chem. SOC. 1967,89 1201. 36 A. J. Kresge Y. Chiang and Y. Sato J. Amer. Chem. SOC.,1967,89,4418. 37 M. A. Matesich J. Org. Chem. 1967,32 1258. 38 A. C. Ling and I‘ H. Kendall J. Chem. SOC.(B),1967,440. 39 R. E. Robertson. Progr. Phys. Org. Chem. 1967,4213.40 E. DE Fabrizio Ricercu xi. 1966,36 1321. 41 E. Hogfeldt and J. Bigteison J. Amer. Chem. SOC. 1960,82 15. 42 Ref. 9 p. 280. 43 A. J. Kresge and R. J. Preto J. Amer. Chem. SOC.,1967,89,5510. 44 A. Streitwieser and J. S. Humphrey J. Amer. Chem. Soc. 1967,89 3767. Reaction Mechanisms I05 deuterium acts as an electron-donator (relative to hydrogen) with steric interactions playing only a minor part in these cases. A similar conclusion is drawn by Karabatsos and his colleagues45 for P-deuteriated systems such as SN1 solvolysis of (CD,),CCl and CD,COCl where hyperconjugative stabi- lisation should be important. Other studies of P-deuterium effects in t-butyl radical formation46 and nucleophilic substitution in allenyl halides4’ have also been interpreted in terms of hyperconjugative rather than steric differences between hydrogen isotopes.Therefore current results by and large serve to temper the recent views of H. C. Brown and his co-workers4’ on the overiding importance of steric effects. Another important development comes from hypotheses that charge de- localisation in the transition state reduces the usual magnitude of both a-and P-deuterium effects observed in limiting (SN1)solvolyses and that this re- duction might therefore be a useful probe for neighbouring-group participation. The case for P-deuterium is the better authenticated. Thus k,lkD ratios for the solvolysis of many 0-deuteriated cyclopropyl and cyclobutyl compounds such as (5) and (6) are much less than the usual value of k,/kD = 1.14 per P-deuterium atom observed for S,1 reactions.This reduction has been identified with substitution uin a common intermediate (7) in which charge ,-fD3 slow -products delocalisation lowers the usual demand for hyperconjugative stabilisation by the P-methyl groups.48 Arguments for a similar reduction of k,,/k ratios for a-deuteriation come from solvolysis studies of 2-arylethyl compounds known to react uia bridged phenonium ions (8). In this case the reduction in k,/kD is associated with the more ‘SN2-like’ transition state. which in turn should cHi-CD2-. -X-(8) 45 G. J. Karabatsos G. C. Sonnichsen C. G. Papaioannou S. E. Scheppele and R. L. Shone J. Amer. Chem. SOC.,1967,89,463. 46 T. Koenig and R.Wolf J. Amer.Chem. SOC.,1967,89 2948. 47 V. J. Shiner and J. S. Humphrey J. Amer. Chem. SOC.,1967,89,622. 48 M. NikoletiC S. BorEiE and D. E. Senko Tetrahedron 1967,23 649. 106 B. C. Challis reduce the usual a-deuterium effects ascribed to the relief of non-bonded interactions by the change from sp3 to sp2 hybridization in S,1 solvoly~is.~~ These suggestions have an important bearing on the much debated incidence of non-classical carbonium ions in the solvolysis of 2-norbornyl compounds. As is shown below for both a-and P-deuteriated species the exo-2-norbornyl brosylates (9) and (11) show significantly reduced k,/kD ratios whereas those for the corresponding endo-isomers (10) and (12) respectively are of the ex- pected magnitude for unassisted solvolysis.50~ 51 It is of importance too that 6-deuteriation of 2-norbornyl brosylates has a markedly different influence on the solvolysis rates of exo-isomers (13) and (14) compared to the endo- isomer (15).52None of these effects are inconsistent with the formation of D&OBs D &OBs D& OBs kJkD 1.14 1.10 1.oo (13)” (14)” (1 5)’’ bridged carbonium ions for the exo-brosylate isomers.Similarly reduced a-deuterium isotope ratios have also been reported for allylic rearrangements via cyclic transition states [equation (4)]. In this case the y-deuterium effect is inverse (kH/kD < 1). It is claimed that such effects may be a useful criterion for intramolecular allylic rearrangement^.^ 49 C. C. Lee and L. Noszk6 Canad. J. Chem. 1966,44,2491.C. C. Lee and E. W. C. Wong J. Amer. Chem. SOC.,1964,86,2752. 51 J. M. Jerkunica S. BorEiE and D. E. Sunko Chem. Comm. 1967 1302. ** B. L. Murr A. Nickon T. D. Schwartz and N. H. Werstiuk J. Amer. Chem. SOC.,1967,89,1731; J. M. Jerkunica S. BorEiE and D. E. Sunko ibid. p. 1732. 53 K. D. McMichael J. Amer. Chem. SOC.,1967,89 2943. Reaction Mechanisms 107 In an extensive compilation of recent data Seltzer and Zavitsas show that the magnitude of a-deterium secondary isotope effects for SN2 reactions are sensibly related to nucleophilic constants for both attacking and leaving groups.54 Acidity Functions and Molecular Basicity.-No major developments were reported in 1967 for either the theory of acidity functions or for their application to mechanistic problems in organic chemistry.For the records however new H values were determined for H2Se0,,55 HClO in mixed ~olvents,’~ and for EtOH-H2S0 mixtures using secondary amine indi~ators.’~ Other work has analysed activity coefficient behaviour of amide indicators (HAfunction) in H2S04,58 and this function also describes the protoQ.tion of heterocyclic N-oxides. Studies of diprotonation of phenylhydrazines aminopyridines and related compounds clearly show that as for motoprotonation of basic species molecular structure plays an important part in determining acidity function behaviour in acidic solvents.60 It is interesting to find that sulphon- amides are predominantly protonated on the nitrogen atom in contrast to 0-protonation for amides themselves indicating that considerable resonance occurs in the neutral species between sulphur &orbitals and the nitrogen lone pair electrons.61 The first direct measurement (using Raman spectro- photometry) of the basicity of methanol and propan-2-01 shows these com- pounds are 50% protonated in HCl at H values of -4.86 and -4.72 res-pectively.62 These figures should be used with discretion however in view of other equilibrium studies by Wells which suggest that both the above alcohols6 and other carbonyl compounds64 may exhibit considerably lower ‘pK’ values in very dilute acid.A substantial difference in the standard states for each set of measurements would account for the discrepancy. The first extensive measurement of the H-function in aqueous hydroxide solutions has been reported using substituted indole indicators ;65 these results lead to pK values for several biologically important pyrrole and indole derivatives.66 Other measurements of H-include those for aqueous dimethyl s~lphoxide~~ for methanol.68 Mechanistic implications of and 54 S.Seltzer and A. A. Zavitsas Canad. J. Chem. 1967,45,2023. ” S. Wasif J. Chem. SOC.(A),1967 142. 56 K. A. Boni and H. A. Strobel J. Phys. Chem. 1966,70,3771. ’’ D. Dolman and R. Stewart Canad. J. Chem. 1967,45,903. L. M. Sweeting and K. Yates Canad. J. Chem. 1966,44,2395. 59 C. D. Johnson A. R. Katritzky and N. Shakir J. Chem. SOC. (4,1967 1235. 6o P. J. Brignell C. D. Johnson A. R. Katritzky N. Shakir H. D. Turhan and G. Walker J.Chem. SOC. (4,1967 1233. 61 R. G. Laughlin J. Amer. Chem. SOC. 1967 SS 4268; F. W. Menger and L. Mandell ibid. p. 4424. 62 R. E. Weston S. Ehrenson and K. Heinzinger J. Amer. Chem. SOC. 1967,89,481. 63 C. F. Wells Trans. Faraday SOC. 1965,61,2194. 64 C. F. Wells Trans. Faraday SOC.,1967,63 147; cf. T. McTigue and J. M. Sime Austral. J. Chem. 1967 20 905. 65 G. Yagil J. Phys. Cheni. 1967,71 1034 1045. 66 G. Yagil Tetrahedron 1967,23 2855. 67 D. Dolman and R. Stewart Canad.J. Chem. 1967,45,911. 68 F. Millot F. Terrier and R.Schaal Compt. rend. 1966 263 C 1529. 108 B. C.Challis acidity function correlations for aromatic nucleophilic substitution have been discussed further and the results are analysed in terms of solvation-model explanations of the type developed earlier for acidic solutions.69 Also there seems to be considerable disagreement between measurements of hydrocarbon acidities in cyclohexylamine70 and dimethyl sulphoxide solvents.25a However relative acidities are similar in the two solvents and most of the discrepancies can be resolved by a suitable readjustment of the pK values for the reference corn pound^.^^^^^*^^ There is a rapidly growing interest in the basicity of molecules in electronic- ally excited states.Experimental studies of several aromatic hydrocarbons show both singlet and triplet states can be up to lo3’ times more basic than the ground in accord with theoretical calculations of lower n-electron localisation energies.73 In one instance the increase in basicity is reflected by a greatly enhanced rate of aromatic hydrogen exchange.74 For heteroaromatic species however the increases in basicity are much smaller7’ and this too accords with theoretical prediction^.^^" Substituent effects in the excited state have also been studied for both aromatic sulphur compounds76 and rn-nitro- aniline~.~’ The results are correlated by Hammett (PO)type equations and discussed in terms of enhanced electronic interactions. Substituent Effects.-Although the combination of resonance and polar (ix. non-resonance) interactions has offered a reasonably satisfactory ex-planation of substituent effects it has been evident for some time that this model requires revision in regard to the character and origin of the polar component.The nature of this revision has become apparent from recent work and is discussed below in connection with related interactions and the interpretation of substituent effects on n.m.r. data. Polar eflects. These have usually been described by the inductive model in which electronic interactions are propagated by successive polarisation of bonds linking the substituent to the reactive site. This polarisation may involve only the o-bond network (o-inductive effects) or in more refined treatments for unsaturated molecules the n-bond network as well (n-inductive effect^).^ * Various critics have assailed the inductive model mainly because of its inability to cope realistically with long range polar interactions i.e. those 69 F. Terrier and R.Schaal Compt. rend. 1967 264 C 465; C. H. Rochester J. Chem. Soc.(@ 1967,1076. 70 A. Streitwieser J. H. Hammons E. Ciuffarin and J. I. Brauman J. Amer. Chem. Soc. 1967 89 59; A. Streitwieser E. Ciuffarin and J. H. Hammons ibid..,p. 63. K. Bowden and A. F. Cockerill Chem. Comm. 1967,989. 72 M. G. Kuz’min B. M. Uzhinov and I. V. Berezin 2hur.jiz. Khim. 1967,41,446; R. L. Flurry and R. K. Wilson J. Phys. Chem. 1967,71 589. 73 R. L. Flurry J. Amer. Chem. Soc. 1966,88 5393. 74 M. G. Kuz’min B. M. Uzhinov G. Sentd’erdi and I. V. Berezin 2hur.jiz. Khim. 1967,41 769. 75 (a)N. Tyutyulkov F. Frater and D. Petkov Theor. Chim. Acta 1967 8 236; (b)R. Cetina D. V. S. Jain F. Peradejordi 0.Chalvet and R. Daudel Compt. rend. 1967 264 C 874. 76 E. L. Wehry J. Amer.Chem. Soc. 1967,89,41. 77 J. P. Idoux and C. K. Hancock J. Org. Chem. 1967,32 1935. ’* (a)M. J. S. Dewar and P. J. Grisdale J. Amer. Chem. Soc. 1962,84 3539 3548; (b)J. N. Murrell S. F. A. Kettle and J. M. Tedder ‘Valence Theory,’ J. Wiley London 1965 ch. 6. Reaction Mechanisms 109 at more than two C-C bond lengths. Instead it has been suggested that long range polar interactions at least are better represented by the field effect in which the electronic perturbation arises from direct electrostatic interaction across space between the substituent bond dipole and the reactive 79 Substantial evidence for the importance of field effects was summarised last year." Even more persuasive evidence comes from two recent investiga- tions by Stock and his co-workers'' on the acidity of several carboxylic acids [(16)+20)] for which the alternative models for polar interactions lead to entirely different predictions.The experimental data for one series of acids (16) are listed below where it can be seen that electron-withdrawing sub- stituents (-C1,-CO,CH etc.) decrease the acidity. The implicit destablisation &$$ \ \ pJyJ \ \ X (19) of the anion the reverse of the 'normal' substituent effect is consistent with the dipolar field but not the inductive model. The same conclusion may be Substituent (X) pK (stat. corrected) H 6-04 c1 6.25 COZCH 6.20 CO,H 5.97 c0,-6.89 drawn from the unusually small destablising influence of the CO -substituent pK(C0 -)/pK(H) = -0-85)."" Stock and his co-workers81b have also examined other acids [(17)+20)] in which the number of potential paths for o-inductive and n-inductive interactions is different.The effect of any sub- stituent is much the same in all these acids suggesting again that polar inter- 79 K. Bowden Canad. J. Chem. 1963,41,2781. Ref. 9 p. 283. *' (a)R. Golden and L. M. Stock J. Amer. Chem. SOC.,1966,88,5928;(b)F. W. Baker R. G. Parish and L. M. Stock ibid. 1967,89 5677. 110 B. C. Challis actions arise from a field effect rather than from any kind or combination of inductive effects. One other important point emerges from these" and related studies:82 the results can only be satisfactorily rationalised if the field asso- ciated with the polarised substituent depends on the direction of this bond which implies that the field is dipolar in character along the lines first suggested by Westheimer and Kirk~ood~~ rather than the point charge envisioned by Dewar and Grisdale more recently.78a This in turn accounts for the failure of the Dewar and Grisdale treatment with 8-substituted f3-naphthoic acids noted earlier by Wells and Adco~k.~~ The relative importance of n-inductive effects remains uncertain.These have been examined further by n.m.r. studies of 4-substituted 3,5-dimethyl- benzenes (21) in which polar effects should predominate because of steric hindrance to resonance. It is claimed the remaining interactions are best X CH3-C-CH, II MeoMe H F+ (3vF+ 3 explained by the dipolar field model with n-inductive effects contributing less than 2.5 %.85 However allowance for n-inductive effects in localised-orbital theory clearly permits a better account of substituent effects on both molecular properties and reactivity in benzene derivatives,86 and a similar conclusion is drawn from spectral studies of iodo- and iodoxy-benzenes.87 Several other experimental studies bear on the discussion above. One is concerned with electron-withdrawal by various cyano-carbon groups which is best acribed to a combination of dipolar field and resonance effects.88 Various suggestions have been advanced to account for the powerful deactiva- tion by the CF sub~tituent,~~ but recent arguments again favour a strong dipolar field effect.g0 These are considerably strengthened by the finding that fluorine hyperconjugation [an alternative explanation ;cf.equation (5)] is not important in carbanion formation a to the CF sub~tituent.~~ However the rapid attenuation of alkyl group polar effects with distance compared to other substituents has been associated with hyperconjugative delocalisation of W. Adcock and M. J. S. Dewar J. Amer. Chem. SOC.,1967,89,379. 83 J. G. Kirkwood and F. H. Westheimer J. Chem. Phys. 1938 6 506; F. H. Westheimer and J. G. Kirkwood. ihid p. 513. 84 P. R. Wells and W. Adcock Austral.. J. Chem. 1965 18 1365. 85 M. J. S. Dewar and Y. Takeuchi J. Amer. Chem. SOC. 1967,89,390. 86 M. Godfrey J Chem. SOC.(4,1967,799. 87 Z. Gakovic and K. J. Morgan J. Chem. SOC.(8, 1967,416. W. A.Sheppard and R. M. Henderson J. Amer. Chem. SOC. 1967,89,4446. 89 W. A. Sheppard J. Amer. Chem. SOC.,1965,87,2410. 'O M. J. S. Dewar and A. D. Marchand J. Amer. Chem. SOC.,1966,88,354. 91 A. Streitwieser A. D. Marchand and A. H. Pudjaatmaka J. Amer. Chem. SOC.,1967,89,694; A. Streitwieser and D. Holtz ibid. p. 693. Reaction Mechanisms (5) charge on the carbon chain rather than o-inductive or dipolar field ex- planations.92 p-n Interactions. Back donation of lone pair electrons or p-z interaction [equation (6)] has often been suggested to account for anomalously low electron withdrawal by polar first row substituents such as -F -OR and -NR2. This donation is usually associated with unsaturated residues and has been cited as such in recent investigations of carbanion stability,93 e.s.r.studies of o-trifluoromethylnitrobenzene radical^,'^ and also to explain the unexpected stability of fluorocarbonium ions [(22t(23)].95 However a recent theoretical reappraisal of polar effects (SCMO-CNDO method) leads to the unusual prediction that o-inductive effects of -F -OR and -NR2 sub-stituents result in charge alternation (24) rather than the usual steady decay (25) of charge along the carbon chain. The alternation is associated with back 6-6+ 66-66+ 6-6+ 66+ 666+ FtCtCtC FtCtCcC (24) (25) donation of lone-pair electrons which suggests this effect may be a feature of saturated as well as unsaturated systems. 96 N.m.r. measurements. The interpretation of n.m.r. chemical shift data has an important bearing on discussions of polar interactions and Dewar has continued his detailed analysis of the problem with studies of substituted 1-and 2-fl~oronaphthalenes.~~ The 19F chemical shifts for these compounds can only be rationalised satisfactorily in terms of resonance and dipolar field interactions if the major contribution to the latter results from lengthwise polarisation of the C-F bond.This would explain why substituent effects of I9F chemical shifts are qualitatively different from those on physical or 92 P. E. Petersen R.J. Bopp. D. H. Chevli E. L. Curran. D. E. Dillard and R.J. Kamat. J. Arwr.. Charti. So<,..1067.89. 5902. 93 J. Hine L. G. Mahone and C. L. Liotta J. Amer. Chem. SOC. 1967,89 5911. 94 L. G. Janzen and J. L. Gerlock J.Amer. Chem. SOC.,1967,89,4902. Chem G. A. Olah R. D. Chambers and M. B. Comisarow,J. Amer. Chem. Soc. 1967,89,1268. 96 3. A. Pople and M. Gordon J. Amer. Chem. SOC.,1967,89,4253. 112 B C. Challis chemical properties of other side chains. Thus Dewar and AdcockS2 conclude that both sets of phenomena cannot be treated by a common scheme involving o-inductive and resonance effects as suggested eaiiier by Taft97 for substituted fluorobenzenes. Two recent discussions of solvent effects in "F n.m.r. cor- relations also bear on this problem.98 Nevertheless there is further evidence of reasonable correlations between 19Fchemical shift data and Hammett oPparameters from studies of carbonyl group electron densities" and heats of formation for Lewis acid adducts'" of benzophenones and also for hydrogen bonding between p-fluorophenols and various acceptors;"' in two cases the results are corroborated by data from other independent measurements.'00.'02 To summarise the situation it seems unlikely that the exact nature of component electronic interactions influencing chemical reactivity will be discernable from 9F n.m.r. measure- ments. This data may however provide reliable estimates of Hammett a,-parameters for benzene derivatives. It is also evident that similar comment applies to 1H85,103 and 13C104 chemical shift data. Here again reasonable correspondence between para-H chemical shifts and 0,-parameters can be obtained,'05 but the correlations break down for substituents at other sites.Dewar and Take~chi~~ conclude that because 'H chemical shifts are more sensitive to long-range magnetic interactions the general treatment of this data in terms of substituent inter- actions may be even less successful than for "F chemical shifts. This does not mean that empirical correlations will have no value as is evident from? statistical treatment of orientation in substituted pheno1s.'O6 Also attempts to account for some of these perturbations have been made for heteroaromatic systems and 'H chemical shifts then c rrelate reasonably well with calculated electron densities.lo7 It is of interest to note too that chemical shifts of side- chain protons may also prove a morg usefuk method of estimating chemical reactivity ; several satisfactory correlations have been noted for phenols,"' 97 R.W. Taft J. Phys. Chem. 1960,64 1805. 98 J. W. Emsley and L. Phillips Mol. Phys. 1966 11 437; H. M. Hutton B. Richardson and T. Schaeffer Canad. J. Chem. 1967,45 1795. 99 R. G. Pewes Y. Tsuno and R. W. Taft J. Amer. Chem. SOC. 1967,89,2391. loo G. S. Giam and R. W. Taft J. Amer. Chem. SOC.,1967,89 2397. lo' D. Gurka R. W. Taft L. Joris and P. von R. Schleyer J. Amer. Chem. SOC.,1967,89 5947. lo* Cf. E. M. Arnett T. S. S. R. Murty P. von R. Schleyer and L. Joris J. Amer. Chem. SOC.,1967 89 5955. lo3 J. C. Schug J. Chem. Phys. 1967 46 2447; J. M. Read and J. H. Goldstein J. Mol. Spectro-scopy 1967,23 179. K. S. Dhami and J. B. Stothers Canad. J. Chem. 1966,4,2855; T. D. Alger P. M. Grant and E. G. Paul J.Amer. Chem. SOC.,1967,89 5397. lo' H. P. Figeys and R. Flammang Mol. Phys. 1967 12 581; D. T. Clark and J. W. Emsley ibid. p. 365. lo6 J. A. Ballantine and C. T. Pillinger Tetrahedron 1967,23 1691. lo' P. J. Black R. D. Brown and M. L. Heffernan Austral. J. Chem. 1967 20 1305 1325; R. J. Chuck and E. W. Randall J. Chem. SOC. (B) 1967 262; E. J. Vincent R. Phan-Tan-Luu and J. Metzger Bull. SOC. chim. France 1966 3537; W. Adam and A. Grimison Theor. Chim. Acta 1967 7 342. lo* J. G. Traynham and G. A. Knesel J. Org. Chem. 1966,31,3350. Reaction Mechanisms carbinols,'Og benzoic acids,'" and styrenes.' ' In these cases either magnetic and electric field effects cancel or they are rapidly attenuated with distance from the nucleus. Linear Free Energy Relationships.-The utility of various substituent-dependent energy parameters to explore the mechanism of organic reactions continues to attract considerable attention.Much of this is concerned with the development of the Hammett and Ingold-Taft relationships but new approaches have been reported. One of the most interesting suggestions comes from Thornton,"' who has outlined a simple theory for predicting substituent effects on transition-state geometry by considering these as linear perturbations of the vibrational potentials for normal co-ordinate motion both parallel with and perpendicular to the motion along the reaction co-ordinate. The one difficulty of course is deciding which mode is the reaction co-ordinate. Nevertheless simple rules for estimating both effects are given and the treatment is discussed in relation to substitution (S,j and elimination (E2j reactions.The influence of solvents on substituent effects has been considered in detail by Tommila''3 using an electrostatic model and the results lead to the con- clusion that electronic interactions are at least partly transmitted through the solvent. A more serious but not unexpected discrepancy is noted by Ceska and Grunwald,' l4 who find appreciable qualitative differences between substituent parameters determined from amine equilibria in water and organic solvents (such as alcohols and acetic acid). This leads to the suggestion that linear free- energy correlations of reactivity for non aqueous solvents should be based on parameters derived from ion-pair formation in acetic acid rather than from ionisation equilibria in water.Other studies also comment on solvent dependent Hammett o-parameters which can be attributed generally to solvation effects on the ionisation equilibria.' ' In view of these findings it is interesting to note that relative reactivities of solid benzoic acids are roughly proportional to their acidities.'I6 Also several further correlations of mass-spectral ion-intensities with Hammett o-para- meters have been reported and it looks as if this technique will be valuable for mechanistic studies of some ion-decompositions in the gas phase. The method first outlined last year,'" is based on the assumption that mass-spectral pro- log R. J. Ouellette D.L. Marks and D. Miller J. Amer. Chem. SOC.,1967,89,913. Y. Kondo K. Kondo T. Takemoto and T. Ikenoue Chem. and Pharm. Bull. (Japan) 1966 14 1332. T. A. Wittstruck and E. N. Trachtenberg J. Amer. Chem. Soc. 1967 89 3803; R. Gurudata J. B. Stothess and J. D. Talman Canad. J. Chem. 1967,45,731. E. R. Thornton J. Amer. Chem. Soc. 1967,89,2915. E. Tommila Ann. Acad. Sci. Fennicae 1967 A No. 139 1. 'I4 G. W. Ceska and E. Grunwald J. Amer. Chem. Soc. 1967,89,1377. C. L. de Ligny Rec. Trau. chim. 1966 85 1114; P. D. Bolton F. M. Hall and I. H. Reece Spectrochim. Acta 1966 22 1825; C. H. Rochester and B. Rossall J. Chem. Soc. (B) 1967 743; A. Fischer G. J. Leary R. D. Topsom and J. Vaughan ihid. p. 686; p. 846. E. A. Myers E. J. Warwas and C. K. Hancock J.Amer. Chem. Soc. 1967,89 3565. 'I7 M. M. Bursey and F. W. McLafferty J. Amer. Chem. Soc. 1966,88,529,4484. 114 B. C.Challis cesses can be regarded as a set of competing consecutive unimolecular decompositions. Thus for the decomposition of a molecular ion M yielding a product ion A the effect of a substituent on the relative abundance ratio 2 (= [A]/[M])is given by equation (7) where X and H refer to substituted and parent compound respectively. Because of the absence of external solvation log ZJZ = po (7) both charge development in the transition state and therefore p values will be generally lower than for equivalent solution processes. Following up on last year's reports,' l7 full details have been published for the decomposition of benzoyl compounds."' It is evident that equation (7) is applicable to the elucidation of reactions leading to the product ion (A)through either a single reaction consecutive reactions or competing reactions when the product ion does not contain the substituent.If the product ion does contain the sub- stituent sensible prr correlations are only possible under low-energy conditions when further decomposition (also substituent dependent) is inappreciable. '' The technique has been used to investigate the mechanisms of the McLafferty rearrangement' in which the nature of the hydrogen migration is speculative. Studies of substituent effects in butyrophenones [equation (S)] show clearly that both electron-withdrawal (p = 2.0) and resonance stabilisation of charge by para-substituents increase the rate suggesting the incursion of two inter- mediates [(26)and (27)] in both of which development of the radical site is the / \ 'X f"0 / I 'I8 M.M. Bursey md F. W. McLafferty J. Amer. Chem. SOC.,1967 89 1; cf. F. W. McLafferty and M. M. Bursey Chem. Comm. 1967,533. (a) F. W. McLafferty and T. Wachs J. Amer. Chem. SOC. 1967,89 5043; (b)ibid. p. 5044. Reaction M echanisrns driving force for rearrangement.' This is partially consistent at least with other findings that the usual rearrangement process is completely inhibited by substituents (such as p-H,NC,H,[CH,],-) on which the positive charge and therefore the unpaired electron may localise. Linear correlations with equation (7) have also been reported for the mass-spectral decomposition of azobenzenes for which C-N bond cleavage is facilitated by electron withdrawal (p = 1-05),'20and for the decarboxylation of arylmethyl carbon- ates.' 21 Despite these results the general applicability of ground state sub- stituent parameters to excited state reactions is questionable particularly in view of Wehry's7 recent investigation which clearly shows that enhanced electronic interactions may feature in the excited state.Other recent work has concerned further extension of the Hammett equation via linear combinations of substituent parameters (on the lines of the Yukawa- Tsuno'22 treatment) to electophilic substitution and rearrangement^,'^^ base-catalysed hydrolysis,' 24 and free-radical reactions.' 25 The latter are shown to correlate well with equation (9) where the second term accounts for resonance interactions; E values are obtained directly from substituent effects on hydrogen abstraction from cumenes.Kinetic studies show the reactivity of various carbonyl compounds towards the hydrated electron correlates with Taft G* parameters and for aldehydes ketones and carboxylic acids p = -0-74. Amides and esters however deviate from this plot on account of resonance delocalisation of the electrophilic centre to either the nitrogen atom or the alkoxy oxygen atom. It is therefore suggested that electrophilic reactivity toward the hydrated electron may be a useful probe for exploring the electronic configuration of molecules.'26 Electrophilic Aromatic Substitution.-Of the many recent developments in these reactions only three topics have been selected for detailed discussion.These are certain aspects of the mechanism reflecting on the structure of possible intermediates substitution of positively charged substrates and re- arrangement reactions in electrophilic hydroxylation. Other significant findings are mentioned briefly below. As predicted in a recent review of reactions in molten salts,'27 the addition of pyrosulphate ions (S2072-) to alkali-metal nitrates increases the con-centration of NOz to synthetically significant concentrations [equation (lo)], + NO3-+ S2OT2-+2 SO,2-+ NOz+ (10) J. H. Bowie G. E. Lewis and R. G. Cooks J. Chem. SOC.(B),1967,621. P. Brown and C. Djerassi,J.Amer. Chem. Soc. 1967,89 2711. "' Y. Yukawa and Y. Tsuno Bull. Chem. SOC.Japan 1959,32,971. lZ3 Y. Yukawa Y. Tsuno and M.Sawada Bull. Chem. SOC.,Japan 1966,39 2274. A. A. Humffray and J. J. Ryan J. Chem. SOC.(B) 1967,468. T. Yamamoto and T. Otsu Chem. and Id. 1967,787. lZ6 E. J. Hart E. M. Fieiden and M. Anbar J. Phys. Chem. 1967,71,3993. ''' W.Sundermeyer Angew. Chem. Internat. Edn. 1965.4222. 116 B. C. Challis and smooth nitration occurs with many aromatic compounds including nitrobenzene. The aromatic substrates were passed into a mixture of doped nitrate salts at 300" via a stream of nitrogen gas.128 This process bears some resemblance to the reaction of arylchloroformates with silver nitrate which produces high yields of o-nitrophenols via rearrangement of the nitrato- carbamate formed initially [equation (1l)] ; with 2,6-disubstituted chloro-formates 4-nitro-products are obtained suggesting an intermolecular mech- anism.' 29 Unexpectedly large amounts of ortho-substitution are reported for the mixed-acid nitration of both neopentylbenzene (45 % o-i~omer)'~' and 0 0 I1 I1 R R R R 2,7-di-t-butylnaphthalene (56% l-isomer).'31 Since earlier studies132 have shown that bulky alkyl substituents usually inhibit o-nitration some special interaction between the reagent and side-chain is clearly indicated. Another recent paper records the first straightforward nitration of pentafluorobenzene by employing an HN03-BF3 mixture in tetramethylene sulphone ; previous attempts to nitrate highly fluorinated aromatic compounds have usually produced oxidation products such as quinones.' 33 Although many chlorination reactions of sulphuryl chloride are catalysed by free-radical initiators 134 there is now definitive evidence that this reagent reacts with reactive aromatic substrates (e.g.anisole and naphthalene) via a heterolytic path with electrophilic attack by molecular SO,CI,. 35 Also kinetic studies show the reaction of cresol with chloramine-T involves a dichloramine-T reagent rather than HOCI as has been presumed for many years; the reaction rate is independent of the cresol concentration because formation of the dimeric reagent is the slow step.'36 Other investigations have concerned hydrogen exchange in methoxy-benzenes12*36 and substituted NN-dimethylanilines.14 38 The isotope effects associated with these reactions are discussed in detail on p. 101. Further lZ8 R. B. Temple C. Fay and J. Williamson Chem. Comm. 1967,966. lz9 M. J Zubik and R. D. Schultz J. Org. Chem. 1967,32 300. D. F. Gurka and W. M. Schubert J. Org. Chem. 1966,31,3416. L. Erichomovitch M. MCnard F. L. Chubb Y. PCpin. and J-C. Richer Canad. J. Chem. 1966,442305. H. C. Brown and W. H. Bonner J. Amer. Chem. Soc.. 1954 76,605. P. L. Coe A. E. Jukes and J. C. Tatlow J. Chem. SOC., (C),1966,2323. 134 Cf. R. Stroh in Houben Weyl's 'Methoden der Organische Chemie' vol. V/3 p. 873. R. Boulton and P. B. D. de la Mare J. Chem. SOC. (B) 1967 1044; P. B. D. de la Mare and H. Suzuki J. Chem. SOC.(C) 1967,1586. 136 T.Higuchi and A. Hussain J. Chem. SOC.(B) 1967 549. Reaction Mechanisms studies of heterogeneous Group VIII metal-catalysed hydrogen exchange 13' including those for heterocyclic substrates,'37b are consistent with the x-dissociative mechanism discussed in last year's report. 38 An interesting development of this process is that similar hydrogen exchange also proceeds under homogeneous conditions with soluble Pt" and Pd" salts.'39 This is of particular value for substituted compounds (e.g. nitrobenzene) that usually 'poison' the conventional heterogeneous metal catalysts. Electrophilic substitution reactions in polyalkylbenzenes are discussed in an excellent review,14' from which it is evident that side-chain substitution in these compounds often results from a rate-determining substitution in the aromatic nucleus followed by rearrangement.It is noteworthy that several further examples of this kind of rearrangement have been recognised both for halogenation of l-methylnaphthalene'41 and p~lymethylbenzenes~~~ and for isotopic hydrogen exchange in anilinium ions. 143 Mechanism. Although there is little doubt that the majority of electrophilic aromatic-substitution reactions involve a two-step process [equation (12)] for which the o-bonded intermediate is stable relative to the transition-state various aspects of this mechanism provoke further inquiry. In only a few cases are the intermediates sufficiently stable to be isolated as with hindered phenols where dienone structures (28) are formed.144 The substituent effects on dienone formation have now been examined and they are remarkably similar to those for electrophilic substitution in phenol.This is good evidence that both reactions have similar o-bonded transition RH OR 137 (a) A. W. Weitkamp J. Catalysis 1966 6 431 ; B. D. Fisher and J. L. Garnett Austral. J. Chem. 1966 19 2299; C. G. McDonald and J. S. Shannon ibid.. 1967 20 297; (b) G. E. Calf and J. L. Garnett Chem. Comm. 1967,306. ''* Cf. Ref. 9 p. 291. 139 (a) J. L. Garnett and R. J. Hodges J. Amer. Chem. SOC. 1967 89 4546; J. L. Garnett and R. J. Hodges Chem. Comm. 1967 1220; (b) ibid. p. 1001. 140 E. Baciocchi and G. Illuminati Progr. Phys. Org. Chem. 1967,5 1. 14' G. Gum P. B. D. de la Mare and M. D. Johnson J.Chem. SOC.(C),1967,1590. 142 V. G. Shubin V. P. Chzhu A. I. Rezvukhin and V. A. Koptyug Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1966 2056. 143 J. R. Blackborrow and J. H. Ridd Chem. Comm. 1967 132. 144 Cf. B. C. Challis Ann. Reports 1965,62,258. 118 B. C.Challis states.14' The intermediate (29) in the reaction of NN-dimethylaniline with tetracyanoethylene is stable enough for n.m.r. and spectral confirmation of its o-bonded str~cture.'~~ Further viability for the concept of o-bonded intermediates comes from several recent reports of phenonium ion formation in solvolysis rea~ti0ns.l~~ Of the many factors that may determine whether k or k is rate-controlling steric compression in the transition state appears to be of prime importance.Thus further evidence of substantial primary isotope effects for the bromination of polyalkylbenzenes is not surprising. 14* Decomposition of the o-bonded intermediate (k2)appears to be slow for the NOz+BF4- nitration of 9-r2H2]-anthracene (kH/kD = 2.6) and it is claimed that spectral measurements show formation of the o-complex is instantaneo~s.'~~ It has also been suggested from studies of acid-catalysed aromatic mercuration that the selectivity rule may not apply to reactions where k2 is the slow step.'" The question of rate-determining n-complex formation in some of these reactions (as a precursor to the o-bonded intermediate) continues to be debated. Much of the controversy centres on the unusually low substrate selectivities obtained from competitive experiments by Olah and his colleagues for aromatic nitration by nitronium salts.' '' Several subsequent experiments have indicated that these low values may arise from partial diffusion control of the reaction rates rather than slow .n-complex formation.' 52 Further kinetic studies of nitration in sulphuric acid support these indications by showing that compounds more reactive than benzene combine with NO2+ on en- counter.lS3 This finding may also explain the apparent breakdown of the additivity principle for the nitration of alkylbenzenes in acetic acid.154 The validity of other results obtained by competitive experiments is also under suspicion.Thus large discrepancies have been reported between the relative reactivities of benzene and toluene for Friedel-Crafts halogenation obtained by competition k(phCH3)/k(phH) 'V 3~6)'~~ and by independent kinetic measure- ments [k(ph,,,)/k(phH) = 381 ' under otherwise similar experimental con-ditions.Although n-complexes are formed between aromatic substrates and suitable electrophilic reagents as is evident from. for example other studies 14' E. Baciocchi and G. I!luminati J. Amer. Chem. SOC. 1967,89. 4017. 146 P. G. Farrell. J. Newton and R. F. M. White J. Chem. SOC.(B),1967,637. 14' G. A. Olah M. B. Comisarow E. Namanworth and B. Ramsay J. Amer. Chem. SOC. 1967 89,711,5259. 14' E. Baciocchi G. Illuminati G. Sleiter and F. Stegel J. Amer. Chem. SOC.,1967 89 125; A. Nilsson Acta Chem. Scand. 1967,21,2423. 149 H. Cerfontain and A. Telder Rec.Trav. chim. 1967,86,371. A. J. Kresge and H. C. Brown J. Org. Chem. 1967,32 745. G. A. Olah S. J. Kuhn and S. H. Flood J. Amer. Chem. SOC.,1961,83,4571; G. A. Olah S. J. Kuhn and S. H. Flood ibid. 1962,84 3687. B. C. Challis Ann. Reports 1966,63 296; ibid. 1965,62 257. R. G. Coombes R. B. Moodie and K. Schofield Chem. Comm. 1967,352. 154 A. Fischer J. Vaughan and G. J. Wright J. Chem. SOC.(B) 1967,368. G. A. Olah S. J. Kuhn S. H. Flood and B. A. Hardie J. Amer. Chem. SOC.,1964 86 1036; G. A. Olah S. J. Kuhn and B. A. Hardie ibid. p. 1055. 156 S. Y. Caille and R. J. P. Corriy Chem. Comm. 1967 1251. Reaction Mechanisms 119 of nitrosation,' 57 the case for their rate-controlling formation in nitration and Fnedel-Crafts halogenation is seriously weakened by the above results.On the theoretical side it has been shown that bromination rates for sub- stituted benzenes' 58 and polynuclear hydrocarbon^'^' correlate with various theoretical estimates of reactivity (e.g.localisation energies) and these results complete the correlations for the commoner electrophilic substitution re- actions. Their significance is of course the implication that o-complex formation is rate-controlling. Also it has been suggested that hyperfine coupling cpnstants (from e.s.r. studies) may prove an accessible measure of chemical reactivity and data for polynuclear hydrocarbons bears out the claim.'6o Positively charged substrates. The incidence of electrophilic substitution in the conjugate acids of aromatic amines was first recognised unambiguously about five years ago for the nitration of anilinium salts.'61 Subsequent evidence has shown this unexpected reaction to be fairly common for aromatic species in concentrated acids and for basic hetero-aromatic compounds under less stringent conditions.Details of the nitration of sulphonium and selenium have now been pub- lished,'62 and the product orientations are listed in Table 3 together with earlier results for the mononitration of other 'onium salts. The implications of TABLE 3 Nitration of 'oniumsalts Product Orientation ( %) Ref ortho meta para ArNMe,' 163 -89 11 ArPMe,' 163 -97 3 ArAsMe,' 163 -96 -4 ArOAr ' 164 -100 ArSMe,' 162 3.6 90.4 6.0 ArSeMe,' 162 2.6 91.3 6.1 these data in terms of substituent interactions are incompletely understood.Arguments have been advanced that electrostatic factors are of overiding importance for the anilinium salts,165 and this conclusion is supported by further theoretical calculations.'66 Presumably the same will be true for other groups but the situation must be modified by the possibility of p-and d-lS7 Z.J. Allan J. Podstata D. Snobl and J. Jarkovsky Coll. Czech. Chem. Comm. 1967,32 1449. lS8 J-E. Dubois and J-P.Doucet Tetrahedron Letters 1967,3413. 159 L. Attschuler and E. Berliner J. Amer. Chem. Soc.. 1967.89.5837. 160 C. P. Poole and 0.F. Griflith J. Phys. Chem. 1967,71 3672. 16' M. Brickman S. Johnson and J. H. Ridd Proc. Chem. SOC.,1962,228. 162 H. M. Gilow and G. L. Walker J. Org. Chem. 1967,32,2580 163 J.H. Ridd and J. H. P. Utley J. Chem. SOC.,1964,24 164 N. N.Nesmayanov T. P.Tolstaya L.S.Isaeva and A. V. Grid Doklady Akad. Nauk S.S.S.R. 1960,133,602. 165 J. H. Ridd Chem. SOC.Spec. Publ. No. 21 1967 p. 149. 166 R. Reynaud Compt. rend. 1967,264 C 1723; D. R. Williams Mol. Phys. 1967,12 33,41. 120 B. C.Challis orbital interactions. For instance electron-withdrawal via d,-p interaction (-M) should lead to enhanced rneta-substitution for all groups other than NMe,' whereas pn-p electron donation (+M) only possible for OMe,+ SMe,' and SeMe2+ would explain the increased tendency for ortho-and para-su bs t it ution. Further evidence of electrophilic substitution in anilinium ions come from isotopic hydrogen exchange in 95% D,S04.In the presence of strongly activating groups (e.g. -0Me) the positive pole appears to have no marked directing effect and exchange rates are remarkably similar for ortho- rneta and para-positions.143 Related experiments show that N-proton exchange with the solvent is much slower than nuclear nitration at the same acidity. This removes any remaining doubt that the nitration may proceed through the microscopic amounts of unprotonated amine at these high a~idities.'~~ Several recent papers have reported on electrophilic substitution in hetero- aromatic cations and salts. These results help to explain unusual products from several reactions arising either from perturbation of the electron distribution or from the blocking of reactive sites in the free organic base.For instance substi- tution of the indole nucleus in concentrated acids occurs predominantly on the aromatic ring rather than the 3-position which is extensively protonated under these conditions.'68 Two recent reviews deal with substitution in six- membered hetero-aromatic cations from the synthetic' 69a and mechanistic standpoint^,'^^' but these are already outdated by further developments. There is clear-cut kinetic evidence that activated pyridinium cations undergo both nitration and hydrogen exchange in concentrated acids. Thus 2,4,6-trimethyl- 2,4-dimethyl- 2,6-dimethyl- and 3,5-dimethoxypyridines all mononitrate as their respective cations. '70 Deactivated isomers however such as nitro- and 2,6-dichloro-pyridine react only as their free bases.70 Also nuclear hydrogen exchange in amino-pyridines occurs via the pyridinium cation,17' and this is consistent with conclusions drawn from the N-nitro-sation of amino-pyridines in concentrated perchloric acid.' 72 These results lead Katritzky and his colleagues170b to predict that basic pyridines (pK > 1)will generally react as cations whereas feebly basic pyridines (PK,< -2.5)will only undergoe substitution as a free base; in both instances the orientation will be to the a-or p-sites depending on other substituents. It is of interest to note however that an alternative mechanism [equation (13)] involving an ylid intermediate (30) has been sugested to account for the smooth exchange at the 2-and 6- positions of pyridine in dilute DCl.17 The reactivity of the S.R. Hartshorn and J. H. Ridd Chem. Comm. 1967 133. W. E. Noland K. R. Rush and L. R. Smith J. Org. Chem. 1966,31,65; B. Bak C. Dambmann and F. Nicolaisen Acta Chem. Scand. 1967 21 1674. 16' (a)R. A. Abramovitch and J. G. Saha Adu. HeterocycZic Chem. 1966,5,229;(b)A. R. Katritzky and C. D. Johnson Angew. Chem. Internat. Edn. 1967,6,608. 170 (a) C. D. Johnson A. R. Katritzky and M. Viney J. Chern. SOC. (B) 1967 1211; (6) C. D. Johnson A. R. Katritzky B. J. Ridgewell and M. Viney ibid. p. 1204. 17' G. P. Bean C. D. Johnson A. R. Katritzky B. J. Ridgewell and A. M. White J. Chem. SOC.(B) 1967,1219. 17* E. Kalatzis J. Chem. SOC.(B) 1967 273 277. J. A. Zoltewicze and C. L. Smith J. Amer. Chem. SOC. 1967,89 3358. Reaction Mechanisms D D (30) pyridine nucleus towards nitration has also been discussed and roughly speaking for several substituted derivatives the base and cation are less reactive than the corresponding derivatives of benzene by factors of lo2 lo3 and lo7,re~pectively.'~' Related investigations of nitration 174 and hydrogen exchange'75 on pyridine-N-oxides show that substitution occurs in the 4-position with the neutral molecule and in the 3-position of the conjugate acid.However an important anomaly remains-under similar experimental conditions nitration preferentially proceeds through the free base whereas the usually more selective hydrogen exchange attacks the more abundant but less reactive con- jugate acid at the 3-position. Qualitative studies reveal a similar anomaly for nitration and hydrogen-exchange of quinoline-N-oxide.' 76 No satisfactory explanation of this phenomenon has been given but Katritzky and J~hnson'~' note that any further destabilisation of the double-charged Wheland inter- mediate (31) for 3-substitution (such as electron-withdrawal by the nitro- group) would make this pathway much less favourable than that for 4-substitution (32).+ (yo2 pj2 I II OH 0 (31) (12) Other pertinent investigations have examined acid-catalysed nitration and hydrogen exchange in . pyridones,' 779 '78 quin~lines,'~~ and pyr~nes'~~ thiapyrones. '78 Collectively these results show that electrophilic substitution occurs in both the free base and the conjugate acids of the N-heterocyclics (depending on the experimental conditions) but only in the free base of the S-and 0-heterocyclic analogues.Thus the relative effects of NH' 0' and S+ in these six-membered rings show an interesting parallelism with those of NH 0 and S in five-membered heteroaromatic cornpo~nds.'~~ Also some unexpected results for hydrogen exchange in 4-pyrimidone~'~' and nitration 17* C. D. Johnson A. R. Katritzky N. Shakir and M. Viney J. Chem. SOC.(B) 1967 1213. 17' G. P. Bean P. J. Brignell C. D. Johnson A. R. Katritzky B. J. Ridgewell H. 0.Tarham and A. M. White J. Chem. SOC.(B),1967,1222. 17' Y. Kawazoe and M. Ohnishi Chem. and Pharm. Bull (Japan),1967,15,826. 177 P. Bellingham C. D. Johnson and A. R. Katritzky J. Chem. SOC.(B),1967 1226. P.Bellingham C. D. Johnson and A. R. Katritzky Chem. Comm. 1967 1042. 179 G. E. Wright L. Bauer and C. L. Bell J. Heterocyclic Chem. 1966,3,440. 122 B. C. Challis of 4-phenylpyrimidine180 may reflect the importance of substitution via their conjugate acids. Electrophilic hydroxylation. An unexpected intramolecular rearrangement of deuterium appears to accompany the peracid hydroxylation of [4-2H]-acetanilide [equation (14)l. NHAc NHAc (7.5%) (14) H D (33) (34) Similar deuterium migration is not observed in other electrophilic substitu- tions (e.g. nitration and bromination) of the anilide thus the key step appears to be smooth formation of the dienone (34) from the cr-bonded intermediate (33).181 This rearrangement has an analogy in the formation of dienones via methyl migration in the related hydroxylation of alkylbenzenes.Although it is just conceivable that the deuteriated product could arise by an inter- molecular process other evidence suggests this is unlikely and also indicates that hydrogen migration may be a common feature of reactions involving phenolic cations such as (33). Thus the acid-catalysed dehydration of 3-chloro-6-deuterio-5,6-dihydroxy-i,3-cyclohexadiene gives 4-chlorophenol containing about 20'4 deuterium in the 3-position [equation (1 5)].183 Also rapid 4-deuteri- ation is reported for 5-hydroxyindole and several of its derivatives in D,O. Other hydroxyindoles however undergo substitution in the aromatic ring at a much slower rate.184 Although it is well established that both pr~tonation'~~ and hydrogen exchange186 occur at the 3-position of the indole nucleus the next most reactive site towards electrophiles is the 5-position.Presumably sufficient protonation occurs here with 5-hydroxyindole for deuterium B. M. Lynch and L. Poon Canad. J. Chem. 1967,45 1431. D. M. Jerina J. W. Daly W. Landis B. Witkop and S. Udenfriend J. Amer. Chem. SOC. 1967,89,3347. lE2 H. Hart P. M. Collins and A. J. Waring J. Amer. Chem. SOC.,1966,88,1005; H. Hart and R. M. Lange J. Org. Chem. 1966,31 3776. lS3 D. M. Jerina J. W. Daly and B. Witkop J. Amer. Chem. SOC.,1967,8!2,5488. J. W. Daly and B. Witkop J. Amer. Chem. SOC.,1967,89 1032. "5 R. L. Hinman and E. B. Whipple J. Amer. Chem. SOC.,1962,84 2534. lS6 B.C. Challis and F. A. Long J. Amer. Chern. SOC. 1963,85,2524. Reaction M echanisms c1 c1 migration to proceed. The significance of this finding in relation to enzymatic hydroxylation which may also involve dienone formation from a cationic intermediate has also been discussed.’*’ ‘13’ S. Udenfriend P. Zaltman-Nirenberg J. W. Daly G. Guroff C. Chidsey and B. Witkop Arch. Biochem. Biophys. 1967,120,413. E

ISSN:0069-3030    

DOI:10.1039/OC9676400099

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

3. (Part ii) REACTION MECHANISMS By H. M. R. Hoffmann (Department of Chemistry University College Gower Street London W.C.1) NUMEROUS books’ dealing with aspects of organic reaction mechanisms were published in 1967. Stable Carbonium Ions.-Thanks to n.m.r. spectroscopy and the use of highly acidic media the chemistry of long-lived carbonium ions continues to develop rapidly. Clearly these stable species arouse much less controversy than the more elusive intermediates in solvolyses and a readable review on these ions has appeared.2 One of the simplest carbonium ions and at the same time the most simple aromatic system is the cyclopropenyl cation (l) which has been obtained by two independent route^.^ Carbonium ion salt (la) shows a single peak at T -0.87 in fluorosulphuric acid; its i.r.spectrum is also com- paratively simple as expe~ted.~’ A useful new route to alkylcarbonium ions (3) .H Sb CIS. CHzCIz* Sb C16--20” HYH H CI Ref. 3a (la) .-.-CI SO; + CO + CH,OH I Ref. 3b H CO,CH H (Ib) (a)B. Capon M. J. Perkins and C. W. Rees ‘Organic Reaction Mechanism 1966‘ Interscience London 1967; (b) ‘Aromaticity,’ Chem. SOC. Special Publ. No. 21 London 1967; (c) Adu. Phys. Org. Chem.,1967,5;(d) Progr. Phys. Org. Chem. 1967,4; 1967,5 ;(e) H. J. Shine ‘Aromatic Rearrangements,’ Elsevier Amsterdam 1967 ; (f)S. Ranganathan ‘Fascinating Problems in Organic Reaction Mech- anism,’ Holden-Day San Francisco 1967 ; (9) A. W. Johnson ‘Ylid Chemistry,’ Academic Press New York 1966; (h) R. W. Hoffmann ‘Dehydrobenzene and Cycloalkynes,’ Academic Press New York 1967; (i) M.P. Cava and M. J. Mitchell ‘Cyclobutadiene and Related Compounds,’ Academic Press New York 1967 ;01 ‘l,.l-Cycloaddition Reactions-The Diels-Alder Reaction in Heterocyclic Syntheses,’ ed J. Hamer Academic Press New York 1967; (k) L. L. Muller and J. Hamer ‘1,2- Cycloaddition Reactions The Formation of Three- and Four-Membered Heterocycles,’ Interscience New York 1967; (I) ‘Organic Reactions’ A. C. Cope ed. Vol. 15 New York 1967; (m)‘Organic Photochemistry,’ 0.L. Chapman ed. Marcel-Dekker New York 1967; (n)‘Reactivityof the Photo- excited Organic Molecule,’ Proc. 13th Conference on Chemistry at Brussels Interscience New York 1967. G. A. Olah Chem. Eng. News 1967 March 27th p. 76. (a)R.Breslow J. T. Groves and G. Ryan,J. Amer. Chem. SOC.,1967,89,5048;(b)D. G. Farnum G. Mehta and R. G. Silberman ibid. p. 5048; (c) a number of substituted cyclopropenyl salts have been described by B. Fiihlisch and P. Burgle Annakn 1967,701 67. H.M. R. Hoffinann R,CH ______t FSOlH -SbFS R,C+SbFSFSOT + H2 (21 (3) - CH3 - (CH31,CH I+ CH3CH2CH2CH H,C/'\CH is based on hydride-ion abstraction from alkanes (2) by the extremely strong acid FS03H-SbF,.4 Thus n-butane and isobutane form exclusively the t-butyl cation (4) which may be recovered after boiling (150")in FS03H-SbF,! At room temperature and in the same medium alkanes with seven or more carbon atoms are also converted into this ion (4). Even paraffin wax and poly- ethylene give (4) obviously by complex isomerisation and fragmentation pro- cesses.Salts of (4) can be crystallised from SO or SOzCIF solutions at -80" and are stable at least to room temperature. n-Pentane and isopentane ionise to the t-pentyl cation (5) while neopentane (6)forms ion (5) only at -20" in FS03H-SbF,-S0,CIF; at +25" in FS03H-SbF, the t-butyl cation (4) is formed with liberation of methane. That ion (4) does not arise from (5) follows from the fact that (5) is stable up to 150" under these conditions. For many purposes the combination FS03H-SbF,-S02C1F is more suitable than the system FS0,H-SbF,-SO, since carbonium ions are more stable in it and may be studied over a wider range of temperatures4 (see also discussion of the 2-phenethyl cation below).In HF-SbF solution and at room temperature the reaction of isobutane with protons is reversible (Scheme l).' In contrast. HF-SbFs (CH,),CH + H+ (CH3),C+ + H2 Scheme I G. A. Olah and J. Lukas J. Amer. Chem SOC. 1967,89,4739. A. F.Bickel C. J. Gaasbeek H. Hogeveen J. M. Oelderik and J. C. Platteeuw Chem. Corn. 1967 634; see also H. Hogeveen and A. F. Bickel ibid. p. 635. Part (ii) Reaction Mechanisms the fate of abstracted hydride ion in FS0,H-SbF is not yet completely clear.4 The 1-methylcyclopentyl cation can be obtained by no less than nine different routes (Scheme 2). Noteworthy is the rearrangement of (7) into (8) 0 t FSO,H -SbFs I Scheme 2 which apparently does not involve a primary carbonium ion intermediate whose formation would be energetically unfavourable.6 Simple alkyldicar- bonium ions (9) in which the ionic centres are separated by two or more saturated carbon atoms have been observed in SbF,-S02.However attempts to generate the di-cation from (10) have led to the allylic species (11) only.7 Perhaps not surprisingly di-cation (12) does exist; it is formed on dissolving acetylacetone in the very strong acid HF-SbF,.* In FS0,H-SbF,-S02 solution at -60"aliphatic alcohols,9a ethers,9b as well as thiols and sulphides" are protonated. These species show well resolved n.m.r. spectra with negligible exchange rates at -60". At higher temperatures cleavage to carbonium ions G. A. Olah J. M. Bollinger C. A. Cupas and J. Lukas J. Amer. Chem. SOC.,1967,89,2692. J.M. Bollinger C. A. Cupas K. J. Friday M. L. Woolfe and G. A. Olah J. Amer. Chem. SOC. 1967,89 156; see also J. S. McKechnie and I. C. Paul ibid. p. 5482. D. M. Brouwer Chem. Comm. 1967 515. (a) G. A. Olah J. Sommer and E. Namanworth J. Amer. Chem. SOC. 1967,89 3576; see also E. F. Mooney and M. A. Qaseem Chem. Comm. 1967,230; (b)G. A. Olah and D. H. O'Brien J. Amer. Chem. SOC.,1967.89. 1725. lo G. A. Olah D. H. O'Brien and C. U. Pittman jun. J. Amer. Chem. SOC.,1967,89,2996. H.M.R. Hoffmann 128 R' X = F,CI ,O,S N ctc. (13) (14) occurs. There is growing evidcnce for the existence of vinylic cations which have been discussed mainly as intermediates in electrophilic additions to triple bonds and in triple-bond participation during solvolysis.'' The number of carbonium ions which are stabilised by a heteroatom according to [(13) *(14)] is rapidly becoming legion. The a-fluorostyryl cation (16) can be obtained readily from (15) as indicated. In analogous fashion ion (17) has been prepared while attempts to obtain (18) and (19) were un- successful.l2 The first stable fluorophenylcarbonium ions (20)+22) have been generated from alcohol and fluoride precursors in FS0,H and FS0,H-SbF,-S02 at -60". "F N.m.r. deshielding in (20)relative to the parent alcohol is particularly pronounced in the para-positions pointing to strong contribu- tio 9 from quinoidal forms. The deshielding pattern of the ring fluorine atoms in (21) and (22)is similar.' That carbonium ions (22)and (16)exist is interesting fi For a review see (a)P.E. Peterson and J. E. Duddy J. Amer. Chem. SOC.,1966,88,4990; (b)see also D. S. Noyce M. A. Matesich and P. E. Peterson ibid. 1967 89 6225; W. M. Jones and F. W. Miller ibid. p. 1960; M. Hanack I. Herterich and V. Votf Tetrahedron Letters 1967 3871 ;however cf. also H. R. Ward and P. D. Sherman,jun. J. Amer. Chem. Soe. 1967,89 1962. G. A. Olah R. D. Chambers and M. B. Comisarow J. Amer. Chem. SOC.,1967,89 1268. l3 G. A. Olah and M. B. Comisarow J. Amer. Chem. SOC.,1967,89 1027; see also R. Filler C. S. Wan& M. A. McKinney and F. N. Miller ibid. p. 1026. Part (ii) Reaction Mechanisms I29 I CH3 CH3 (15) (16) FScb ';.c /'bFS -etc. F I H F (A8=6.21ppm) (21) (A6=40-1ppm) (20) since the parent benzyl cation as well as the styryl cation have remained elusive.A full paper on ring-substituted benzyl cations has appearedl4 and n.m.r. spectra of aryl carbonium ions have been discus~ed.~~ A rich variety of protonated carbonyl compounds has been observed by n.m.r. spectroscopy. In media such as HF-SbF5 at -20" protonated acetalde- hyde is present as a mixture of syn-(23) and anti-(24) isomers16-18 in a ratio of ca. 5:1. The dramatic difference in JCH,OH (Jamti> .Isyn)as well as the analysis of fine structure leave no doubt that it is the syn-isomer (23) which is the more stable.16,17 As the methyl group in acetaldehyde is replaced by bulkier sub-stituents formation of the syn-isomer is very much preferred.l6. l7 Protonated ketones have also been investigated.and n.m.r. spectra suggest that the l4 J. M. Bollinger M. B. Comisarow C. A. Cupas and G. A. Olah J. Amer. Chem. SOC.,1967 IE9 5687. l5 D. G. Farnum J. Amer. Chem. SOC. 1967,89 2970. l6 G. k Olah D. H. O'Brien and M. Calin J. Amer. Chem. SOC. 1967,89 3582. '' D. M. Brouwer Rec. Trav. chim. 1967,86 879; H. Hogeveen ibid.,p. 696. l8 M. Brookhart,G. C. Levy and S. Winstein J. Amer. Chem. SOC. 1967,89 1735. 130 H.M.R. Hofiann positive charge resides mainly on oxygen (25) the contribution of resonance form (26) being Protonated formic acid exists in the two forms (27) and (28); the ratio (27):(28) is ca. 1 :2 and corresponds to a statistical population of the two forms. Hence their stability must be similar.20 Treatment ,H t-5-47 C-5.74 H R R \+ / + -R?-OH R'?=OH (25) (261 H \ HC+ \, 7 H (27) (28) of carbon monoxide with HF-SbF solution under 10 atmos.pressure did not give any detectable amount of the formyl cation.2' The properties of pro-tonated esters in strongly acidic media are of interest in connection with the mechanism AAcl and AALl of ester hydrolysis; n.m.r. spectra indicate that ion (29) alone is present; no (30) or diprotonated species can be detected. Unfortunately one is still left with the choice of several mechanisms for uni- molecular acid hydrolysis since the ion (30) could be present in very small concentration or else the concerted process (Scheme 3) might apply.22 Pro- tonated dicarboxylic acids and anhydride^,^ acetyl- and benzoyl-pyridinium ions,24 1-formyl and l-a~etyl-azulene~~ have been studied.l9 G. A. Olah M. Calin and D. H. O'Brien J. Amer. Chem. SOC. 1967,89,3586. 2o G. A. Olah and A. M. White J. Amer. Chem. SOC.,1967,89 3591. see also H. Hogeveen Rec. Trau. chim. 1967,86,809. 21 H. Hogeveen A. F. Bickel C. W. Hilbers E. L. Mackor and C. MacLean Rec Trav. chim. 1967,86 687. 22 H. Hogeveen Rec. Trau. chim. 1967 86 816; G. A. Olah D. H. O'Brien and A. M. White J. Amer. Chem. SOC.,1967,89 5694. 23 G. A. Olah and A. M. White J. Amer. Chem. SOC. 1967,89,4752. 24 G. A. Olah and M. Calin J. Amer. Chem SOC. 1967,89,4736. 25 D. Meuche D. Dreyer K. Hafner and E. Heilbronner Helv. Chim. Acta 1967 So 1178. Part (ii) Reaction Mechanisms OH R-c'=O + RdH R'C:4+ + R+ \.OH (31) (321 The methoxycarbonium ion (33) which has been generated from chloro- methyl methyl ether in SbF,-S02 at -60" represents the first stable primary alkoxycarbonium ion.26 The first stable alkyldiazonium ion the 2,2,2-trifluoro- ethyldiazonium ion (35) has been obtained from diazoalkane precursor (34) as shown. Protonation of (34) at the terminal nitrogen atom to give (36) was not detected in the 19Fn.m.r. ~pectrum.~' The reactions of aliphatic diazo- compounds with acids have been reviewed;28 protonated imines have been obtained.29 Halogen-bridged ions have long been recognised as intermediates in certain electrophilic additions and solvolysis reactions to account for the observed stereospecific course and enhanced rates.The ions (37a-c) have now been observed directly on dissolving various 2,3-dihalo-2,3-dimethylbutanesin SbFS-S02 at -60". The n.m.r. spectra reveal the expected ability I > Br > C1 of sustaining positive charge. The fluorine derivative did not form the bridged ion ;instead the rapidly equilibrating pair [(38) +(39)J was observed even at -90°.30 One of the first examples of carbon participation in solvolysis was observed in the phenonium system studied extensively by Cram Winstein and their co-workers. Olah and his group have now shown that the substituted phen- onium ion (41b) can be prepared by direct aryl participation from its precursor 26 G. A. Olah and J. M. Bollinger J. Amer. Chem. SOC. 1967.89.2993;for other alkoxycarbonium ions see H.Hart and D. A. Tomalia Tetrahedron Letters 1967 1347; J. F.King and A. D. Allbutt ibid. p. 49. 27 J. R. Mohrig and K. Keegstra J. Arner. Chem. SOC.,1967,89 5492. 28 R A. More O'FerraU ref l(c),p. 331; see also W. Kirmse and K. Horn Chem. Ber. 1967,100 2699. 29 G.A. Olah and P. Kreienbuhl J. Amer. Chem SOC. 1967,89 4756. 30 G.A. Olah and J. M. Bollinger J. Amer. Chem. SOC.,1967,89,4744. E* 132 H. M.R. Hoffiann + C F3 CH =N=NH / \+/ \ CH, YC X (37a X=CI 1 (37b X=Br) (37c X=I 1 (40).The n.m.r. spectrum of ion (41b) is very simple showing only three types of protons the cyclopropane protons at z 6.53 (relative area 4) the methoxy- protons at T 5.75 (relative area 3) and the AB ring quartets at z 1.88 (relative area 4).31The AB quartets are well separated stressing the benzenonium ion character of (41b) and excluding Brown’s rapidly equilibrating ions (classical or 7~-bridged),~~ which should have phenyl character.In order to appreciate the driving force for phenyl participation (and also some of the experimental difficulties in this field) it is instructive to consider the earlier incorrect report33 that a phenonium ion was formed from threo-and erythro-3-phenylbutan-2-01 (42)in FS03H-SbF5-S02. Under these conditions the benzene ring is sulphin- ated to give the diprotonated ion (43).349 35 Clearly two factors account for the failure to observe a bridged ion in this case :(i) participation by the unsubsti- tuted phenyl group is not sufficiently powerful and (ii) participation is less urgent at a secondary than at a primary centre.Sulphination can be suppressed 31 G. A. Olah M. B. Comisarow E. Namanworth and B. Ramsey J. Amer. Chem. SOC. 1967 89 5259. ’’H. C. Brown R. Bernheimer C. J. Kim and S. E. Scheppele J. Arner. Chem SOC. 1967,89,370. 33 G. A. Olah and C. U. Pittman jun. J. Amer. Chem SOC. 1965,87 3507 3509. 34 M. Brookhart F. A. L. Anet and S. Winstein J. Amer. Chem. SOC.,1966,88,5657; M. Brookhart F. A. L. Anet D. J. Cram and S. Winstein ibid. p. 5659; see also footnote 33 in ref. 35. 35 G. A. Olah C. U. Pittman jun. E. Namanworth and M. B. Comisarow J. Amer. Chem SOC. 1966,88 5571. Part (ii) Reaction Mechanisms 0CH R I I CH2CH2CI (40) (4la:R=H 1 (4lb:R= OCH,) CH3CH-kHCH3 CH3CH-CHCH (42) (431 (43a 1 in the system FSO,H-SbF,-SO2C1F.A great deal of kinetic evidence points to the intermediacy of phenyl-bridged ions.36 Remarkably 2-phenethyl toluene-p-sulphonate is solvolysed 3040 times faster than ethyl tosylate at 75" in trifluoroacetic acid while the corresponding ratio in formic acid is only 2.1 :1 Thus trifluoroacetic acid must be one of the best ionising solvents yet known for solvolytic ~tudies.~' It should be borne in mind that the geometry of the phenonium ion in which the cyclopropyl grouping intersects the six- membered ring at a right angle is not unique; spirodienone (43a) is an inter- mediate of the abnormal Claisen rearrangement and cyclopropylcarbinyl cations prefer the bisected conformation (44).It would be irlteredting l&ooompare these two ions with the hypothetical spiro-cation (45); in this case unlike that of cation (41a) the lowest unoccupied molecular orbital of the cationic system is antisymmetric with respect to the plane through the cyclopropyl ring. Carbonium-Ion Rearrangements and Nucleophilic Substitutions.-The inter-mediates and transition states in various 1,2-rearrangements have been 36 D. J. Cram and J. A. Thompson J. Amer. Chem. SOC. 1967,89 6766; J. L. Coke ibid. p. 135; C. C. Lee and R. J. Tewari Canad. J. Chem. 1967,45 2256; C. C. Lee and B.-S. Hahn ibid. p. 2129; C. C. Lee and L. Noszk6 ibid. 1966 44 2481 2491; C. A. Kingsbury and D. C. Best Tetrahedron Letters 1967 1499; R. Leute and S. Winstein ibid.p. 2475; H. M. R. Hoffmann J. Chem. SOC. 1965 6762. 37 J. E. Nordlander and W. G. Deadman Tetrahedron Letters 1967,4409. H. M. R. Hoflann described by molecular orbital theory.38 Spurred on by the unique tenfold degeneracy of b~llvalene,~' a number of degenerate rearrangements have been reported for carbonium ions and it seems likely that an exciting new chapter of carbonium ion chemistry is about to begin. Doubly degenerate carbonium ions have of course been known for a few years and the ion [(38) + (39)] mentioned above represents just one of several examples. Another case also from the work of Olah4 is ion [(46)+ (47)],in which all five methyl groups are scrambled rapidly even at -180".The barrier for the methyl shift seems to be less than 2-3 kcal./mole and yet there is apparently no tendency to form the static nonclassical ion (48).Even more spectacular is the degeneracy of certain bicyclic and polycyclic systems. Solvolysis of (49) in unbuffered formic acid at reflux yielded a formate in which only 10 +_ 2% deuterium had stayed at C-9 while the remainder was scrambled over the molecule. Thus under forcing conditions the nine carbon atoms of the 9-homocubyl cation (50) achieve complete eq~ivalence.~' The homododecahydryl cation (51) represents a 0 .D (52) (53) '' N. F. Phelan H. H. Jaffk and M. Orchin J. Chem. Educ. 1967,44 626. 39 W. von E. Doering B. M. Ferrier E. T. Fossel J. H. Hartenstein M. Jones jun. G. W. Klumpp R M. Rubin and M. Sunders Tetrahedron 1967,23,3943.40 P. von R. Schleyer J. J. Harper G. L. Dunn V. J. Dipasquo and J. R. E. Hoover J. her. Chem. SOC. 1967,89 699; corrigenda p. 2242; see also J. C. Barborak and R. Pettit ibid. p. 3080. Part (ii) Reaction Mechanisms potentially twenty-one-fold degenerate ion whose synthesis should challenge even the brightest organic chemist.40 After heating [l-l4C)naphthalene in benzene with moist aluminium trichloride for 2 hr. at 60” the radiolabelled carbon was found to be distributed statistically over the carbon skeleton of naphthalene ; this phenomenon has been termed “automeri~ation”.~~ Win-stein and his co-worker~~~~ have described a novel degenerate five-carbon scrambling of the 7-norbornadienyl cation (52). On warming this ion in fluoro- sulphuric acid to -50” atoms C-1 C-6 C-5 C-4 and C-7 become equivalent as shown in (53).A detailed n.m.r. analysis suggests the mechanism in Scheme 4 D Scheme 4 with the first two steps as follows a 1,2-shift by either electron-donating vinylic carbon (C-2 or C-3) to C-7 generates a bicyclo[3,2,0]heptadienyl cation which ring expands immediately either forward (shift of C-3 from C-4 to C-5) or backward (shift of C-2 from C-7 to C-1). Apparently in solvolysis in acetic acid or aqueous acetone the [3,2,0)-system does not leak into the [2,2,1]- cation (nor does the reverse apply).42b Thus the potentially different behaviour of fleeting solvolytic intermediates and long lived carkonium ions should be borne in mind. The possibility of a five-fold degenerate ion [(54) +(55) +etc.] wherein the cyclopropyl ring revolves about the perimeter of the larger ring has also been con~idered.~~ For the first time the energy barrier to “bridge flipping” in various 7- norbornadienyl cations has been determined ;the n.m.r.spectra suggest that all three cations are unsymmetrical [56a-c) +(58a-c)] with flipping barriers of b 19.6 12.4 and <7.6 kcal./mole respectively. Thus the barrier is lowered by an electron-donating group at C-7 which stabilises the symmetrical ion (57b c) but has no appreciable effect on the unsymmetrical species. At higher temperatures the bicycloaromatic (see below) 7-norbornadienyl cation re-arranges to the tropylium ion (59a-c) by a mechanism which remains to be el~cidated.~~ 41 A.T. Balaban and D. FBrcaSiu J. Amer. Chem. SOC. 1967,89 1958. 42 (a) R. K. Lustgarten M. Brookhart and S. Winstein J. Amer. Chem. SOC. 1967 89 6350; (b)Footnote 8 ref. 42a. 43 D. W. Swatton and H. Hart J. Amer. Chem. SOC. 1967,89 5075. 44 M. Brookhart R. K. Lustgarten and S. Winstein J. Amer. Chem. SOC. 1967,89 6352. H.M. R. Hofiann (56a R=H 1 ( 570-c 1 (580-c) (59a-c) (56b:R=CH,) (56~: R=Ph 1 The trishomocyclopropenyl cation (68) was first formulated by Winstein to account for the rate and stereochemical course in the solvolysis of cis-3-bicyclo[3,l,0]hexyl toluene-p-sulphonate (69).45 Compared with other non-classical systems the observed rate enhancement ( 50) is moderate. Extended Huckel calculations indicated that the bridging carbon in the hypothetical cation (66) is stabilised more by bending toward the cyclopropyl group than Relative o.4 I 10'' 6 10" loi4 rates (60) (61) (62) (63) (641 (65) p 01s 45 S.Winstein E. C. Friedrich R. Baker and Y. Lin Tetrahedron 1966 Suppl. No. 8 621 and earlier papers. Part (ii) Reaction Mechanisms to the double bond.46 Two research groups4’ have now demonstrated in- dependently that the tricyclic toluene-p-sulphonate (62) solvolyses lOI4 times faster than its saturated analogue (61);thus the system (62)and (65)“*share the record for nr-participation in solvolysis ! The strikingly different reactivity of (62) and (60) shows again (cf. ions (41a) and (44) above) that cyclopropane interacts only ricr its edge (but not with its “face”) with carbonium ion centre^.^' As regards the different reactivity of (62) and (69) it seems clear that ion (67) is formed much more readily because the trishomocyclopropenyl system in (67) is locked from the beginning into a chair by the bridging ethano-group while considerable energy must be expended to bend the nearly planar five-membered ring in (69) into the chair-like form (68),which is required for maximum orbital overlap.47b Other trishomocyclopropenyl-cation intermediates have been in~estigated.~” However a stable species had remained elusive at the time of writing.That a cyclopropyl group need not necessarily assist ionisation more effec- tively than a double bond can be seen for the toluene-p-sulphonates (70) and (71) which are solvolysed at nearly the same rate in acetic acid.50 Presumably the reduced requirement for participation in the bicyclo[3,3,1]-system as well (70) (71) as the less favourable geometry determine this difference.In the acetolysis of exo-(72) and endo-3-bromobenzenesulphonate(74) more than 90 ”/ bicyclic olefin (73) is formed and both compounds solvolyse some 950 times faster than cyclohexyl bromobenzenesulphonate. In this case relief of nonbounded 46 R. Hoffmann Tetrahedron Letters 1965,3819. 47 (a) H. Tanida T. Tsuji and T. hie J. Amer. Chem SOC. 1967 89 1953; (b) M. A. Battiste C. L. Deyrup R. E. Pincock and J. Haywood-Farmer ibid. p. 1954. 48 S. Winstein and C. Ordronneau J. Amer. Chem. SOC. 1960,82 2084. 49 (a)For further recent studies of the stereoelectronic requirements in cyclopropyl participation see B.Halton M. A. Battiste R. Rehberg C. L. Deyrup and M. E. Brennan J.Amer. Chem. SOC.,1967 89 5964; R. E. Pincock and J. Haywood-Farmer Tetrahedron Letters 1967,4759; (b)W. Broser and D. Rahn Chem. Bm.,1967,100 3472. M. A. Eakin J. Martin and W. Parker Chem. Comm. 1967,955. H.M.R. Hofiann repulsion between the C-3 and C-7 groupings seems responsible for the rate enhan~ement.~ Winstein’s concept of homoconjugation and homoaromaticitys2 discussed above for trishomocyclopropenyl cations has clearly wide applicability particularly to high-energy species (ions excited states and transition states).s3a Conceptually elegant is the extension to “spiroconjugation” and “bicyclo- aromaticity” which has been propounded in three stimulating papers.s3* s4 In Goldstein’s papers4 the concept of aromaticity is no longer tied to planar molecules but clearly taken into the third dimension; as pointed out by this author on the basis of simple MO considerations ions (75)-(78) and higher ”bicycloaromatic‘ 4n systems ”antibicycloarornatic” 4m + 2 systems analogues with 4n x-electrons are expected to possess enhanced thermodynamic stability (h-bicycloaromaticity ’) relative to an appropriate reference compound.Ions (79H82) on the other hand possess 4m + 2 n-electrons and should be antibicycloaromatic in complete reversal of the Hiickel rule. Goldstein has summarised the experimental evidence which is as yet quite meagre ion (75) is the most simple bicycloaromatic species ;regarding potentially antibicyclo- aromatic ions anion (79) could not be obtained on treatment of norbornadiene with amylsodium.Instead only cyclopentadienylsodium and acetylene were observed as primary products (Scheme 5)’’ Furthermore barbaryl toluene-p- sulphonate (83) was solvolysed only in a sluggish reaction despite the neigh- bowing cyclopropyl grouping and two double bonds. Therefore cation (80) ” J. P. Schaefer and C. A. Flegal J. Amer. Chem Soe. 1967,89 5729. ” S. Winstein ref. l(b),p. 5; S. Winstein M. Ogliaruso M. Saki and J. M. Nicholson J. Amer. Chem. SOC. 1967,89 3656; see also J. M. Brown,Chem. Comm. 1967 638. ” (a) H. E. Simmons and T. Fukunaga J. Amer. Chem. Soc. 1967 89 5208; (b)R. Hoffmann A.Imamura and G. D. Zeiss ibid. p. 5215. ” M. J. Goldstein J. Amer. Chem. Soc. 1967,89 6357; see also M. J. Goldstein and B. G. Odell ibid. p. 6356. ” R. A. Finnegan and R. S. McNeeq J. Org. Chem. 1964,2!# 3234. Part (ii) Reaction Mechanisms Scheme 5 cannot be particular stable and all other attempts to generate it were fruitless.3g Chlorination of (84) under mild conditions in ether yielded only rearranged product (85).56Following Goldstein bicycloaromaticity should be measured relative to a reference compound which possesses the same number of trigonal carbon atoms and n-electrons. For example the relative stability of (75) could be gauged by that of the unknown cations (86) and (87); the number k of methylene groups in the reference has to be chosen such as to minimise differ- ences between the o-bond interactions in the bicycloaromatic and the reference.(84) 4 H (86) (87) '' A. S. Kende and T. L. Bogard Tetrahedron Letters 1967 3383. H. M. R. Hohann For a lucid exposition of spiroconjugation the interested reader is referred to the two original papers,53 which should provoke a further flood of experi- mental work. It has been suggested53" that spiroconjugation might extend to kinetic phenomena such as the enhanced rates of solvolysis of certain sulphates and phosphates. The possibility of homoconjugation in certain cumulenes has also been con~idered.~' More than twenty-five different papers have appeared dealing with the norbornyl cation in one way or another; of these only about five seem directly relevant to the solvolytic behaviour of the parent norbornyl cation.An interest- ing model system has been selected by Corey and Glass5* who synthesised the tricyclic exo488) and endo-(89) sulphonates. These two compounds preserve ti (88) (89) Waqncr -Meerrsin rcarranqcrnen H (881 (921 H OH (94) (93) s7 H. Fischer and H. Fischer Chem. Ber. 1967,100,755. s8 E. J. Corey and R. S. Glass J. Amer. Chem. SOC. 1967,89 2600; for a similar approach see R Baker and J. Hudec Chem. Comm. 1967,929. Part (ii) Reaction Mechanisms to a maximum extent the geometry of the norbornyl skeleton and particularly the exact environment at the reaction zone C-1 C-2 and C-6. As expected Wagner-Meerwein rearrangement of the model compound (88) (i.e.,.shift of C-6 from C-1 to C-2) yields (92) which has been estimated to be some 6-7 kcal./mole more strained than the starting material [qualitatively this addition- al strain may be appreciated readily by comparison with the hypothetical cis-and trans-pentalene derivatives (93) and (94)]. If the ion derived from exo-norbornyl toluene-p-sulphonate (90) were to be formulated as a classical ion the rate of ionisation of tricyclic em-sulphonate (88) and (90) should be essentially the same; likewise the rate of solvolysis for endo-norbornyl toluene- p-sulphonate (91)and model (89)would be expected to be similar. The measured rate constants are strikingly in favour of the bridged-ion mechanism 2-endo-norbornyl toluene-p-sulphonate (91) and its tricyclic analogue (89),for which bridging has never been postulated show similar reactivity.However the rate of acetolysis for the tricyclic exo-sulphonate (88)is severely suppressed relative to (90) as one would expect if a bridged nonclassical norbornyl cation is formed from (90) under these conditions. The y-deuterium isotope effects” as well as the activation volume6’ for the solvolysis of exo- and endo-norbornyl bromobenzenesulphonate are also consistent with the intermediacy of a bridged cation. eido -attack (98) (99:R= OH OEt 1 s9 B. L. Murr A. Nickon T. D. Swartz and N. H. Werstiuk J. Amer. Chem. Soc. 1967 SS 1730; J. M. Jerkunica S. BorEiC and D. E. Sunko ibid. p. 1732; Chem. Comm. 1967 1302.6o W. J. le Noble B. L. Yates and A. W. Scaplehorn J. Amer. Chem SOC.,1967,89 3751. 142 H. M.R. Hofiann That the stereospecific exo capture of a norbornyl cation does not necessarily require a bridged precursor has been emphasised by several authors.61 Many reactions of bicyclo[2,2,l]heptenes including electrophilic nucleophilic and free-radical additions as well as 1,3-dipolar cycloadditions proceed with stereo- specific exo-attack.62 It throws an interesting sidelight on the heat of the argument that similar observations had been reported by Alder as early as 1935;63u these have been generally referred to as Alder exo-rule in the German 1iteratu1-e.~~~ An interesting attempt to rationalise this rule has been made by S~hleyer.~~ As an example consider the 1,3-dipolar cycloaddition of phenyl azide to norbornene which yields adduct (96).It may be seen readily from the sideview (97) of norbornene that the hydrogen atoms H-1 and H-2 as well as H-3 and H-4 are partially eclipsed (dihedral angle 20");this is an unfavourable conformation. In reaching the transition state for endo-attack the two hydrogen atoms H-2 and H-3 would have to be bent upwards and the torsional angle to be decreased even further with a concomitant increase in torsional strain. The alternative attack from the exo-side however should be favoured since in approaching the transition state the H-2 and H-3 hydrogens must be bent in endo-direction and torsional strain is relieved. The Alder exo-rule has been excellently reviewed by Klumpp and his co-worker~,~~~ who have also probed experimentally for its limitations.The 3,2-hydride shift observed for the long-lived 2-norbornyl cation can be frozen out in the n.m.r. spectrum by lowering the temperature. However certain substituted norbornyl cations which are usually tertiary and of the C-2 alkylated and arylated type may suffer 3,2-carbon and hydrogen shifts more extensively or even exclusively.64* 65 These shifts appear to proceed with a preference for exo,exo-migration although the first clear example of a 3,2- endqendo-hydride shift has just come to light.66 Presumably bridged tertiary ions are not involved here and in the opinion of the writer Schleyer's torsional hypothesis64 should be considered. Bridgehead-substituted compounds are free from the frustrating ambiguities so often encountered in alicyclic compounds and have long enjoyed special 61 H.C. Brown Chem. Eng. News 1967 Febr. 13th p.87. A. F. Thomas R. A. Schneider and J. Meinwald,J. Amer. Chem SOC. 1967,89,68;H. C. Brown and K.-T. Liu ibid. p. 466 3898 3900; P. von R. Schleyer ibid. p. 3901; see also G. D. Sargenf Quart. Rev. 1966,20 344. (a)K. Alder and G. Stein Annulen 1935,515 185; ibid. J936,525 183 221 ;(b)G. W. Klumpp A. H. Veefkind W. L. de Grad and F. Bickelhaupt Annulen 1967,706,47. 64 P. von R Schleyer J. Amer. Chem. SOC.,1967,89 699; see also ibid. p. 701. 65 C. J. Collins and B. M. Benjamin J. Amer. Chern. SOC.,1967,89,1652; C. J. Collins V. F. Raaen B. M. Benjamin and I. T. Glover ibid.p. 3940; J. A. Berson J. H. Hammons A. W. McRowe R. G. Bergman A. Remanick and D. Houston ibid. p. 2561 ;J. A. Begion A. W. McRowe R. G. Bergman and D. Houston ibid. p. 2563; J. A. Berson and R. G. Bergman ibid. p. 2569; J. A. Berson A. W. McRowe and R. G. Bergman ibid. p. 2573 ;J. A. Berson R. G. Bergman J. H. Hammons and A. W. McRowe ibid. p. 2581 ;J. A. Berson J. H. Hammons A. W. McRowe R. G. Bergmann A. Remanick and D. Houston ibid. p. 2590. 66 A. W. Bushel1 and P. Wilder jun. J. Arner. Chem SOC. 1967,89 5721 ;for another possible example see R. P. Lutz and J. D. Roberts,ibid. 1962,84 3715. Part (ii) Reaction Mechanisms attention in mechanistic studies.67 A spectrum of bridgehead reactivities has emerged and 1-chlorobicyclo[ l,l,l]pentane (98) reacts with particular ease; in 80% aqueous ethanol at 25" this compound is three times more reactive than t-butyl chloride and 1014 times more reactive than 1-chloronorbornane.Unlike its higher homologues compound (98) fragments to give 3-methylene- cyclobutanol and its ethyl ether (99);one possible driving force for this reaction is relief of strain.68 The solvolysis of 8,9-dehydro-2-adamantyl toluene-p- sulphonate has been ~tudied.~' The first saturated bridgehead Grignard reagents (100H102) have been prepared. Compound (102) decomposes in refluxing ether in an ElcB-type reaction (Scheme 6). Not surprisingly the corresponding lithium compound with the greater carbanionic character of the bridgehead is less stable. In striking contrast to the fully fluorinated Grignard reagent (102) com- pounds (100) and (101) are stable in refluxing ether.The reduced stability of Grignard reagent (102) has been ascribed to inside-cage transmission of the F2' MgX F2&) F F2@F2 MgX F2 MgX F2 (100a:X=BrI (IOla-bI (100b X=I I (102a-b 1 lx- Scheme 6 67 R. C. Fort jun. and P. von R. Schleyer Adv. AIicyclic Chem. 1966 1 284; G. J. Gleicher and P. von R. Schleyer J. Amer. Chem. SOC. 1967 89 582; T. McAllister 2. DoleSek F. P. Lossing R. Gleiter and P. von R. Schleyer ibid. p. 5982. K. B. Wiberg and V.Z. Williams,jun.,J. Amer. Chem. SOC.,1967,89 3373. 69 J. E. Baldwin and W. D. Foglesong .I.Amer. Chem. SOC.,1967,89,6372. 144 H.M.R. Hoffmann dipole associated with the carbon fluorine bond of the other bridgehead'O as indicated in formula (102).Perfluorobicyclo[2,2,l]heptanes provide just one example for the marked effect of perfluoroalkyl groups on chemical reactivity. Various workers have proposed that fluorine not only exerts a normal electron-attracting inductive effect but in addition is capable of a conjugative interaction (103b) involving carbon-fluorine no-bond resonance. Since a bridgehead carbanion is forced to remain pyramidal hyperconjugation according to (103b) should be appreci- ably diminished in such a system. 1H-Undecafluorobicyclo[2,2,l]heptane (104) undergoes base-catalysed tritium exchange some five times more readily than tris(trifluoromethy1)methane (105) and it has been suggested that there is no need to invoke such hyperconjugation.If we accept this argument then the tris(trifluoromethy1) anion should have a pyramidal (rather than planar) structure like tri~(trifluoromethy1)amine.~'In the Reporter's opinion fluoro- hydrocarbon (106) would have provided a somewhat better model in this study for the same reason which determines the contrasting reactivity of (101) and (102). According to Bredt's rule a bridgehead double bond cannot exist in a bicyclic system unless the rings are large enough to accommodate the double bond without excessive strain. Bicyclo[3,3,l]non-l-ene (107) has now been obtained by two independent routes. This compound constitutes the smallest ring system yet prepared with a bridgehead double bond; on standing in air it poly- merises.F--c.=-c\/F -C-CNF 'FF 'F ( I07 (I08 1 70 S. F. Campbell J. M. Leach R.Stephens and J. C. Tatlow Tetrahedron Letters 1967,4269. A. Streitwieser jun. and D. Holtz J. Amer. Chem. Soc. 1967,89,692;for a second independent piece of evidence against fluorine hyperconjugation see A. Streitwieser jun. A. P. Marchand and A. H. Pudjaatmaka ibid. p. 693. ''(a) J. A. Marshall and H. Faubl J. Amer. Chem. SOC. 1967 89 5965; (b)J. R. Wiseman ibid. p. 5966. Part (ii) Reaction Mechanisms Neighbouring group participation and rearrangements in cyclopropyl- methyl cyclobutyl and homoallyl systems have been reviewed.73 Since Whitmore described the first authentic neopentyl derivatives the solvolysis of these compounds has been studied with unusual intensity.It is generally accepted and quoted in most textbooks that these reactions entail complete rearrangement of the neopentyl skeleton under carbonium-ion conditions. Fraser and the writer have now analysed the products formed from neopentyl toluene-p-sulphonate in ethanol-water mixtures at 130”. In pure water as much as 10% neopentyl alcohol is formed aside from other products. The distribution of products is solvent dependent and the most simple mechanism embraces at least three intermediates A neopentyl ion-pair the t-pentyl cation and a dimethylcyclopropane precursor.74 Solvolytic studies in water75 and the interpretation of activation parameter^'^ have been reviewed.77 Bedevilling all interpretations of rates particular in water as a solvent is the general question :‘How much do changes in the energy of the initial state (including solvent-solute interactions) and how much do changes in the energy of the transition state contribute to a particular rate ratio?’78 Following Robertson77 this difficulty may be illustrated with the road- grading analogy A road-builder who wishes to lower the height of a hill may do so by (a)scraping offthe top of the hill (b)filling in the valley or perhaps most frequently in real life by (c) a combination of the two operations.lb Fig.1. The road grading analogy It should be pointed out that the internal strain of a molecule which is so frequently invoked as a contributor to the energy of the initial state can be estimated from the measurement of the energy evolved in the thermal re-arrangement of that molecule.Such measurements have been made possible 73 M. Hanack and H.-J. Schneider Angew. Chem. Znternat. Edn. 1967,6,666. 74 G. M. Fraser and H. M. R Hoffmann Chem. Comm. 1967 561; for an elegant demonstration of the role of the counterion in reactions of diazoneopentane with acid see W. Kirmse and K. Horn Tetrahedron Letters 1967 1827. ’’ E. Buncel and P. R. Bradley Canad. J. Chem. 1967 45 515; A. Queen ibid. p. 1619; B. N. Hendy W. A. Redmond and R. E. Robertson ibid. p. 2071; H. S. Golinkin I. Lee and J. B. Him J. Amer. Chem SOC.,1967,89 1307; J. G. Martin and J. M. W. Scott Chem. and Ind. 1967 665. 76 G. Kohnstam ref. l(c) p. 121. ’I7 R. E. Robertson Progr. Phys. Org. Chem. 1967,4 213.(a)See Ann. Reports 1965,62,238; (b)H. M. R. Hoffmann J. Chem. Soc. 1965 6753 6762. 146 H. M.R. Ho_f)Fnann with a temperature-programmed differential calorimeter. Particular attention should be paid to the work of 0th’’’ who by using this instrument has even elucidated the complex steps in the thermal rearrangement of hexamethyl- prismane to hexamethylbenzene. When solving kinetic equations for multistep reactions (e.g. SN1 El and ElcB reactions) one usually assumes that the reactive intermediate (say A) is present in a stationary state (dA/dt = 0). The exact solutions of the kinetic equations however may deviate and criteria for recognising such deviations have been discussed.” The SN2’ reaction is a bimolecular nucleophilic substitution involving allylic rearrangement.That such a reaction involves attack of the nucleophile syn to the leaving group (108) is generally accepted and can also be rationalised by simple HMO theory. For a similar substitution in a system of five carbon atoms (with two conjugated double bonds) one has reached full circle and an anti-relationship of entering and leaving group (cf. SN2 reaction) is predicted. Nucleophilic substitution at an optically active tertiary carbon has received relatively little attention. Displacements in the 2-phenyl-2-butyl system involve an asymmetric ion-pair or its equivalent (formed in the rate-limiting step) which is attacked by the nucleophile in the product-determining step with predominant inversion of configuration.” Similarly the product distribution from the phenyldimethylcarbinyl system implicates the c~unterion,’~ as expected from earlier The principle of hard and soft acids and bases has been discussed for multi- centre (mainly catalysed) reactions8’ and for organic reactions in general.86 When applied with care the concept allows one to appreciate many otherwise unconnected facts ; however the principle does not necessarily invalidate earlier explanations.For example Pearson and Songstad’s reinterpreta- tion86p87 of the ratio koTs/kBr7” has been criticised by Trahanovsky and Doyle.88 Acetolysis of 5-hexenyl toluene-p-sulphonate at 100” leads to open (66 %) and cyclic (34 %) products while the corresponding bromide forms 84 % open and only 16 % cyclic product.Since the olefinic double bond is clearly a soft base and acetic acid is hard one would expect less cyclisation in the acetolysis of the ‘hard’ toluene-p-sulphonate. The results can be understood on the basis that the reaction of the toluene-p-sulphonate involves the more 79 J. F. M. Oth Lecture held at the ‘Symposium on Small Rings’ Louvain Belgium Sept. 1967. K. Frei and H. H. Giinthard Helv. Chim. Act4 1967 SO 1294. W. Drenth Rec. Trav.chim. 1967,86 318. L. H. Sommer and F. k Carey J. Org. Chem. 1967,32 2473 800; see also R. R. Sauers and D. H. Ahlstrom ibid. p. 2233. 83 R. L. Buckson and S. G. Smith J. Org. Chem. 1967,32,634. D. J. Cram and M. R. Sahyun J. Amer. Chem. SOC.,1963,85,1257;M. Cocivera and S. Winstein ibid. p. 1702; P. S. Skell and W.L. Hall ibid. p. 2851. ’’ B. Saville Angew. Chem. Internat. Edn. 1967,6 928. 86 R. G. Pearson and J. Songstad J. Amer. Chem. SOC. 1967,89 1827. R. G. Pearson and J. Songstad J. Org. Chem. 1967,32,2899. W. S. Trahanovsky and M. P. Doyle Chem. Comm. 1967 1021; see also J. Amer. Chem. SOC. 1967,89,4867. Part (ii) Reaction Mechanisms 147 ionic transition state which is also more prone to participation. Other k,,/k, ratios have been determined and discussed.89 Olefin-forming Eliminations.-Sicher and his collaborators continue to make important contributions to the mechanism of bimolecular eliminations. In two detailed papers the syn-anti dichotomy has been described for Hofmann eliminations from cycloalkylammonium and sulphonium salts.In these eliminations the cis-cycloalkenes (n = 5-14 16) are formed by an anti-elimination while the trans-cycloalkenes arise from a syn-route. Remarkably trans-cycloalkenes predominate in rings greater than se~en-rnembered.~~* 91 As an example of an elimination from an open-chain derivative the quarternary ammonium bases (109) and (110) have been studied. Each diastereomer may potentially yield several olefins; the &,trans-pairs shown account for 92 5 % of the products in each case. If (109) were to give the trans-olefin (111) by a syn-elimination and the cis-olefin (112) by the anti-route then these two reactions must proceed with loss of deuterium and should therefore show a H D (I09I H Scheme 7 distinct isotope effect. On the other hand supposing the syn -+ trans and anti + cis dichotomy applies to the corresponding pair (113) and (114)formed from (110) then no pronounced isotope effect would be expected since deuterium is retained in the products.For a variety of conditions e.g. the systems MeOK-MeOH and Bu'OK-Bu'OH it has been found that (113) and (1 14) are indeed formed without appreciable isotope effect (kH/kD 0.9-1-2) 89 D. D. Roberts and J. G. Traynham J. Org. Chem. 1967,32,3177. 90 J. Sicher and J. Zhvada Coll. Czech. Chem. Comm.,1967,32,2122. 9' J. Zkvada and J. Sicher Coll. Czech. Chem. Comm. 1967,32 3701. 148 H. M.R. Hofiann whereas the isotope effect in the analogous reaction of (109) is clearly dis- cernible (kJkD 2.34.7). Thus the original premise is correct i.e.cis-olefins (112) and (114) arise by the anti-route and trans-olefins (111) and (113) by syn-elimination either largely or excl~sively.~~ Sicher’s dichotomy applies also to the formation of cis-and trans-cycloalkenes from cycloalkyl bromides (n = 5-14 16) in Bu‘OK-Bu’OK but not in EtOK-EtOH.93 Several con- clusions follow. Firstly syn-eliminations are much more common than has been hitherto suspected. Medium rings are particularly prone toward this reaction mode open-chain compounds less so and six-membered rings least.94 Secondly syn-eliminations are favoured under Hofmann conditions i.e. with C-H bond breaking in the lead. Typically one uses a powerful base in a solvent which supports ion-pairing ; thus it is visualised that the alkoxide counterion assists the departure of the leaving group electrophilically as shown in Scheme 7.Finally syn-elimination may also intrude when C-X bond breaking is in the lead ; for example solvolytic eliminations from cyclodecyl toluene-p-sulphonates follow the syn-route predominantly if not excl~sively.~~ The challenging question as to the origin of Sicher’s syn-anti elimination dichotomy remains to be answered. Much reinterpretation of earlier data such as k,/k isotope effects Hammett p-values and cis-trans olefin ratios seems imminent . In the light of Sicher’s work a preparative observation by Traynham and collaborator^^^ deserves particular interest When cyclodecyl chloride was treated with Bu‘OK-Me2S0 an olefin mixture (ca. 60 %) was obtained which consisted of almost pure (ca.97%) cis-cyclodecene. On the other hand with lithium dicyclohexylamide in hexane solvent a remarkable reversal occurred ; in this system which seems designed to induce syn-elimination according to Scheme 7 trans-cyclodecene was formed in 96 % purity. syn-Eliminations in the flexible systems discussed above should be dis- tinguished from those occurring in more rigid substrates which cannot attain the anti-periplanar array of H-C-C-X necessary for an anti-elimination. Not surprisingly em-2-norbornyltrimethylammonium hydroxide yields nor- b~rnene~~ in a ‘torsionally enforced’ syn-elimination. In strongly basic media menthyltrimethylammoniumhydroxide forms up to 27 % menth-3-ene by the ~yn-route.~~ In competing substitution and elimination of primary alkyl substrates toluene-p-sulphonates generally give much more product of substitution than the corresponding bromides.Putting this another way koTJkB,(SN2) and 92 M. PBnkova J. Sicher and J. ZBvada Chem. Comm. 1967 394. 93 J. Zivada J. KrupiEka and J. Sicher Chem Comm. 1967 66. 94 M. Svoboda J. Zhvada and J. Sicher Coll. Czech. Chem. Comm. 1967,32 2104. 95 J. G. Traynham D. B. Stone and J. L. Couvillion J. Org. Chem. 1967,32 510; see also G. Wittig and R. Polster Annalen 1958,612 102. 96 (a) J. L. Coke and M. P. Cooke jun. J. Amer. Chem. SOC. 1967 89 2779; (b)These authors appear to have overlooked that practically the same results were described earlier by C. W. Bird R C. Cookson J. Hudec and R. 0.Williams J.Chem. SOC. 1963.410. 97 M. A. Baldwin D. V. Banthorpe A. G. Loudon and F. D. Waller J. Chem. SOC. (B),1967 509. Part (ii) Reaction Mechanisms 149 kOT$kBr(E2) are markedly different from the values observed for pure substi- tution and pure elimination. The observed distortion of koTs/kBr points to an unusual feature of the transition state in these competing processes and a merged mechanism has been proposed;98 such a mechanism has also been considered by Sicher and Zavada" for the elimination of cycloalkyltrimethyl- ammonium salts in Bu'OK-Bu'OH. A novel elimination mechanism the E2cB mechanism in Ingold's termi- nology has been uncovered by Schlosser and Ladenberger;99 on treatment with an organolithium base cis-styryl chloride (115) is converted into the lithium acetylide (1 17).In the first slow step cl-metallated styryl chloride (1 16) The E2c8 mechanism is formed (which can also be identified as an intermediate in tetrahydrofuran at low temperature). In the following fast step a second molecule of base effects dehydrochlorination to (117). The alternative carbenoid route i.e. loss of lithium chloride and hydride shift to give phenylacetylene (118) can be ruled out.99 The ElcB mechanism has been reviewed by McLennan'" and also dis- cussed by the writer.l0' For eliminations in the 2-phenethyl system the ratios koTs/k, increase as the P-proton is rendered more acidic by electron- withdrawing groups (Table). Clearly as C-H bond-breaking (process 'h') increases and the transition state is shifted toward the nearly-ElcB extreme C-X bond-breaking (process 'x') increases in attenuated fashion.Thus the ElcB character of these concerted reactions should be visualised as process 'h'-'x' and it may be appreciated why in general an E2 reaction does not leak into the ElcB route (Scheme 8) merely when the acidity of the P-proton 98 G. M. Fraser and H. M. R. Hoffmann J. Chem. SOC.(B),1967,425. 99 M. Schlosser and V. Ladenberger Chem. Ber. 1967 100 3877 3893 3901 loo D. J. McLennan Quart. Rev. 1967 21,490. lo' H. M. R. Hoffmann Tetrahedron Letters 1967,4393. H.M.R. HofJinann TABLERatios koTs/kBr for E2 reactions in the 2-phenethyl system RC6H4*CH2CH2X Bu*oK~Buto~RC6H4*CH~H2 R kOTslkBr p-Me0 0-15 H 0.22 p-c1 0.44 m-Br 1.19 P-NO2 1.57 is enhanced.'02 As a theoretical handrail for predicting ElcB reactions one should scrutinise the microscopic reverse of the slow step of a potential ElcB reaction ;lo' such nucleophilic additions of typical leaving groups to olefins are rare.However these additions do occur to certain carbenes benzynes other high-energy olefins (cf. Scheme 6) and a,P-unsaturated ketones (cf. The El& mechanism I1 k (slow1 2. -c-c-x 7--II k-i Scheme 8 Michael addition). It is precisely for the formation of these 'olefins' that the ElcB mechanism has been proposed."' The reactions following some carbonyl-methylene condensations furnish instructive examples of ElcB reactions. For instance treatment of 3-nitrobenzaldehyde (1 19) with (120) in the presence of pyridine gives a violet solution (Amax 550 mp) owing to the intermediate carbanion (121); after a few minutes the reaction solution turns cloudy with precipitation of a mixture of (123) and 'olefin' (122).Significantly on adding piperidine to a warm solution of (123) in ethanol the same transient colour (kmax.550 mp) is observed and a mixture of compounds (122) and (123) crystallises on cooling.'03" Because of its insolubility in ethanol olefin (122) can be isolated; in other cases clear proof of an ElcB mechanism is usually more difficult since the olefin formed tends to dimerise and polymeri~e,~~~" as expected for a compound of comparatively high energy."' Presumably the Mannich-Robinson reaction proceeds via an El CBstep also.' 03' lo2 G.M. Fraser and H. M. R. Hoffmann J. Chem. SOC.(B),1967,265. lo' (a) G. Schwenker Arch. Pharrn. 1966,299 131 ;(b)ibid. 1965,298 826. Part (ii) Reaction Mechanisms (122) &OH -& CN (121) A terminology for 1,3-eliminations (Scheme 9) has been proposed by Nickon and Werstiuk.lo4 These authors envisage four principal structures of the transition state and demonstrate two such arrangements experimentally -A-C X-A-0-C-Y + X'Y-Scheme 9 For the concerted formation of nortricyclene (124) from exo-norbornyl toluene-p-sulphonate (90) in Bu'OK-Bu'OH the exo-sickle arrangement (125a) is preferred to the W geometry (125b). For the corresponding reaction of endo-norbornyl toluene-p-sulphonate (91) the initial U geometry (126a) is favoured over the endo-sickle arrangement (126b).Other related reaction^,'^' the Favorsky rearrangement,'06 and the Ramberg-Backlund rearrangement O7 A. Nickon and N. H. Werstiuk J. Amer. Chem. SOC. 1967,89 3914 3915 3917. lo' S. J. Cristol and B. B. Jarvis J. Amer. Chem. ioc. 1967,89 401; ibid. 1966,88 3095. lo6 J. F. Pazos and F. D. Greene J. Amer. Chzm. SOC.,1967,89 1030; H. R. Nace and B. A. Olsen J. Org. Chem. 1967,32 3438; H. 0.House and F. A. Richey jun. ibid. p. 2151; G. W. K. Cavil1 and C. D. Hall Tetrahedron 1967,23 1119; W. Reusch and P. Mattison ibid. p. 1953; C. Rappe and L. Knutsson Acta Chem Scad. 1967,21 163; N. Schamp and W. Coppens Tetrahedron Letters 1967 2697. lo' L. A. Paquette and L. S. Wittenbrook J. Amer.Chem. SOC.,1967,89 4483. 152 H.M.R. Hofiann YOTS H-oTs H H OTs 01s have been Studied. Fragmentation reactions have been reviewed by Grob and Schiess. O8 Nucleophilic Displacement of Vinylic and Acetylenic Halogen.-Until recently three mechanisms have been discussed for the displacement of vinylic and acetylenic halogen (Cl Br and I); threse three mechanisms may be illustrated for the displacement of chlorine from an alkylchloroacetylene (i) Direct Displacement R-CeC-QI &R-CEC-NU 2- (ii) a-Addition and p-elimination R-C'C-CI __C R-C~C-NU 2-(iii) Attack on halogen followed by direct displacement R-C&& 2-[RCEC-+ NU-CI ] -R-CGC-NU Of these three routes the first is considered unlikely and the third viable only in special cases.A new mechanism has been described by Viehe in a lecture :log (iv) P-Addition a-elimination and 'onium rearrangement. R )=2 CI -R-CEC-NU Nu N\ c=c-1 Y lo* C. A. Grob and P. W. Schiess Angew Chem. Internat. Edn. 1967 6 1. Io9 H. G. Viehe Lecture held at the 'Symposium on Small Rings,' Louvain Belgium Sept. 1967; see also H. G. Viehe and S. Y. Delavarenne results reported in H. G. Viehe Angew Chem. Internat. Edn. 1967,6 767. Part (ii) Reaction Mechanisms For example the addition of thiophenolate ion to t-butylchloroacetylene (127) yields the sulphide (128)and has been considered to proceed via mechanism (iv). The last two steps of this mechanism ie. ar-elimination and 'onium rearrange- ment can be observed in other systems also; for instance treatment of (129) Ph S-Na* Bu'-CEC-CI ___f (I271 (I291 (130) with base gives (130) and it remains to be seen whether the phenyl residue is the migrating group ('Fritsch-Buttenberg-Wiechell reaction') or the dimethyl- amino-grouping or both groups.109 Several other questions remain to be answered:"' (i) What in general is the migratory aptitude of groups in the 'onium rearrangement? (ii) Is this rearrangement stereospecifically cis trans or nonstereospecific? (iii) Is a carbene or a carbenoid involved in the rearrange- ment? (iv) Is the new mechanism applicable to displacements in certain halo- olefins and halobenzenes? Truce and his coworkers110 have shown that in contrast to recent claims displacement of chlorine in activated vinylic halides by amines entails complete retention of configuration ;mechanistic details remain to be elucidated.Cyc1oadditions.-A novel cycloaddition principle the 1,4-dipolar cyclo- addition has been clearly exposed by Huisgen in a Since its generalisation by Huisgen in 1959 the 1,3-dipolar cycloaddition has proved to be a fruitful synthetic principle. In the past year alone some thirty papers have dealt with this reaction type and two reviews have appeared.Il3 The 110 W. E. Truce J. E. Parr and M. L. Gorbarty Chem. and Ind. 1967 660. R. Huisgen Lecture held at the 'Symposium on Heterocyclic Chemistry,' Reinhardsbrunn Thuringia DDR Oct. 1967. The lecture will appear in full in Zeitschrift fi Chemie 1968. For published examples of 1,Cdipolar cycloadditions see R.Huisgen M. Morikawa K. Herbig and E. Brunn Chem. Ber. 1967,100 1094; R. Huisgen K. Herbig and M. Morikawa ibid. p. 1107; M. Morikawa and R. Huisgen ibid. p. 1616; E. Winterfeldt and H. Radunq ibid. p. 1680; C. Szhtay and L. Novhk ibid. p. 3038; A. Gomes and M. M. Joullit Chem. Comm. 1967 935. 'I3 R. Huisgen Helu. Chim. Acta 1967,50,2421; R. Huisgen ref. l(b) p. 51. 154 H. M. R. Hofiann Woodward-Hoffmann rules allow concerted photochemical 2 + 2 cyclo-additions but thermally induced reactions should proceed stepwise (Scheme 10). In the wake of this prediction mechanistic interest in the reactions of II + /I t .n F] Or Scheme 10 electron-rich with electron-deficient olefins has been revived.In one of the first detailed investigations of such a cycloaddition Woodward and his collaborators demonstrated that the reaction between diphenylketen (131) and ethoxyacetylene (132) at -25" produced the adducts (134) and (137). The formation of (137) was elegantly rationalised via the dipolar intermediate (133) and the spiro-conjugated species (135) which was visualised to collapse to f phv EtO' Ph,C=C=O + EtOCrCH (131) (132) OEt OEt (135) (136) (l40a X=CI 1 (140b X=OAc) Part (ii) Reaction Mechanisms norcaradiene derivative (136).'l4 Shortly afterwards Huisgen and his co- workers observed a stereospecific addition of ketens onto enol ethers and proposed a concerted mechanism ;'' similar stereospecific reactions were reported by Martin et At the time of writing this Report most authors favoured a spectrum of transition states which was thought to range from a near-concerted' l7 to a stepwise' mechanism' l9 depending on the individual reaction.It should be mentioned that one may replace ketens by vinylic cations in this type of cycloaddition. For instance aluminium trichloride- catalysed trimerisation of but-2-yne yields hexamethyl Dewar benzene in several stages,12' and a similar mechanism appears to account for the forma- tion of aromatic hydrocarbons from alkynes and trifluoroacetic acid.' la For the 2 + 2 cycloaddition of l,l-dichloro-2,2-difluoroethylene to con- jugated dienes a di-radical intermediate had been implicated by the work of Bartlett and his collaborators.'" A recent development is the recognition that transition-metal catalysts may profoundly affect the mechanistic course of cycloadditions.For example the thermally induced 2 + 2 combination (or its microscopic reverse) may become concerted in the presence of metal catalysts.'22 Quadricyclane (138) isomerises smoothly to (139) below 0" in the (1411 (142) (143) presence of ca. 2 mole "/ rhodium(I) palladium(n) and platinum@) complexes ; in the absence of these metals the reaction proceeds with a half-life greater than 14 hr. at 140°.'23 The Reppe synthesis of cyclo-octatetraene (Le. the cyclisation of four molecules of acetylene in the presence of a nickel catalyst) has been considered to proceed in concerted fashion;'24 since a thermally induced metal-free 2 + 2 + 2 + 2 cycloaddition cannot be concerted the '14 J.Druey E. F. Jenny K. Schenker and R. B. Woodward Helv. Chim. Acta 1962,45 600. R. Huisgen L. Feiler and G. Binsch Angew. Chem. Znternat. Edn. 1964,3 753. 'I6 J. C. Martin V. W. Goodlett and R. D. Burpitt J. Org. Chem. 1965,30 4309. 'I' W. T.Brady and 0.H. Waters J. Org. Chem. 1967,32,3703; W. T. Brady and H. R. O'Neal ibid. p. 2704 617. R. Gompper W. Elser and H.-J. Miiller Angew. Chem. Znternat. Edn. 1967 6 453; W. E. Truce D. J. Abraham and P. Son J. Org. Chem. 1967,32,990;L. A. Paquette and M. Rosen J. Amer. Chem. SOC. 1967,89,4102; A. S. Kende Tetrahedron Letters 1967 2661. 'I9 G. Opitz Angew. Chem. Znternat. Edn. 1967,6 107; F. Effenberger and G. Kiefer ibid. p. 951 ; E.V. Dehmlow Chem Ber. 1967,100,3260; I. Fleming and M. H. Karger J. Chem. SOC.(C) 1967,226. W. Schafer and H. Hellmann Angew Chern. Znternat. Edn. 1967,6 518. P. D. Bartlett L. K. Montgomery and B. Seidel J. Arner. Chem. SOC. 1964 86 616; L. K. Montgomery K. Schueller and P. D. Bartlett ibid. p. 622; P. D. Bartlett and L. K. Montgomery ibid. p. 628; see also J. D. Roberts and C. M. Sharts Org. Reactions 1962,12 1. lZ2 F. D. Mango and J. H. Schachtschneider J. Amer. Chem. SOC. 1967,89 2484. 123 H. Hogeveen and H. C. Volger J. Amer. Chem. SOC. 1967 89 2486. G.N. Schrauzer Angew. Chem. Znternat. Edn. 1964,3 185. F H. M.R. Hoffinann transition metal has been considered to provide an orbital pathway for a one- step reaction.lZ2 Electrocyclic reactions.Cyclopropyl-ally1 cation transformations represent the most simple type of electrocyclic reactions and much further work is in accord with the Woodward-Hoffmann rules.'25 If the cyclopropyl cation is stabilised by a second cyclopropyl group as in (140a) ring-opening need not be the exclusive reaction; for example acetolysis of (140a) in the presence of silver acetate yields some 40 % unrearranged product (140b)'26 [the marked stability of the cyclopropylcarbinyl cation (44) has been mentioned above]. What appears to be the first example of the reverse reaction i.e. disrotatory closure of an allylic cation to a cyclopropyl derivative has been reported by Corey and Pirkle.' 27 Bicyclo[2,2,0]pyran-2-one (141) the major product from U.V. irradiation of 2-pyrone forms the tricyclic compound (143) when dissolved in an aprotic solvent at room temperature.Presumably carbon- oxygen fission leads to the substituted cyclobutenyl cation intermediate (142) which is stabilised by 1,3-x-intera~tion'~* and perhaps also by the bidentate interaction with the carboxylate counterion. Covalent collapse at the central carbon of the allylic system furnishes the final product (143).127 Woodward and Hoffmann predicted12' that the opening of cyclopropyl anions to allylic anions should be conrotatory if thermally induced and photochemically disrotatory. By using the isoelectronic aziridines (144) as model13' for a Ma02C C0,Me MU02C H Ar Ar Me0,C H Me0,C C02Me truns 4145 1 cis41451 lZ5 U. Schollkopf K.Fellenberger M. Patsch P. von R. Schleyer T. Su and G. W. Van Dine Tetrahedron Letters 1967 3639; M.S. Baird and C. B. Reese ibid. p. 1379; T. Ando H. Yamanaka S. Terabe A. Horike and W. Funasaka ibid. p. 1123; T. Ando H. Yamanaka and W. Funasaka ibid. p. 2587; L. Ghosez P. Laroche and G. Slinckx ibid. p. 2767; L. Ghosez G. Slinckx M. Glineur P. Hoet and P. Laroche ibid. p. 2773; W. Kutzelnigg ibid. p. 4965; G. H. Whitham and M. Wright Chem. Comm. 1967 294; S. R. Sandler J. Org. Chem. 1967,32,3876; for some possibly anomalous results see W. Kirmse and H. Schiitte,J. Amer. Chem. SOC. 1967,89 1284. lZ6 J. A. Landgrebe and L. W. Becker J. Amer. Chem. SOC.,1967,89 2505. 12' E. J. Corey and W. H. Pirkle Tetrahedron Letters 1967 5255. lZ8 See T. J. Katz and E.H. Gold J. Amer. Chem. SOC.,1964,% 1600. lZ9 R. B. Woodward and R. Hoffmann J. Amer. Chem. SOC.,1965,87 395. Part (ii) Reaction Mechanisms cyclopropyl anion Huisgen and coworker^'^ have provided experimental evidence for this prediction. On heating to loo" cis-(144) is in equilibrium with a small concentration of tran~(145)which can be trapped by efficient dipolarophiles (e.g. dimethyl acetylenedicarboxylate) before leaking into cis-(145).Similarly the azomethine ylid cis-( 145) can be generated stereospecifically from trans-( 144) while the photolytic processes are disrotatory. Opening of cyclobutene to butadiene is symmetry-allowed if conrotatory for the thermal and disrotatory for the photochemical reaction (Scheme 1 l).'" Some time ago dibenzotricyclo-octadiene (146) was reported to form dibenzocyclo- octatetraene (148)on refluxing in o-dichlorobenzene (b.p.180') for 4-5 dR Scheme II (146 1 (I47 1 (I48 1 At room temperature in tetrahydrofuran and in the presence of a molar amount of silver tetrafluoroborate the same isomerisation is complete within 10 sec. The o-xylelene derivative (147)can be trapped in the presence of maleic anhydride as the Diels-Alder adduct (149). Presumably the metal ion and (146) form a complex in which the sterically preferred disrotatory opening is now an allowed process.133 For the opening of the cyclobutenone derivative 30 That amines might not be altogether satisfactory models for carbanions is suggested by work of F. A. L. Anet R. D. Trepka and D.J.Cram J. Amer. Chem SOC.,1967,89,357; see also A. Ratajczak F. A. L. Anet and D. J. Cram ibid. p. 2072. 13' R. Huisgen W. Scheer and H. Huber J. Amer. Chem. SOC. 1967,89 1753. 13' M. Avram D. Diny G. Mateescy and C. D. Nenitzescy Chem. Ber. 1960,93 1789. lJ3 W. Merk and R. Pettit J. Amer. Chem. SOC. 1967,89,4788; see also ibid. p. 4787. H. M.R. Hofmann 0 Ph RG R' Ph M-" Ri MeOD (IS01 Ph H RTy H (150) the terms 'conrotatory' and 'disrotatory' no longer seem to be meaning- ful.' 34 Interestingly thermal and photochemical ring-opening still proceed stereoselectively as indicated; the reason for this finding is not yet clear.'34 The conversion of cis,cis-cyclo-octa-l,3-diene (151)into bicyclo[4,2,00)oct-7-ene (153) on irradiation is a reaction with ample precedent.However the same reaction can also be effected by photosensitisers; in this case (151) is isomerised to the strained cis,trans-cyclo-octa-1,3-diene (152) first which cyclises thermally to (153) in conrotatory fashion. The isomerisation of bicyclohexenyl in the presence of photosensitiser is likely to follow a similar course.'35 The isomeri- sation4' (valence tautomerism) of cycloheptatrienes and n~rcaradienes"~" 134 J. E. Baldwin and M. C. McDaniel J. Amer. Chem. Soc. 1967,89 1537. lJs R. S. H. Liu J. Amer. Chem Soc. 1967 SS,112. IJ6 (a)G. Maier Angew. Chem. Internat. Edn. 1967,6 402; (b)see also E. Ciganek J. Amer. Chem. SOC.,1967,89 1454; T. Mukai H. Kubota and T.Toda Tetrahedron Letters 1967 3581.Part (ii) Reaction Mechanisms 159 and the related processes for benzene oxide and ~xepine'~~ have been reviewed. [161Annulene (154) is conformationally highly mobile. On warming the two cisoid triene systems in (155) are closed to (157) in disrotatory outward disrotatory inward fashion ; similarly the two (photochemically induced) conrotatory closures to (158)proceed in an alternate sense.138 The thermal A A -hY W (155 1 (1571 hv ___t (156) (158) cyclisation of tetraenes to cyclo-octatrienes is a known rea~tion,'~' but the stereochemistry had not been investigated. A preliminary report suggesting a conrotatory closure for cis,cis,cis,trans-8-decadienehas now appeared.I4' Sigmatropic rearrangements. In a concerted thermal 1,3-sigmatropic shift the migrating hydrogen can establish bonding interactions with C-1 and C-3 only if the rearrangement is antarafacial (159).14' A transition state such as (159) is not readily accessible on geometric grounds and therefore such 1,3- hydrogen shifts are rare.14' If however the migrating atom is a carbon atom a suprafacial shift might be possible (160).14' Such a shift must necessarily entail inversion of configuration of the migrating carbon.'42 Some elegant experiments pertinent to these predictions have been described by Berson and his coworker^.'^^ Heating (161) in decalin solution to above 300" gives (162) in which the deuterium and acetoxy group are now both syn to each other.'The (1 61) -+(1 62) rearrangement necessarily suprafacial thus occurs with highly specific inversion of the migrating group C-7.This result is difficult to rationalise in terms of a stepwise mechanism passing over an 137 E. Vogel and H. Giinther Angew Chem. Internat. Edn. 1967,6,385. 13' G. Schroder W. Martin and J. F. M. 0th Angew. Chem Internat. Edn. 1967,6 870. 13' W. Ziegenbein Chem. Ber. 1965,98 1427; H. Meister ibid.,1963,96 1688. 140 E. N. Marvell and J. Seubert J. Amer. Chem. SOC. 1967 89 3377; however see R. Huisgen A. Dahmen and H. Huber ibid.,p. 7130 for a highly stereospecific cyclisation of three geometrically isomeric decatetraenes. R. B. Woodward and R. Hoffmann J. Amer. Chem. SOC. 1965,87,2511. J. A. Berson and G. L. Nelson J. Amer. Chem. SOC. 1967,89 5503; see also J. A.Berson and R. J. Wood ibid.,p. 1043. H. M. R. HofJinann 4fq-0 +;Ac LH,qH AcO D OAc H intermediate in which the C-7-C-1 bond is broken but no significant bonding of C-7-C-3 exists. Such a process would be expected to result in retention or randomisation of configuration. Apparently the preferred approach to the transition state is by compression of the C-2-C-1-C-7 angle and torsion about the C-5-C-6 and C-6-C-7 bonds until oppositefaces of C-7 can bond simultaneously to C-1 and C-3 as in (160). That progress along the reaction co-ordinate should consist of this complex set of motions demonstrates the predictive power of orbital symmetry consideration^."^^ Thermal 1,5-sigma- tropic shifts in medium rings have been investigated further.'43* 144 143 A.P. ter Borg H. Kloosterziel and Y. L. Westphal Rec. Trao.chim. 1967,86,474; K. W. Egger J. Amm. Chem. SOC. 1967,89 3688; J. K. Crandall and L.-H. Chang J. Org. Chem. 1967 32 532; J. K. Crandall and R. J. Watkins Tetrahedron Letters 1967 1717. 144 I thank Mr. J. B. Cresswell for his superb drawings.

ISSN:0069-3030    

DOI:10.1039/OC9676400125

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

4. PHOTOCHEMISTRY By A. C. Day (The Dyson Perrins Laboratory Oxford) RECENT general textbooks,’ volume 4 of Advances in Photochemistry and the first volume of a new review series (Organic Photochemistry) all attest to the continued rapid growth of this subject. Literature surveys have appeared for 1965 and 1966,2and during 1966-1967 there have been numerous reviews on specific topics. A wide-ranging yet particularly detailed review by Warrener and Bremner4 discusses systematically the photochemistry of unsaturated systems. It is impossible to overstress the importance of photophysical considerations in mechanistic organic photochemistry ;and relevant topics which have recently been reviewed include excited states’ and fluorescence lifetimes6 of aromatic N.J. Turro ‘Molecular Photochemistry,’ Benjamin New York 1965; J. G. Calvert and J. N. Pitts jun. ‘Photochemistry,’ Wiley New York 1966; R. 0. Kan ‘Organic Photochemistry,’ McGraw-Hill New York 1966; D. C. Neckers ‘Mechanistic Organic Photochemistry,’ Reinhold New York 1967. B. Capon M. J. Perkins and C. W. Rees ‘Organic Reaction Mechanisms 1965,”Interscience London 1966 p. 285; ibid. ‘Organic Reaction Mechanisms 1966,’ Interscience London 1967 p. 369. (a) Olefins G. J. Fonken Org. Photochem. 1967 1 197; (b) Conjugated dienes and trienes R. Srinivasan Ado. Photochelfi. 1966 4 113; (c) Polyenes M. Mousseron ibid. p. 195; (d) Intra- molecular cycloadditions of non-conjugated olefins W. L. Dilling Chem. Rev. 1966 66 373; (e) Cycloaddition reactions 0.L.Chapman and G. Lenz Org. Photochem. 1967,1,283 and R. Steinmetz Fortschr. Chem. Forsch. 1967 7 445; v) Carbonyl compounds R. B. Cundall and A. S. Davies Progr. Reaction Kinetics 1967 4 149 J. N. Pitts jun. and J. K. S. Wan in ‘The Chemistry of the Carbonyl Group,’ ed. S. Patai Wiley London 1965 p. 823 and K. Tokumaru A. Sugimori T. Akiyama and T. Nakata J. SOC. Org. Synthetic Chem. (Japan) 1966 24 1183; (9) Small-ring carbonyl compounds A. Padwa Org. Photochem. 1967,1 91 ; (h) Cyclohexenones and cyclohexa- dienones P. J. Kropp ibid. p. 1 and K. Schaffner Adu. Photochem. 1966 4 81; (i) Photo-Fries reaction :V.I. Stenberg Org. Photochem. 1967,1 127; (j)Quinones J. M. Bruce Quart. Rev. 1967 21 405; (k) Photocyclization of stilbenes F. R. Stermitz Org.Photochem 1967 1 247; (I) Photo-chromism and reversible photoisomerisation E. Fisher Fortschr. Chem. Forsch. 1967 7 605 ; (m)Troponoid compounds D. J. Pasto Org. Photochem. 1967,1,155 and K. F. Koch Adv. Alicyclic Chem. 1966,1 257; (n)Photoalkylation D. Elad Fortschr. Chem. Forsch. 1967 7 528; (0)Photo-oxbation :M. Pap ibid. p. 559; (p) Photo-oxidation :K. Gollnick and G. 0.Schenck in ‘1,CCyclo- addition Reactions,’ ed. J. Hamer Academic Press New York 1967 p. 255 and M. Niclause J. Lemaire and M. Letort Adv. Photochem. 1966,4 25; (4)Diazirines H. M. Frey ibid. p. 225; (r) Chemiluminescence F. McCapra Quart. Rev. 1966,20,485 ;R. F. Vassil’ev Progr. Reaction Kinetics 1967 4 305; (s) Nucleic acids K. L. Wierzchowski Postepy Biochem. 1967 13 127; L. Musajo Chimica e Industria 1967,49,131; (t)Flavins S.Paszyc Postepy Biochem. 1967,13,161. R. N. Warrener and J. B. Bremner Rev. Pure Appl. Chem. (Australia) 1966 16 117. W. A. Noyes jun. and I. Unger Adv. Photochem. 1966,4,49. J. B. Birks and I. H. Munro Progr. Reaction Kinetics 1967 4 239. 162 A. C. Day molecules energy transfer,' the triplet state,** and photochemistry and re- action kinetics." General.-Kearns and his co-workers have developed the phosphorescence excitation technique for the measurement of singlet -+ triplet (So + T) absorption spectra and recorded So + T spectra for aromatic hydrocarbons ketones and aldehydes. l1 The method is simple in practice highly sensitive and much less influenced by impurities than other methods of determining So-+ T spectra.Transitions involving n,n* states are less susceptible to heavy atom effects than are those involving x,n* states;12 and by use of heavy-atom solvents it is possible to distinguish between S+T,,,%* and S+K,%*transitions in the phosphorescence excitation method. Media containing heavy atoms (including xenon) also promote non-radiative S-+ T intersystem crossing,13 and Cowan and Drisko have provided a chemical demonstration of this effect. l4 In the photodimerisation of acenaph-thylene the cis-dimer is formed predominantly via an excited singlet state or excimer whereas the trans-dimer is formed uia the triplet state. Irradiation of Ph wh \ 0 h-0-co (3) (4) F. Wilkinson Adv. Photochem. 1964 3 241; Quart. Rev. 1966 20 403; R.G. Bennett and R. E. Kellogg Progr. Reaction Kinetics 1967 4 215. S. K. Lower and M. A. El-Sayed Chem. Rev. 1966,66,199. 'The Triplet State,' Proceedings of a Symposium held at the American University of Beirut 1967 ed. A. B. Zahlan G. M. Androes H. F. Hameka J. H. van der Waals F. W. Heineken C. A. Hutchinson jun. and G. W. Robinson Cambridge 1967. lo 'Photochemistry and Reaction Kinetics,' ed. P. G. Ashmore F. S. Dainton and T. M. Sugden Cambridge 1967. R. F. Borkman and D. R. Kearns Chem. Comm. 1966 446; D. R. Kearns and W. A. Case J.Amer. Chem. Soc. 1966,88,5087; A. P.Marchetti and D. R. Kearns ibid. 1967,89,768; W. Rothman and D. R. Kearns Photochem. and Photobiol. 1967,6,775. l2 M. A. El-Sayed J. Chem. Phys. 1964,41 2462. l3 A.R. Horrocks T. Medinger and F. Wilkinson Photochem. ad Photobiol. 1967,6 21. l4 D. 0.Cowan and R. L. Drisko Tetrahedron Letters 1967 1255; J. Amer. Chem. SOC. 1967 89 3068. Photochemistry R$3 R (9) a:R=H a:R=H b R = CO,H b R = CO,H acenaphthylene in solvents containing ethyl iodide or n-propyl bromide produced much greater yields of trans-dimer than were obtained in solvents containing only light atoms.14 In contrast no heavy-atom effect was detected in the dimerisation of coumarin” or the Type I1 fission of aliphatic ketones.16 These reactions both involve n,n* states and the absence of a heavy-atom effect is thereforeI2 understandable. The use of optical rotatory dispersion measurements coupled with flash photolysis has been suggested as a possible method of studying structural changes occurring on electronic excitation as well as the absolute configura- tion of excited states.Preliminary work showed that excited states of (+)-and (-)-benzoin give roughly mirror-image 0.r.d. curves.17 Full details of a colour test for compounds having short-lived triplet states have appeared.18 This is based on the photochemical valence tautomerisation of 2,3-epoxy-2,3-diphenyl-indanone (1) to the diphenylbenzopyrylium oxide (2). Excitation of the benzophenone chromophore in 3-(a-naphthyl)-5a-androstan- 17p-yl p-benzoylbenzoate (3) results in intramolecular transfer of triplet energy to the naphthalene residue which then phosphoresces. The analogous 9’-carbazolylacetate behaved similarly.1g Chemiluminescence in- ’ H.Morrison H. Curtis and T. McDowell J. Amer. Chem. SOC.,1966,88,5415; cf. H. Morrison and H. Curtis Abstr. 151st Meeting Amer. Chem. SOC. March 22 1966 K55. l6 P. J. Wagner J. Chem. Phys. 1966,452335. l7 P. A. Carapellucci H. H. Richtol and R. L. Strong J. Amer. Chem. SOC.,1967,89,1742. E. F. Ullman and W. A. Henderson jun. J. Amer. Chem. Soc. 1967,89,4390. l9 R. A. Keller and L. J. Dolby J. Amer. Chem. SOC.,1967,89,2768. * Application of the appropriate orbital symmetry rules6 to the concerted thermal reversion of an anti-4a.4b-dihydrophenanthreneto the corresponding stilbene leads to an impossible situation ! F* 164 A. C.Day volving intramolecular energy transfer has been observed by White and Roswell:20 e.g.oxidation of the luminol analogue (4) leads via the singlet excited state of the dicarboxylate di-anion to chemiluminescence at the fluorescence wavelengths of 9,lO-diphenylanthracene. Several observations by Hammond’s group demonstrate that efficient inter- molecular transfer of triplet energy requires close contact between donor and acceptor. The efficiency of benzophenones as sensitisers2 and metal complexes as quenchers22 is reduced by the. introduction of bulky alkyl groups. Asym- metric induction was found in the photosensitised cis-trans isomerisation of 1,2-diphenylcyclopropaneswith an optically active ~ensitiser.~~ Cautions have appeared concerning the use of diene~~~ and triphenylene2 in triplet-transfer studies. Care is also necessary in the general Stern-Volmer treatment of quenching processes.26 ‘Topochemical’ studies of the relationship between crystal structure and solid-state photochemistry have been extended to heterocyclic analogues of cinnamic acid2’ and to muconic acid derivatives.28 Irradiation of poly(viny1 cinnamate) produces spectral changes in the U.V.spectrum similar to those observed in the photosensitised cyclodimerisation of ethyl inna am ate.^' Two groups have used polymeric phenyl ketones as heterogeneous photosensiti~ers.~~ Comparative studies of the photochemical behaviour of molecules in solution and adsorbed on silica gel have been reported.31 Mercury-sensitised photolytic degradation coupled with gas chromato- graphy has been investigated as a tool for organic structure determinati~n.~~ Several photochemical techniques have been described.33 Olefms.”304 Two recent interesting developments relate to very simple olefins.’O E. H. White and D. F. Roswell J. Amer. Chem. SOC. 1967,89,3944. W. G. Herkstroeter L.B. Jones and G. S. Hammond J. Amer. Chem. SOC. 1966,884777. 22 A. J. Fry,R. S. H. Liu and G. S. Hammond J. Amer. Chem. SOC. 1966,88,4781. ” G. S. Hammond and R. S. Cole J. Amer. Chem. SOC. 1965,87,3256. 24 L. M. Stephenson D. G. Whitten G. F. Vesley and G. S. Hammond J. Amer. Chem SOC. 1966,88,3665,3893; S. D. Andrews and A. C. Day Chem. Comm. 1967,477. ’’ W. M. Hardham and G. S. Hammond J. Amer. Chem. SOC.,1967,89,3200 footnote 27. 26 R. J. Campbell E. W. Schlag and B. W. Ristow J. Amer. Chem. SOC.1967 89 5098; P. J. Wagner ibid. p. 5715. ” M. Lahav and G. M. J. Schmidt J. Chem. SOC. (B) 1967,239; cf. J. Rennert E. M. Ruggiero and J. Rapp Photochem. and Photobiol. 1967,6,29. ” M. Lahav and G. M. J. Schmidt J. Chem. SOC. (B) 1967,312. ’9 H. G. Curme C. C. Natale and D. J. Kelley J. Phys. Chem. 1967,71,767. 30 R. Searle J. L. R. Williams J. C. Doty D. E. deMeyer S. H. Merrill and T. M. Laakso Makromol. Chem. 1967,107,246; P. A. Leermakers and F. C. James J. Org. Chem 1967,32,2898. 31 J. L. Ruhlen and P. A. Leermakers J.Amer. Chem. SOC. 1967,89,4944; T. R. Evans A. F. Toth and P. A. Leermakers ibid. p. 5060; C. Balny and P. Douzou Compt. rend. 1967,264 C 417. 32 R. S.Juvet jun. R. L. Tanner and J. C. Y.Tsao J. Gas Chromatog. 1967,5 15. 33 S.D.Cohen M. V. Mijovic G. A. Newman and E. Pitts Chem. and Ind. 1967,1079; D. Bryce- Smith J. A. Frost and A. Gilbert Nature 1967,213,1121; D. A. Warwick and C. J. H. Wells J. Sci. Instr. 1967 44 483; J. H. Allen and J. F. McKellar Lab. Practice 1967 16 991. * The phenomenology of organic photochemistry presents some difficulty and it has been decided to systematise this year’s Report according to compound rather than reaction type. This naturally leads to some dispersal of closely related material e.g. on cycloadditions. Cross-referencing within the compound divisions will it is hoped mitigate this situation somewhat. The ‘orthogonal’ classi- fication into reaction types presents at least asmany problems. Photochemistry 165 1- Alkylcyclohexenes and 1-methylcycloheptene undergo a light-induced ionic reaction in alcohols containing aromatic sensitisers such as benzene toluene or ~ylene.~~’ 35 Products are the corresponding exocyclic olefin and tertiary ether formed by Markovnikov addition across the double bond.For example menthene (5)irradiated in 0-deuteriomethanol containing benzene gave the deuteriated olefin (6)and stereoisomeric ethers (7).34In two cases,349 36 molecular rearrangements characteristic of cationic species were observed and an ionic mechanism involving protonation has been proposed. 34 Reaction is very slow with 1-methylcyclopentene and is not observed at all with 1- methylcyclo-octene acyclic or highly strained 01efins.~~’ In methanol containing xylene norbornene gave only dimers and products characteristic of radical reactions e.g.(8).37 From the premise that an olefinic n,n* triplet prefers to exist in the orthogonal conformation Kr~pp~~. 37 has rationalised the photochemical behaviour of cyclic olefins. Triplets of larger-ring cyclic olefins and acyclic and exocyclic olefins can decay to cis-or trans-olefin with an efficiency which precludes alternative processes. In cyclohexenes and cyclo- heptenes the orthogonal triplet (or derived trans-olefin) relieves excessive strain mainly by protonation although hydrogen-atom abstraction becomes possible in an aprotic solvent.34 In strained olefins the orthogonal conformation is impossible and the triplet state may therefore be sufficiently long-lived or energetic to take part in intermolecular processes of a radical nature.Cyclo- pentenes are apparently a borderline case between the last two categories. Formally analogous reactions have been observed with phenols ;3 mechanistic-ally though these reactions probably involve the well known enhancement of phenol acidity in the singlet excited state. In favour of this explanation is the close similarity in isomer distribution between products from photocyclisa- tions and ‘dark’ acid-catalysed reactions of o-allyl- and o-b~t-3’-enylphenols.~~~ Irradiation of tetramethylethylene for long periods with unfiltered light from a medium pressure mercury lamp gives octamethylcyclobutane in reasonable yield.39 Reaction involves the n,n*singlet state for which dimerisa- tion appears to be fast enough to compete with other deactivation processes (e-g.,intersystem crossing).This is in marked contrast to the n,n*triplet situation in simple olefins where deactivation is so rapid that dimerisation cannot com- pete. In fact sensitised dimerisation is commonly only observed in cases of exceptional strain (cf. ref. 37) e.g. 1,2,3-triphenyl~yclopropene,~~ bridged and intramolecular ‘caging’ processes.41b The dimerisation of 34 P. J. Kropp J. Amer. Chem. SOC. 1966 88 4090; P. J. Kropp and H. J. Krauss ibid. 1967 89 5199. 35 J. A. Marshall and R. D. Carroll J. Amer. Chem. SOC.,1966,88,4092. 36 J. A. Marshall and A. R. Hochstetler Chem. Comm. 1967 732. ” P. J. Kropp J. Amer. Chem. SOC.,1967,89,3650. 38 (a) W. M. Horspool and P. L. Pauson Chem.Comm. 1967 195; (b)G. Frhter and H. Schmid Helv. Chim. Acta 1967 50,255. 39 D. R. Arnold and V.Y. Abraitys Chem. Comm. 1967,1053. 40 C. Deboer and R. Breslow Tetrahedron Letters 1967,1033 ;H. Diirr ibid. p. 1649. *’ Znter ah (a) H. D. Scharf Tetrahedron 1967 23 3057; (b) C. G. Chin H. W. Cuts and S. Masamune Chem. Comm. 1966 880; J. C. Barborak and R. Pettit J. Amer. Chem. SOC. 1967,89 3080. 166 A. C.Day tetramethylethylene is of some significance since examples involving simple non-conjugated olefins are rare.42 As a (2 + 2)7t type cycloaddition it is photo- chemically allowed on orbital symmetry grounds.43 The tetracyclononene (9a) undergoes ‘caging’ to the saturated compound (1Oa) on direct photolysis in ether.44 This internal cycloaddition of an ethylenic bond to a cyclopropane must involve singlet states since triplet sensitisation with acetone gave only dimers( 1 l) together with an acetone-addition pr~duct.~’ The carbonyl-containing analogue (9b) gives the ‘cage’ isomer ( Analogues of the photochemical valenq isomerisation of bicyclo[2,2,l]hepta- diene to q~adricyclene~’ have been described.48 The reaction of the (less strained) cyclohexa-1,4-diene (1 2) follows a different course giving the isomer (13) presumably by the sequence shown.49 This rearrangement which bears a formal resemblance to some light-induced reactions of cycl~hexadienones,~” is one example of a general photochemical reaction in which a divinylmethane is converted into a vinylcycl~propane.~~ Most of the examples listed by Zimmerman et a1.” are relatively complex but its seems possible that the mercury-photosensitised rearrangement of acyclic 1,4-dienes to vinylcyclo- propanes’ proceeds similarly.An example is the rearrangement of 3,3-dimethylpenta-l,6diene (14) to the cyclopropane (16) and isomeric diene (15); (1 5) is also convertible into (16) on sensitised irradiation. Meinwald and Smith have suggested a homolysis-recombination mechanism for reactions of this kind.’ la Other products of mercury-sensitised rearrangement of 1,4-dienes are bicyclo[2,1,0]cyclopentanes5 and bicycle[ l,l,l]pentanes.’ lb Srinivasan and Carlough’ lb studied the sensitised photolysis of several 1,4- 1,5- and 1,6-dienes and obtained in all cases except cyclo-octa-1,5-diene (vide inpa) a mixture of two internal adducts a bicyclo[n,2,0]alkane (‘parallel adduct’) and a bicyclo[n 1 llalkane (‘crossed adduct’).The crossed adduct :-parallel adduct ratio was found to be dependent on chain length values of 0.10 233 and 0-04 being found for penta-1,4-diene (17) hexa-1,5-diene (18) and hepta-1,6- diene (19) respectively. To explain the selectivity of these reactions Srinivasan and Carlough suggested that cycloaddition is a two-step process and that the 42 G. S. Hammond N. J. Turro and A. Fischer J. Amer. Chem. SOC.,1961,83,4674;J. R. Chesick ibid. 1963,85 3718. 43 R. Hoffmann and R. B. Woodward J. Amer. Chem. SOC. 1965,87,2046. 44 E. Wiskott and P. von R. Schleyer Angew. Chem. 1967,79,680 (Angew. Chem. Internat. Edn. 1967,6 694).4s H.-D. Scharf and G. Weisgerber Tetrahedron Letters 1967 1567. 46 C. F. Huebner E. Donoghue L. Dorfman E. Wenkert W. E. Streth and S. W. Donely Chem. Comm. 1966,419; P. K. Freeman and D. M. Balls J. Org. Chern. 1967,3& 2354; H. Prinzbach and D. Hunkler Angew. Chem. 1967,79,232 (Angew. Chem. Internat. Edn. 1967,6,247). 47 G. S. Hammond P. Wyatt C. D. DeBoer and N. J. Turro J. Amer. Chem. SOC.,1964,86,2532. 48 E. Payo L. Cortts J. Mantech C. Rivas and G. de Pinto Tetrahedron Letters 1967 2415; H. Prinzbach and J. Rivier ibid. p. 3713. 49 W. Reusch and D. W. Frey Tetrahedron Letters 1967 5193. H. E. Zimmerman R. W. Binkley R. S. Givens and M. A. Sherwin J. Amer. Chem. SOC. 1967,89 3932 and refs. therein cited. (a) J. Meinwald and G. W.Smith J. Amer. Chern. SOC.,1967,89 4923; (b) R. Srinivasan and K. H. Carlough ibid. p. 4932. 167 Photochemistry c -a:-a preferred initial step is formation of afiue-membered ring cf. (20) (21) and (22). The second step then gives the preferred adducts (23) (24) and (25). Formation of the alternative adducts cannot involve biradicals containing a five-membered ring. Cyclo-octa-l,5-diene (26) gives solely ( >97 %) the crossed adduct (27),”’ via a biradical analogous to (21).52 Internal cycloadditions of acyclic trienes have been discussed in similar terms by Hammond and L~u.~~ Photoisomerisa-tion of o-divinylbenzene (28) gives (29) not the expected bicyclo[2,1,1]- compound (30).Deuterium-labelling studies showed that the reaction involves a rearrangement of the carbon skeleton rather than hydrogen rnigrati~n.’~ A different type of behaviour is shown by the 1,5-diene (31).Photolysis at 254 52 I. Haller and R. Srinivasan J. Amer. Chem. SOC.,1966,88 5084;cf. J. E.Baldwin and R. H. Greeley ibid.,1965,87 4514. 53 R.S. H. Liu and G. S. Hammond J. Amer. Chem. SOC.,1967,89,4936. 54 M. Pomerantz J. Amer. Chem. SOC.,1967,89,694;J. Meinwald and P. H. Mazzocchi ibid. p. 696. 168 A. C. Day mp gives the cis-trans isomers (32) and (33). The position of the deuterium in the products rules out a Cope rearrangement and the reaction appears to involve a concerted 1,3-shift of the 3-deuterioallyl group without allylic in~ersion.~~ Numerous electro~yclic~~ reactions involving conjugated olefins have appeared.The highly strained cis-trans-diene (34) is an intermediate in the photosensitised conversion of cis-cis-cyclo-octa-l,3-diene(35)into the bicyclic compound (36) in boiling benzene. The cis-trans-diene (34) can be isolated in 85 % yield from photolyses conducted at a lower temperature and it is quanti- tatively converted into (36) at 80” by a thermally all~wed,’~ conrotatory (30) (28) H (35) (37) (38) ’’ R.F. C.Brown R C. Cookson and J. Hudec Chem. Comm. 1967,823. 56 R. B. Woodward and R. Hoffman J. Amer. Chem. SOL 1965,87,395; H.C.Longuet-Higgins and E.W. Abrahamson ibid. p. 2045. Photochemistry 169 process.57cis-cis-Octa- 1,3,5,7-tetraene has been detected in the flash photolysis of cyclo-octa-1,3,5-triene in solution.58" In previous (preparative) studies only the bicyclic and tricyclic isomers (37) and (38) had been obtained.58b Cyclohexa- 1,3-diene photoisomerises initially to hexa-1,3,5-triene ; prolonged photolysis leads to a 1 :1 mixture of bicyclo[3,1,0]hex-2-ene (39) and 3-vinyl~yclobutene.~~" The cyclohexadiene-+bicyclo[3,l,O]hexene conversion which seems to be rather common,4* 6o has been discussed in terms of several mechanisms desig- nated by Meinwald and MazzocchiSb paths a b and c(see formulae).Photolysis of the labelled triene (40) gave the bicyclohexene (41) labelled as shown. Path c can therefore be eliminated since it would have given uiu the symmetrical intermediate (42),both (41) and the isomer with deuterium and the indicated hydrogen inter~hanged.~ Two photoisomerisations of cycloheptatriene are 9b known valence tautomerisation to bicyclo[3,2,0]hepta-2,6-diene (43) and a 1,7-shift of hydrogen.Molecular orbital considerations show that the 1,7-shift is allowed in the first excited state.61* 62 A similar 1,7-shift of an alkyl group has been observed in the photolysis of the trimethylcycloheptatriene(44) which Q '' R. S. H. Liu J. Amer. Chem. SOC.,1967,89 112. (a) T. D. Goldfarb and L. Lindqvist J. Amer. Chem. SOC.,1967,89,4588 ;(b) 0.L. Chapman G. W. Borden R. W. King and B. Winkler J. Amer. Chem. SOC.,1964,86 2660; W. R. Roth and B. Peltzer Angew. Chem. Internat. Edn. 1964 3 440; 1.Zirner and S. Winstein Proc. Chem. SOC. 1964,235.59 J. Meinwald and P. H. Mazxocchi J. Amer. Chem. SOC.(a) 1966,88,2850; (b)1967,89 1755. 6o W. G. Dauben and J. H. Smith J. Urg. Chem. 1967,32,3244. 61 R. B. Woodward and R. Hoffmann J. Amer. Chem. SOC.,1965,87,2511. G. W. Borden 0.L. Chapman R. Swindell and T. Tezuka J. Amer. Chem. Soc. 1967,89,2979. 170 A. C. Day gives (45)and (46)as primary photo product^.^^ In 7-substituted cyclohepta- trienes the relative importance of the two processes depends on the nature of the substituent.62* 64 Substituent dependence is also shown in the photo- chemistry of 7,8-dimethylenecyclo-octa-1,3,5-trienes(47). The unsubstituted compound (47; R = H)gave (48),whilst the chloro-compound (47;R = C1) gave (49)and (50) i.e. chlorine deactivates the em-diene and the compound behaves like a conjugated cyclo-~ctatriene.~~ Intriguing thermal and photochemical inter-relationships exist between a series of compounds of general formula (CH),, e.g.bullvalene (51),the 9,lO-dihydronaphthalenes (52) and (53) and Nenitzescu's hydrocarbon (54).The formulae indicate those interconversions which seem at the present time to be definitely established.66 The tetracyclic compound (59,a conceivable inter- mediate in some of these transformations still resists detection. The identifica- tion of the (4n + 2)-hydrocarbon cyclodecapentaene (56),seems secure but its geometry is as yet unknown.67 The photochemistry of the (CH) series is not yet so rich in detail. Barrelene (57) undergoes photosensitised isomerisation to semibullvalene (58) and cyclo-octatetraene.68 Studies" with deuteriated barrelene show that the conversion of (57) into (58)involves the divinylmethane- vinylcyclopropane rearrangement referred to above.Photoadditions across the ethylenic bond have been reported with cyclic ethers,69 f~rmamide,~' ~hloramine,'~~ nitro soar nine^,^' N-chl~rourethane,~~" and haloacetic acids.73 Most of these seem to be straightforward radical reactions some of synthetic value ;and rather similar reactions with saturated compounds have been described74 (cf. refs. 3n and 30). 63 L. B. Jones and V. K. Jones J. Amer. Chem. SOC. 1967,89 1880. 64 A. P. ter Borg E. Razenberg and H. Kloosterziel Chem. Comm. 1967 1210. 65 J. A. Elix M. V. Sargent and F. Sondheimer 1. Amer. Chem. SOC.1967 89 180 5081; cf. ref. 58b. 66 M. Jones jun. J. Amer. Chem. SOC. 1967 89 4236 and refs. therein cited; G. Schroder and J. F. M. Oth Angew. Chem. Internat. Edn. 1967,6,414. 67 E. E. van Tamelen and T. L. Burkoth J. Amer. Chem. SOC. 1967,89 151. 68 H. E. Zimmerman and G. L. Grunwald,J. Amer. Chem. SOC. 1966,88 183; cf.,J. P. N. Brewer and H. Heaney Chem. Comm. 1967,811. 69 I. Rosenthal and D. Elad Tetrahedron 1967,23 3193. 70 J. Rokach and D. Elad J. Org. Chem. 1966,31 4210; cf. M. Pfau and R. Dulou Bull. SOC. chim. France 1967,3336. 71 Y. L. Chow C. Colon and S. C. Chen J. Org. Chem. 1967,32,2109. 72 (a) K. Schrage Tetrahedron Letters 1966 5795; Tetrahedron 1967 23 3033; (b) Y. Ogata Y. Izawa and H. Tomioka Tetrahedron 1967,23 1509. 73 N. Kharasch P.Lewis and R. K. Sharma Chem. Comm.,1967,435. 74 R. C. Cookson J. Hudec and N. A. Mirza Chem Comm.,1967,824; C. Pac and S. Tsutsumi Bull. Chem. SOC. Japan 1966,39 1926; Y. Shigemitsu T. Tominaga T. Shimodaira Y. Odaira and S. Tsutsumi ibid. p. 2463. Photochemistry Y = H; Z = C1 orY = C1:Z = H (52) z (51) A A '"\ A t (53) (55) (57) (58) Photocycloadditions involving olefins are discussed in the sections on car- bony1 compounds aromatic compounds and quinones. Carbonyl Compounds. 3f-h.-The photochemistry of very simple carbonyl compounds many in the vapour phase has received attention.75 It has been '' (a) Inter al. B. A. Degraff and J. G. Calvert J. Amer. Chem. SOC.,1967,89,2247; P. J. Wagner ibid. 1966,88 5672; J.C. W. Chien ibid. 1967,89 1275; E. K. C. Lee J. Phys. Chem. 1967,71,2804; E. K. C. Lee and N. W. Lee ibid. p. 1167; R. E. Rebbert and P. Ausloos J. Amer. Chem. SOC. 1967 89 1573; M. J. Yee Quee and J. C. J. Thynne Trans. Faraday SOC. 1967 63 1656; 1966,62 3154; (b)P. J. Wagner J. Amer. Chem. SOC. 1967,89 2503; Tetrahedron Letters 1967 1753. 172 A. C.Day suggested that the Type-I1 photofragmentation of ketones may be a concerted six-centre process in the singlet-excited state but a two-step process initiated by abstraction of y-hydrogen in the triplet state.75b* 76 The vapour-phase photolysis of a series of ketones n-C,H COR has been studied correlations between structure and efficiency of the Type I1 fission were found.76 Ketones are photoreduced by amine~~~ Contrary and trib~tylstannane.~’~ to e~pectation,~~ singlet-excited acetone is less reactive in hydrogen abstraction than the triplet by a factor of 1000.75b Benzophenones with an o-alkyl substituent give photoenols on irradiation.Further examples have been reported this year.’’ Enolic species of a different kind e.g. ‘isobenzpinacol’ (59) have been detected spectroscopically in the photolysis of benzophenone in isopropyl alcohol and other hydrogen-donating solvents.79 (60) %c 76 C. H. Nicol and J. G. Calvert J. Amer. Chem. SOC.,1%7,89,1790. 77 S. G. Cohen and J. I. Cohen J. Amer. Chem. SOC.,1967 89 164; S. G. Cohen and R. J. Baumgarten ibid. p. 3471. 78 P. J. Wagner and G. S. Hammond J. Amer. Chem. SOC.1966,88 1245. 79 G. 0. Schenck M. Cziesla K. Eppinger G. Matthias and M. Pap Tetrahedron Letters 1967,193. Photochemistry 173 The photoisomerisation of ketones containing a cyclopropane ring con- jugated with carbonyl to ap-unsaturated ketones involves specific breakage of that cyclopropane bond which overlaps best with the carbonyl x-bond. Thus Dauben and Shaffer have found that in solution n,x* excitation of a series of alkylated bicyclo[4,1,0]heptan-2-ones results in fission of the 1-7 bond (60)-*(61)-*(62). In no case was 1-6 cleavage observed." When as in (63) both ap-bonds in the cyclopropane ring overlap equally with carbonyl cleavage gives the more highly substituted biradical (64).809 Cyclopropane ring opening however only competes with cleavage of the 2-3 bond [cf.(60)] when C-3 is unsubstituted or C-7 is substituted.80 A similar but slightly less specific cyclopropane cleavage has been observed for bicyclo[3,1 ,O] hexan-2- one in the vapour phase.82 An elegant labelling study by Beugelmans defines the stereochemistry of rearrangement of a 3,5-cyclosteroidal ketone (65). Photolysis gave a mixture of the two unsaturated ketones (66) and (67) by respectively 4a+3 hydrogen and 4p+5 deuterium shifts.83 A discussion of the U.V. spectra of cyclopropyl ketones which is of relevance to the foregoing has been given by Dauben and Bere~in.'~ The photochemical rearrangement of ap-epoxy-ketones to P-diketonesy38 (68)-+(70) is characterised by an unusual order of migratory aptitudes of p groups (Ph,CH and PhCHz > H > RCH > Me % Ph).85 The initial cleavage of the C-0 bond to (69) resembles the cyclopropane cleavage already discussed.Recent studies'' seem to favour a concerted mechanism for the rearrangement of (69) to (70) rather than two-step fragmentation in- volving a caged-radical pair (71) although there is some evidence to suggest that (69) may be diverted to (71) at least in part when the group p can form a particularly stable radical.85" The fission of ap-bonds in cyclopropyl and epoxy- ketones has a formal analogy in the x*-assisted elimination of the substituent in a-substituted ketones as (72).86 For the photolysis of py-epoxy ketones see Padwa et 'O W. G. Dauben and G. W. Shaffer Tetrahedron Letters 1967,4415. R.E. K. Winter and R. F. Lindauer Tetrahedron Letters 1967,2345. '' L. D. Hess and J. N. Pitts jun. J. Amer. Chem. SOC.,1967,89,1973; L. D. Hess J. L. Jacobson K. Schaffner and J. N. Pitts jun. ibid. p. 3684. " R. Beugelmans Bull. SOC.chim. France 1967,244. 84 W. G. Dauben and G. H. Berezin J. Amer. Chem. SOC.,1967,89 3449. 85 (a) C. S. Markos and W. Reusch J. Amer. Chem. SOC. 1967 89 3363; (6) refs. cited in (a) especially H. Wehrli C. Lehmann P. Keller J. J. Bonet K. Schaffner and 0.Jeger Helu. Chim. Acta 1966,49 2218. 86 Literature cited in ref. 82 and C. L. McIntosh P. de Mayo and R. W. Yip Tetrahedron Letters 1967,37; Y. Saburi K. Minami and T. Yoshimoto J. Chem. SOC. Japan 1967,88,557. '' A. Padwa D. Crumiine,R. Hartman and R. Layton J. Amer.Chem SOC. 1967,89,4435. 174 A. C.Day Photolysis of cyclobutanone in the vapour phase involves Type I fission to the biradical CH CH CH CO which may fragment to keten and ethylene or lose carbon monoxide. Decarbonylation leads to cyclopropane in a vibra- tionally excited state which may be collisionally deactivated or rearrange to propene.” A third reaction which becomes competitive with these processes in solution is cyclisation without decarbonylation to give an oxacarbene. Thus 2,2-dimethylcyclobutanone(73) photolysed in methanol gives (75) via (74).” The process is structurally specific cleavage only occurring at the more highly substituted a-bond. The importance of the oxacarbene pathway relative to keten and cyclopropane formation increases with increasing substitution at the a-carbon atoms.” An attempt by Hostettler to trap with olefins an oxacarbene in the photolysis of 2,2,4,4-tetramethylcyclobutanonesfailed perhaps because of steric hindrance,” but an analogous oxacarbene (76) from benzocyclobutenedione did undergo carbenoid addition to olefins.” The reaction is not confined to cyclobutanones for the strained ( +)-cyclocampha-none (77) in cyclohexene gave the adduct (78).’ An oxacarbene may also be involved in the photolysis of ( f)-fenchone (79) in aqueous ethanol to give (80).93 Photolysis of 3-methylenecyclobutanone in a glass at -196” gave triplet *’ H.0.Denschlag and E. K. C. Lee J. Amer. Chern. SOC. 1967,89 4795; R. J. Campbell and E. W. Schlag ibid. p.5103. 89 N. J. Turro and R. M. Southam Tetrahedron Letters 1967,545. H. U. Hostettler,Helu. Chim. Actu 1966,49 2417. 91 H. A. Staab and J. Ipaktschi Tetrahedron Letters 1966,583. 92 P. Yates and L. Kilmurry J. Amer. Chem. SOC.,1966,88,1563. 93 P. Yates and A. G. Fallis Tetrahedron Letters 1967 4621 ;cf. G. E. Gream J. C. Paice and C. C. R. Ramsay Austral. J. Chem. 1967,20,1671. Photochemistry 60 (79) @PhPh Ph4 (86) Ph a:R=Me h:R = Ph R (88) (89) (a Ar = p-C6H4.CN) (b Ar = p-C6H4*OMe) trimethylenemethane identified by its e.s.r. spectrum.94 Stereospecific transfer of the em-hydrogen atom occurs in the photolysis of carvone camphor in methanol e.g. (81)+(82).95 A heavy-atom effect (see General Section) has been observed in a study of the photolysis of non-enolisable ~-diketonesg6-somewhat surprising in a reaction which probably involves n,n* states.Mesityl oxide is per se photostable but when irradiated with 1849 8 light in methanol or isopropyl alcohol it gives several products in a reaction which is probably initiated by photodecomposition of the Vinylic esters derived from dimedone undergo photohydrolysis in an aqueous medium probably by an initial hydration of the ethylenic bond.98 The stereochemistry of addition of alcohols to ap-unsaturated ketones to give P-alkoxy-ketones has been studied.99 94 P. Dowd and K. Sachdev J. Amer. Chem. SOC. 1961,89,715. 95 J. Meinwald R. A. Schneider and A. F. Thomas J. Amer. Chem. SOC.,1967,89,70. 96 H.Nozaki Z. Yamaguti T.Okada R. Noyori and M. Kawanisi Tetrahedron 1967,23,3993. ’’)N.C.Yang and Do-Minh Thap J. Org. Chem. 1967,32,2462. ’* P.de Mayo and J. S. Wasson Chem. Comni..1967,970. 99 B.J. Ramey and P. D. Gardner J. Amer. Chem. SOC.,1967,89,3949. 176 A. C. Day Bridged py-unsaturated ketones (83) are isomerised by light to the bicyclic cyclobutanones (84). loo Cyclo-oct-4-enone undergoes photoinduced cleavage of the 2-3-bond to give the biradical (85) which subsequently recyclises to form 3-vinylcyclohexanone as major product. l The cleavage is unprecedented in solution photochemistry-though reminiscent of the z*-assisted cleavages noted above86-and is probably a conformational effect. The interest in cyclo hexenones and cross-conj ugated cyclo hexadienone~~~ continues unabated.The 'type A' rearrangement of cyclohexenones exemplified by the conversion of 4,4-dimethylcyclohexenone(86a) into (87) is not followed by the 4,4-diphenyl compound (86b) which gives instead the stereoisomeric products (88) formed by phenyl migration."' 'Type A' rearrangement though predominates in the rearrangement of 3,5-diphenylcyclohexenone(89). The difference in behaviour between (86b) and (89) was ascribed to the ability of the phenyl groups to stabilise different intermediates in the two cases though steric factors may also be important."' Both p-methoxyphenyl and p-cyano- phenyl migrate in preference to phenyl in the photolysis of the 4,4-diaryl- cyclohexenones (90a) and (gob) which suggests that C-C=C-0 is more helpful than the dipolar form C-C=C-O-as a guide in predicting reactivity + in this series.lo4 The cyclohexadienone (91) is converted by light into the bicycle[3,l ,O] hexenone (92) without aryl migration.Irradiation of the bicyclo- hexenone (92) gives a mixture of the isomeric phenols (93a) and (93b) i.e. phenyl migrates in preference to p-cyanophenyl. O5 Preferential migration of phenyl with (92) provides a sharp contrast to the case of cyclohexenone (90a)lo4 above. Different chemical behaviour is of course to be expected of electronically excited species and the corresponding ground-state intermediates and Zimmerman OH Ph Ar 6 (93b) (91) loo W. F. Erman and H. C. Kretschmar J. Amer. Chem. SOC. 1967,89,3842. lo' K. J. Crandall J. P.Arrington and R. J. Watkins Chem. Comm. 1967 1052. lo2 H. E. Zimmerman and J. W. Wilson J. Amer. Chem. SOC.,1964,86,4036. H. E. Zimmerman and D. J. Sam J. Amer. Chem. SOC. 1966,88,4905. lo* H. E. Zimmerman R. D. Rieke and J. R. Scheffer,J. Amer. Chem. SOC. 1967,89,2033. H. E. Zimmerman and J. 0.Grunewald J. Amer. Chem. Soc. 1967,89,5163. Photochemistry has suggested that the excited state of (92) suffers electron demotion to aground- state zwitterion (94) before migration. In the rearrangement of the cyclo- hexenones (go) aryl migration might precede electron demotion and thus involve species of essentially biradical as opposed to ionic character.lo5 The zwitterionic intermediate (95) supposed to be implicated in the photochemical rearrangement of 4,4-diphenylcyclohexadienone to the corresponding bi-cyclo[3,l,0]hexenone has been generated non-photochemically.It rearranged spontaneously to the expected product.lo6 As hydrocarbon analogues of cyclohexenones and cyclohexadienones the exo-methylene compounds (96) and (97) have been investigated photochemically by Zimmerman and his co-workers. Like the ketone (86b) (96) rearranged with phenyl migration giving (98).'07" The triene (97) underwent a similar phenyl shift to give (stereo- selectively) the product (99) and not the product of 'type A' rearrangement which might have been expected by analogy with the behaviour of 4,4-diphenyl-cyclohe~adienone.'~~~ Of course the analogy is rather a superficial one for the ketones react through their n,n* triplets whilst n,n* singlets were shown to be involved in the photochemistry of the hydrocarbons (96) and (97).lo7 The photochemistry of cyclohexadienones containing t-butyl groups displays some new features attributable to steric effects.lo8 The representation of carbonyl n,n* states has been discussed by Taylor who has argued for a modified convention (100) and discussed its applica- bility to cyclohexadienones.logThe mnemonic value of representations such as (100) in stressing correspondences between mass spectra and photochemical processes'l0 is obvious; but it is not clear to the Reporter that they offer any &h R -+-R,c=o .- Ph hh (100) x+Y (98) B- xQY Hz (99) A- MeOH-"Gy' \ / z z (104) (101) (102) (103) lo6 H. E. Zimmerman D.Dopp and P. S. Huyffer J. Amer. Chem. SOC.,1966,%8,5352. lo' (a) H. E. Zimmerman and G. E. Samuelson J. Amer. Chem. SOC. 1967 89 5971; (b) H. E. Zimmerman P. Hackett D. F. Juers and B. Schroder ibid. p. 5973. lo* B. Miller and H. Margulies J. Amer. Chem. SOC. 1967 89 1678; B. Miller ibid. p. 1690; T. Matsuura and K. Ogura ibid. pp. 3846 3850. log G. A. Taylor Chem. Comm. 1967,896. 'lo Inter alia N. J. Turro,D. S. Weiss W. F. Haddon and F. W. McLafferty J. Amer. Chem. SOC. 1967 89 3370; A. L. Burlingame C. Fenselau W. J. Richter W. G. Dauben G. W. Shaffer and N. D. Vietmeyer ibid. p. 3346; C. Djerassi and B. Zeeh Chem. and Ind. 1967,358; M. M. Bursey L. R. Dusold and A. Padwa Tetrahedron Letters 1967,2649. 178 A. C.Day advantage in interpretative or predictive power over other valence-bond representations of photochemical mechanisms (e.g.cf. refs. 102-107). Two photochemical paths are available to conjugated cyclohexadienones (A) reversible ring-opening to a cis-keten which may then react with a protic solvent and (B) rearrangement to a bicyclo[3,1,0]hexenone.11' Collins and Hart studied a series of methyl-substituted cyclohexadienones (101 ; X Y Z = H or Me) and found the course of photolysis to be markedly dependent on the number and position of the methyl groups e.g. (101 ; X = Y = Z = Me) follows path B giving(l04; X = Y = 2 = Me),112 (101; X = Z = H Y = Me) follows path A and (101 ; X = Z = Me; Y = H) reacts by both pathways. It was further shown that the cis-ketens (102) react with methanol to give cis-fly :&-unsaturated esters (103) which may subsequently photoisomerise to the corresponding trans-isomers.There was no evidence for cis-trans isomerisation of the ketens (102) under the reaction conditions."' .".c;. * EtO CN (109) (110) The photodimerisations of cyclopentenone' and cyclohexenone1'4 occur by triplet mechanisms. The relative proportions of head-to-head and head-to- tail dimer in both cases are subject to 'polar solvent effe~ts'.''~~"~ The photocycloaddition of cyclopentenone to cyclohexene may involve a higher triplet state having ET-73 kcal./mole (ETfor the lowest triplet is ca. 61 kcal./mole).' ' The photochemistry of troponoid compounds is discussed in a separate section. Since late 1966 most of the aliphatic photocyc1oadditions3' reported between unlike molecules have been of a preparative nature and embody no new fundamental advance.Oxetan formation occurs in the light-induced reactions of olefins with the P. M. Collins and H. Hart J. Chem. SOC.(C),1967 1197 and refs. therein cited. 'I2 cf. H. Hart and R. K. Murray,jun. J. Org. Chem. 1967,32,2448. 'I3 P. E. Eaton and W. S. Hurt J. Amer. Chem. SOC.,1966,88 5038; J. L. Ruhlen and P. A. Leer-makers ibid. p. 5671. E. Y. Y. Lam D. Valentine and G. S. Hammond J. Amer. Chem. SOC. 1967,89 3482. 'I5 P. de Mayo J.-P. Pete and M. Tchir J. Amer. Chem. SOC.,1967,89 5712. Photochemistry 179 carbonyl groups of benzophenone,' diethyl mesoxalate,' ' ethyl cyano- formate [to give with for example 1,l-diphenylethylene the oxetan (lOS)],' '* acetyl cyanide,' l9 and ap-acetylenic ketones.' 2o The last case contrasts with that of ap-ethylenic ketones which usually react at the carbon-carbon double bond to give cyclobutanes.In all of these cases the orientation of addition to an unsymmetrical olefin is that expected for an initial attack by carbonyl oxygen (n~* triplet) in such a way as to give the more stable biradical. Nucleo- philic attack by carbonyl in the n,~*singlet state seems to be implicated in the oxetan formation between ketones and trans-l,2-dicyanoethyleneor maleic anhydride.' 21 Ketones give spiro-oxetans with allenes ; e.g. acetone gives two 2:1 adducts (106) and (107) with tetramethylallene.'22 Ketenimines give 1-imino-oxetans with aromatic ketones.123 Cyclobutane formation has been reported between various components maleic anhydride with olefins' 24 and dienes,' 25 acrylonitrile with indene,'26 and chromone with 01efins.'~~ Butadiene gives only cyclobutanes (108) with or-acetoxyacrylonitrile on unsensitised irradiation but additionally the 1,4-adduct (109) on photosensitisation.' 28 Light-induced cycloadditions to 3-acetoxycyclopentenones provided a convenient route to seven-membered rings in the synthesis of stipitatonic acid' 29a and ( f)-p-himachalene.' 29b Cycloadditions and cyclodimerisation of vinylene carbonate (1 10)offer routes to lY2-dihydroxy- and 1,2,3,4-tetrahydroxycyclobutanes,respectively.'30 The photochemistry of 0x0-sulphides,' 31 silyl ketones,'32 and imonium salts133 has received attention.Irradiation of degassed solutions of a variety of a-substituted cinnamic and crotonic acids gives the isomeric p-lactones ; analogous amides similarly give p-lactams. 34 Aromatic Compounds.-Last year Bryce-Smith and Longuet-Higgins' M. Ogata and H. Kanb Chem. and Znd. 1967,321. M. Hara Y. Odaira and S. Tsutsumi Tetrahedron Letters 1967,2981. Y. Odaira T. Shimodaira and S. Tsutsumi Chem. Comm. 1967 757. Y. Shigemitsu,Y.Odaira and S. Tsutsumi Tetrahedron Letters 1967 55. M. J. Jorgenson Tetrahedron Letters 1966 5811. N. J. Turro P. Wriede J. C. Dalton D. Arnold and A. Glick J.Amer. Chem. Soc. 1967,89,3950. H. Gotthardt R. Steinmetz and G. S. Hammond Chem. Comm. 1967,480; D. R. Arnold and A. H. Glick ibid.1966 813; H. Hogeveen and P. J. Smit Rec. Trao. chim. 1966,85 1188. IZ3 L. A. Singer and G. A. Davis J. Amer. Chem. SOC. 1967,89,941. R. L. Cargill and M. R. Willcott J. Org. Chem. 1966 31 3938; W. Metzner H. Partale and C. H. Krauch Chem. Ber. 1967,100,3156. H.-D. Scharf Tetrahedron Letters 1967 4231; H.-D. Scharf and F. Korte Chem. Ber. 1966 99 1299. 126 J. J. McCullough and C. W. Huang Chem. Comm. 1967 815. J. W. Hanifin and E. Cohen Tetrahedron Letters 1966 5421. W. L. Dilling and J. C. Little J. Amer. Chem. SOC.,1967,89,2741; W. L. Dilling ibid. p. 2742. (a)G. L. Lange and P. de Mayo Chem. Comm. 1967,704; (b)B. D. Challand G. Kornis G. L. Lange and P.de Mayo ibid. p. 704. 130 W. Hartmann and R. Steinmetz Chem. Ber. 1967,100,217. 131 P. Y.Johnson and G. A. Berchtold J. Amer. Chem. SOC. 1967,89,2761. 13' A. G. Brook and J. M. Duff,J.Amer. Chem. Soc. 1967,89,454; H. G. Kuivila and P. L. Maxfieid J. Organometallic Chem. 1967 10 41. 133 G. Adam C. Horstmann and K. Schrieber Chem. Ber. 1967,100,1753. 134 0.L. Chapman and W. R. Adams J. Amer. Chem. SOC. 1967,89,4243. 135 D. Bryce-Smith and H. C. Longuet-Higgins Chem. Comm. 1966,593. 180 A. C.Day 2531A Benzene L . (113) 1 (112) SCHEME rationalised the known photochemistry of benzene as illustrated in Scheme 1. Initial excitation gives the lowest excited singlet state (’ &) which may undergo intersystem crossing to the triplet state (3B1u). Orbital correlation diagrams showed that the excited singlet may pass adiabatically into the singlet form of (11l) the triplet into the triplet states of (1 12) and (1 13).The intermediate (1 11) readily accounts for the formation of fulvene benzvalene (114) and 1,3-adducts with olefins e.g. (1 15) from a 1,2-disubstituted olefin. Prismane Dewar benzene and 1,2-adducts with olefins are derivable from (1 12) and 1,4-adducts from (113). The transposition of ring atoms which can be observed with substituted benzenes could occur by 1,2-shifts in the reversible conversion of benzene into benzvalene and by both 1,2-and 1,3-hifts through prismane. Leading references to the experimental basis for these conclusions are readily a~ailable,’~~ and only the more recent work is discussed here. Photolysis of benzene vapour at 1470 A causes extensive fragmentati~n,’~’ but at longer wavelengths fulvene is formed.’38 Benzvalene (1 14) was not detected in vapour phase photolyses but irradiation of liquid benl e at 2537 8 gives both benzvalene and f~1vene.l~’ Photolysis of o-xylene vapoufi,in Zen ’R (114) (1 15) (1 16) (117) (118) (119) (120) 136 Ref.135 and I. 0.Sutherland,Ann. Reports 1966,63 398; cf. also ref. 2 (1966 p. 379). 13’ W. M.Jackson J. L. Faris and B. Donn J. Phys. Chem. 1967,71,3346. 13’ H.R. Ward J. S. Wishnok and P. D. Sherman jun. J. Amer. Chem. SOC. 1967 89,162; L. Kaplan and K. E. Wilzbach ibid. p. 1030. 13’ K. E. Wilzbach J. S. Ritscher and L. Kaplan J. Amer. Chem. SOC. 1967,89 1031. Photochemistry X (=9 a-X = OAc b:X =OH (124) (125) the vacuum ultraviolet or at 2537 A gives chiefly m-xylene possibly via di-methylbenzvalene along with smaller quantities of other hydrocarbons includ- ing p-xylene and benzocy~lobutene.'~~ A detailed study of the light-induced cycloaddition of maleic anhydride to benzene has a~peared,'~' and of the maleic anhydride-hexamethylbenzene system (which does not undergo 1,2-photocycloaddition).142 Photo-oxidation of liquid benzene gives trans trans-hexa-2,4-dienedial and a C1,-dialdehyde OCH -[CH=CH] CHO.Possible intermediates are the 1,2-adducts with oxygen (1 16) and (1 17).'43 Analogously Perrins and Simons have detected acyclic tetraenes spectroscopic- ally when benzene is irradiated in the presence of chloro-olefins or benzonitrile in the presence of trimeth~lethylene.'~~ The photochemical reaction between benzene and butadiene gives a complex mixture of products including the highly reactive trans-olefin (1 18) which may dimerise or react with a second molecule of diene to give the 2 1 adduct (119) in non-photochemical steps.In the presence of nitric oxide (1 18) isomerises to the more stable cis-isomer. 14' Formation of (1 18) is readily rationalised as involving the reaction of transoid butadiene with the biradical (1 13). Biradical(ll3) may also be involved in the photochemical 1,6addition of amines to benzene.'46 Pyrrole gives the 1 -cyclo- hexadienylpyrrole (1 2O),' 460 whilst simple primary and secondary amines react at nitrogen to give 3-alkylaminocyclohexa-1,4-dienes.'46b Since N-methylpyrrole failed to react it was proposed that compound (120) is formed by intramolecular hydrogen abstraction in the intermediate (121).The possibility that these reactions involve attack on ground-state benzene by R ficould be excluded since no traces of the corresponding aminobenzenes were found 146 140 H. R. Ward J. Amer. Chem. Soc. 1967,89,2367. 141 W. M. Hardham and G. S. Hammond J. Amer. Chem. Soc. 1967,89,3200; cf. D. Bryce-Smith B. Vickery and G. I. Fray J. Chem. Soc. (C),1967 390. 142 Z. Raciczewski J. Chem. Soc. (B),1966,1142 1147. 143 Kei Wei J.-C. Mani and J. N. Pitts jun. J. Amer. Chem. Soc. 1967,89 4225. 144 N. C. Perrins and J. P. Simons Chem. Comm. 1967,999. 14' K. Kraft and G. Koltzenburg Tetrahedron Letters 1967,4357; cf.ibid. p. 4723. M. Bellas D. Bryce-Smith and A. Gilbert Chem. Comm. 1967 (a)p. 263; (b)p. 862. 182 A. C. Day Benzene undergoes photochemical reactions with acetic acid and aqueous phosphoric acid to give the exo-adducts (122a) and (122b) re~pectively.'~'" To explain inter alia,147b the stereospecificity of the reaction the carbonium ion (123) has been proposed as an intermediate.148 This could be formed either by protonation of the species (1 11) discussed above or by direct photogxcitation of a-protonated benzene. The cation (123) should be attacked by nucleophiles preferentially in the exo sense and thus accounts for the stereochemistry of the products.'48 It is not yet clear whether any relationship exists between these reactions and the photosensitised protonation of 01efins~~-~~ noted earlier.Other discussions relevant to the photochemistry of benzene concern the geometry of the triplet and a hypothetical intermediate having the topology of a Mobius strip."' Hexamethylbicyclo[2,2,0]hexa-2,5-diene (hexamethyl Dewar benzene) is converted by light into hexamethylprismane (124)'' and small amounts of the bicyclic isomer (125). ''lb Further examples of light-induced valence isomerisation will be found in the sections on heterocyclic and troponoid compounds. Dianthracene the 9,lO-photodimer of anthracene dissociates when irradiated in the solid state to give a new sandwich crystal form of anthracene which has a yellow-green excimer fluorescence.' 52 Cross-adducts analogous to dianthra- cene have been prepared by irradiation of mixtures of substituted anthra- cenes.lS3Also analogous to dianthracene is the bridged system formed by reversible p hotoisomerisation of 1,2-di-(9'-an t hry1)ethane.'54 [2,2JParacyclo-naphthane (126) is converted by light into the heptacyclic hydrocarbon (127) ('dibenzoequinene'). lS5Transannular bonding of this kind was not observed in the photolysis of [2,2]paracyclophane; cleavage of one of the saturated bridging groups occurred instead. lS6 The photolysis of sodium cyclopentadienyl in tetrahydrofuran-t-butyl alcohol gives in low yield a 1 :1 mixture of the meso-and (+)-isomers (128). Deuterium-labelling implied that the excited anion abstracts a hydroxy hydrogen atom from the solvent. Subsequent protonation gives the cyclopent- 2-enyl radical which dimerises.'' 14' (a) E. Fahrenhorst and A. F. Bickel Tetrahedron Letters 1966 5911; (b) L. Kaplan J. S. Ritscher and K. E. Wilzbach J. Amer. Chem. SOC. 1966,88,2881. 148 D. Bryce-Smith A. Gilbert and H. C. Longuet-Higgins Chem. Comm. 1967,240. 14' G. C. Nieman and D. S. Tinti J. Chem. Phys. 1967,46,1432. 150 E. Fahrenhorst Tetrahedron Letters 1966,6465; 6.footnote in ref. 148. 15' (a)D. M. Lemal and J. P. Lokensgard J. Amer. Chem. SOC. 1966 88 5934; W. Schiifer R. Criegee R. Askani and H. Griiner Angew. Chem. Internat. Edn. 1967 6 78; W. Schafer and H. Hellmann ibid. p. 518; (b)H. Hogeveen and H. C. Volger Chem. Comm. 1967 1133. 152 E. A. Chandross and J. Ferguson,J. Chem. Phys. 1966,45 3564. 153 H.Bouas-Laurent and R. Lapouyade Compt. rend. 1967 264 C 1061. 154 R. Livingston and K. S. Wei J. Amer. Chem. SOC.,1967,89 3098. 155 H. H. Wasserman and P. M. Keehn J. Amer. Chem. SOC.,1967,89,2770; cf. ibid. 1966,88,4522. R. C. Helgeson and D. J. Cram J. Amer. Chem. SOC. 1966,88 509. E. E. van Tamelen J. I. Brauman and L. E. Ellis J. Amer. Chem. SOC.,1967,89 5073. Photochemistry The photochemical reduction of a phenol by sodium borohydride has been reported. Sodium bisulphite and dithionite were also effective. '58 Photosubstitution. A recent lecture described contributions made by Havinga's group to aromatic photochemical substitution. '59 Aromatic nitro-compounds undergo substitution at the ortho-and para- positions when irradiated in liquid ammonia with unfiltered light.Nitrobenzene yields p-nitroaniline and a smaller amount of the ortho-isomer. p-Chloro- nitrobenzene gives 5-chloro-2-nitroaniline and by displacement of chlorine p-nitroaniline. rn-Chloronitrobenzene is attacked at the 4-and 6-positions.' 6o Preferential substitution at the positions ortho and para to the nitro-group is consistent with earlier observations on the light-induced reaction of nitro- anisoles with amines.16 'In contrast displacement of the methoxy-group meta to the nitro-group occurred in the reactions between methylamine and 4-nitro- veratro1e,162a and between hydroxide ion and m-nitroanisole.' 62b Attack meta to the nitro-group has also been observed recently in the photochemical reactions of cyanide ion with p-and m-nitroanisoles in the latter case with displacement of meth0~y.I~~ Nucleophilic attack at the meta-position was discussed by Letsinger and McCain in terms of a bicyclic intermediate e.g.(129) for the reaction of m-nitroanisole with cyanide.'63 A formal analogy is discernible between protonated prefulvene (123) and representations such as (130) [which could give (129) with cyanide ion] and it therefore seems worth considering whether any useful correlation can be drawn between these 15* J. A. Waters and B. Witkop J. Amer. Chem. SOC. 1967,89 1022; cf. T. Tokuyama S. Senoh T. Sakan K. S. Brown,jun. and B. Witkop ibid. p. 1017. lS9 E. Havinga R. 0. de Jongh and M. E. Kronenberg Helo. Chim. Acta 1967 50 2550; E. Havinga 2nd IUPAC International Symposium on Photochemistry Enschede Holland 16-22 July 1967 Pure Appl.Chem. to be published. A. van Vliet M. E. Kronenberg and E. Havinga Tetrahedron Letters 1966,5957. M. E. Kronenberg A. van der Heyden and E. Havinga Rec. Trao. chim. 1966,85,56. (a) M. E. Kronenberg A. van der Heyden and E. Havinga Rec. Trao. chim. 1967 86 254; (b)R.0.de Jongh and E. Havinga ibid. 1966,85,275. 163 R. L. Letsinger and J. H. McCain J. Amer. Chem. Soc. 1966,88,2884. 184 A. C.Day NO reactions and the photoinduced addition of acetic acid etc. to benzene. However a satisfactory rationale of substituent effects will not be easy to reach without more experimental data in view of complexities such as the dependence of the reaction path on the nucleophile,'6'* 162a the formation of complexes between benzene and arnine~,'~~ and the light-induced 1,4-addition of amines to benzene noted above.In a study of photoinduced hydrolysis striking differences have been observed between substituted pyridines and the corresponding nitrobenzenes. 3-Bromo- pyridine undergoes photohydrolysis in dilute base easily whereas m-bromo- nitrobenzene is unreactive. Conversely 3-methoxypyridine hydrolyses photo- chemically much less readily than m-nitroanisole. ''' Havinga has stressed that care is necessary in interpreting differences of this kind.'59* '" The reason is obvious but merits repetition. The quantum yield of a photochemical reaction with respect to a given product is a measure of the efficiency of that reaction compared with all other pathways available to the excited species.Therefore comparisons based on the quantum yields for the formation of analogous products from different reactants are meaningless in absence of a detailed knowledge of the reaction mechanism and the lifetimes of the relevant excited states. Monosubstituted benzenes such as fluorobenzene chlorobenzene and anisole undergo photonucleophilic displacement of the substituent in the presence of alkoxide ion cyanide ion and piperidine. '"Displacement of the nitro-group occurs in the light-induced reaction between piperidine and 4-nitropyridine-1 -oxide.' 67 Replacement of the trimethylamino-group by hydrogen when substituted trimethylanilinium salts are irradiated in methanol involves free radicals.The effect of different anions suggests that the initial step is usually charge-transfer excitation of the associated ions. '68 Recent studies of aromatic photosubstitution also include the fol!owing photochlorination of nitro-compounds with hydrochloric acid,' 69 alkylation 164 H. Kehiaian Abstracts of Chemical Society Anniversary Meeting Exeter 3-6 April 1967 C 12 quoted in ref. 146b. 165 G. H.D. van der Stegen E. J. Poziomek M. E. Kronenberg and E. Havinga Tetrahedron Letters 1966,6371. 166 J. A. Barltrop N. J. Bunce and A. Thomson J. Chem. SOC.(C) 1967,1142. "'R. M. Johnson and C. W. Rees J. Chem. SOC.(B),1967 15. 16* T.D. Walsh and R. C. Long J. Amer. Chem. SOC.,1967,89,3943. 16' R.L. Letsinger and G. G. Wubbels J. Amer. Chem. SOC.1966,88 5041. Photochemistry of polycyclic aromatic compounds by lithium alkyls,' 70 photochemical de- alkylati~n,'~oxidative coupling of phenols,' 72 reactions of diary1 ketones ' with phenols,173 and photoarylation.'74 The 9-anthroate ion in aqueous solution is converted by light into the anion of 9-hydroxyanthracene and carbon monoxide. A mechanism analogous to the photochemical rearrangement of vinyl and aryl nitro-compounds to nitrites is proposed. 17' Photo-Fries rearrangement. Two mechanisms have been proposed for the photochemical rearrangement of aryl esters to 0-and p-hydroxyaryl ketones (i) a cyclic mechanism and (ii) homolysis of the acyl-0 bond to give two radicals which may recombine within the solvent cage to ketone e.g.(133) from a p-substituted ester (131) (path a in Scheme 2) or diffuse apart and give O*COR1 0 [0 -R'cO'] a OCOR' hv R2 R2 R2 (132) 033) 1 (fll/ h from cage diffusion 0 6-0 R2 R1 RZ R2 (135) SCHEME 2 phenols e.g. (135) as cleavage products (path l~).~' For p-tolyl acetate (131; R' = R2 = Me) Sandner and Trecker have now shown that the quantum yield of p-cresol (135; R2 = Me) is reduced by an increase in the viscosity of the medium whilst formation of (133; R' = R2 = Me) is insensitive to vis- cosity changes. An analogous effect was caused by oxygen. '76 This behaviour is consistent with mechanism (ii) provided that the radicals ArO and RCO recombine to give (133) only within the solvent cage (132). (The insensitivity to external effects commonly observed of caged radical pairs is well known).Sandner and Tre~ker'~~ give a somewhat less economical interpretation of their results. 17* H. J. S. Winkler R. Bollinger and H. Winkler J. Urg. Chem. 1967,32 1700. T. Matsuura and Y. Kitaura Tetrahedron Letters 1967 3311. J. M. Bobbitt J. T. Stock k Marchand and K. H. Weisgraber Chem. and Ind. 1966,2127. 173 H.-D. Becker,J. Org. Chem. 1967,32,2115,2124. P. W. Jeffs and J. F. Hansen J. Amer. Chem. Soc. 1967,89 2798; S. M. Kupchan and R. M. Kanojia Tetrahedron Letters 1966 5353. 17' A. W. Bradshaw and 0.L. Chapman J. Amer. Chem. SOC.,1967,89 2372. 17' M. R. Sandner and D. J. Trecker J. Amer. Chem. SOC. 1967,89 5725. 186 A. C.Day Side reactions which may accompany the photo-Fries rearrangement are decarboxylation (ArO*COR -+ArR) and decarbonylation (ArO- COR -+ ArOR).177a Photolysis of the optically active ester (136) gives optically inactive ether (137) and optically active hydrocarbon (138).' 77bFinnegan and Knutson suggested that the ether (137) is formed by decarbonylation of RCO within the solvent cage to give a new caged radical pair [cf.(134)] in which inversion of R is faster than radical recombination. The retention of optical activity in (138) was taken to imply a concerted mechanism for decarb~xylation.'~~ Steric and electronic effects in the photo-Fries rearrangement have been studied. '78a No isotope effect was observed in the rearrangement of p-methoxy- phenyl [1-'4C]acetate.1 78b /Me Me 0-CO-CH 'Et (139) (140) (141) The rearrangement of aryl cinnamates has been de~cribed.'~' Aryloxy- acetones180a and alkyl aryl ethers180b undergo reactions analogous to the photo-Fries rearrangement.Other reactions involving aromatic side-chains which have been reported include the photolysis of 2,4-dinitrophenylamino-a~ids'~ la and aryl phos- phates,l8lb and the use of photosensitive protecting groups.181c Stilbenes. Triplet stilbene has at last been detected. It was produced by flash photolysis of trans-stilbene in EPA glass at 77"~ (lifetime 2.2 x sec.).'82 The curious multiple maxima reported in the Saltiel plot for photosensitised 177 R. A. Finnegan and D. Knutson (a) Chem. Comm. 1966 172; (b)J Amer. Chem. SOC. 1967 89 1970.17* (a) G. M. Coppinger and E. R. Bell J. Phys. Chem. 1966 70 3479; (b) L. Schutte and E. Havinga Tetrahedron 1967,23,2281. 179 H. Obara and H. Takahashi Bull. Chem. SOC.Japan 1967,40 1012. 180 (a)J. Hill Chem. Comm. 1966 260; (b) D. P. Kelly J. T. Pinhey and R. D. G. Rigby Tetra-hedron Letters 1966 5953; cf. also enamides N. C. Yang and G. R. Lenz ibid. 1967,4897. (a)D. J. Neadle and R. J. Pollitt J. Chem. SOC.(C),1967,1764;(b)A. J. Kirby and A. G. Varvoglis Chem. Comm. 1967,405; (c)J. A. Barltrop P. J. Plant and P. Schofield ibid. 1966 822. 183 G. Heinrich H. Blume and D. Schulte-Frohlinde Tetrahedron Letters 1967,4693. Photochemistry 187 cis-trans isomerisation of stilbene' 3a do not have the significance originally attributed to them. The latest version is a more or less typical Saltiel plot having a single maximum in the region of ET= 50 k~al./mole.'~~~ Evidence for the intervention of triplet states in the unsensitised photoisomerisation of stilbene has been advanced.184 Pyrylium salts have been used as photosensitisers for the isomerisation of ~tilbene,'~ and SCF-MO calculations on stilbene have been reported.' The reversible photocyclisation of cis-stilbene to 4aY4b-dihydrophenan- threne exemplifies a general class of photochemical reactions3k applicable to a wide variety of compounds of the type Ar*X=Y*Ar'. Additionally it has interest as one of the simplest photochromic system^.^' Muszkat and Fi~cher'~~ have studied in great' detail the reversible photoisomerism of 1,2-diphenylcyclopentene (1 39) to the dihydrophenanthrene (140) and the oxidation of the latter by molecular oxygen to the phenanthrene (141).The photoisomerisation involves singlet states and on the basis of orbital symmetry arguments56 the anti-configuration as shown in (140) has been predicted for the dihydr~phenanthrene'~' (conrotatory process).* Synthetically stilbene photocyclisations are normally carried out under conditions which permit oxidation of the dihydro-compound to the phenan- threne. Iodine has often been used as oxidant ;but cupric halides in the presence of air are particularly effective for the cyclisation of m-disubstituted stil-benes,'88 and selenium radicals for the synthesis of sterically hindered phen- an throne^.'^^ In the presence of diphenyl diselenide 1,2-di-(a-naphthyl)- ethylene is converted by light quantatitively into picene.189 Examples reported recently are of considerable diversity and illustrate the great versatility of the reaction for the synthesis of polycyclic aromatic and heterocyclic compounds. P-Styrylnaphthalene cyclises solely at the a-position to give the benzophenanthrene (142) and a variety of P-naphthyl analogues of stilbene behave ~irnilarly.'~~ 9-Styrylphenanthrenes (143 ; R = H or Me) cyclise at the 10-position to give benzo[g]chrysenes (144).190a o-Terphenyl gives triphenylene in 88 % yield when irradiated in the presence of iodine.Ig1 The metacyclophane (145) undergoes reversible photoisomerisation to the coloured hexaene (146) which is rapidly converted by oxygen into the di- ls3 (a) G.S. Hammond J. Saltiel A. A. Lamola N. J. Turro J. S. Bradshaw D. 0. Cowan R. C. Counsell V. Vogt and C. Dalton J. Amer. Chem. Soc. 1964,86 3197; (b)W. G. Herkstroeter and G. S. Hammond ibid. 1%6,88,4769. lS4 K. A. Muszkat D. Gegiou and E. Fischer J. Amer. Chem. Soc. 1967,89,4814. R. Searle J. L. R. Williams D. E. DeMeyer and J. C. Doty Chem. Comm. 1967,1165. lS6 P. Borrell and H. H. Greenwood Proc. Roy. Soc. 1967 A 298 453. K. A. Muszkat and E. Fischer J. Chem. Soc. (B) 1967,662; cf. K. A. Muszkat D. Gegiou and E. Fischer Chem. Comm. 1965,447. 188 D. J. Collins and J. J. Hobbs Austral. J. Chem. 1967,20 1905. E. J. Levi and M. Orchin J. Org. Chem. 1966,31,4302. 190 (a) W. Carruthers J. Chem. Soc. (C) 1967 1525; (b)M.Scholz M. Miihlstadt and F. Dietz TetrahedronLetters 1967,665. 19' T. Sato. Y. Goto and K. Hata Bull. Chem. Soc. Japan 1967,40,1994. G 188 A. C.Day \/ (142) (143) (144) hv -@ -hv' / (145) (146) (147) Ph Ph Ph HO \/ dR2 (148) hydropyrene (147).192 Related systems which undergo light-induced cyclisa- tion are tetraphenylcy~lopentadienone,'~~ tetraphenylcyclopentenone,'94 and the hydroxylactone (148)8f cis-3-benzoyl-2,3-diphenylacrylic acid. lg2 H. Blaschke and V. Boekelheide J. Arner. Chern. SOC. 1967 89 2747; cf. H.-R. Blattmann D. Meuche E. Heilbronner R. J. Molyneux and V. Boekelheide ibid. 1965,87 130. 19' N. Toshima and I. Moritanj Bull. Chem.SOC.Japan 1967,40,1495; Tetrahedron Letters 1967 357; I.Moritani and N. Toshima ibid. p. 467. 194 I. Moritani N. Toshima S. Nakagawa and M. Takushiji Bull. Chem SOC.Japan 1967,40,2129. 195 G. Rio and J. C. Hardy Bull. SOC.chirn. France 1967,2642. Photochemistry 189 Applications in the heterocyclic field have included the oxidative photo- cyclisation of 4-styrylpyrimidine to benzo[f]quinazoline,' 96 and 4-styryl- pyridine (4-stilbazole) to benz[h]isoquinoline and the related reaction of 4-styrylquinoline. lg7 The last two reactions which were conducted in cyclo- hexane also give cyclohexyl-substituted products e.g. (149a) and (149b) from 4-styrylpyridine.' 97 Quantum yields have been measured for the photo- isomerisation of isomeric 1,2-dipyridylethylenes.'g* Examples containing furan and thiophen rings have also been reported.' 99 For example compound (150) cyclises to give (151) in good yield.1,2-Di-(2'-furyl)ethylene gives a cyclisation product analogous to (1 5 1) in very poor yield 199a unless cupric chloride is employed as oxidant.'99b Photocyclisation is also observed when the -CH=CH group of stilbene is replaced by the CH-N -or N=N groups. Benzylideneaniline is converted into 9-a~aphenanthrene;~OO and azobenzenes are converted into benzo[c]cin- nolines. Photocyclisations of azobenzenes require strongly acidic conditions and part of the azo-compound is reduced during the reaction to give benzidines as by-products. Reactions which bear some formal relationship to these cyclisations are the photochemical conversion of diphenylamine and diphenyl ether into carbazole and dibenzofuran respectively,202 and the photocyclisation of ben~anilides~~~ and anilides of substituted acrylic In the last case a deuterium- labelling study established that 1,3-shifts of hydrogen occur in the conversion of the presumed intermediate or excited state (1 52) to 3,4-dihydrocarbo~tyrils.~~~ 2,2'-Di(phenylethynyI)biphenyl is converted by light or thermally into the (152) 196 C.E. Loader and C. J. Timmons J. Chem. SOC.(C),1967,1343. 197 C. E. Loader and C. J. Timmons J. Chem. SOC.(C) 1967,1457. H.-H. Perkampus G. Kassebeer and P. Miiller Ber. Bunsengesellschafi Phys. Chem. 1967 71,40. 199 (a) C. E. Loader and C. J. Timmons J. Citem. SOC.(C),1967 1677; (b) R. M. Kellogg M.B. Groen and H. Wynberg J. Org. Chem. 1967,32,3093. 2oo C. E. Loader and C. J. Timmons J. Chem. SOC.(C) 1966 1078; cf. V. M. Clark and A. Cox Tetrahedron 1966,22,3421. '01 N. C. Jamieson and G. E. Lewis Austral. J. Chem.,1967,20,321; C. P. Joshua and G. E. Lewis ibid. p. 929; G. E. Lewis and J. A. Reiss ibid. p. 1451 and refs. therein cited. '02 H. Stegemeyer,Naturwiss. 1966,53 582; CJ ref. 3k. 203 B. S. Thyagarajan N. Kharasch H. B. Lewis and (in part) W. Wolf Cheni. Comm. 1967,614. 204 P. G. Cleveland and 0.L. Chapman Chem. Comm. 1967 1064. 190 A. C. Day polycyclic hydrocarbon (1S3).205 Examples where photocyclisation according to the stilbene pattern might have been expected but where alternative reac- tions occurred have been reported in connection with some heterocyclic syntheses.2o Quinone~~j-The structure of the photodimer of p-benzoquinone has been reinve~tigated.~” The photodimerisation and photoreduction of 1,2-naphtha-quinone in various media have been studied.208 The light-induced reaction between p-quinones and aldehydes to give acylquinols probably involves attack by acyl radicals on the q~inone.~” Several photoadducts between p-dioxen and 1,Znaphthaquinones have been described. They are believed to have the cis-configuration (1S4).21 Full details have appeared of Bryce-Smith’s work on the photocycloaddition of olefins to p-benzoquinone. Addition occurs at the carbonyl group to give spiro-oxetans e.g. cyclo-octene gives the adduct (155) when irradiated in benzene in the presence of p-benzoquinone.The adducts readily undergo dienone-phenol rearrangement to hydroxy-coumarans when treated with acid.2l1 The light- induced addition of diphenylacetylene to p-benzoquinone gives the adduct (156) presumably uia the intermediate spiro-oxeten (157).21 The photo- addition of diphenylacetylene and other alkynes to methoxy-p-benzoquinone provides an instructive contrast illustrating the marked influence of substi- tuents in the photochemistry of quinones. The products were cyclobutenes (158a<) formed by attack upon a carbon-carbon double The photocycloaddition of conjugated dienes to quinones has been studied by Barltrop and He~p.~~~ Chloranil l,Cnaphthaquinone and 2,S-dimethyl-p- benzoquinone reacted with simple dienes to give adducts containing a cyclo- butane ring e.g.(159; R = H or Me) from the reaction of chloranil with 205 E. H. White and A. A. F. Sieber Tetrahedron Letters 1967,2713. 206 N. C. Yang A. Shani and G. R. Lenz J. Amer. Chem. Soc. 1966,88 5369; G. R. Lenz and N. C. Yang Chem. Comm. 1967,1136; K. H. Grellmann and E. Tauer Tetrahedron Letters 1967,1909. ’*’ E. H. Gold and D. Ginsburg J. Chem. SOC.(C),1967,15. 208 J. Rennert S. Japar and M. Guttman,Photochem. and Photobiol. 1967,6,485. ’09 J. M. Bruce D. Creed and J. N. Ellis J. Chem. SOC.(C) 1967,1486. 210 W. M. Horspool and G. D. Khandelwal Chem. Comm. 1967,1203. ‘I1 D. Bryce-Smith A. Gilbert and M. G. Johnson J. Chem. SOC.(C) 1967 383. ’12 H. E. Zimmerman and L. Craft Tetrahedron Letters 1964,2131 ;D.Bryce-Smith G. I. Fray and A. Gilbert ibid. p. 2137. 213 S. P. Pappas and B. C. Pappas Tetrahedron Letters 1967 1597. ’14 J. A. Barltrop and B. Hesp J. Chem. SOC. (C) 1967 1625. 191 Photochemistry (157) (158) (159) a R' = RZ = Ph b:R' = R2 = Me c R' = Ph R2 = H or R' = H R2 = Ph I butadiene and 2,3-dimethylbutadiene7 respectively [cf. also (1 60)]. A number of other products were obtained in most cases notably the 2 1 adduct (161) and the spirodihydropyran (1 62) from 1,4-naphthaquinone and 2,3-dimet hyl-butadiene. The structures of the products with the exception of (162) can all be rationalised in terms of intermediate biradicals such as (163). The spiro- compound (162) probably arises from attack of carbonyl oxygen on the diene.21 These and earlier examples cited by Bruce3' and Barltrop and He~p,~'~ illustrate two competing tendencies in quinone photochemistry attack on carbonyl oxygen versus attack at a ring carbon atom.The opposing tendencies as typified by the difference in behaviour between chloranil and p-benzoquinone have received some rationalisation from molecular orbital calculations.214 Heterocyclic Compounds.-The phototransposition of ring atoms in benzenes (see section on aromatic compounds) has an analogy in the photo- isomerisation of pyrazine to pyrimidine.2 ' Methyl derivatives of pyrazine behave similarly when irradiated at 2537 A in the vapour For (a)F. Lahmani N. Ivanoff and M. Magat Compt. rend. 1966,263 C 1005;(b)F.Lahmani and N. Ivanoff Tetrahedron Letters 1967,3913.192 A. C. Day example 2,5-dimethylpyrazine gives 4,6- and 2,5-dimethylpyrimidine and 2-methylpyrazine gives 4- and 5-(and possibly also 2-) methylpyrimidine. Singlet n,n* states seem to be involved. The observed products are all derivable from the pyrazines by 1,2-shiftsY and Lahmani and Ivanoff have therefore suggested that phototransposition occurs through a benzvalene (114) analogue. The light-induced rearrangement of substituted thiophens involves similar transpositions within the thiophen ring as demonstrated in 14C labelling experiments. A series of paperszi6 describes the thiophen rearrangements in detail and the following generalisations emerge (a) 2-arylthiophens re-arrange irreversibly to 3-arylthiopnens ; (b) the major process occurring in 2-phenylthiophens containing phenyl methyl or deuterium substituents is interchange of C-2 and C-3 without concomitant interchange of C-4 and C-5; and (c) in 3-phenylthiophens containing phenyl methyl or deuterium sub- stituents C-2 and C-4 are interchangeable and also C-2 may rearrange to become C-5 and C-5 may rearrange to become C-4.In the case of 4-substituted 3-phenylthiophens a second possibility is rearrangement to a 3-substituted 2-phenylthiophen. Wynberg and his collaborators have rationalised these observations in terms of structures of the valene type e.g. (164) and related canonicals involving the d-orbitals of the sulphur for the C-2-C-3 trans-position in 2-phenylthiophen. Structures such as (1 64) may of course represent some (electronically or vibrationally) excited species.Alternative structures such as (165) are less attractive for various even though two analogies are known. Thus the azirine aldehyde (1 66) has been characterised as an intermediate and actually isolated in the photochemical rearrangement of 3,5-diphenylisoxazole to 2,5-diphenyloxa~ole.~'~ Also cyclopropene-2-carbaldehyde seems to be implicated in the vapour-phase mercury-photo- sensitised decarbonylation of furan to propyne and cyclopropene since one of the minor products of this reaction is the aldehyde (167) which could well have been formed by Diels-Alder addition of the cyclopropene aldehyde to furan.218 Furan decomposition is however a rather complex process since in addition vinylketen can be detected by trapping experiments with methanol." Benzophenone undergoes photocycloaddition to furan to give a 1 :1 adduct and two 2:l adducts the stereochemistry of which has been determined.219 Pyridine is reversibly photohydrolysed in aqueous base to 5-aminopenta-2,4-dienal.220A convenient new synthesis of cyclobutadieneirontricarbonyl(168) from a-pyrone has been described.22' Photolysis of this complex gave free 'I6 H.Wynberg R. M. Kellogg H. van Driel and G. E. Beekhuis J. Amer. Chem. SOC. 1967,89 3501 and preceding papers. 217 E. F. Ullman and B. Singh J. Amer. Chem. SOC.,1966,88 1844. R Srinivasan J. Amer. Chem. SOC.,1967,89 1758,4812. 219 M. Ogata H. Watanabe and H. Kanb Tetrahedron Letters 1967,533; J. Leitich ibid. p.1937; G. Evanega and E. B. Whipple ibid. p. 2163; S. Toki and H. Sakurai ibid. p 4119. 220 J. Joussot-Dubien and J. Houdard Tetrahedron Letters 1967,4389. 221 M. Rosenblum and C. Gatsonis J. Amer. Chem. SOC. 1967,89,5074. Photochemistry Ph 12 5aph -4 As+ v-(164) Ph 3q S' H COzMe I 9 0 1 C0,Me Fe (CO) (170) (168) (1 69) cyclobutadiene.222 The photochemical alkylation of nitrogen-heterocycles by acetic and acidic methanol and has received attention. The electronic states of azoalkanes have been discussed theoretically.224 The photolysis of 2,3-diazabicyclo[2,2,l]hept-2-ene in the vapour phase gives vibrationally excited bicyclo[2,1,0]pentane as initial Cycloaddition of diazomethane to cyclobutenes containing a conjugated carbonyl or cyano- group followed by photolysis of the adduct provides a convenient route to bicycl0[2,1,0]pentanes.~~~Starting with dimethyl acetylenedicarboxylate and 2-diazopropane a similar sequence gave the bicyclobutane (169).227 The photolysis of several pyrazolenines has been reported.228 The photolysis of a 2,3-dihydropyrazine with ring-contra~tion,~~~ and of a 3-benzoylazetidine with ring-expan~ion~~' have been described.The photosensitised extrusion of sulphur dioxide from the cis-and trans-dimethylsulpholenes (1 70) gives hexa-2,4-dienes by a conrotatory process as 222 W. J. Tyerman M. Kato P. Kebarle S. Masamune 0.P. Strausz and H. E. Gunning Chem. Comm. 1967,497. 223 (a) H. Nozaki M. KatB R. Noyori and M. Kawanishi Tetrahedron Letters 1967 4259; (b)M.Ochiai and K. Morita ibid. p. 2349. 224 M. B. Robin R. R. Hart and N. A. Kuebler J. Amer. Chem. SOC.,1967,89 1564. 225 T. F. Thomas C. I. Sutin and C. Steel J. Amer. Chem. SOC. 1967,89 5107. 226 T. H. Kinstle R. L. Welch and R.W. Exley J. Amer. Chem. SOC.,1967,89 3660. 227 M. Franck-Neumann Angew. Chem. Internat. Edn. 1967,6 79. G. L. Closs L.R. Kaplan and V.I. Bendall J. Amer. Chem. SOC.,1967,89 3376; A. C. Day and M. C. Whiting J. Chem. SOC.(B) 1967,991. 229 P. Beak and J. L. Miesel J. Amer. Chem. SOC.,1967,89,2375. 230 A. Padwa R. Gruber and L. Hamilton J. Amer. Chem. SOC.,1967,89 3077. 194 A. C. Day expected for concerted fragmentation of an electronically excited sulpho- lene.23la Disrotatory fragmentation occurs (as predicted) in the thermal decomposition of the sulpholenes (170).231bHuisgen and his co-workers have elegantly demonstrated that the thermal cleavage of aziridines to azomethine ylids is a conrotatory process whilst photochemical cleav.age is disrotatory.The ylids were generated in the presence of dimethyl acetylenedicarboxylate as 1,3-dipoIarophile and thereby stereospecifically trapped as 3-pyrrolines e.g. the cis-ester (172) was obtained in the thermally induced cleavage of the trans-aziridine (171).232 Aziridines thus behave in the way predicted for a system isoelectronic with the cyclopropyl anion.56 Carbenoid fragmentations of three-membered heterocyclic rings are deferred to the last section. Ar Ar (17 1) X =C0,Me;Ar =p-C,H,*OMe (172) 0 0 (174) hv /\ Q -(173) qCN + H UN -C(CN)z CN (177) (175) (176) Interest in the photochemical rearrangements of heterocyclic N-oxides continues.As with simple nitr~nes,~~~ the most common process is isomerisa- tion to an oxaziridine which may subsequently rearrange; but loss of the N-oxygen atom occurs in some cases. Such photodeoxygenations have been found with N-oxides in the ~yridine,~~~ and pyrida~ine~~~ pyra~ine,~~’ series. 231 (a)J. Saltiel and L. Metts J. Amer. Chem. Soc. 1967,89 2232; (b)W. L. Mock ibid. 1966,88 2857; S. C. McGregor and D. M. Lemal ibid. p. 2858. 232 R. Huisgen W. Scheer and H. Huber J. Amer. Chem Soc. 1967,89 1753. 233 For recent examples see L. S. Kaminsky and M. Lamchen J. Chem.Soc. (C) 1966 2295; J. B. Bapat and D. St. C. Black Chem. Comm. 1967 73. 234 J. Streith B. Danner and C. Sigwalt Chem. Comm. 1967 979. 235 N. Ikekawa and Y. Honma Tetrahedron Letters 1967 1197. 236 M. Ogata and K. Kanb Chem. Comm. 1967 1176. Photochemistry 195 (Azoxybenzene yields azobenzene as major product on photosen~itisation.~~~) Recent studies with N-oxides of quinolines isoquinolines quinoxalines quinazolines and phenanthridines provide examples of ring expansion to ring c~ntraction,~~’ oxazepines and o~adiazepines,~~~ and 1,2-shifts of oxygen.240 The quinoxaline di-N-oxide (1 73) photoisomerises with ring contraction to the benzimidazole derivative (174).241 The various types of rearrangement product can all be rationalised in terms of intermediate oxaziridines.The ylid (1 73 an N-oxide analogue rearranges photochemically to the aziridine (1 76) and 2-(2’,2’-dicyanoviny1)pyrrole( 177).242 Troponoid Compounds3’”-Further studies of the photodimerisation of tropone have appeared.243. 244 Spectroscopic evidence has been presented favouring the trans-configuration (178) for the (4 + 2)-dimer; this could be formed either concertedly from a ‘Mobius’ tropone or by stepwise trans- cy~loaddition.~~~~ The (6 + 4)-dimer (179) may also be formed by non-243n9 concerted cycloaddition. 2440 Sensitisation and quenching experiments have shown that tropone dimerisation occurs by a triplet mechanism.244a Bicyclo[5,1 ,O]octa-3,5-dien-2-one (2,3-homotropone) gives mainly isomeric products on irradiati~n.~~’ Two paths are available for the photoisomerisation of a-tropolones and their derivatives.As exemplified for a 5-substituted methyl ether (180) the possibilities are (i) 2,5-bonding to give (18 l),which may subsequently rearrange to (182) and (ii) 4,7-bonding to give (183). Path (i) is favoured for simple a-tropolones and their derivatives whilst path (ii) occurs in the colchicine series.’’” Mukai and his co-workers have described the first examples of path (ii) amongst simple monocyclic a-tropolones. 5-Phenyltropolone methyl ether (180; R = Ph) followed path (ii) exclusively on photolysis yielding compound (183 ; R = Ph). 5-Phenyl- and 5-chloro-tropolone and 5-chloro- tropolone methyl ether (180; R = C1) gave products by both pathways.246 The methyl ether of 6-phenyltropolone similarly gave analogues of (181) and (183); a further product in this case was the dimer (184).247 The light-induced 237 R.Tanikaga K. Maruyama R. Goto and A. Kaji Tetrahedron Letters 1966,5925. 0. Buchardt and J. Feeney Acta Chem. Scand. 1967 21 1399; 0.Buchardt Tetrahedron Letters 1966 6221 ; 0.Buchardt C. Lohse A. M. Duffield and C. Djerassi ibid. 1967 2741 ; C.Kaneko S. Yamada I. Yokoe and M. Ishikawa ibid. p. 1873;C. Kaneko and S. Yamada ibid. p. 5233. 239 0. Buchardt J. Becher and C. Lohse Acta Chem. Scand. 1966 20 2467; J. Streith H.K. Darrah and M. Weil Tetrahedron Letters 1966 5555. C. Kaneko I. Yokoe and M. Ishikawa Tetrahedron Letters 1967 5237; E.C.Taylor and G. G. Spence Chem. Comm. 1966,767.241 M. J. Haddadin and C. H. Issidorides Tetrahedron Letters 1967 753. 242 J. Streith and J.-M. Cassal Compt. rend. 1967,264,C 1307. 243 T. Tezuka Y. Akasaki and T. Mukai Tetrahedron Letters 1967,(a) p. 1397;(b) p. 5003; (c) cf. T. Mukai T. Tezuka and Y. Akasaki J. Amer. Chem. SOC. 1966,88 5025. 244 (a) A. S. Kende and J. E. Lancaster J. Amer. Chem. SOC. 1967,89 5283; (b) cf. A.S. Kende ibid. 1966,88 5026. 245 L. A. Paquette and 0.Cox J. Amer. Chem. SOC.,1967,89,1969. ’*’ T.Mukai and T. Miyashi Tetrahedron 1967 23 1613; T. Mukai and T. Shishido J. Org. Chem. 1967,32,2744. 247 T. Mukai T. Miyashi and M. C. Woods Tetrahedron Letters 1967,433. G* 196 A. C.Day (178) (179) a hv - R R (181) (182) rearrangement of tetra-0-methylpurpurogallin (185 ; R = OMe) to methyl 6,7,84rimethoxynaphthoate has received further attention.248 The desmethoxy- analogue (185; R = H) behaved differently giving the photoproduct (186).249 Miscellaneous.-Mixed solvents containing carbon disulphide have been recommended for dye-sensitised photo-oxidation~.~~~ Photo-oxidations of trop~lones~~ and several heterocyclic compounds252 have been reported.1,4-Dialkoxyanthracenes give 1,Cphoto-oxides (1 87) which on continued illumination isomerise to diepoxides (188).25 The 1,4-photo-oxide of 1,4-dimethoxy-5,8-diphenylnaphthaleneis the first such derivative to be isolated 248 0.L. Chapman and T. J. Murphy J. Amer. Chem. Soc. 1967,89 3476; cf. E. J. Forbes and R. A. Ripley J. Chem. SOC.,1959,2770.249 E. J. Forbes and J. Griffiths J. Chem. SOC.(C),1966,2072. E. J. Forbes and J. Grifiths Chern. Comm. 1967,427. 251 E. J. Forbes and J. Griffiths Chem. Comm. 1966,896;J. Chem. SOC.(C) 1967,601. 252 C. Dufraisse G. Rio and A. Ranjon Compt. rend. 1967 265 C 310; C. S. Foote M. T. Wuesthoff S. Wexler I. G. Burstain R. Denny G. 0.Schenck and K.-H. Schulte-Elte Tetrahedron 1967,23,2583. 253 J. Rigaudy N. C. Cohen and Nguyen Khim Cuong Compt. rend. 1967,264 C 1851. Photochemistry I97 in the naphthalene series. It reverts slowly to the parent hydrocarbon at room temperature.254 Cyclopropanes containing phenyl substituents undergo heterolytic photo- cleavage in protic solvents as well as the previously observed fragmentation to carbene~.~~~ A similar heterolytic reaction is the photochemical alcoholysis of oxiran~.~~~ New examples of the carbenoid photofragmentation of aryl- oxirans illustrate further the generality of the reaction.*''I Diphenylcarbene \/ (187) IOR ROT0 (188) NEt (189) (190) has been detected by e.s.r. and luminescence techniques in the photolysis of triphenyl- and tetraphenyl-oxiran at 77°K.258 Irradiation of the oxaziridine (189) with 2537 8 light in the presence of diethylamine gives the azepine (190) presumably by fragmentation of the oxaziridine ring to phenyl nitrene and cyclohexanone.259 Photolyses of 2-ben~oylaziridines~~' and a thi-iran-l-oxide26 have been described. Papers have appeared on the photochemistry of N-nitroso-compounds262 triphenylbor~n,~~~" triphenylmethyl hal- and sodium tetra~henylborate~~~~ 254 J.Rigaudy C. Delttang and J. J. Basselier Compt. rend. 1966,263 C 1435. 255 C. S. Irving R. C. Petterson I. Sarkar H. Kristinsson C. S. Aaron G. W. Griffin and G. J. Boudreaux J. Amer. Chem. SOC. 1966,88,5675. 256 K. Tokumaru Bull. Chem. SOC.Japan 1967,40,242. '"P. C. Petrellis H. Dietrich E. Meyer and G. W. Griffin J. Amer. Chem. SOC.,1967,89 1967; P. C. Petrellis and G. W. Griffin Chem. Comm. 1967,691. 258 A. M. Trozzolo W. A. Yager G. W. Griffin H.Kristinsson and I. Sarkar J. Amer. Chem. SOC. 1967,89 3357. '" E. Meyer and G. W. Griffin Angew. Chem. Internat. Edn. 1967,6,634. 260 A. Padwa and L. Hamilton J. Amer. Chem. SOC.,1967,89,102. 261 D. C. Dittmer G. C. Levy and G. E.Kuhlmann J. Amer. Chem. SOC.,1967,89,2793. 262 L. P. Kuhn G. G. Kleinspehn and A. C. Duckworth J. Amer. Chem. SOC. 1967 89 3858; 0.E. Edwards and R. S. Rosich Canad. J. Chem. 1967 45 1287; Y.L. Chow and A. C. H. Lee Chem. and Ind. 1967,827; T. Axenrod and G. W. A. Milne Tetrahedron Letters 1967,4443. 263 (a)J. L. R. Williams P. J. Grisdale and J. C. Doty J. Amer. Chem. SOC.,1967 89 4538; (b) J. L. R. Williams J. C. Doty P. J. Grisdale R. Searle T. H. Regan G. P.Happ and D. P. Maier ibid. p. 5153; J. L. R. Williams J. C. Doty P. J. Grisdale T. H. Regan and D. G. Borden Chem. Comm 1967. 109. I98 A. C. Day ides264 ~ulphones~~~ thiocyanates and isothiocyanates266 and a cyanatel"' i~ocyanate~~ b and nitrile Pyrimidine bases of biochemical interest have received considerable attention268 during 1967.Phosphorescence and e.s.r. studies show that the phosphorescence of native DNA originates in the thymine residues.z69 Thymine dimers and dihydrothymine have been isolated from DNA after photoly~is.~~~ vestigated.27 ' The photolysis of S-S bonds in proteins has been in-264 H. G. Lewis and E. D. Owen J. Chem. SOC.(B) 1967,422. 26s N. Kharasch and A. I. A. Khodair Chem. Comm. 1967,98. U. Mazzucato G. Beggiato and G. Favaro Tetrahedron Letters 1966 5455; G. Favaro and U. Mazzucato Photochrm. and Photobiol. 1967,6,589. 267 (a) M. Hara Y. Odaira and S. Tsutsumi Tetrahedron Letters 1967 1641; (b)J. S. Swenton ibid. p. 2855; (c)G. Just and W. Zehetner ibid. p. 3389. 268 N. Camerman S. C. Nyburg and D.Weinblum Tetrahedron Letters 1967,4127; E. Sztumpf- Kulikowska D. Shugar and J. W. Boag Photochem. and Photobiol. 1967,6 41 ;V. I. Danilov ibid. p. 233; D. Elad C. Kriiger and G. M. J. Schmidt ibid. p. 495; I. von Wilucki H. Matthaus and C. H. Krauch ibid. p. 497; M. Charlier and C. HBlbne ibid. p. 501; H. Becker J. C. LeBlanc and H. E. Johns ibid. p. 733; C. L. Greenstock I. H. Brown J. W. Hunt and H. E. Johnson Biochem. Biophys. Res. Comm. 1967,27,431; J. Eisinger and A. A. Lamola ibid. 1967,28 558; V. I. Danilov 0.V.Shramko and G. G. Dyadyusha Biokhimiya 1967,12,544. 269 R. 0.Rahn R. G. Shulman and J. W. Longworth J. Chem. Phys. 1966,452955. 270 T. Yamane B. J. Wyluda and R. G. Shulman Proc. Nat. Acad. Sci. U.S.A. 1967 58 439; A.A. Lamola and T. Yamane ibid. p. 443. 271 S. Risi K. Dose T. K. Rathinasamy and L. Augenstein Photochem. and Photobiol. 1967,6 423 ;K. Dose ibid. p. 437.

ISSN:0069-3030    

DOI:10.1039/OC9676400161

出版商:RSC    

年代:1967

数据来源: RSC