ISSN:0069-3030    

DOI:10.1039/OC98178FX001

出版商:RSC    

年代:1981

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC98178BX003

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

2 Physical Methods and Techniques Part (ii)N.M.R. Spectroscopy By R. F. M. WHITE Department of Chemistry City of London Polytechnic Jewry Street London EC3N 2EY 1 Introduction As in last year's article' this report concentrates on the use of ideas and techniques as means of solving chemical problems. Changes in some of the section headings reflect changes in the topics chosen for consideration. As in previous years the illustrations selected have not been narrowly restricted and in addition to organic applications examples drawn from studies of organometallic and inorganic com- pounds are included. 2 Chemical Shifts and Coupling Constants A computer program for the direct determination of the carbon backbone of molecules from natural abundance I3C n.m.r.data has revealed2 that in the earlier literature there is an erroneous assignment for C-3 and C-6 in (+)-limonene (1); the revised assignment gives C-3 at 30.9 and C-6 at 30.6 p.p.m. Natural abundance C-I3C one-bond coupling constants have been determined for patchoulo13 and 1,2-didehydrotestosterone17-O-a~etate;~ in both cases simple analysis of the spectral data was supported by the computer program CABSA Connectivity by AB Satellite Analysis. A summary of all known up to the begining of 1J13c-13c 1980 have been presented in a diagramatic form.5 ' R. F. M. White Annu. Rep. Prog. Chem. Sect. B 1980,77 3. ' R. Richarz W. Ammann and T. Wirthlin J. Magn. Reson. 1981 45 270. A. Neszrnelyi and G. Lukacs J. Chem. SOC. Chem. Commun. 1981,999. G.Lukacs and A. Neszrnelyi J. Chem. SOC. Chem. Commun. 1981 1275. P. E. Hansen and V. Wray Org. Magn. Reson. 1981,15 102. 15 16 R. F. M. White The 13C chemical shifts obtained6 under uniform conditions for selected secondary aliphatic compounds have been used in a linear regression analysis and two- parameter relationships describing substituent effects have been calculated. In aldopyranoses 'J13C-1H(-170 Hz) for an equatorial C-H adjacent to a ring oxygen exceeds that of a comparable axial C-H by about 10 Hz. When the ring oxygen is replaced by sulphur both orientations of the C-H bond have almost the same value' of this one-bond coupling constant. The 'H and I3C spectra of 5-thio-~-glucopyranose have been analysed* the 'H spectrum showing that two anomers are present in the ratio 85 :12 (a:@) in solution in 2H20 or 2H20-DMS0.The proton-proton vicinal coupling constants provide evidence that the ring of 5-thio-a-D-glucopyranose is slightly puckered compared with that of a-D-glucopyranose. The smaller C-S-C angle deforms the ring in such a way that the carbon atoms are further from the average plane. The I5Nfrequencies have been measured' for ""0 "'NO "NH3 15N2,and HC"N in the gas phase and for neat liquid Me"N02. From the temperature dependence at various densities it is possible to reduce the measured frequencies to the zero pressure limit at 300K providing a basis for an absolute nitrogen shielding scale. A number of N-alkyl formamides show" two resonances in their 15 N n.m.r. spectra the cis- and trans-isomers only slowly interconverting about the CO-N bond.For most of the compounds examined the "N shift in the cis-isomer was to high field of that in the trans- exceptions being when the N-substituent was was a tertiary alkyl group. For all the compounds studied was greater for 1J15N-H the trans-isomer. Substituent effects in "N chemical shifts have been determined" for a number of substituted pyridines and pyrimidines and (on the p.p.m. scale) are about three times more sensitive towards substituents than are 13C shifts; substituent shifts for 15N are additive and are useful for assignment purposes. The 1sN coupling constants to 13C and 'H have been reported'* for 2-(hydroxymethyl)-5- methoxy-1 -phenyl-4-pyridone (2) values of being consistent with the 1J15NT13c 0 nitrogen being sp2 hybridized.The molecule resembles a hybrid between an aromatic system and an a,@-unsaturated ketone; the authors expressed the opinion that there is a definite bias towards aromatic character. On protonation I3C shifts do not change much whereas 15Nshifts undergo a large change. A. Ejchart Org. Mugn. Reson. 1981 15 22. 'V. S. Rao and A. S. Perlin Curbohydr. Res. 1981,92 142. J. B. Lambert and S. M. Wharry J. Org. Chew. 1981,46 3193. C. J. Jameson A. K. Jameson D. Oppusunggu S. Wille P. M. Burrell and J. Mason J. Chem. Phys. 1981 74 81. lo H. Nakakishi and J. D. Roberts Org. Mugn. Reson. 1981 15 7. W. Stadeli and W. Von Philipsborn Org. Mugn. Reson. 1981,15 106. '* C. A. Kingsbury M. D.Cliffton S. Rajan D. L. Durham and J. H. Looker Heterocycles,1981,16,343. Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy 17 OR" R = Me,But,or H R' = Me. R = MeorEt R' (3) Both 13C and 'lo n.m.r. spectra have been ~btained'~ for 2-alkoxytetrahy- dropyrans (3) ten of which were in the form of cis- and trans-isomeric pairs; when both isomers were available a consistent pattern of shifts was observed. In the trans-isomers where the alkoxy-group is exclusively or predominantly axial both the ring oxygen and alkoxy-oxygen are more shielded than the corresponding atoms in the (equatorial) cis-compounds. The three carbons attached to oxygen atoms are also shifted upfield in the axial isomers. Substituent effects and hydrogen- bonding interactions in 170n.m.r.spectra have been investigated for mono- and poly-substituted acetophenones and benzaldehydes the 170 shifts being14 highly sensitive to electronic perturbation induced by substituents. Rate constants for carbonyl oxygen exchange in H2170-dioxane solvent are reported for several acetophenone derivatives. Magnetic non-equivalence of diastereotopic oxygen atoms has been detected by means of 170 n.m.r. spectroscopy. Oxidation of phenyl 1-phenylalkyl sulphides with iodobenzene trichloride and H2170 gives" dias-tereomeric phenyl 1-phenylalkyl ['70]sulphoxides (4) of known configurations (RRISS)-(4) and (RS/SR)-(4). Further oxidation with rn-chloroperoxybenzoic acid gives the [I6O,'70]sulphones (5) in the forms (RR/SS)-(5)and (RS/SR)-(5) and Ph Ph I I PhSOCH PhS02CH I I R R (4;R = Me or Et) (5) a solvent-dependent chemical shift of 4 to 10 pqm.between the diastereotopic oxygen atoms was found for these sulphones. Both diastereomers of cyclic 2'- [170 '80]deoxyadenosine 3',5'-monophosphate have been synthesized and configurations at phosphorus determined. The 170spectra show16 that the chemical shift 1J31p-170 and the line widths are different for the diastereomers the chemical shift being large enough for two resonances to be detected in ihe 31P decoupled 17 0 spectrum of a racemic mixture. The compound previously thought to be a hydroxyphosphazene has been shown17 to exist in solution as an oxophosphazene (6).The product of condensing (6) with P3N3ClPh5 has an oxygen bridged structure with 3Jp-o-p= 37.9 Hz the largest value so far obtained for this coupling.l3 R. D. McKelvey Y. Kawada T. Sugawara and H. Iwamura J. Org. Chem. 1981,46,4948. l4 T.E.St. Amour M. I. Burgar B. Valentine and D. Fiat J. Am. Chem. SOC.,1981,103 1128. lJ K. Kobayashi T. Sugawara and H. Iwamura J. Chem. SOC.,Chem. Commun. 1981,479. 16 J. A. Coderre S. Mehdi P. C. Demou R. Weber D. D. Traficante and J. A. Geret. J. Am. Chem. SOC.,1981,103 1870. " D. S.Rycroft V. R. Miller C. D. Schmulbach and R. A. Shaw Phosphorus Sulfur 1981,10 121. R. F.M. White Isomeric mixtures of compounds Me,M(CH :CHMe)4-, with M = Si or Pb and It = 0 to 3 have been studied18 by 'H 13C 29Si and *"Pb n.m.r. spectra. The results obtain from isomers Me3M(C3H5) were used to calculate shifts for the other compounds.The lead derivatives gave good agreement between observed and calculated values but for silicon they diverged with decreasing n because of steric factors. The existence of the two tin isotopes "'Sn and '19Sn simplifies determina- tion" of the coupling constant between isochronous tin atoms. In (Me,Sn),CH 2Jsn-Sn is negative and the evidence points to this also being so in most of the trimethyl tin-(Group IV and V) compounds examined the results obtained for a number of stannylamines suggest a dependence of 2Jsn-Sn on the Sn-N-Sn bond angle and the geometry of the nitrogen atom. General features in the 'H spectra of 2-substituted 1,3-dithians (7) suggest that when X = SnMe or PbMe there should be observable coupling between H-4e and the metal atom.In the 270MHz 'H spectra of these derivatives H-4e and H-4a give well resolved multiplets2' in which the former is easily identified from the couplings to other ring hydrogen atoms; it also shows satellites due to "7*119Sn or 207Pb whereas the metal to H-4a coupling was not identifiable and was presum- ably small. If the long range metal-proton coupling is favoured by a zig-zag arrangement the metal groups must favour a predominantly equatorial orientation. Vicinal couplings (,JC4between 13C and 117~119Sn or 207Pb are dihedral-angle dependent; in the cyclohexyl compounds an axial SnMe group gives 3JSn-C of 10 to 12 Hz whereas the equatorial group gives a coupling constant of 65 Hz.The conformationally homogeneous trans-5 -t-butyldithian-derivatives were prepared. In the tin compound 3JSn-C = 32.4 Hz was comparable with the coupling of 28 Hz found for (7;X = SnMe,); when M = Pb 3J207'pb-13c = 38 Hz. The chemical shifts low temperature spectra and JM-13c in 2,2-disubstituted derivatives indicate that the M-Me3 group (M = Si Ge Sn or Pb) has a large preference (A > 2.0 kcal mol-') for the equatorial orientation at position 2 in the 1,3-dithians a preference that is larger than in the corresponding M-cyclohexyl compounds. T. N. Mitchell and H. C. Marsmann Org. Mugn. Reson. 1981 15 263. l9 W. Biffar T. Gasparis-Ebeling H. Noth W. Storch and B. Wrackmeyer J. Mugn. Reson. 1981 44 54. 2o G. M. Drew and W. Kitching J. Org. Chem.1981,46,558 Physical Methods and Techniques-Part (ii) N.M.R. Spectroscopy 19 Relatively narrow line widths-of 50 to 150 Hz have been obtained” in the 33S (I= 3/2 abundance 0.74%) n.m.r. spectra of a number of sulphones and sulphonic acids. The range of 33Sshifts is wide enough to allow their use for quantitative analysis particularly in the examination of petroleum. Resonances from ”Fe (I= 1/2 abundance 2.2%) in organo-iron complexes mainly of the type Fe(CO),(diene) show a range of 3000 p.p.m. in chemical shifts,” mostly to high frequency of Fe(C0)’. Values of were from 23 to 32 Hz ‘J57~~-13~ for carbonyl carbon atoms whereas for carbon m-bonded to iron this coupling was less than 6Hz. Values of have been obtained from the 13C spectra of 2J13c-13c highly 13CO-enriched metal carbonyls.At low temperatures the rearrangement of the carbonyl groups of (8) is frozen so that separate peaks (intensities 1:2) 0 C (8) arise from axial and equatorial groups. At -100 “C the spectrum of a 65% 13C0 enriched sample has carbonyl peaks split into 5 and 3 lines because of the different isotopomers present. The five-line feature arises from overlap of a singlet (no 13C in the basal position) with a doublet (one basal 13C) and with a triplet (two basal 13 C) while three lines arise from overlap of a singlet and a doublet. Values of 2J13c-13c are 30-35 Hz for trans-carbonyl groups while the cis-couplings are an order of magnitude smaller. 125 Te n.m.r. has been usedz4 to follow the stereochemistry of orthotelluric acid solvolysis in hydrogen fluoride.When (H0)6Te is dissolved in 48% aqueous HF the product is cis-(H0)4TeFz; further fluorination in anhydrous HF leads to equal proportions of sym- and asyh- (HO),TeF,. 3 Isotope Shifts The deuterium-decoupled ‘H n.m.r. spectrum of a mixture of pentadeuteriobenzene and benzene showed” a chemical shift for (’HS)benzene 0.0032 p.p.m. to low field of benzene. Such a down-field 2H isotope shift is uncommon and is attributed to the increased electron density affecting the n-electron ring current. Downfield ’H isotope shifts have been detected26 over three bonds in monodeuterated cyclopen- tane and over four bonds in cyclopentane. The fate of deuterium in biosynthesis can be followed directly by 2H n.m.r.or indirectly from the 13C spectra. The indirect method has been used with monitoring of 13C directly bonded to 2H, the resonance of such a carbon atom being shifted to high field (the a-shift) and showing coupling to deuterium. In this form the method suffers disadvantages of a reduced signal to ” R. Faure E. J. Vincent J. M. Ruiz and L. LCna Org. Magn. Reson. 1981 15,401. ‘’ T.Jenny W. Von Philipsborn J. Kronenbitter and A Schwenk J. Organomet. Chem. 1981,205,211. 23 S. Aime and D. Osella J. Chem. SOC., Chem. Commun.. 1981 300. 24 W.Totsch P. Peringer and F. Sladky J. Chem. SOC., Chem. Commun. 1981 841. 25 T.Yonemitsu and K. Kubo J. Chem. SOC.,Chem. Commun. 1981,309. 26 R.Aydin and H. Guenther J Am. Chem. SOC.,1981,103 1301. R.F.M. White noise ratio of the shifted resonance because of poor relaxation signal multiplicity and loss of nuclear Overhauser effect (n.0.e.). A variation of the indirect method has been suggested2' in which 13Cin a position @ to the deuterium is monitored. The @-shift is upfield and as 2J2H-13c is negligible the proton-decoupled spectrum gives a singlet for the 13Cpeak monitored and difficulties arising from poor relaxation and n.0.e. are avoided. A comparison between the direct 'H n.m.r. and the indirect @-isotope shift methods has been made by measurement of incorporation of deuterium into the polyketide 6-methylsalicyclic acid. Poor resolution is often a limitation of 'H n.m.r a problem which becomes worse as the molecular size increases; on the other hand the @-shift method offers better resolution which is not lost with increased molecular size.Exchange of 'H for 'H in carbohydrate hydroxy-groups also causes 13Cisotope shifts. Parameters have been determined empirically for a number of such isotope shifts and these may be used to calculate shifts to assist in the assignment of 13C spectra. The 13Cn.m.r. of psicose has been re-examined2* by the differential isotope shift (DIS) to clarify some of the assignments for this ketose which in aqueous solution exists as a mixture of a-and p-furanose and pyranose forms. In the DIS technique samples of the isotopomers (a) all -02H in 'H20 solution and (6)all -OH in H20 are contained in co-axial n.m.r. tubes. When the isotope shift was large enough two lines were obtained for each 13Cresonance and comparison of calculated and observed shifts clarified assignments for psicose.Deuterium isotope effects in I3Cn.m.r. have been usedz9 to investigate tautomeric equilibria in enamine ketones which may exist in the three forms (9a b and c) although no evidence has been observed for (9c). Samples of compounds [(lo) (ll) and (12)] were examined in co-axial n.m.r. tubes 2H,0 being added to one half and H20 to the other. An isotope shift was observed for C-2 in (lo) whereas in (11)and (12) both C-1 and C-2 showed isotope shifts. As deuterium isotope shifts are usually detect- able over not more than three bonds it was concluded that forms (9a) and (9b) are present in equilibrium for (11)and (12) and that there is no tautomeric equilibrium for (10).This is consistent with the conclusion that the double bond prefers to be em-to a five-membered ring system for P-diketone derivatives. 0JrJ/ 0 (10) (11) (12) 27 C. Abell and J. Staunton J. Chem. SOC.,Chem. Commun. 1981,856. 28 K. M. Valentine L. W. Doner and P. E. Pfeffer Carbohydr. Res. 1981,96 293. 29 G. M. Coppola R. Damon A. D. Kahle and M. J. Shapiro J. Org. Chem. 1981 46 1221 Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy Double isotopic substitution ("C and l80) has been used3' in an investigation of the thermal and the acid-catalysed rearrangements of trispiro (tricycl0(3.3.3.0'*~) undecane-2,2';8,2";9,2-tris[oxirane]} (13) to the triether 2,5,14-trioxahexa-c yclo[ 5.5.2.1.2 .4*10 O4*l7O10*17]heptadecane (14).(13) (14) The trioxide (13) labelled with 13C (90%) and l80(45%) in one of the three epoxide groups showed two resonances with intensities 55(low field) :45(high field) in part of its 13C n.m.r. spectrum. The rearrangement to (14) could proceed either by breaking 13C-'80 bonds (15; pathway 1)with loss of the l80isotope-shifted 1 (15) peak or by breaking oxygen-quaternary carbon bonds (16; pathway 2) with retention of '*O shifted peaks and hence two 13C resonances with relative intensities 55:45. The products of both the thermal and the acid catalysed reactions showed 18 0 shifted peaks hence the reactions proceeded by pathway 2. (16) Mono- and di-labelled '80-acetals [(17)-(19)] show3' additivity of isotope shifts in their 13Cn.m.r.spectra. Thus the shifts at C-2 for the doubly labelled compounds (19a-c) are from 0.046 to 0.055 p.p.m. twice those for the singly labelled analogues (Ma+) which are from 0.023 to 0.028 p.p.m. It is usually found that "0 P-isotope shifts are not resolvable however the ethylene glycol derived acetals (18) and (19) are favourable cases in which this shift can be resolved. These show that within experimental error the &shifts (ca. 0.007 p.p.m.) are also additive. 'O S. A. Benner,J. E. Maggio and H. E. Simmons 111 J. Am. Chem. SOC.,1981,103 1581. 31 R.N.Moore J. Diakur T.T. Nakashima S. L. McLaren and J. C. Vederas J. Chem. SOC.,Chem. Commun. 1981,501. R. F.M. White *o= 180 (a) R1 = Me(CH&,R2 = H (b) R'-R2 = -(CH&- (c> R' = R2 = Me(CH2)3 The l80isotope shifts in 13C spectra have been used in the in~estigation~~ of the acid-catalysed exchange of oxygen between water and [l-13C,'802]acetic acid the isotope shift facilitating the direct observation of the three different 0-isotopomers of acetic acid.In water the exchange proceeds by the pathway shown in Scheme 1to give (20) the slow stage involving a tetrahedral intermediate formed by nucleophilic attack of H20 on RC02H2+. A similar use has been made of "0 induced shifts in the "N n.m.r. spectrum of the nitrite ion. Here33 the isotope shift depends on the number of **Oatoms in the ion and is about 0.138 p.p.m. per '*O; in the nitrate ion the corresponding shift is 0.056 p.p.m.per l80.The acid catalysed exchange of oxygen between water and "0 nitrous acid can be monitored by "N n.m.r.. At pH 6.26 and 28°C a sequential mode of oxygen atom exchange was found in agreement with a mechanism postulated earlier on the basis of indirect evidence. 4 Interactions in Solutions Reactions of phenylphosphonic acid and phenylphosphonic dichloride with the strong acid solvents 100% H2S04,25% and 65% oleum and chlorosulphuric acid have been inve~tigated~~ by 31P n.m.r. which shows that initial protonation of the phosphoryl oxygen varies with solvent and solute and except for 100% sulphuric acid may be followed by sulphonation of the aromatic group and/or condensation of the phosphonic acid or solvolysis of the dichloride. Slow chlorination occurs for phenylphosphonic acid dissolved in chlorosulphuric acid.Attempts to prepare the mono-chloride PhPClO(0H) did not yield the pure compound but the mixtures obtained reacted in a way that was consistent with the mono-chloride participating in the reactions mentioned above. 32 J. M. Risley and R. L. Van Etten J. Am Chem. SOC.,1981,103,4389. 33 R. L. Van Etten and J. M. Risley J. Am.Chem. SOC.,1981,103,5633. 34 K.B.Diilon M. P. Nisbet and T. Waddington J. Chem. SOC., Dalton Trans. 1981,212. Physical Methods and Techniques -Part (ii) N.M.R.Spectroscopy 23 As the temperature of the sample is lowered to -74 "C the 'H n.m.r. spectrum of 1-methoxycarbonylpiperidine(21) in FS03H shows3' the appearance of a broad peak at 66.7 (1 H) and an increase in the complexity of the signal from the C-2 protons.In contrast the amide (22) gives a sharp (1H) peak at 69.2 (a shift characteristic of 0-protonation) and no change in the C-2 proton signal. It was concluded that (21) is predominantly N-protonated in fluorosulphuric acid. The variation of 13C chemical shifts with acidity in sulphuric acid indicated that a switch to predominant 0-protonation occurs at lower acidities. Me-C-N'3 (21) (22) Low temperature 'H and 13C n.m.r. spectra that in super-acid media (SbF,-FSO,H) the mono- and dialkyl-substituted thioanisoles investigated were protonated at the sulphur atom. A doublet at about 63.2 can be assigned to the methyl thio-group while the integrals and multiplicities of peaks indicate that the signal due to H' on sulphur is hidden in the aromatic region of the spectrum.In the 13C spectrum the carbon atom of the methyl thio-group is most affected by protonation showing an upfield shift of about 20 p.p.m. In 1:1SbFs-FS03H solvent 2,6-dimethyl-y-pyrone (23; X = 0)and its sulphur and nitrogen analogues (23; X = S or NMe) are doubly protonated on the exocyclic oxygen atom37 while the 4-thione derivative is singly protonated at the exocyclic sulphur atom. 0 (23) N.m.r. line-shape analysis has been to study the kinetics of proton exchange between fluorosulphuric acid and protonated solute bases in super-acid media. The rates of proton exchange have been used to show that over the range from 0 to 90 mol% of SbF the acidity increases monotonically despite the low concentration of acidic protons in such solutions.The value of Ho for 90mol% SbFS is about -26.5 which is the most acidic liquid Bronsted acid measured so far though there are indications that mixtures of hydrofluoric acid with an excess of SbFs may be even stronger acids Differences in pK between the bases examined were also derived. In these media water appears to be a slightly weaker base than acetone the solute species produced being very different from the hydrated oxonium ion formed in aqueous media. The effects of solvent temperature and concentration on 170and 14N n.m.r. spectra have been determined39 for some simple amides. Hydrogen bonding causes 35 P. J. Battye J. F. Cassidy and R. B. Moodie J. Chem. SOC.,Chem.Commun. 1981 68. 36 M. Eckert-MaksiC J. Chem. SOC.,Perkin Trans.2 1981 62. 37 V. Gold and T. Mah J. Chem. SOC., Perkin Trans.2 1981 812. 38 V. Gold K. Laali K. P. Morris and L. Z. Zdunek J. Chem. SOC., Chem. Commun. 1981,769. 39 M. I. Burgar T. E. St. Amour and D. Fiat J. Phys. Chem. 1981,85 502. 24 R.F.M. White relatively large changes in amide 170 chemical shifts increased hydrogen bonding being associated with an increase in shielding. Smaller variation is observed for 14 N paramagnetic shifts being observed on increased proton donation of the directly bonded amide N protons. At elevated temperaturFs 14N-lH coupling constants were found to be 60 * 5 Hz (formamide) and 67 f5 Hz (N-methyl formamide). In addition to solvent polarity-polarizability and solvent-to-solute hydrogen bond- ing effects solute-to-solvent hydrogen bonding by the second protons of self- associated formamide leads to "N shifts being dependent on the solvent hydrogen bond acceptor basi~ity;~' self association of N-monoalkyl amides is sufficiently strong for this effect to be negligible.The results of infrared 'H and I9F n.m.r. spectroscopic on solutions of alkali-metal fluorides in amides! support the finding from LCAO-MO-SCF calculations estimating the strength of the amide- fluoride ion hydrogen bond at about 148 kJ mol-' this is the second strongest hydrogen bond known. The possible chair conformations of myo-inositol can be described as equatorial (24) (having five equatorial -OH groups) or axial (25).Changes in the 31P spectrum of myo-inositol hexaphosphate with pH have been interpreted4* in terms of conformational change with degree of protonation. Above pH 12 the equatorial structure [cf,(24)] is preferred while below this the axial conformation [cf. (25)] is favoured until pH 5 when on addition of a seventh proton there is a reversion to the equatorial structure. There is evidence that at lower pH the axial conformation is again favoured. Nitration of N,Ndimethylaniline with H"NO in 85-90'/0 sulphuric acid shows evidence43 of "N chemically induced dynamic nuclear polarization in the puru-nitro-product from which it is concluded that at least part of this product is formed by a reaction involving radical pairs probably the nitrous acid-catalysed reaction.Depending on the direction of the reaction 15N02-'4N02 exchange reactions of the ion (26) with nitric acid gives44 either enhanced absorption or emission in the kMMe* MefJMe Me NO2 (26) *O M. J. Kamlet C. Dickinson and R. W. Taft J. Chem. SOC.,Perkin Trans. 2 1981 353 41 J. Emsley D. J. Jones J. M. Millet R. E. Overill and R. A. Waddilove J. Am. Chem. Soc. 1981 103,24. 42 J. Emsley and S. Niazi Phosphorus Sulfur 1981 10,401. 43 J. H. Ridd and J. P. B. Sandall J. Chem. SOC.,Chem. Commun. 1981,402. 44 P.Helsby and J. H. Ridd J. Chem. SOC.,Chem. Commun. 1981,825. Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy 25 15 N spectrum showing that some if not all of the isotopic exchange proceeds via the radical cation (27).ye2 MQMe'. Me (27) 5 Solid-state Studies In last year's article' examples were given of the splitting into broadened doublets observed in the spectra of solid state samples in which 13C atoms are bonded to nitrogen. Magic-angle sample-spinning is unable to eliminate 13C-14N dipolar interactions because of the 14N quadrupole moment. Natural [14N]glycine has an asymmetrically split aC resonance4' while 15N labelled material gives a 13C solid- state n.m.r. spectrum with sharp lines for both of the carbon atoms. Using a 13C-14N bond length of 1.49 A gives the calculated value of the 14N quadrupole coupling constant as 1.18MHz with an asymmetry parameter of 0.54. From the 13C-13C and 13C-14N dipolar splittings in the 13C n.m.r.spectrum of a single crystal sample of gly~ine~~ bond $stances rC4 = 1.543 f 0.008 A rC-N = 1.509 f 0.009 A and the bond angle CCN 111.1f 1.0" were obtained. The 14N spectrum was used to find the 14N quadrupole coupling constant (1.18 f 0.01 MHz) and the asymmetry parameter (0.54 f 0.01); Vzzis approximately along the C-N bond and V is almost perpendicular to the CCN plane. In the vapour phase and in solution the diazoles pyrazole and imidazole exist in tautomeric equilibria (28) and (29). Proton exchange in imidazole has not been slowed down sufficiently for separate signals to be observed for protons H-4 and H-5, for carbons C-4 and C-5 or for nitrogens N-1 and N-3. The high resolution solid-state 13C n.m.r. spectra of pyrazole and imidazole each show4' three carbon resonances the shifts for pyrazole being similar to those obtained for pyrazole in solution at low temperatures.The shifts for solid imidazole compare with those for N-methylimidazole in CDC13 (C-2 137.6 C-4 129.3 and C-5 119.7 p.p.m.); the assignments for solid imidazole are given in (30). (30) 45 J. G. Hexem M. H. Frey and S. J. Opella J Am. Chem. Soc. 1981,103,224. 46 R. A. Haberkorn R. E. Stark H. Van Willigen and R. G. Griffin J.Am. Chem. SOC.,1981,103,2534. '' J. Elguero A. Fruchier and V. Pellegrin J. Chem. Soc. Chem. Commun. 1981 1207. 48 M. Begtrup R. M. Claramunt and J. Elguero J. Chem. Soc.. Perkin Trans. 2,1978,99. R. F.M. White Introduction of a paramagnetic impurity has been suggested49 as an effective aid to obtaining 13C spectra from solid-state biomolecules which as pure compounds may have unfavourably long proton-relaxation times.Small (i.e. about 0.4%) amounts of Cu2' do not produce any observable shift or broadening in the 13C spectra of imidazole uracil thymine and cytosine.H,O whereas spectra could be obtained easily from the doped samples. These authors also report that imidazole shows three 13C peaks as a result of freezing out of tautomerism in the solid state. Assignment of the 13C peaks on the basis of broadening by the 14N quadrupole moments gives the correct assignment for C-2 but other factors prevent an exact assignment of the C-4 and C-5 resonances on this basis. A procedure has been described" for the preparation of unstable carbonium ions and their subsequent manipulation for high-resolution solid-state n.m.r.at about 77 K. The s-butyl cation EtCHMe' could be obtained in the solid state by high vacuum co-deposition of EtCHMeCl and SbFs on a liquid-nitrogen cooled surface. At the temperature of formation any 13C label was scrambled over the four-carbon linear unit but scrambling is too slow to cause coalescence of the s-butyl signals at -60 "C. Rearrangement to t-butyl cation occurs to a limited extent under the conditions of ion formation. Over the temperature range -60 to -190 "C there was no evidence for a static structure suggesting that the barrier for the degenerate hydride shift Me-CH-CH2-Me + + Me-CH2-CH-Me + may be less than 2.4 kcal mol-'. Comparison of 29Si n.m.r.spectra obtained for solid silicon polymers with results for the liquid state showS1 no evidence of special solid-state effects on 29Si chemical shifts so that relationships between shifts and molecular structure obtained for liquid samples can be used for the interpretation of solid-state 29Si spectra. This should offer detailed information about structural units in siloxane resin networks. The 29Si n.m.r. spectra of aluminosilicates show distinct signals for five possible numbers of AlO tetrahedra connected to the SiO tetrahedra in building units Si(OSi)4-n(OAl)n and a number of studies of zeolite structures have been rep~rted.~~-~~ Zeolites containing five-membered rings of silica tetrahedra give a 29Si peak at about -113 p.p.m. whereas other zeolites which do not contain this building unit give55 resonances in the range -80 to -106 p.p.m.6 Conformational Studies The crystal structure of the dinuclear complex (31)shows that the six-membered cyclic ligand adopts a boat conformation. The 'H n.m.r. spectrum of the compound cooled below -30°C is that predicted for a static structure that is three signals 49 S. Ganapathy A. Naito and C. A. McDowell J. Am. Chem. Soc. 1981,103 6011. "P.C. Myhre and C. S. Yannoni J. Am. Chem. Soc. 1981,103,230. '' G. Englehardt H. Jancke E. Lippmaa and A. Samoson J. Orgunomet. Chem. 1981,210 295. 52 G. Engelhardt E. Lippmaa and M. Magi J. Chem. SOC.,Chem. Commun. 1981,712. 53 J. M. Thomas L. A. Bursill E. A. Lodge A. K. Cheetham and C. A. Fyfe J. Chem. SOC.,Chem.Commun. 1981,276. "J. Klinowski J. M. Thomas M. Audier S. Vasudevan C. A. Fyfe and J. S. Hartman J. Chem. SOC. Chem. Commun. 1981,570. 55 J. B. Nagy J. P. Gilson and E. G. Derouane J. Chem. SOC.,Chem. Commun. 1981,1129. Physical Methods and Techniques -Part (ii) N.M.R. Spectroscopy +s'cYs s-c \I Pt ;Pt=Pt< I 101-I __. -Pt /Qt-T /4 a\ S-S -/ LS' from the different Pt-Me groups and two AB quartet patterns (intensities 2 1) from the two types of ring-methylene In addition all these peaks show coupling to one or two 19'Pt nuclei. Between -30 and +30"C signals from the Pt-Me trans-to the bridging chlorine atoms coalesce to a single line and the two AB quartets coalesce to a single AB quartet coupling to '"Pt being maintained throughout.These changes are interpreted in terms of a series of intramolecular 1,3-shifts in which the dinuclear metallic moiety commutes between pairs of sulphur atoms (32) a change in ring conformation from boat to chair almost certainly being involved. The "N n.m.r. spectrum of 5-azacytidine (33) in DMSO consistss7 of peaks at 207.9 (N-1 doublet 5 Hz); 184.4 (N-3 doublet 5 Hz); 159.2 (N-5 doublet of 4' Ribosyl (33) doublets 5 and 12 Hz); and 273.4 p.p.m. (N-4' triplet 91 Hz)from external H"NO in D20. Selective proton decoupling showed that the 5 Hz splittings of N-3 and N-5 each involve a single proton of the NH2 group indicating that rotation about the C-4-N-4' bond is slow. At 52°C the couplings to N-3 and N-5are lost as a s6 E.W. Abel M. Booth K. G. Orrell G. M. Pring andT. S. Cameron J. Chem. SOC.,Chem. Commun. 1981 29. '' J. D. Roberts G. R. Sullivan P. P. Pang and N. J. Leonard J. Org. Chem. 1981,46 1014. R.F. M. White result of either rapid rotation or intermolecular exchange of N-4' protons. If the former is the reason then AG" for the rotational process would be about 17 kcal-mol-' which is in the range observed for rotation of dimethylamino-groups in other cytosine derivatives. Like cytidine Sazacytidine is protonated largely at N-3. Increasing pressure leads to a low-field shift in the 'H spectrum of "N enriched formamide dissolved in acetones8 indicating a concomitant increase in hydrogen bonding. From variable pressure spectra the activation volume for rotation A V" (= dAG*/dp) was found to be (3.5 f 0.5) x m3 mol-' a value explained by steric effects.Breaking of a hydrogen bond is accompanied by an increase in volume of about 4 X m3 mol-'; if amide rotation involved simultaneous breaking of hydrogen bonds between amide and solvent molecules the expected value of A V" would be significantly larger than that actually observed. The observations9 of s7Fe satellites in the 13Cspectra of Fe2(C0)6S2 syn-and ~nfi-Fe~(CO)~(sMe)~ isomers and Fe3(CO)12 has given further understanding of the dynamic behaviour of these molecules in which carbonyl exchange occurs. Among the molecules containing one 13C atom in the carbonyl groups 4.4% will have one s7Fe atom; in half of these there will be a one-bond separation '3C-57Fe and in the other half there will be a two-bond separation 13C-Fe-s7Fe.Possible mechanisms of carbonyl exchange are (a) a polytopal rearrangement within each Fe(CO) moiety or (b) exchange between metal centres. For (a),the '7Fe satellite sub-spectrum would be two doublets with separations 'J57Fe-13c and 2J57Fe-13c. As the latter coupling is probably small the satellites arising from it would probably be hidden. If on the other hand (6) is the exchange process the satellite sub- spectrum should be one doublet only with a maximum separation of 3[1J57Fe-13c + 2J57Fe-13c 3. The values obtained for the sulphur containing compounds above range from 29.3 to 26.9 Hz favouring mechanism (a). '* J. Hauer G. Volkel and H.-D. Ludemann Chem. Phys.Letr. 1981,78,85. s9 S. Aime and D. Osella J. Organornet. Chem.,1981 214 C27.

ISSN:0069-3030    

DOI:10.1039/OC9817800015

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

2 Physical Methods Part (iii) High-pressure Chemistry By N. S. ISAACS Department of Chemistry University of Reading Whiteknights Reading Berkshire RG6 2AH With some 60 groups carrying out aspects of chemistry at pressures in the kbar range this topic must be regarded as one of major importance. Two principal objectives may be sought in carrying out reactions under pressure in solution. Rates may be measured as a function of pressure and the volume of activation AV* obtained according to equation (1) (AV* is defined as the difference in partial molar volumes of the transition state and reagents).* This is most usefully combined with a measurement of the overall volume change for the reaction to give a ‘volume profile’ which is proving to be a unique probe into the nature of the transition state.A similar relationship holds for equilibria equation (2) where AV is the volume change for the reaction. Pressure will therefore favour a process that involves a diminution in volume and conversely. The other aspect of interest is to employ this principle for preparative chemistry in order to bring about reactions that have a large negative value of AV* but which will not occur at 1 bar. Much activity around the world is evident on both fronts. -RT[d In k/dp]p+o = AV* (1) -RT[d In K/dp]p+O= AV (2) The subject has been thoroughly reviewed recently in two general review articles and data compilation^^.^ in which some 1500 volumes of activation and volumes of reaction of both organic and inorganic reactions are collected.Pressure effects on enzymic reactions,’ on displacements from co-ordination com- pound~,~*’ and upon isotope effects’ are among more specialized articles. The 50th Commemorative Volume of Reviews of Physical Chemistry of Japan is devoted to review articles on Modern Aspects of Physical Chemistry at High Pressure; topics * The units of molar volume throughout this article are cm’ mot-’. * ‘High Pressure Chemistry’ ed. H. Kelm Reidel Amsterdam 1978. ’ N. S.Isaacs ‘Liquid Phase High Pressure Chemistry’ Wiley Chichester 1981. T. Asano and W. J. le Noble Chem. Rev. 1978,78,407. W.J . le Noble and H. Kelm Angew. Chem. Znt. Ed. Engl. 1980 19,841. E. Morild ‘Theory of Pressure Effects on Enzymes’ Adv. Protein Chem. 1981,34,93.G. A.Lawrance and D. R. Stranks Acc. Chem. Res. 1979,12,403. T.W.Swaddle Inorg. Chem. 1980,14 3203. N.S.Isaacs ‘Effect of Pressure on Kinetic Isotope Effects’ in ‘Isotopes in Organic Chemistry’ Vol. 6 in press. 29 N. S. Isaacs of interest to organic chemists treated here include pressure effects on conforma-ti~n,'~ micelle f~rmation,~' volume profiles," kinetic and thermodynamic parameter^,'^ electro~triction,~' inorganic reactions," radical and biophysical chemistry.'" The proceedings of the 7th AIRAPT conference" contains much information concerning current interests. N.m.r. techniques and results at high pressure have also been reviewed." There are no physical measurements which it seems cannot be carried out under high pressure.However the problem remains of interpreting high-pressure data and extracting the mechanistic information which is latent in values of AV' or AV and their dependence on temperature solvent or other variable. Values of AV* and A are often partitioned into two components A V,,the intrinsic volume change i.e. change in van der Waals volumes of reagents in conversion into products or transition states and A V, the electrostrictive volume change comprising the change in volume of the solvation sphere. Ionogenic reactions are expected to show a large negative value of A V that may mask A Vi,while the latter may be the quantity most easily interpreted in terms of mechanism. Asano has derived an expression [equation (3)] whereby AV; may be obtained as the intercept of a linear plot,'* where k, k are rates at P and 1 bar respectively K is the compressibility of the solvent and B its constant in the Tait equation.As an instance the Menshutkin reaction between Et3N and EtI has AV* = -54 whereas the calculated AVi = -8.7 it follows that AV; = -45.3 consistent with an SN2displacement accom- panied by large charge separation. Analysis of actual molar volumes of alkanes in terms of three contributions from the molecular van der Waals volume V, the void volume V (space between molecules) and expansion volume V, has led to the conclusion that it is the latter which accounts for the increase in volume on cyclization rather than void volume at the centre of the ring which is sometimes assumed to be the case.13 Association of two molecules with no change in charge is accompanied by a reduction in volume.The formation of Meisenheimer complexes between nitroaryl ethers and methoxide in methanol is no exception but the overall volume changes are much less negative as a result of delocalization of charge and relaxation of the tight solvation sphere around the methoxide ion.l4 In water the attachment of OH- is accompanied by a much larger release of electrostricted solvent. Hydrogen bonding causes only a small volume change for example association of phenol in carbon tetrachloride is accompanied by A e = -2.4,15 whereas charge-transfer interactions between tetracyanoethylene and arenes is much more variable -3 to (a)E. Whalley Review of Physical Chemistry of Japan 50th Commemorative Edition 1980 119; (b) H..W.Offen ibid. p. 97; (c)W. J. le Noble ibid. p. 207; (d)B. S. El'yanov and E. M. Vasilvitskaya ibid. p. 169; (e) S. D. Hamann ibid. p. 147; (f) R. Van Eldik and H. Kelm ibid. p. 185; (g) M. Zhulin ibid. p. 217; (h)K. Heremans ibid. p. 259. lo B. Vodar and P. Marteau Proc. AIRAPT Conf. Le Creusot 1979. A. E. Merbach and H. Vanni Helv. Chim. Acra 1977.60.1124. T.Asano Rev. Phys. Chem. Jpn. 1979 49 109. l3 T. Asano Rev. Phys. Chem. Jpn. 1979,49 56. M. Sasaki N. Takisawa F. Amita and J. Osugi J. Am. Chem. SOC.,1980 102,7268. Is C. Josefiak and G. M. Schneider J. Phys. Chem. 1980,84 3004. Physical Methods and Techniques -Part (iii) High Pressure Chemistry 31 -15 if reliance is placed upon the Benesi-Hildebrand method for evaluating equilibrium constants for association.l6 Ion pairing of small highly charged ions is well known to bring about a large increase in volume owing to a reduction in solvation but the association of large delocalized ions such as (1)and (2) in water at least shows a small reduction indicating little change in the solvation pattern.17 Release of the solvation shell around Group IA cations is evident when they are complexed by cryptands and related polydentate ligands. The better the fit between ion and complexing agent the more positive the value of AV." [2,2,2]-Cryptand for example shows a maximum value of A V for complexation of K' and Rb' (+16) and a minimum for Li' (+3). There is much interest in complexation by cyclodextrins as the pressure effect of the equilibrium formation of inclusion complexes with some fluorescent indicators has been considered to model the behaviour of ligand-protein complexes1g thereby permitting a deeper understanding of these processes.Many proton-transfer reactions have been examined under high pressure both as equilibria and as rate processes. The latter are frequently fast enough to require the use of stopped-flow P-or T-jump techniques. Further work on reaction (4) NMe, / 02N~CH,N02 (4) -+ NH=C \ NMe, / 02N0CHN02 H2&C \NMe, -originally studied by Caldin,20 has been reported. Volumes of activation have been determined for both proton- and deuteron-transfer by stopped-flow analysis in CH2C1 as AV*(H) = AV*(D) = -15 but in toluene AV*(H) = -18 AV*(D) = -24.21*22 The isotopic effect has been shown' to be associated with reactions in which hydrogen transfer partakes of quantum mechanical tunnelling and is equivalent to a kinetic isotope effect which is diminished at high pressure.In this l6 T. Nakayama Rev. Phys. Chem. Jpn. 1979,49,25. R.K.Williams J. Phys. Chem. 1981,85 1795. N. Morel-Desrosiers and J.-P. Morel J. Am. Chem. SOC. 1981 103 4743. l9 P. M. Torgerson H. G. Drickamer and G. Weber Biochemistry 1979.18 3079. 'O C.D. Hubbard C. J. Wilson and E. F. Caldin J. Am. Chem. SOC. 1976,98 1870. M. Sasaki N. Sugimoto and J. Osugi Chem Lett. 1980 887. 22 N. Sugimoto M. Sasaki and J. Osugi submitted to Bull. Chem. SOC. Jpn.; personal communication. 32 N.S. Isaacs case the effect amounts to kH/kD=11.9 (1 bar) 9.3 (1 kbar) and is similar to effects previously Pressure can affect conformational equilibria favouring the conformer of lowest volume.9a Recently a Raman investigation on conformational equilibria in 1-bromoalkanes has shown that the gauche conformer at C-1-C-2 is lower in volume than the trans by 0.6-1.3°h.24 This is true also of 1,1,2-trichloroethane,2s The conformational preference about the N-CO bond of a cyclic amide depends upon the ring size above 15-membered rings the trans-geometry is favoured by pressure whereas below the cis-isomer is preferred.26 The volume of activation for conformational rotation in dimethylacetamide has been obtained from the pressure dependence of the ‘H n.m.r.signals of the CH groups. In many aprotic solvents AV* == 10 whereas in water the value becomes ~0.~’ The explanation tendered seems to imply that specific solvation of the acetamide occurs in the non-protic solvents requiring an increase in volume during rotation. Water however forms an open hydration shell within which the acetamide may rotate and undergo conformational change with little or no increase in volume. It seems possible that some sort of compensating effect is also operating. However in water A V*rapidly increases upon the addition of small quantities of ionic solutes or urea known to have drastic effects upon solvent structuring which are held to confirm this analysis. N.m.r has also been used to probe the rotational volume requirements of the ribose-base bond of a nucleoside using (3) as a model.’’ The small value of AV* in water is taken as evidence that rotation occurs in a number of small discrete steps.H OVNV0 (3) cis-Azocompounds can be generated by photolysis under pressure and their thermal isomerization to the trans- compounds studied kinetically. Mechanisms suggested are either by inversion (both aromatic rings remaining parallel) or by rotation (in which they become orthogonal). Isomerization of the conjugated and dipolar 4-amino-4’-nitroazobenzenes has been used as a probe into the mechanism (Scheme 1). Inversion should require only a small change in volume as the dipole remains intact throughout but rotation by decoupling the aryl rings would result in a loss of the dipole in the transition state and should result in a larger positive value of AV*.The experimental value obtained is very solvent dependent 0 in hexane but -22 in benzene suggesting a change of mechanism from inversion to 23 N. S. Isaacs K. Javaid and E. Rannala J. Chem. SOC.,Perkin Trans. 2 1978.709. 24 D. J. Gardiner R. W. Jackson and B. P. Straughan J. Chem. SOC.,Chem. Commun. 1981 159. ’’Ref. 2,p. 174. 26 A. Sera H. Yariada and H. Masaki Chem. Lett. 1980 1533. 27 G. Volkel J. H.iuer and H.-D. Ludemann,Angew. Chem. In?. Ed. Engf. 1980 19,945. 28 G.Klimke J. Hauer H.-D. Ludemann and W. Pfleiderer,J. Chem. Res. (S),1981,80. Physical Methods and Techniques -Part (iii) High Pressure Chemistry NR2 transition state Scheme 1 rotation respecti~ely.*~*~~ A 3,3'-bridged azobenzene which cannot undergo inversion for steric reasons has AV' = -22 and the rotational mechanism is inferred.30 Cycloadditions are invariably associated with large negative volumes of reaction and activation and have been extensively in~estigated.~ Pressure effects on several new types have been recently reported.Addition to a pyridinium betaine (4) resembles in its volume profile a Diels-Alder reaction (AV' and AV both -30 to -40),despite the loss of the dipolar character of the betai~~e.~' Ar / 00-7ZOOEt Et02CJ-J EtOH N I AV' = -36 Ar AV = -37 (4) Scheme 2 Intramolecular Diels-Alder reaction of (5) shown in Scheme 3 is found to have A V*= A which is typical of many intramolecular examples although slightly less 29 T.Asano. J. Am. Chem. SOC.,1980,102,1205 T. Asano T. Okada S. Shinkai K. Shigematsu Y.Kusano and 0.Manabe J. Am. Chem. Soc. 1981 103,5161. 31 N. S. Isaacs and P. Van der Beeke J. Chem. SOC.Perkin Trans. 2 in press. N. S. Isaacs negative (as is also the entropy of a~tivation).~~ This may be held to support the view that the association of two molecules brings about a volume reduction of ca. -10 and the formation of a covalent bond a further reduction of the same magnitude. Only the former quantity is absent from the intramolecular reaction. Ph' (5) AV' = -25 AV = -25 Scheme 3 The ene reaction has also received attention and the reaction of (6) with (7) exemplifies the existence of a generally tight product-like transition An exception to this pattern appears to be the dimerization of 2,3-dimethylbuta-1,3-diene (8).34In this case the volume of activation is considerably less negative than the volume of reaction suggesting a transition state which does not lie close to products.The entropy of activation is correspondingly less negative than usual ( -28 cal K-' mol-' compared to -41 for isoprene dimerization). It is suggested that in this case a 2-step diradical mechanism may be operating or at least is in competition with the normal concerted process. This seems the more likely since this diene is known to have an energy barrier to the cis-coplanar conformation required for reaction. Considerable interest is being shown in the use of high pressures for synthetic purposes taking advantage of the large negative activation volumes of cycloaddi-tion reactions.In such work the highest pressures attainable are used frequently in the 10-15 kbar range. Some examples of recent successes include the single-step syntheses of thia- and selena-fulvalenes (Scheme 4),35*36 synthesis of cantharadin (9) (Scheme 5),37 and synthesis of cyclobutanones (Scheme 6).38 32 N. S. Isaacs and P. Van der Beeke Tetrahedron Lett. in press. 33 M. Papadopoulos and G. Jenner Tetrahedron Lett. 1981,22,2773. 34 G. Jenner and J. Rirnrnelin Tetrahedron Lett. 1980,21,3039. 35 Y.Okarnoto and P. S. Wojciechowski J. Chem. SOC.,Chem. Commun. 1981,669. 36 J. E.Rice and Y. Okarnoto J. Org. Chem. 1981,46,446.37 W. G.Dauben C. R. Kessel and K. H. Takernura J. Am. Chem. SOC., 1980,102,6893. 38 N.S.Isaacs and P. Van der Beeke unpublished work. Physical Methods and Techniques -Part (iii) High Pressure Chemistry 35 -RCECR' + 2CX R = COOEt,R' = H R X X R (9) cis and rrans Scheme 4 Scheme 5 Ph,C=C=O 30 "C + 0-ph-m 0 Scheme 6 The use of high pressure techniques in the synthesis of heterocycles has been reviewed. 39 The complex addition-elimination reaction between dibutylamine and benzo- quinone (Scheme 7) is extremely pressure-dependent and evidently proceeds via a dipolar intermediate which may be reached by an electron tran~fer.~' .yBu2 '-# H -'.-.* 0 0 I ;HBu, Bu,N bNBU2 0 0-AV*(M~CN) = -67 Reagents i HNBu,; ii p-benzoquinone Scheme 7 Pressure studies of additions of methanol to nitriles forming iminoethers have also been reported and are moderately favoured by pressure.41 Furan will undergo '' K.Matsumoto T. Uchida and M. Acheson Heterocycles 1981 16 1367. M. Sasaki M. Bando Y. Inagaki F. Amita and J. Osugi J. Chem. SOC. Chem. Commun. 1981 725. 41 H. Inoue Rev. Phys. Chem. Jpn. 1978,48 105; ibid. 1979,49,95. 36 N. S. Isaacs a Friedel-Crafts reaction with a,&unsaturated carbonyl compounds at 100 “Cand atmospheric pressure with very poor yields. At 3 kbar 75% of 3-(2-furyl)pro- panoate may be obtained from the addition of a~rylate.~~ On the other hand dissociative processes will often be in evidence by their positive volumes of activa- tion for example the retro-aldol reaction shown in Scheme 8.43When catalysed by OH- AVS = +6.9,whereas catalysis by ethylamine is accelerated by pressure.These figures point to rate-determining steps which are different in type. Similarly the loss of nitrogen from a diazonium ion is accompanied by an increase in volume of +8 to +10 cm3 mol-’ a value in accordance with the formation of an aryl cation and dinitrogen as initial prod~cts.~~*~~ 2CH3COCH3 CH2COCH2 I ,C-CH3 OH H3C ‘CH2-c-/ \ CH3 NR2 Scheme 8 Solvolytic reactions have received as might be expected a great deal of attention by workers studying high-pressure reactions and dozens of activation volumes have been recorded. Unfortunately the results are not always straightforward since in most cases both Vi and V make major contributions to the experimental value and little attempt has been made to disentangle these effects.Seldom can one use an experimental volume of activation as a criterion of mechanism in this field at the present although undoubtedly valuable information is there. Values of A V* are usually negative but systems undergoing sN1 and sN2 mechanisms tend to be indistinguishable e.g. solvolyses of Me-OTos and But-Br in aqueous acetone have AV* = -17 and -24 respectively. Some additional examples do little to clarify the It is shown however that such data may be extremely temperature dependent; the acetolysis of 2-aryl-2-propyl tosylate (presumably by an ionization mechanism) has a maximum negative volume of activation at 74 0C.26The difficulties experienced in this field seem on the whole to point to the inability to interpret complex solvation effects.Line broadening at high pressure of the ‘H n.m.r. spectrum of the cation (lo) which undergoes rapid degenerate rearrangement (Scheme 9) yields an activation volume A V* = 8.7 in agreement with the charge dispersal in the transition 42 G. Jenner J. Rimmelin F. Antoni S. Libs and E. Schleiffer Bull. SOC.Chem. Fr. 11 1981,65. 43 F.Gr~nland and B. Anderson in ref. 10 807. 44 T.Kuokkanen Finn. Chem. Lert. 1980 189. 45 T.Kuokkanen Finn. Chem. Lett. 1980 192. 46 0.C. Kwun J. R. Kim and J. C. Ryu Taehan Hwahakhoe Chi 1981,25,152. 47 V.Nunes and M. Calado Rev. Port. Quim. 1979 21 139. 48 J. Hwang S.Yoh and J. Jee Taehan Hwahakhoe Chi 1980,24,150. 49 W.J. le Noble S. Bitterman P. Staub F. R. Meyer and A. E. Merbach J. Org. Chem. 1979,44,3263. Physical Methods and Techniques -Part (iii) High Pressure Chemistry 37 Me0 OMe Me0 4-(10) Scheme 9 Valuable information concerning 1-and 2-bond fission of peroxides azo-compounds and other radical precursors has been published by Neuman," in which the solvent-cage effects are considered particularly important. An examin- ation of pressure effects on rates and product yields from the decomposition of azocumene (11)gives evidence that the unstable 'semibenzene' products (12)result from cage recombination of geminate radical pairs rather than by diffusion apart followed by attack on a molecule of the azo-compo~nd.~~ AV' = +5 ArCMe,N=NCMe,Ar -ArdMe + N, 11 AVt = +6 ArCMe H E.s.r.of radicals has been successfully adapted to high-pressure work and the volumes of activation for radical reactions can now be studied directly. The radical dissociations in equations (5) and (6) have been studied in this way.52 The less- positive value for the phosphorus radical is taken as indicating an earlier transition state for bond fission than that of iminyl radical since the latter reaction is much more endothermic. The kinetic isotope effect of the internal hydrogen-atom transfer of the radical (13) has been measured by e.s.r. at high pressure. This reaction has a very curved Arrhenius plot and shows characteristics of a tunnelling reaction.It is of interest therefore that this reaction is a further example of one whose isotope effect diminishes with pressure or volumes of activation are i~otope-dependent.~~ BU'*C=& + BU" + BU'CN AV' = +3 (5) Bu'O+(OEt) + But' + O=P(OEt) AV* = +0.2 (6) CL3 L "P2Me *"'VC\ I ,Me B~tQ~'Me AV'(H) = +5.3 Me + \ AV*(D) = -1.2 \ Bu' But (13) L = H or D " R. C. Neuman Acc. Chem. Res. 1972,5,381. '' R. C. Neuman and M. J. Amrich J. Org. Chem. 1980,454629. 52 P. R. Marriott and D. Griller J. Am. Chem. Soc. 1981,103 1521. 38 N. S. Isaacs Radical recombination reactions are normally associated with a decrease in volume. It is surprising to learn therefore that several radicals generated by a laser flash apparently undergo recombination as judged by spectrophotometric observations at rates which diminished with pressure.53 In some instances both dissociation and recombination processes were studied and both assigned values of A V’ of the same sign.The meaning of these results is far from clear and indeed have been queried.54 There is considerable current interest in pressure effects upon reactions of biological although often their complexity renders interpretation somewhat difficult. Detailed study of reactions catalysed by isolated enzymes with simple substrates offers a better chance of elucidating some details of the mechan- isms. For example hydrolysis of pentyl and heptyl benzoates by a-chymotrypsin has been studied under pressure.55 Michaelis-Menten kinetics were observed and the Michaelis constant Kappand the catalytic constants k,, were evaluated giving volumes of dissociation of the enzyme-substrate complex of -7 and volumes of activation for the rate-determining step of -20.The latter was consistent with that expected for formation of two hydrogen bonds from the substrate and the co- ordication of serine-195 to give the tetrahedral intermediate together with the breaking of hydrophobic interactions inferred to be involved in the deacylation step. A similar type of reaction is the hydrolysis-aminolysis of long-chain p-nitrophenyl esters which serves as a model system for enzymic proteolytic processes.56 Activation volumes for displacement of p-nitrophenoxide by four competing processes were evaluated separately [by buffer OH- RNH2 and (RHN2)2] i.e.nucleophilically assisted aminolysis. Also the effect of chain length of both amine and ester on AV’ values led to the conclusion that there is a contribution to the activation volume of about +1 cm3mol-’ for each pair of methylenes on the two chains in contact owing to hydrophobic interactions. Finally mention must be made of current interest in the pressure effects upon micelle formation of detergent molecules. The solubilization effects of these micro- emulsions stabilized by a combination of hydrophobic and polar forces make such systems of more than scientific interest. The critical micellar concentration (c.m.c) is an easily measured property representing the saturation concentration of the free solute and increases to a maximum with pressure usually in the 2-5 kbar range depending upon the compo~nd.~’ The method of measurement i.e.electrical conductivity or addition of naphthalene as a fluorescent probe may however lead to conflicting values.’’ Micellar stability is assessed from the volume and compressi- bility of the solution both above and below the c.m.c. It appears that micelles are more compressible than solutions containing dispersed molecule^.^^ Micellar size however is relatively unaffected by pressure. ” A. I. Yasmenko I. V. Khudyakov A. P. Darmanjan V. A. Kuzmin and S. Claesson Chem. Scr. 1981,49. 54 W. J. le Noble personal communication. ’’ Y. Taniguchi and K. Susuki Bull. Chem. SOC.Jpn. 1980,53 1709. E. Morild and G. Aksnes Acta. Chem. Scand Ser. A 1981,35 169. 57 Y. Taniguchi and K. Susuki Rev. Phys. Chem. Jpn. 1980,49,91. S. D. Hamann Rev. Phys. Chem. Jpn. 1978,48,60.

ISSN:0069-3030    

DOI:10.1039/OC9817800029

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

3 TheoreticaI Chemist ry By H.S. RZEPA Department of Chemistry Imperial College of Science and Technology London S W7 2AY 1 Introduction A further volume in the series of Specialist Periodical Reports’ and the proceedings of a conference on computational theoretical organic chemistry’ provide reviews of recent advances in this field and Clementi3 has described in detail one approach to the study of large chemical systems of biological importance using quantum mechanical methods. Two interesting developments are the increasingly common study of organometallic systems and their reactions by ab initio SCF-MO techniques (see below) and the setting up of computer data bases for ab initio molecular wave function^^" and their literature citation^.^' 2 Advances in Theoretical Techniques Geometry Optimization.2a-Schaefer and co-workers’ have extended their study of analytical energy derivatives (i.e.with respect to the nuclear co-ordinates) by formulating expressions for the gradients of open-shell/CI wavefunctions illustrat- ing the technique by calculating the geometry and the vibrational frequencies of the ‘Al and 3B1states of methylene. The difference in the zero-point energies between the two states was then calculated and used in an estimate of the ‘most probable’ value of 41 f 6 kJ mol-’ for the singlet-triplet energy difference. Developments in theoretical techniques seem often to be reflected in calculations on this species! Improvements to the algorithms for calculating energy derivatives continue to be made6a and in one particularly elegant example6’ molecular symmetry was used to reduce the time taken for the ab initio analytical evaluation of the Hessian (i.e.the second derivative or force constant) matrix [cf.Annu. Rep. Bog. Chem. Sect. B 1980 77 161. The time was found to be proportional to 1/N ‘Theoretical Chemistry’ ed. C. Thompson (Specialist Periodical Reports) Royal Society of Chemistry 1981 Vol. 4. (a) H. B. Schlegel Nato Adu. Study Inst. Set. Ser. C Comput. Theor. Org. Chem. 1981 67 129 J. D. Goddard ibid. p. 161; (b)R. A. Poirier ibid. p. 15; (c)M. A. Robb and R. H. A. Eade ibid. p. 21. E. Clementi Lecture Notes in Chemistry. Vol. 16 Computational Aspects for Large Chemical Systems Springer-Verlag New York 1980. (a)R. A. Whiteside J.S. Binkley R. Krishnan D. J. DeFrees,,H. B. Schiegel and J. A. Pople Carnegie Melfon Quantum Chemistry Archive Carnegie-Mellon University Pittsburgh PA (USA) 1980; (6)Y. Osamura S. Yamabe F. Hirota H. Hosoya S. Iwata H. Kashiwagi K. Morokuma M. Togasi and S. Obara J. Chem. Znf. Comput. Sci. 1981,21 86. Y. Osamura Y. Yamaguchi and H. F. Schaefer J. Chem. Phys. 1981,75 2919. (a) L. R. Kahn J. Chem. Phys. 1981 75 3962; (a) T. Takada M. Dupuis and H. F. King ibid. 1981,75 332. 39 H. S. Rzepa where N is the order of the point group corresponding to the molecular symmetry and in this way the vibrational frequencies of ethane could be calculated in ca. 20 times less time than more conventional methods. It is also important to use efficient energy-minimization algorithms for the optimization of molecular geometries.Iterative algorithms based on variable-metric methods [such as the Davidon- Fletcher-Powell (DFP) or the generally more robust Broyden-Fletcher-Goldfarb-Shanno (BFGS) procedure],' in which only a local approximation to the inverse Hessian matrix is evaluated were foundga to be faster or at least equal in speed to the force relaxation method of Pulay,g6 where the inverse Hessian is obtained via numerical differentiation of the first derivatives. The force relaxation method may yet prove to be superior if the inverse Hessian is obtained analytically as in the example of ethane above. BasisSets.26-New minimum and extended basis sets for use in ab initio calculations continue to be recommended for specific applications.The Schleyer groupg have developed STO (4-31+G) sets in which extra p functions are added to all the first row atoms and which are claimed to be suitable for calculations involving anionic species. Such basis sets were used inter alia in studying proton affinities. Morokuma's group" have shown how effective core potentials (ECP) for the first row elements can be used in the calculation of geometries and energies of molecules and the transition states for their interconversion. They used a STO 31G valence basis set in conjunction with analytical gradients and found that the energies differed by less than 8 kJ mol-' and the geometries by less than 4 pm or 0.5" from the values obtained using the all electron 4-31G basis set.This approach promises to decrease significantly the time taken by ab initio calculations. The last year has seen a proliferation of basis sets for elements of higher rows of the periodic table such as for Rb Sr In-Xe,"" Sc-Cu,"* and the transition metals of the first and second series,"' greatly facilitating studies involving organometallic species. Electron Correlation.2c-This immensely difficult problem continues to attract interest. The CASSCF (Complete Active Space SCF) method has been developed12 and is an MCSCF scheme [cf. Annu. Rep. Prog. Chern. Sect. B 1980 77 171 in which no selection of individual configurations is required. This selection problem has also been tackled using many-body-perturbation theory. A coupled-cluster- singles-doubles (CCSD) method is reported13 to be a good approximation to a 'full' Configuration Interaction (CI) approach in which th 7 number of terms is limited to a function of N6 (where N is the number of basis functions).These types of ' R. W. H. Sargent ACSSymp. Ser. 1980,124,37. (a) F. Bernardi A. Bottoni and G. Tonachini J. Mof.Sfrucr. Themhem. 1981 86 123; (6)P. Pulay G. Fogarasi F. Pang and J. E. Boggs J. Am. Chem. SOC.,1979,101,2550. J. Chandrasekhar J. G. Andrade and P. von R. Schleyer J. Am. Chem. SOC.,1981,103 5609. S. Obara K.Kitaura and K. Morokuma Theor. Chim. Actu 1981,60 227. (a)W.J. Pietro E. S. Blurock R. F. Hout W. J. Hehre D. J. DeFrees and R. F. Stewart Inorg. Chem. 1981,20 3650; (b)A. K. Rappe T. A. Smedley and W.A. Goddard J. Phys. Chem. 1981 85 2607;(c) I. Hyla-Kryspin J. Demuynck A. Strich and M. Bknard J. Chem. Phys. 1981 75 3954;(d)W. R. Wadt J. Am. Chem. SOC.,1981,103,6053. l2 P. E. M. Siegbahn J. Almlof A. Heiberg and B. 0.Roos J. Chem. Phys. 1981,74,2384. l3 (a) G.D. Purvis and R. J. Bartlett J. Chem. Phys. 1982,76 1910;(b)R. J. Bartlett Annu. Reu. Phys. Chem. 1981 32,359. Theoretical Chemistry 41 methods enable in some cases more than 98% of the correlation energy to be recovered although at considerable computational expense. Semi-empirical and Other More Approximate Methods.-Thie114" has presented a modified version (MNDOC) of the MNDO method which has been parameterized for the elements C H N and 0.The problem of electron correlation is dealt with explicitly rather than in the averaged parametric manner of MNDO and for molecules in which electron correlation effects are not unusual MNDOC was found to give results similar to MNDO.Significant differences were found for e.g. cyclo-butadiene at a square (D4h) geometry where MNDOC predicts the singlet state to be ca. 42 kJ mol-' more stable than the triplet apparently in violation of Hundt's rule (!) but in agreement with ab initio calculations. This is purely a correlation effect since the two states are formally degenerate in the NDDO approximation upon which MNDO is based. Another interesting result was that the most stable geometry of [18]annulene is predicted to have D6h rather than D3hSYX'nmetry. Although this agrees with the X-ray structure it differs from other semi-empirical and ab initio calculations that have been made using uncorreluted wavefunctions [cf.Annu.Rep. Prog. Chem. Sect. B 1980,77,22]. It is ironic that current SCF-MO theory has such problems in dealing correctly with the types of molecules with which molecular orbital theory at the Huckel level began! Thie114' has also applied the MNDOC method to the study of excited states finding that although vertical excitation energies are calculated to be too low by cu. 1eV singlet-triplet energy separations are predicted very well. A related in~estigation'~' into the use of split valence shell basis sets at the NDDO level was not encouraging yielding results that were no better than the original MNDO procedure for the prediction of molecular energies.MNDO parameters for aluminium have been reported'' and the results of calculations for species containiig lithium have been cited,I6 although the parameters have unfortunately not yet been published. A method based on a new formalism for U/Tseparability (QCFF/PI) has been parameterized for molecules containing nitrogen and oxygen" and it appears to be useful in calculating the equilibrium conformations electronic spectra vibra- tional spectra and resonance Raman spectra of large biological chromophores and for studying the effect of solvent or protein environment on these properties. One curious property of semi-empirical methods and CND0/2 in particular is the high degree of charge alternation predicted for polar molecules.An experi- mental study was found not to support this although an indication was given that the MNDO method may give more physically reasonable results.18 3 Electronic Structure and Geometries of Molecules Conformations and Intermolecular Interactions.-Molecular mechanics calcula- tions seem capable of quite subtle indications of molecular deformations and 14 (a) W. Thiel J. Am.Chem. SOC.,1981,103,1413 1420; (b) A.Schweig and W. Thiel ibid. p. 1425; (c) W. Thiel Theor. Chim. Acra 1981,59,191. L. Davis R.M. Guidry J. R. Williams M. J. S. Dewar and H. S. Rzepa J. Compur. Chem. 1981,2 1s 433. 16 J. Chandrasekhar and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1981,260. 17 A.Warshel and A. Lappicirella J. Am.Chem.Soc. 1981,103,4664. 18 R. D. Stolow P. W. Samal and J. W. Giants J. Am. Chem. SOC.,1981,103 197. H. S. Rzepa reactivities. Ab initio calculations had indicated [cf.Annu. Rep. Prog. Chern. Sect. B 1980,77,21] that norbornadiene has non-planar double bonds with the hydrogen atoms on the endo-side of the molecule and this finding is also now reproduced by MM2 molecular mechanics calculati~ns.'~ This suggests that these deformations to not occur as a result of orbital repulsions for which the MM2 method is of course not parameterized. Other applications of this method include a study of the conformation of valerolactone,20 in which the calculated rotational constants of the half-chair form were found to be in better accord with those obtained from the microwave spectrum than those calculated for the boat form.A study2' (using the molecular mechanics method) of the formation of acylchy- motrypsins as a model for enzyme-substrate interactions involved considering more than 40000 non-bonding interactions alone! Ac-L-Trp-X (X = NH,) is a good substrate binding some lo5times faster than the D-isomer which corresponds to a difference in free energy between the two species at the transition state of ca. 29 kJ mol-'. The calculations indicated that coulombic terms for the three major hydrogen bonds accounted for 20 kJ mol-' of this energy while van der Waals repulsions accounted for 8-13 kJ mol-'. This study is claimed to be the first success- ful computer evaluation of enzyme-substrate specificity! The force field of uracil has now been calculated using an ab initio procedure and a STO-3G basis set with the force constants being corrected to '4-31G basis set quality'.22" Many uncertainties in the force field of benzene appear to have been resolved using 4-21G basis set calculations,22b and sxh studies of the force fields of medium-sized molecules are now becoming fairly routine.As well as being of great help in applications such as the molecular mechanics calculations cited above they can often help to interpret complex experimental spectra. Neutral Species.-Several challenges to experimentalists have been made on the basis of theoretical calculations. Mislow and co-workersZ3 find no theoretical support for the surprisingly short (147 pm) central C-C bond length in hexaphenylethane suggested by a recent X-ray structural determination on a related compound.The molecular mechanics and MNDO methods both predict a value of ca. 168 pm. A class of bridged 'hyperstable' olefins which should display decreased reactivity of the bridgehead double bond (e.g.a smaller than normal heat of hydrogenation) have been [i.e. (l)]and molecular mechanics calculations have indicated l9 U. Burkert Angew. Chem. Int. Ed. Engl. 1981 20 574. T. Philip. R. L. Cook T. B. Malloy N. L. Allinger S. Chang and Y. Yuh J. Am. Chem. Soc. 1981 103,2151. D. F. DeTar J. Am. Chem. Soc. 1981,103 107. 22 (a) Y. Nishimura M. Tsuboi S. Kato and K. Morokuma J. Am. Chem. Soc. 1981 103 1354; (b) P. Pulay G. Fogarasi and J. E. Boggs J.Chem. Phys. 1981,14 3999. 23 E. Osawa Y. Onuki and K. Mislow J. Am. Chem. Soc. 1981,103,7475. 24 W. F. Maier and P. von R. Schleyer J. Am. Chem. Soc. 1981,103,1891. Theoretical Chemistry H H (4) several unknown polycyclic compounds containing 3,4 or 5-membered rings which should be reasonably stable [e.g. (2)and (3)JZ5 Schleyer and his group in a continuing search for compounds containing planar tetraco-ordinate carbon found that (4) was predicted by the MNDO method to prefer a planar geometry. Unfortunately an X-ray study of 2,6-dimethoxyphenyl-lithium showed it to be hexameric and not dirneric.I6 A MNDO studyz6 of [2.2.2.2]paddlane (5) revealed an extraordinary central C-C bond of length 148pm. Planar tetraco-ordinate carbon is sp2 hybridized with a lone pair of electrons in the p orbital perpendicular to the molecular plane and a LUMO with S symmetry located on the substituents.The central bond in (5) is formed from the appropriate overlap of pairs of these orbitals to give a cr bond with some S (!) character i.e. correspondingly shorter than a normal C-C bond. However the energy of this system was so high that it is unlikely to be detected experimentally. S~haefer~~~ has thrown down the gauntlet regarding the structure of dimethyl silaethene. Ab initio calculations using a very large basis set and a substantial configuration interaction (CI) predicted a bond length of 169pm for the Si=C bond compared with the surprisingly long exrerimental value of 183 pm. Morokuma and co-workers independently calculated this bond to be 173 pm long finding also a Si=C vibrational wavenumber (about 1000cm-') in good agreement with that observed.28 Schaefer has also challenged276 a suggestion based on experi- mental evidence that (6) rearranges rapidly at low temperatures to the silylene (7)z7c with an implied barrier of less than 20 kJ mol-'.Ab initio calculations on the parent silaethene using a large basis set and extensive CI indicated a barrier to a [1,2]-hydrogen shift of 170 kJ mol-'! The possibility that addition of one methyl group to silaethene would account for this discrepancy was considered to be remote implying that this reaction may not be a simple hydrogen shift. Many other calculations of species containing silicon continue to be 25 P.Gund and T. M. Gund J. Am. Chem. SOC.,1981,103,4458. 26 E. U. Wurthwein J. Chandrasekhar E. D. Jemmis and P. von R. Schleyer Tetrahedron Lett. 1981 22 843. '' (a) Y. Yoshioka J. D. Goddard and H. F. Schaefer J. Am. Chem. SOC.,1981,103,2452;(b) Y. Yoshioka and H. F. Schaefer ibid. p. 7366;(c) R. T. Conlin and D. L. Wood ibid. p. 1843;T. J. Drahnak J. Michl and R. West ibid. p. 1845. '* K.Morokuma S. Nagase and M. Hanamura Tetrahedron Lett. 1981,22 1813. 29 (a) R. A. Poirier and J. D. Goddard. Chem. Phys. Lett. 1981,80 37;(b) M. H. Lien and A. C. Hopkinson ibid. p. 114;(c)M. S. Gordon and R. D. Koob J. Am. Chem. SOC.,1981 103 2939; (d) M. S. Gordon and J. A. Pople ibid. 1981 103 2945; (e)G.Trinquier and J.-P. Malrieu ibid. p. 6313.44 H. S. Rzepa ~~ H H H \ / .. 1 Si-C -H B Me/si=c\ /\ A H Me H Ab initio calculations indicate3' that the unknown and formally aromatic com- pounds (8) and (9) have about 70% of the resonance energy of the isoelectronic carbocyclic cations and a STO-3G study31 of the recently synthesized dodecahe- drane indicated that although a proton could easily pass through one of the faces into and out from the centre this would not be possible for Li'. Although theoretical calculations are often of great help in the study of molecules with unusual bonding Schaefer and co-workers have shown how difficult it can be to interpret wavefunctions in simple bonding An MCSCF calculation with 13 413 configurations indicated that ozone has 23% biradical character compared with a value of 5.2% obtained using 'only' 6825 configurations and 46% such character obtained using a simple SCF/2 X 2CI procedure.The first value was suggested to be the most soundly based and it was also emphasized that the MCSCF procedure is readily applied to the analytical calculation of first and second deriva- tives enabling the facile optimization of molecular geometries and th,e vibrational analysis of stationary points so located. Such an analysis has often proved useful. Despite being formally aromatic (10) was in this manner to actually correspond to the transition state for racemization of the chiral species (ll)! Charged Species.-Homoaromaticity in cyclic carbanions is considered by several groups of workers to be an over-rated phenomenon.MNDO and STO-3G calcula-tions on (12) show no evidence of the effect and it has been concluded34 that such effects may only arise in carbonium ions. Simple Huckel theory has been used to study the resonance energy in delocalized polycarbanions and oxocarbanion~.~~ Although such theory includes no terms for electron repulsion which is known to be especially important in anionic species it was concluded that of the oxocar- banions only the deltate dianion was highly 30 K. Krogh-Jespersen D. Cremer J. D. Dill J. A. Pople and P. von R. Schleyer J. Am. Chem. Soc. 1981,103,2589. 31 R. L. Disch and J. M. Schulman J. Am. Chem. SOC.,1981,103,3297. 32 W. D. Laidig and H. F. Schaefer J Chem. Phys. 1981,74 3411. 33 E.E. Waali J. Am. Chem. Soc. 1981,103,3604. 34 (a)E. Kaufmann H. Mayr J. Chandrasekhar and P. von R. Schleyer J. Am. Chem. Soc. 1981,103 1375; (b)J. B. Grutzner and W. L. Jorgensen J. Am. Chem. Soc. 1981,103 1372. '' (a) R.B. Bates B. A. Hess C. A. Ogle and L. J. Schaad J. Am. Chem. Soc. 1981 103 5052; (6) J. Aihara ibid. p. 1633. Theoretical Chemistry An interesting conflict has appeared over whether fluorine can stabilize car- banions by hyperconjugation. Apel~ig,~" found that ab initio 4-31G calculations predict (13a X = F) (in which hyperconjugation is possible) is 47 kJ mol-' more stable than (13b) (in which it is not). A similar difference in energy was found for X = CF,. This and the results of a Mulliken population analysis was taken as evidence that both F and CF can stabilize carbanions by hyperconjugation.However Streitwieser and co-w~rkers~~~ have criticized the Mulliken population analysis as being inter alia basis-set dependent. By carrying out a specific integra- tion of the calculated electron density they show that a H-C bond is as effective as a F-C bond if charge transfer is taken as a criterion of hyperconjugation. They conclude that the concept of negative fluorine hyperconjugation is not significant and that the calculated barrier to rotation in (13a) is more correctly explained in terms of a charge-induced polarization of the appropriate (T bond rather than actual charge transfer to the fluorine. 0 (134 (13b) (14) A great deal of attention has been devoted to other ways of stabilizing carbanions.In the system XCH; substituents such as X = Li BH2 CF etc. exert a significant stabilizing effect whereas X = NH2 OH F CH have only a weak stabilizing Anions of the type (14) have been to be stabilized by 'dipole interactions' and an even greater effect was shown if lithium was present (15) being 55 kJ mo1-I more stable than (16). In systems such as LiCH2X (X = OH or NH2) stabilization is achieved by the formation of a non-classical carbene-like species (17).37c The Schleyer group3* has investigated the rather elusive acyl carbanions using MNDO and ab initio 4-31+G calculations. These indicated that bridgehead aldehydes such as 1-adamantyl carboxaldehyde aromatic aldehydes and disub- stituted formamides were best suited for experimental studies of proton abstraction from CHO groups.Clark39 has rationalized the experimental e.s.r. observation that whereas the radical anions formed from CH3X (X= F or Cl) have three-fold symmetry the analogous silicon compounds have lower symmetry with one unique hydrogen. Ab initio 3-21G calculations indicated that the most stable form of CH3XL was best considered as a loose complex between CH; and X-and that the silicon compound had a 'T' structure (18). A suggestion has been made that the observed exchange between CN- and CH3CN is not a simple S,2 substitution but involves nucleophilic addition/elimina- tion with an intermediate [1,2]methyl shift. MNDO calculation^^^ indicate that this (a)Y. Apeloig J. Chem. SOC.,Chem. Commun.1981 396; (b)A. Streitwieser C. M. Berke and G. W. Schriver Tetrahedron (Suppl. 9),1981,37,345. 37 (a)A. Pross D. J. DeFrees B. A. Levi S. K. Pollack L. Radom and W. J. Hehre J. Org. Chem. 1981,46 1693; (b) N. G. Rondan K. N. Houk P. Beak W. J. Zajdel J. Chandrasekhar and P. von R. Schleyer ibid. 1981 46 4108; (c) T. Clark P. von R. Schleyer K. N. Houk and N. G. Rondan J. Chem. SOC.,Chem. Commun. 1981,579. 38 J. Chandrasekhar J. G. Andrade and P. von R. Schleyer J. Am. Chem. SOC.,1981,103,5612. 39 T. Clark J. Chem. SOC.,Chem. Commun. 1981,515. 'O J. G. Andrade T. Clark J. Chadrasekhar and P. von R. Schleyer Tetrahedron Lett. 1981 22 2957. H.S. Rzepa (15) (16) (17) (18) may be the least favourable pathway. The route shown in Scheme 1 is about 68 kJ mol-' lower in energy and makes an interesting suggestion for experimental study.'C HH H-*j Scheme 1 The mechanism of nucleophilic reactions that proceed via electron transfer to form radical intermediates has been investigated using ab initio 3G and 4-31G calculations using CHzNO; + CICHzNOz as a m0de1.~' The species resulting from electron transfer (CICHzNO;) was predicted to dissociate spontaneously to CH,NO; and C1-. A simple point-charge model was used to represent the effects of solvation by a dipolar solvent and the activation energy for electron transfer (in which the nitro-groups act as relays) was estimated at about 104 kJ mol-' or less. An important study of gas-phase SN2reactions using ab initio wavefunctions has shown4* that whereas the geometry of the transition state correlates very well with the heat of reaction (i.e.the Bell-Evans-Polanyi-Leffler-Hammond effect) the activation energy as is well known does not. The calculations enabled a quantitative test of the Marcus theory of such reactions which emerged with flying colours. Interestingly these authors observe that the calculated gas-phase behaviour of HOO- was entirely normal discussing the implications of this finding in terms of the so called a-effect. Lest the impression be given that studies of positively charged species have been neglected the impressive study by the Pople group Qf the C1-C3 carbocations should be The geometries have been optimized at the 6-31G* basis set level correlation energy corrections have been made at the MP4 level and zero- point-energy terms have been included.This definitive study of simple carbocations is claimed to predict relative energies to an accuracy of at least *20 kJ mol-'. Carbenes and Open-Shell Species.-Borden and Da~idson~~ have discussed some of the considerable theoretical problems associated with the quantitative calculation of wavefunctions of diradicals illustrating this with species such as cyclobutadiene 41 B. Bigot D. Roux and L. Salem J. Am. Chem. SOC.,1981,103 5271. 42 S.Wolfe D. J. Mitchell and H. B. Schlegel J. Am. Chem. SOC.,1981 103 7692 7694. K. Raghavachari R. A. Whiteside J. A. Pople and P. von R. Schleyer J. Am. Chem. Soc. 1981, 103,5649. 4* W. T. Borden and E. R. Davidson Ace. Chem. Res.1981,14,69. Theoretical Chemistry 47 cyclopentadienyl cation trimethylenemethane and cyclopropenyl anion. Despite a common belief that the square singlet (*Big) ground state of cyclobutadiene is obliged by the Jahn-Teller theorem to distort to a rectangular species this is not formally correct. They show that distortion can occur by mixing with a higher 'A1 state (uia a vibration of bl symmetry) and such a 'second-order Jahn-Teller effect' can either merely reduce the force constant for this vibration (as in tetra-t-butylcyc- lobutadiene) or actually make it negative (as in cyclobutadiene itself). A detailed consideration of the problems involved in calculations of cyclopropenyl anion makes illuminating reading. The singlet is unlikely to prefer a planar geometry much less one of D3hsymmetry.Related problems were enco~ntered~~ in a study of the degree of dipolar character in methylenecyclopropene. Using ab initio MCSCF wavefunctions calculated at the geometries predicted by the MNDO method states were found for the twisted species (19) that had both diradical and pronounced zwitterionic character. The calculated properties and energies of these states were highly sensitive to the method of calculation although in general the MCSCF approach was concluded to be more reliable than the SCF/CI method. The four basic types of n-orbital initialized photoreactions of ketones (hydrogen- atom abstraction cycloaddition to double bonds a-cleavage and electron transfer) have been analysed using 'natural' orbital correlations.The first two examples belong to a class in which a new (T bond is formed to the ketone whilst in the second two examples a (T bond possessed by the ketone is ruptured. The role of triplet states and charge-transfer states in these reactions was Organometallic Species.-Schaefer has reviewed the state of the art regarding non-correlated ab initio wavefunctions for metals prognosticating that the 1980s will see a large increase in such calc~lations.~~ To illustrate what is currently feasible Schaefer quotes his group's study of (20) (21) and (22) in which the calculated geometries showed good agreement with the experimental structures. An orbital correlation diagram for (20) is presented which has interesting pedagogic possibility.:0.' H U That such calculations can be of genuine interest to organic chemists is illustrated by the theoretical study of trimethylenemethane-bis(phosphine)Pdcomplexes car- ried out by the Fenske-Hall procedure.48 Trost and co-workers had found that the species (23)was formed rather than the isomeric (24),contrary to chemical intuition regarding the relative stability of primary and secondary carbanions. The calcula- tions confirmed that indeed (23) is the more stable this being rationalized in terms of the bonding of the trimethylenemethane fragment with the metal. A cyano-group 45 R. P. Johnson and M. W. Schmidt J. Am. Chem. Soc. 1981,103,3244. 46 B. Bigot A. Devaquet and N. J. Turro J. Am. Chem. SOC.,1981,103,6. 47 (a)H. F. Schaefer J.Mol. Struct. Theochem. 1981 1 117; (b) R. M. Pitzer J. D. Goddard and H. F. Schaefer J. Am. Chem. SOC.,1981 103 5681. 48 D. J. Gordon R. F. Fenske T. N. Nanninga and B. M. Trost J. Am. Chem. SOC.,1981,103 5974. H. S. Rzepa > -,I,' 7' -7 Pd Pd' 4 '\\ L' 't LL (23) (24) perhaps not so surprisingly was predicted to show similar regioselectivity whereas a CF3group was predicted to be much less selective. Morokuma and co-~orkers~~ have illustrated a much more rigorous theoretical approach to the study of organometallic reactions e.g. Pt(PH3) +H2 = Pt(H)2(PH3)2. The calculations were carried out using a double-6 basis set with a relativistic effective core potential and a large 67 000 configuration CI. The transi- tion state for the insertion reaction was located with the aid of analytically calculated gradients and characterized by a vibrational analysis.Zero-point energy corrections led to a value of 34 kJ mol-' for the activation energy corresponding to a very early transition state in which the H-H bond was only stretched by 4%. This is no doubt the forerunner of many such calculations on organometallic systems. 4 Dynamic Processes and Energy Hypersurfaces The MIND0/3C4H4 potential energy surface has been studied by the Dewar group." This surface proved to be of immense complexity and no less than 19 minima corresponding to closed-shell species with no biradical character together with many of the transition states interconnecting them were located! Although some of these species are probably artifacts of the MIND0/3 method several isomers of surprisingly low energy were suggested as being worthy of further study (e.g.25). Such a comprehensive study of the surface of even a relatively small molecular system is still largely beyond an ab initio approach and the use of semi-empirical methods as 'pathfinders' is an obviously useful technique. H (25) A paper by Schaefer and co-w~rkers'~ on an ab initio study of the decomposition of glyoxal to CO and H2 makes very interesting reading. A symmetrical (Cz,) stationary point was located at four basis set levels; STO-3G 3-21G double-[ and double-6 +polarization and a vibrational analysis was carried out for each basis. The STO-3G stationary point had three imaginary frequencies (corresponding to the Hessian matrix having 3 negative eigenvalues) the 3-21G had only one (and was thus a genuine transition state) whereas the two double-6 basis sets had two 49 K.Kitaura S.Obara and K.Morokuma J. Am. Chem. Soc. 1981 103,2891. H. Kollmar F. Carrion M. J. S. Dewar and R. C. Bingham J. Am. Chem. Soc. 1981,103 5292. Y. Osamura H. F. Schaefer M. Dupuis and W. A. Lester J. Chem. Phys. 1981.75 5828. Theoretical Chemistry 49 such vibrations the lower one of ca. 96i cm-' corresponding to an out of plane deformation. The properties of the energy surface are seen to change qualitatively with basis set and such a result serves to emphasize the importance of carrying out a vibrational analysis on any putative transition state.Inclusion of an extensive CI (but without reoptimization of the geometries) reduced the calculated barrier by 75 kJ mol-' to ca. 250 kJ mol-'. The vibrational analyses were not then repeated using such correlated wavefunctions leaving open the question of whether the use of CI could have an effect similar to the change of basis set noted above. The transition state for the dissociation of formaldehyde to CO and H2has been located5' using a double-[ + polarization basis set both with and without the inclusion of CI and characterized by a vibrational analysis. In each case only one imaginary vibration was found although no simple relation between the magnitudes of the SCF and SCF/CI transition state vibrational frequencies was observed. The theor- etical barrier to the reaction was estimated at 364 f 21 kJmol-' compared with a barrier of ca.351 kJ mol-' estimated by the same method for the formation of hydroxycarbene by a [1,2]hydrogen shift. A vibrational analysis of the latter system enabled several predictions pertinent to the possible spectroscopic detection of HO(H)C to be made. A vibrational analysis of substituted chloroethanes and their transition states for dehydrochlorination using the MNDO method enabled the 'H/'H and 35C1/37C1 kinetic isotope effects to be calculated and compared with experiment and the '*C/13C isotope effects to be predicted.53 Although good agreement with the observed isotope effects was found the calculated structures of the transition states differed significantly from previously suggested models.These had been established largely by fitting to observed spectroscopic and kinetic data and included many rather arbitrary approximations. It was concluded that a more reliable and consistent approach may be to test quantum mechanical models against experiment rather than to derive models from kinetic and spectroscopic data alone. Kato and Morokuma have presented an impressive study of the unimolecular reactions of vinyl Ab initio calculations at the 4-31G basis set level were used to determine geometries and to carry out vibrational analyses whereas energies were determined at the 6-31G*/Cl level of theory. Ten elementary reactions such as cis-trans isomerization hydrogen migration and a/aor a/P elimination of HF were investigated on both the So and Tl surfaces in order to better understand the photosensitized photodecomposition of this species.Large basis set effects were observed in a study of the fragmentation of (26) to give CO and ethe~~e.~~ The stationary point of CZvsymmetry was characterized as a true transition state at the 4-3 1G basis set level by a vibrational analysis and the calculated barrier (including correction for zero-point energy differences) was 127 kJ mol-' compared with 360 kJ mol-' using a minimal STO-3G basis. Inclusion of CI had little effect on this barrier although the authors then go on to suggest that a biradical intermediate involving C-0 bond cleavage is only 91 kJ mol-' above (26) and that the transition state leading to this biradical is only a little higher in energy! This apparent 52 J.D. Goddard Y. Yamaguchi and H. F. Schaefer J. Chem. Phys. 1981,75 3459. s3 H. S. Rzepa J. Chem. SOC.,Chem. Comrnun. 1981,939. s4 S. Kato and K. Morokuma J. Chem. Phys. 1981,74,6285. 55 D. Feller E. R. Davidson and W. T. Borden J Am. Chem. Soc. 1981,103 2558. H. S. Rzepa contradiction tends to suggest that a vibrational analysis of the C,,stationary point corresponding to the use of a correlated (i.e. SCF/CI or MCSCF) wavefunction may lead to an entirely different result. Such behaviour has in fact been observed at the MNDO The uncatalysed rearrangements of (27)to (28) were predicted to be concerted reactions involving synchronous attack by the nitrogen lone pair on the acyl carbon atom and concomitant C-0 bond cleavage via a planar transition state.However for the analogous acyl migration in (29) the transition state at the uncorrelated RHF/SCF level was found to be significantly non-planar. A simple 3 x 3 CI at this geometry was found to have little effect on the calculated energy [cf.(26)above] but reoptimization of this geometry using a correlated (UHF) wavefunction changed the character completely. The geometry became planar corresponding to a biradical intermediate and nut to a transition state. A 3 x 3 CI at this geometry showed a large lowering of the energy. One problem in such studies is in selecting the appropriate configurations to be used in the CI since those configurations important at the starting estimate of the geometry (i.e.that predicted using RHF/SCF theory) may not be at all important at the geometry eventually predicted using SCF/CI theory. (26) (27) (28) (29) Several studies have been concerned with the correlation of parameters derived from linear free-energy relationships with calculated electronic proper tie^.^' Non-additive substituent effects in some highly hindered pyridines were found to corre- late very well with geometrical distortions calculated using the MIND0/3 method57a and the best set of ‘normal’ Hammett substituent constants that reflect purely the inductive effect of the substituent have been deduced by a comparison with ab inifiu calculations of the charge distrib~tion.~’~ Finally we note several rather less rigorous methods that have been proposed to enable simple rationalization of regio and stereochemistry in organic reactions.Fukui and ~o-workers~~ have shown how the canonical MOs of the fragments of a composite reacting system can be transformed into a set of paired reagent and reactant hybrid MOs so as to condense the intermolecular overlap density. This is an extension of the well known Frontier Orbital approach to include the contribution of all the relevant MOs. It is hoped that the extent of localization and direction of these hybrid MOs will offer a clear and simple distinction between favourable and unfavourable reactions. Burgess and Li~tta~~ have extended the frontier orbital method to include just the major u-T interactions and have shown how this simple approach (which does not require any computer calculations) can often rationalize observed stereochemistry.56 H. S. Rzepa Tetrahedron 1981 37 3107. 57 (a)J. I. Seeman R. Galzerano K. Curtis J. C. Schug and J. W. Viers J. Am. Chem. SOC.,1981 103 5982; (6) E. R. Vorpagel A. Streitwieser and S. D. Alexandratos J. Am. Chem.SOC.,1981,103 3777. (a) K. Fukui N. Koga and H. Fujimoto J. Am. Chem. Soc. 1981 103 196; (b) H. Fujimoto N. Koga M. Endo and K. Fukui Tetrahedron Lett. 1981 22 3427. 59 E. M. Burgess and C. L. Liotta J. Org. Chem. 1981,46 1703.

ISSN:0069-3030    

DOI:10.1039/OC9817800039

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (i)Pericyclic Reactions By G. B. GILL Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Introduction Pericyclic reactions continue to be an extremely popular and profitable area of study. Major preoccupations of much of this work have been the synthesis of natural products or exploration of synthesis methodology as in the study of tandem sigmatropic rearrangements. Details of such applications are to be found elsewhere in Annual Reports. Ene and Diels-Alder reactions of singlet oxygen with olefins and 1,3-dienes have been reviewed.' The zwitterionic peroxide mechanism is the model whereby the regioselectivity of the hydroperoxidation of tetra-substituted olefins by '02can be rationalized.2 However ab initio MO (4-3 1G) calculations on the '02-propene reaction affords results that are interpreted as a preference for a concerted mechan- ism involving a transition state such as (l).3Both ab initio and semi-empirical calculations predict that perepoxides are too high in energy to qualify as reaction intermediates as are zwitterions for weakly polar alkenes.Although 1,4-diradical intermediates are calculated to be more stable reactions of such species should be non-stereospecific yet should show significant regioselectivities contrary to experi- mental observations for the '02-olefin reactions Hence these calculations appear to exclude all but the concerted mechani~m.~ Evidence has been presented that reactivity and regioselectivity in these reactions may be the subject of conforma- tional control through differences in rotational barriers.' The desirable perpen- dicular conformation (2) with <1234 = 90" is intermediate in energy between the more stable eclipsed conformation (3) <1234 = O" and the more energetic staggered conformation (4) <1234 = 180".Experimental and calculated rotational barriers for Me groups substituting propene are increased by gem-Me substitution (Me2C=CH2) unchanged by trans-Me substitution [(E)-MeCH=CHMe] and decreased for cis-Me substitution [(Z)-MeCH=CHMe].For 2-methylbut-2-ene (5) the percentage H abstraction and rotational barriers (in parentheses kcal ' H. H. Wasserman and J. L. Ives Tetrahedron 1981,37 1825;A.A. Gorman and M. A. J. Rodgers Chem. SUC.Rev. 1981,10 205.C. W.Jefford Helv. Chim. Acta 1981,64 2534. K.Yaniaguchi T.Fueno I. Saito T. Matsuura and K. N. Houk Tetrahedron Lett. 1981,22 749. K.Yamaguchi S.Yabushita T. Fueno and K. N. Houk J. Am. Chem. Suc. 1981,103,5043. ' K. N. Houk J. C. Williams jun. P. A. Mitchell and K. Yamaguchi J. Am. Chem. Suc. 1981,103,949. 51 G. B. Gill 7-4 H--0 Q Me 53(0.47) MeAMe 40(0.61) 7(2.22) (5) mol-’) are as indicated. Thus the greater reactivity of alkyl groups on the disub- stituted side may arise from the relative ease of achieving the necessary perpen- dicular conformations; since all groups lower the rotational barrier of a cis-methyl this explanation of the ‘cis-eff ect’ is quite general. Chiral prosthetic groups have been employed as direct probes of the geometry of the transition state of the Diels-Alder reaction and co-operativity in asymmetric induction is demonstrated to be a direct result of the concertedness of the reactiom6 The relative rate of enantiomer formation (k,/k,) in the presence of two chiral groups will be the product of the rates in the presence of one chiral group that is the linear free-energy corre,lation has the form k,/kp = kalka2/kBlkD2.This relationship is demonstrated by the measured product ratios for the quantitative cycloaddition of 1,3-diphenylisobenzofuranto dialkyl fumarates (6) to give the adducts (7) (R’,R2 = Me,Me Me,l-bornyl or I-borny1,l-bornyl). (6) (7) Ab initio SCF-MO calculations utilizing a STO-3G basis set have been applied for example to 7-hydroxynorbornadiene in which it is demonstrated that the olefinic C-H bonds are bent out of the C-1-C=C-C-4 plane in the endo direction (1.3” for the syn-H’s and 0.9” for the anti-H’~).’~From parallels of molecular distortions and .n-facial stereoselectivities the following general rule has been propo~ed:’~ ‘attack of a reagent at an unsaturated site occurs such as to minimize antibonding secondary orbital interactions between the critical frontier molecular orbital of the reagent and those of vicinal bonds’.Restricted Hartree- Fock calculations on the fragmentation of carbenadioxolane (8) indicate that the L. M. Tolbert and M. B. Ali J. Am. Chem. Soc. 1981,103,2104. (a)N. G. Rondan M. N. Paddon-Row P. Caramella and K. N. Houk J. Am. Chem.Soc. 1981,103 2436; (b) ibid. p. 2438. Reaction Mechanisms -Part (i) Pericyclic Reactions (8) diradical formed by cleavage of just one C-0 bond lies at least 8.8 kcal mol-' below the transition state for concerted fragmentation and it is concluded that the cleavage of one C-0 bond in (8) is a lower-energy process than concerted fragmentation.8 E (9) E = C02Me (10) Deuterium substitution confirms that cis-transfer of two endo-hydrogen atoms occurs in the thermal rearrangement of (9) at 130-160°C. The product (10) was isomerized to (9) when isolated and heated separately.' Thermochemical factors appear to rule out a free-radical mechanism and thus (9) + (10) is an example of the rarely observed concerted hydrogen-transfer rearrangement process k = (2.0 f 0.1) x lop6s-' at 130°C with AH' = 35-39 kcal mol-' and AS*= 2-1 1e.u.Dyotropic rearrangements are likewise comparatively uncommon. The lactones (11;X Y = 0)and lactols (11;X Y = H OH) underwent the indicated formal [2,2] dyotropic rearrangements upon thermolysis at 350 "C (n = 1or 2).1° 2 Cycloadditions and Cycloreversions A further review of intramolecular cycloaddition reactions as applied to the total synthesis of steroids has appeared." Although the stereoselectivities of the additions of acrylic acid derivatives (H,C=CHR; R = CHO C02Me or CN) to cyclopenta- diene in homogeneous solution are largely insensitive to changes in solvents and many catalysts the endo exo ratios are altered by the presence of a heterogeneous carrier substance.'* Thus for example the homogeneous thermal addition of acrolein (R = CHO) to cyclopentadiene affords a ca. 75 :25 mixture of endo exo D. Feller E. R. Davidson and W. T. Borden J. Am. Chem. Soc. 1981,103,2558. J.-P. Hagenbuch B. Stampfli and P. Vogel J. Am. Chem. SOC.,1981,103 3934. lo K. Satb Y. Yamashita and T. Mukai Tetrahedron Lett. 1981,22 5303. T. Kametani and H. Nemoto Tetrahedron 1981 37 3. '* H. Parlar and R. Baumann Angew. Chem. Znt. Ed. En& 1981,20 1014. G.B. Gill (4 + 2) adducts; in the presence of neutral alumina the ratio is altered to ca. 51 :49. In the related reaction of methyl acrylate (R=C02Me) the ratio for the homogeneous reaction ca. 75 :25 is altered to ca. 97 :3. In no case however was the endo preference overturned.Although (i) Lewis acid catalysis of Diels-Alder reactions is well documented (ii) the improvements in stereo- and regio-selectivities thus gained have found wide use in synthesis and (iii) the ca. lo6 rate accelerations have enabled many such processes to be conducted at relatively low temperatures the inherent need for specific complex formation between the Lewis acid and the dienophile does introduce a limitation the catalysed reactions being largely limited to those involving oxygen-bearing electron-deficient dienophiles. Alternative methods for providing rate enhancements are now available. The first of these utilizes the 'hydrophobic effect'; the results obtained for the rates of the (4 + 2) cycloaddition of N-ethyl maleimide to 9-hydroxymethylanthracene are i1lu~trative.l~ At 45 "C the rate con- stants (x lo5M-' s-') obtained for the reaction in various solvents were isoctane (796),butan-1-01 (666),methanol (344),acetonitrile (107),and water (22 600).The decrease in rate with increasing solvent polarity with a large increase for the heterogeneous aqueous system seems to implicate molecular aggregation due to the hydrophobic effect.With less bulky dienes/dienophiles the addition of p-cyclodextrin to provide a hydrophobic cavity provides an enhancement of rate over that of the water reaction; likewise the addition of lithium chloride to increase the hydrophobic effect also provided a rate enhancement (in this case however it is not clear from the data provided if the effect of LiCl may not at least in part be due to Lewis acid catalysis by Li+).In a quite different approach Diels-Alder re- actions have been accelerated by converting dienophiles into their corresponding cation radicals the particularly useful feature here being that neutral and electron- rich dienophiles are those that are most readily ionized to cation radi~a1s.l~ Thus treatment of cyclohexa-1,3-diene with tris(pbromopheny1)aminium hexachloro-stibnate [(~-Brc~&)~Nt SbC16-] in CH2C12 at 0 "C afforded in 15 min a 70% yield of the (4 + 2) adducts (endo:em = 5 :1); the thermal addition proceeds poorly (30%) giving a 4 1 product mixture in 20 h at 200°C. Similar reaction of cyclohexa-1,3-diene with 2,5-dimethylhexa-2,4-dienein the presence of the cation- radical salt gave the adducts (12; endo:exo = 4 :3).This result indicates a prefer- ence for the formation of the cation radical from the more alkylated diene and an unusually low sensitivity of the (4 + 2)t process to steric effects. A possible reaction mechanism is DP + Ar,Nt +Ar,N + DPt DPt + D +At At + DP +A + DP? (D = diene DP = dienophile and A = adduct). l3 D. C. Rideout and R. Breslow J. Am. Chem. Soc. 1980 102,7816. *' D. J. Bellville D. J. Wirth and N. L. Bauld J. Am. Chem. Soc. 1981 103 718. Reaction Mechanisms -Part (i) Pericyclic Reactions The reluctance of 1-azadienes to undergo cycloaddition reactions is emphasized by the work of two groups. l5 Reaction of the pentachloro-1 -azacyclopentadiene (13)with vinyl acetate gave only the (4 + 2) adduct resulting from the participation of the 2-azacyclopentadiene (14) a result ascribed to a less favourable enthalpy change for the 1-relative to the 2-azadiene Diels-Alder reactions.15a Whereas (13) functions as a dienophile in the addition to cyclopentadiene it reacts predominantly as a diene with cyclohexa-1,3-diene and with acyclic dienes such as trans-pipery- lene and then by way of the valence isomer (14).15’ As the HOMO energies of the hydrocarbon dienes are very similar the change in role of the azacyclopen- tadiene system from two- to four-electron cycloaddition component is thought to be determined largely by steric factors.The intramolecular (4 + 2) addition of N-acyl-1-aza- 1,3-dienes to olefinic double bonds has been successfully employed in the preparation of endocyclic enamines for example (15; n = 1 or 2).“ Asymmetric induction in Diels-Alder reactions has obvious synthetic applications and therefore continues to attract attention (see also ref.6). The observed asym- metric induction for (4 + 2) additions to (E)-buta-1,3-dienyl (S)-0-methylmande- late is a function of the dienophile and may be due to the importance of charge transfer intermediates in the r-stacking arrangement (16). The addition of juglone to this diene under B(OAc) catalysis proceeds with virtually quantitative enan- tioselectivity.” The results of (4 + 2) cycloadditions of anthracene to chiral dienophiles in which the functional groups are at ‘concave sites’ as in (17) have been compared with literature data for the corresponding menthyl methyl fumarates (functional groups at a ‘convex site’) stereoselectivities being appreciably greater for the former dienophiles (R = CH,Ph or CONPh) in both thermal and AlC1,- catalysed reactions.l8 The preferred topography for the transition states involves bonding with the diene on the side of the dienophile C=C in (17) remote from the methyl group at C-1. Chiral induction in the Diels-Alder addition of the menthyl acrylates (18; R = H or Ph) to cyclopentadiene has been found to depend upon the auxiliary chiral group and particularly the Lewis acid used to catalyse the reacti~n.’~ The adducts formed from the thermal (4 + 2) addition of unsymmetrical electron- rich dienes and methoxy benzoquinones or naphthoquinones are those in which the more nucleophilic diene terminus becomes bonded to the non-methoxylated l5 (a) M.E. Jung and J. J. Shapiro J. Am. Chem. Soc. 1980 102 7862; (b)B. Kh. Rammash C. M. Gladstone and J. L. Wong J. Org. Chem. 1981,46 3036. l6 Y.-S. Cheng F. W. Fowler and A. T. Lupo jun. J. Am. Chem. Soc. 1981,103,2090. l7 B. M. Trost D. O’Krongly and J. L. Belletire J. Am. Chem. Soc. 1980 102 7595. G. Helmchen and R. Schmierer Angew. Chem. Znt. Ed. Engl. 1981 20 205. l9 W. Oppolzer M. Kurth D. Reichlin and F. Moffatt Tetrahedron Lett. 1981 22 2545; see also W. Oppolzer M. Kurth D. Reichlin C. Chapuis M. Mohnhaupt and F. Moffatt Helu. Chim. Acru 1981 64 2802. G.B. Gill (16) (17) (18) carbon for example (19) + (20) +(21) through the minimization of repulsive steric eff ectsS2' Experimental results bear out the expected high dienophilic reac- tivities of the benzoquinone derivatives (22; X = 0or NPh).21 MNDO calculations indicate that C-2 and C-3 have the largest LUMO coefficients while the coefficients at C-1 and C-4 are larger than those at C-9 and C-11.The adducts are found to possess structures in accord with the predicted regioselectivity (addition to C-2 C-3) and stereoselectivity (endo with respect to the benzoquinone moiety). (19) (20) (21) @; 0 (22) A very unusual stereospecific addition of tetracyanoethylene to substituted cyclo- propanone thioacetals has been uncovered.22 Because of the observation of the formation of a coloured charge-transfer complex when the co-reactants are mixed it is suggested that TCNE approaches the cyclopropane ring in (23) at the face where the SMe group is situated.The two products formed from (23) and TCNE namely (24) and (25) could arise from the two alternative HOMO/LUMO combi- nations indicated that is by a formal [,2 + ,2,] cycloaddition. Among the very many examples of the application of cycloaddition reactions to synthesis that have been published this year a paper by Wender and Ho~bert~~ deserves specific mention since it breaks relatively new ground. It concerns the use of intramolecular 1,3-~hotoaddition of olefins to arenes in synthesis leading to the formation of three new rings and up to six new stereocentres. The paper provides an important analysis of the stereoelectronic features of these photo-cycloadditions 2o I.-M.Tegmo-Larsson M. D. Rozeboom and K. N. Houk Tetrahedron Lett. 1981 22 2043; I.-M. Tegmo-Larsson M. D. Rozeboom N. G. Rondan and K. N. Houk ibid. p. 2047; M. D. Rozeboom I.-M. Tegmo-Larsson and K. N. Houk J. Org. Chem. 1981,46 2338. 21 K. Kanematsu S. Morita S. Fukushima and E. dsawa J. Am. Chem. SOC., 1981,103,5211 ;S. Yoshino K. Hayakawa and K. Kanernatsu J. Org. Chem. 1981,46 3841. 22 P. G. Wiering J. W. Verhoeven and H. Steinberg J. Am. Chem. SOC.,1981 103 7675. 23 P. A. Wender and J. J. Howbert J. Am. Chem. SOC. 1981 103 688. Reaction Mechanisms -Part (i) Pericyclic Reactions NC CN NC CN Me CN CN OMe SMe bearing upon their potential use in synthesis and the validity of the arguments presented is demonstrated by an elegant four-step synthesis of (*)-a-cedrene from a simple benzoid precursor.3 Ene Reactions Regio- and stereo-selective syntheses of cyclic natural products by intramolecular cycloadditions and ene reactions have been reviewed.24 The stereoselective introduction of steroid side chains at C-17 and C-20 has been achieved by two groups with the aid of Lewis acid-catalysed intermolecular ene additions to the A17‘*O’ ster~id.~~**~ Thus for example the Et,AlCl-catalysed addition of methyl propiolate to the (17Z)-ethylidene steroid proceeds as expected by a-attack as shown in (26) and sets the stereochemistry of the C-20 carbon in the natural onf figuration.^^ Other enophiles can be added similarly and reaction at a prochiral unsaturated centre in the enophile proceeds with good stereoselectivity.26 The transfer of axial chirality into kentral chirality is observed in the thermal intramolecular ene reaction of the optically active y-allenic aldehyde (27) by way of the endo transition-state geometry indi~ated.~’ 24 W.Oppolzer Pure Appl. Chem. 1981 53 1181. 25 W. G. Dauben and T. Brookhart J. Am. Chem. SOC.,1981,103,237. 26 A. D. Batcho D. E. Berger S. G. Davoust P. M. Wovkulich and M. R. UskokoviC Helv. Chim. Acta 1981,64 1682. 27 M. Bertrand M. L. Roumestant and P. Sylvestre-Panthet Tetrahedron Lett. 1981 22 3589. G.B. Gill R R I R I ~I (28) R = Me OAlR,,Cl,-, (291 2 equiv. Me,AICI "x"' OAl R,,Cl,-, OAlR .C12 -,, (31) b=/d (30) (32) The use of alkylaluminium halides as Lewis acids (with Brgnsted base reactivity) for promoting ene reactions has been further explored by Snider's group.28a The precise choice of alkylaluminium halide its quantity and the reaction temperature can have an important bearing upon the reaction pathway.Thus for example treatment of 2,6-dimethyl-5-heptenal (28) with 1equivalent of Me2AlC1 at -80 "C affords the ene adduct (29) presumably by the concerted mechanism indicated.28b With 2 equivalents of Lewis acid the more electrophilic aldehyde-(Me2A1C1)2 complex can form and the production of the chloro-alcohol (31) implicates the intermediacy of the carbonium ion (30). Use of the more acidic Lewis acid MeAlCl at -80 "C on the other hand afforded mainly (32) which is probably formed from (30) by way of two 1,2-hydride shifts.The methyl a-cyanoacrylate-Me,AlCl com-plex reacts with alkenes to give ene adducts dihydropyrans and cyclobutanes by way of the initially formed zwitterion (33); 2 1adducts are formed by the intercep- tion of (33) with another molecule of methyl a-cyanoacrylate.28C The proton- scavenging action of the alkylaluminium halides is frequently crucial. It allows for example observation of the ene-type addition of aryl sulphinyl chlorides to alkenes (Et ,AlC1 catalysis). 28d Kinetic hydrogen-isotope effects have been studied in order to determine any differences in mechanism between thermal and SnC1,-catalysed ene reactions of oxomalonic esters (34; E = C02Me or C02Et).29 Primary isotope effects are high (kH/kD = 3.3) in the thermal reactions (130°C) and negligible (kH/kD = 1.1)in the catalysed cases (25 "C) even where intramolecular H/D competitions are available.Concerted mechanisms with variations in C-C bond formation and C-H(D) bond breaking are proposed. The pressure dependence (1-1325 bars) of the rate constant (1.39 -5.75 x 1 m-'s-') for the ene addition of dimethyl oxomalonate (34; E = C0,Me) to hex-1-ene reveals that AV' = -28 to 28 (a)B. B. Snider D. J. Rodini M. Karras T. C. Kirk E. A. Deutsch R. Cordova and R. T. Price Tetrahedron 1981 37 3927; (b) M. Karras and B. B. Snider J. Am. Chem. SOC.,1980 102 7953; (c) B. B. Snider and G. B. Phillips J. Org. Chem. '-981 46 2563; (d) B. B. Snider J. Org. Chem.1981,46,3155. 29 L. M. Stephenson and M. Orfanopoulos J. Org. Chem. 1981,46 2220. Reaction Mechanisms -Part (i) Pericyclic Reactions 59 -31 cm3 mol-' (with hV = 27.0 f 0.5 cm3 mol-') confirming that the reaction is entirely ~oncerted.~' The thermal (120 "C) reactions of diethyl oxomalonate (34; E = C0,Et) and butyl N-(ptoluenesulphony1)iminoacetate (15)with enolizable ketones have been in~estigated.~~" The y-0x0-a-hydroxy-diestersand y-oxo-a- tosylamino-esters thus obtained may be regarded as arising from the formal ene additions of the above enophiles to the ketone enolates; there are precedents for such processes in the chemistry of 1,2,3-tricarbonyl systems (e.g. alloxan). The full paper on the thermal and Lewis acid-catalysed additions of (35) to olefins has now appea~ed;~" simple routes to a-amino-acids and y,S-unsaturated a-amino acids are outlined.Acylnitroso compounds (RCONO) are conveniently generated in situ by the decomposition of their (4 + 2) adducts with 9,lO-dimethylanthracene and are potent ene- and dien-~philes.~~ Thus formation of MeCONO in refluxing cyclo- hexene (an olefin of very moderate ene reactivity) afforded the hydroxamic acid ene adduct (36) in 85% yield.32" The reactions follow the expected regiochemistry of ene additions and appear to be kinetically controlled processes. An intramolecular variant of the affords a general approach to the cis-fused octahydroindole skeleton of Amaryllidaceae alkaloids and a synthesis of (f)-crinane is Reaction of diphenylphosphinodithioicacid with nitriles takes place in two steps the first being a formal ene reaction involving four heteroatoms as indicated by (37).33 Treatment of ethyl a-methylsulphinylacetate with trifluoroacetic anhydride OAIMe2CI fo2Bu N 0 EKE Tos/ R3 (1equiv.) in trifluoroacetic acid solution affords the Pummerer reaction intermediate (38) which may be intercepted by added alkenes to give ene adducts for example (39) from ~ent-l-ene.~~ The HOMO energy of 1,l'-bicyclopropylidene is about 0.59 EV higher than that of the (very ene reactive) parent methylenecyclopropane.30 M. Papadopoulos and G. Jenner Tetrahedron Lett. 1981 22 2773 31 (a)0.Achmatowicz jun. and M. Pietraszkiewicz Tetrahedron Lett. 1981 22,4323; (b)J.Chem. SOC. Perkin Trans. 1 1981 2680. 32 (a)G. E. Keck R. R. Webb and J. B. Yates Tetrahedron 1981 37 4007; (b)G. E. Keck and R. R. Webb J. Am. Chem. SOC.,1.981 103 3173. See also P. Horsewood G. W. Kirby R. P. Sharma and J. G. Sweeny J. Chem. SOC.,Perkin Trans 1 1981 1802 and G. W. Kirby and J. G. Sweeny ibid. p. 3250. 33 S. A. Benner Tetrahedron Lett. 1981 22 1855. 34 Y. Tamura H.-D. Choi H. Maeda and H. Ishibashi Tetrahedron Lett. 1981 22 1343 60 G.B. Gill + + MeSCHC0,Et +B MeS=CHC02Et + Addition to tetrachlorocyclopropene occurs at 80 "C to give (41) which is presum- ably derived from (40) the product of an ene-type addition involving chlorine-atom transfer.35 4 Sigmatropic Rearrangements The [ 1,5] shifts of H and substituent groups primarily in cyclic unsaturated systems have been reviewed.36 A [1,5] shift of a silyl group from oxygen to carbon has been observed; on heating (42) to 120 "C (1h) smooth rearrangement occurred to give (40) (41) (42) the a,P-unsaturated ester." The normal direction of the [1,5] homodienyl H shift (43a) (44a) in which strain effects shift the equilibrium to favour the acyclic diene can be reversed if the allylic C atom from which the H-transfer occurs in (43) is substituted by an OH group i.e.(43b) $ (44b) $ (45).38 Thermochemical calculations reveal that (43b) -+ (45) is exothermic (AAH = -8.8 kcal mol-') whereas for (43a) + (44a) the process is endothermic (AAH = +6.4 kcal mol-'). The procedure can be employed for stereoselective cyclopropane synthesis.(43) a; X = H (45) b;X=OH Substituent effects in aliphatic Claisen rearrangements have been studied and the results rationalized in terms of HMO For the rearrangement of a monosubstituted ally1 vinyl ether (46) the predictions are that the rearrangement should be accelerated (relative to the unsubstituted case) for a donor substituent at positions 1 2 or 3 and an acceptor substituent at positions 2 3 or 4. A chair conformation is assumed except in the case of an acceptor substituent at position 4 when a half-chair conformation is predicted to lower AH' for reaction. The 35 W. Weber U. Behrens and A. de Meijere Chem. Ber. 1981,114 1196. 36 V.A.Mironev A. D. Fedorovich and A. A. Akhrem Usp. Khim. 1981,50 1272. 37 G. Anderson D. W.Cameron G. I. Feutrill and R. W. Read TetrahedronLett. 1981,22,4347. 38 F.-G. Klarner W. Rungeler and W. Maifeld Angew. Chem. Int. Ed. Engl. 1981 20 595. 39 C. J. Burrows and B. K. Carpenter J. Am. Chem. SOC., 1981 103,6983,6984. Reaction Mechanisms -Part (i) Perk yclic Reactions regioselectivities of the photochemical [1,7] sigmatropic shifts and electrocycliz- ations of substituted cycloheptatrienes can be explained in terms of an excited singlet-state model in which a 90" rotation about a terminal bond is accompanied by 'sudden polarization' to form a zwitterionic species.4o Thus for a 2-substituted cycloheptatriene the presence of a pentadienyl anion-stabilizing substituent (accep- tor) favours polarization as indicated in (47) hence leading to 1,4-electrocyclization and 7 + 1sigmatropic shift.On the other hand a donor 2-substituent destabilizes the pentadienyl anion thereby favouring the cross-conjugated polarization of (48); this should lead to 3,6-electrocyclization and 7 + 6 sigmatropic shift. Secondary kinetic hydrogen-isotope effect at the side-chain p carbon atom in phenyl allyl sulphide and the effect of para-substituents on rate have been examined in a study of nucleophilic catalysts of the thio-Claisen rearrangement.41 The results (k,/k = 1.05 and log kpPx/kH = 0.250') support the mechanism involving nucleophilic triggering of the sigmatropic rearrangement. (46) (47) (48) The recently (1980) reported (PhCN),PdCl,-catalysed Cope rearrangement of acyclic 1,5-dienes by way of complexed cyclohexyl carbonium ion as the proposed intermediate is now thought instead to occur by way of the bis(q3-allyl) complex (49).42Thus treatment of hexa-1,5-diene vapour with an aqueous solution of (PhCN),PdCl, CuCl, and CuCl at 60°C in a stream of excess oxygen gave substantial catalytic conversion to acetone (Wacker oxidation).The organo- aluminium reagents Et,AlSPh or Et,AlCl-PPh have been found to be effective for promoting the Claisen rearrangement of allyl vinyl ethers to y,&unsaturated aldehydes and ketones without competing nucleophilic addition to the carbonyl carbon atom. With Et,Al for example both Et and H addition to the aldehyde carbon atom occur.43 The Cope rearrangement of 1,5-dienes possessing an acyl group at the 2-position is strongly accelerated by protonic and Lewis acids e.g.(50) +(51).44 pPd x-(yJ+(y-\/ /\ CI CI Me Me (49) (50) (5 1) 40 T. Tezuka 0.Kikuchi K. N. Houk M. N. Paddon-Row C. M. Santiago N. G. Rondan J. C. Williams jun. and R. W. Gandour J. Am. Chem. Suc. 1981,103 1367. 41 H. Kwart W. H. Miles A. G. Horgan and L. D. Kwart J. Am. Chem. Suc. 1981,103 1757. 42 R. Hamilton T. R. B. Mitchell and J. J. Rooney J. Chem. Suc. Chem. Commun. 1981,456;see also J. Muzart P. Pale and J.-P. Pete ibid. p. 668. 43 K. Takai I. Mori K. Oshima and H. Nozaki Tetrahedron Lett. 1981,22 3985. 44 W. G.Dauben and A. Chollet Tetrahedron Lett. 1981 22 1583. 62 G. B. Gill Removal of the development of repulsive interactions in a boat transition state for the Cope rearrangement leads to a marked reduction in the activation energy.Thus both (52)45 and (53)46 undergo [3,3] shifts with remarkably low activation energies; the rearrangement of (52) is reversible. (52) (53) The alkoxide-accelerated [1,3]migration (54) -+ (55) is 75% intramolecular and 25YO intermolecular. When allowance is made for the intermolecular process rearrangement is found to occur with at least 65% retention of configuration. Although fragmentation-recombination within a solvent cage cannot be ruled out the above observation is consistent with the hypothesis that 'any substituent will accelerate a forbidden pericyclic reaction more than it accelerates the corresponding allowed reaction' because of the strong interaction with the essentially antiaromatic orbital array at the transition state (i.e.high HOMO and low LUM0).47The fraction of the rearrangement of the cyclononatrienols (56; R = Me or Et) which passes through the oxy-Cope pathway increases (relative to [1,3] rearrangement) as the donor properties of the oxygen atom are decreased (K' -+Na' -+ H) with a reduction in rate of is~merization.~~ The Li' and K' salts of 2-vinylcyclobutanols undergo [1,3] rearrangement at 25-70 "C,giving cyclohexen-3-01 derivative^.^^ The suprafacial-inversion pathway predominates when the alkoxide substituent and vinyl group are trans whereas the (slower) suprafacial-retention mode is involved when these two groups are cis to one another.The means whereby ring expansion by eight carbon atoms can be achieved by alkoxide-accelerated [5,5] or two consecutive [3,3] shifts have been dis~losed.~" The [2,3] sigmatropic rearrangement of the cis-and trans-isomers of spirocyclic sulphonium ylides (57) gives respectively cis and trans doubly bridged S-heterocyclic ethylene~.~~ -)Ph .() 0-K+ -0:. ekY ?H K+-O ':LPh 0 tQ= H / (54) (55) (56) (57) 45 K.B. Wiberg M. Matturro and R. Adams J. Am. Chem. SOC.,1981,103,1600. 46 Y. Tobe F. Hirata K. Nishida H. Fujita K. Kimura and Y. Odaira J. Chem. SOC., Chem. Commun. 1981,786. 47 M. T. Zoeckler and B. K. Carpenter J. Am. Chem. SOC.,1981,103,7661. 48 G. D. Crouse and L. A. Paquette Tetrahedron Lett. 1981 22 3167. 49 R. L. Danheiser C. Martinez-Davila and H.Sard Tetrahedron 1981 37 3943. 50 P. A. Wender and S. McN. Sieburth Tetrahedron Lett. 1981 22 2471; P. A. Wender S. McN. Sieburth J. J. Petraitis and S. K. Singh Tetrahedron 1981 37 3967. 51 V. Cere C. Paolucci S. Pollicino E. Sandri and A. Fava J. Org. Chem. 1981 46 486; see also ibid. p. 3315. Reaction Mechanisms -Part (i) Pericyclic Reactions 5 Electrocyclic Reactions Replacement of an olefinic carbon atom in cyclobutene by an N-oxide imino function has a relatively small effect on the activation energy for ring opening without alteration of the stereochemical pathway; thus (58) + (59) with AH*= 27 f 1kcal mol-’ and AS* = -2 f3 e.u.” Dehydrochlorination of N-chloroazetidine in the vapour phase over silica-supported Bu‘OK at 94 “C affords Me 70 phF N<m 2 + PhXNyMe H Me H CO I NEt (58) (59) 1-azetine almost quantitatively which on flash vacuum pyrolysis is converted into 2-a~abutadiene.~~ Eliminative processes have been employed to generate oxete and 3-phenyloxete both of which ring-open at 25°C to the corresponding a,@-unsaturated aldehydes (ti = ca.8.4 hand 14 h re~pectively).~~ Infrared multiphoton excitation has been employed successfully for the selective isomerization of 2- methyl- and 2,3-dimethylbuta-1,3-diene to the corresponding cyclobutenes (respec- tively AH = +10.6 and +9.6 kcal mol-1 and AS = -3.9 and -3.6 e.u.). By shifting the irradiating frequency the cyclobutenes can be efficiently and cleanly ring- opened.” Vibrational excitation of the gound electronic state of cis-3,4-dichloro- cyclobutene by an intense infrared laser leads to a substantial increase in the yield of the symmetry-forbidden products; precise mechanisms are not yet knowns6 Irradiation of the enedione (60) at 77 K in a hydrocarbon glass with light of wavelength 240-400 nm afforded CO and a labile compound identified from its spectral characteristics as norcaradiene (61).s7The kinetics of the isomerization of (61) into cycloheptatriene were measured at ca.100 K by monitoring the increase in optical density at 261 nm giving good first-order behaviour with k = (6.0 0.3) 1011e(-6500*1000)/RT . Estimates for the room temperature (25°C) isomerization give a rate constant of 1 x lo7s-’ with AS* = -4.5 e.u. and AG* = 7.2 kcal mol-’.Whereas 1,3,5-cyclo-octatriene is favoured with respect to its valence isomer cis,cis-l,3,5,7-octatetraene,the bicycle (62) is more stable than its tricyclic conrota- tion product (63). Presumably the inherent instability of the central cyclobutadiene ring in (63) is responsible for the differences in behavio~r.~’ Further details on the photochromic behaviour in the electrocyclization of substituted (E)-3-furylethyl- idene(isopropy1idene)succinic anhydrides (i.e. fulgides) have appeared,59 as have 52 M. L. M. Pennings D. N. Reinhoudt S. Harkema and G. J. van Hummel J. Am. Chem. SOC.,1980 102,7570. ” J. C. Guillemin J. M. Denis and A. Lablanche-Combier J. Am. Chem. SOC.,1981 103,468. ” L. E. Friedrich and P. Y.-S. Lam J. Org. Chem. 1981,46 306. ” J.L. Buechele E. Weitz and F. D. Lewis J. Am. Chem. SOC.,1981 103 3588. 56 C. R. Mao N. Presser L.-S. John R. M. Moriarty and R. L. Gordon J. Am. Chem. SOC.,1981 103 2105. 57 M. B. Rubin J. Am. Chem. SOC.,1981,103,7791. H. Meier T. Echter and 0.Zimmer Angew. Chem. Znt. Ed. Engl. 1981,20 865. 59 H. G. Heller and S. Oliver J. Chem. SOC.,Perkin Trans. 1 1981 197; P. J. Darcy H. G. Heller P. J. Strydom and J. Whittall ibid. p. 202. 64 G. B. Gill (60) (61) (62) (63) the results on the thermal isomerization of cyclic cis,truns,cis-trienes where it is demonstrated that there is a structure-reactivity correlation between the size of the ring and the type of product observed.60 Because of the coiled geometry the proximity of the Me group to the n-system in truns,cis,truns-cyclononatriene(64) has an acidifying effect on its H atoms.The HOMO of the derived anion (KH in THF containing 18-crown-6 at 20 "C)has an in-phase relationship with the closely located terminal p-IT lobes and the system is ideally set up for antarafacial electrocyclization which affords bicyclor4.3. lldeca- 2,4-diene by way of the bicyclic anion (65).61With the ethyl and benzyl analogues of (64) the exocyclic group at C-10 in the final product is syn to the diene unit. The vinylogous sesquifulvalene (66) underwent thermal electrocyclization (E = 100 kJ mol-' log A = 12.0) perispecifically in the symmetry-forbidden conrotatory mode to give (67) which is rapidly isomerized to aromatic [1,5] H-shift products. The trans-stereochemistry of (67) was verified by X-ray crystallographic analysis of its adduct with dimethyl acetylenedicarboxylate.62 6o W.G. Dauben D. M. Michno and E. G. Olsen J. Org. Chem. 1981,46,687. " L.A.Paquette and G. D. Crouse J. Am. Chem. Soc. 1981,103,6235. H. Prinzbach H. Bingmann A. Beck D. Hunkler H. Sauter and E. Hadicke Chem. Ber. 1981 114 1697.

ISSN:0069-3030    

DOI:10.1039/OC9817800051

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (ii) Polar Reactions By D.G. MORRIS Department of Chemistry University of Glasgow Glasgow GI2 8QQ 1 Introduction The present Report is concerned with nucleophilic substitution carbo-cations carbanions the reactivity of carbonyl groups and of esters and elimination and addition reactions together with miscellaneous topics. A review on pyramidal carbo-cations’ and one on long-lived cations2 have appeared. Several constructive papers have been published in this challenging area which may be witnessing a period of enhanced interest following a period of ennui probably brought about by norbornyl cation overkill. Gas-phase reactions of anions have been the subject of two reviews devoted to bimolecular aspects3 and to the use of the flowing afterglow technique as a m~nitor.~ A review with emphasis on the number of stages involved in nucleophilic vinylic substitution has also been publi~hed.~ 2 Nucleophilic Substitution A number of papers have been concerned with the position of the transition state during SN2reactions at saturated carbon.From entropies of activation and standard entropies of the dissociated ions in the Menschutkin reaction of Me3N with EtI in a variety of solvents it has been concluded6 that in contrast to a recently expressed and cited estimation the transition state is early. Based on the response of the rate to substitution by methyl at the a carbon it has been concluded’ that bond-making and -breaking at the central carbon are synchronous. In the hydrolysis of methyl perchlorate standard Gibbs energies of transfer after correction for cavity and non-electrostatic effects indicate that the C-0 bond is longer than expected in the transition state which is indicated to be ‘looser’ than its counterpart for MeI.8 By means of Grunwald-Winstein plots regions of decreasing rate with increasing ionizing power of the solvent are observed in the hydrolysis of MeOClO in aqueous ’ H.Schwarz Angew. Chem. Znt. Ed. Engl. 1981 20 991. G. A. Olah Chem. Scr. 1981,18,97. ’N. M. M. Nibbering Recl. Trav. Chim. Pays-Bas 1981,100 297. C. H. De Puy and V. M. Bierbaum Acc. Chem. Res. 1981,14,146. ’ Z. Rappoport Acc. Chem. Res. 1981,14,7. M. H. Abraham and A. Nasehzadeh J. Chem. SOC. Chem. Commun. 1981,905. ’D. N. Kevill J.Chem. SOC. Chem. Commun. 1981,421. M. H. Abraham and A. Nazehzadeh Tetrahedron Lett. 1981.22 1929. 65 D. G.Morris DMSO mixtures;' the leaving group is better solvated in dipolar aprotic solvents than in water. In the regions of decreasing rate with increasing ionizing power of the solvent the authors claim the first reported solvolysis by nucleophilic substitu- tion of an initially neutral substrate. It has been concluded" that in a vinylogous SN2'reaction bond formation to C-1 and bond cleavage from C-5 are mutually trans as in (l),in a concerted but non-synchronous process whereas syn stereochemistry is predicted for a fully concerted reaction." (1) Solvolyses (in CF3C02H CH3C02H and 80% aq. EtOH) have been effected on 2-adamantyl benzenesulphonate that is labelled with l80, and they were so monitored that the "0 content of the alkoxyl and sulphonyl moieties of the original ester could be determined the latter by a novel (and outlined) technique." Accord- ingly it was shown that the minimum fraction of internal return lay in the range 0.69-0.84 in the above solvents; this is in contrast to cited reports in which it is proposed that ion-pair return is not appreciable.The rate of bonding between the cation and the solvent is probably controlled by separation of the intimate ion-pair. However in a slightly later paper it is stated,12 with respect to the solvolysis of 2-adamantyl tosylate that 'internal return of substantial magnitude (>5-fold effect on k,)is absent'. The formation of a bridged carbo-cation from the tetracyclic ester (2) is postulated to occur with appreciable strain increase whereas a classical counterpart would cause minimal deformation at C-2.For the tertiary substrate (2) and its epimer the ratio of rate constants ke,,/kendo is 164 in 60% aqueous acetone (broadly similar to values in related tertiary systems) whereas the corresponding ratio for (3) is much lower i.e. 7.4 in 80% aqueous acetone with the main contributory reason being the diminished reactivity of the exo-e~ter.'~ From buffered acetolysis of (4) 2% of endo-acetate was produced this being ca. 90 times more than that formed in the corresponding reaction of exo-norbornyl tosylate. Thus acetolysis of (2) is R' (2) R' = R2 = Me,X = pNB (3) R' = R2 = H,X = Bs (4)R' = H,R2 = Me,X = Ts D.N. Kevill and A. Wang J. Chem. SOC.,Chem. Commun. 1981,83. lo W. S. Murphy and B. O'Mahony Tetrahedron Lett. 1981 22 585. It C. Paradisi and J. F. Bunnett J. Am. Chem. SOC.,1981,103,946. *'T. W. Bentley C. T. Bowen D. H. Morten and P. von R. Schleyer J.Am. Chem. SOC.,1981,103,5466. l3 J. E. Nordlander J. R. Neff W. B. Moore Y. Apeloig D. Arad S. A. Godleski and P. von R. Schleyer Tetrahedron Lett. 1981 22 4921. Reaction Mechanisms -Part (ii) Polar Reactions considered to involve a localized ion-pair in contrast to the delocalized species from exo-norbornyl tosylate. The role of ion-pairs in biological systems has been detailed in two investigations. Thus farnesylpyrophosphate synthetase catalyses the condensation between isopen- tenyl pyrophosphate and geranyl pyrophosphate (5) wherein cleavage of the C-0 bond in (5) is a discrete step yielding a geranyl cation-pyrophosphate ion-pair which then (see Scheme 1) alkylates isopentenyl pyr~phosphate.'~" By means of C,HI PP-J J I Scheme 1 (5) that was labelled with C-'80-P it was shown that ion-pair return does not occur with randomization of the "0 It is contended that ion-pair return does take place but is not detected on account of topological constraints which impede relative motion of the two ions io the enzymic reaction.Farnesyl pyrophosphate (6) is converted into nerolidyl pyrophosphate (7) by cell-free enzymes from Gibberella fujikuroi ; the transformation is a net suprafacial process1s involving an ion-pair in which the three non-bridge oxygens of the pyrophosphate are able to scramble.The reaction sequence is then completed by transformation of (7) into cyclonerodiol (8) by means of all-trans addition of water across the vinyl and the central double-bonds. The E-isomer (9) in trifluoroethanol (TFE) or ethanol gave the appropriate aromatic ether (10) or (11)in 26 or 6% yield respectively uia an intermediate T OPP**HOH +-PH OH (7) (a) C. D. Poulter P. L. Wiggins and A. T. Le J. Am. Chem. Soc. 1981 103 3926; (6) E. A. Mash G. M. Gurria and C. D. Poulter J. Am. Chern. Suc. 1981 103 3927. D. E. Cane R. Iyengar and M.-S. Shiao J. Am. Chern. Suc. 1981,103,914. D.G.Morris phenonium ion (13)whose intermediacy has been confirmed16 by interception with excess bromide ion in TFE-H20dioxan to give the bromide (12). R Mel$ Me Me Meo (10) R = OCH2CF3 (11) R = OCHZCH3 (12) R = Br A gas-phase investigation of nucleophilic displacements has been made by means of a y-radiolytic technique which permits isolation and structure determination of the neutral end-products following the generation of the halonium ions (14).”The gaseous ions are produced in the presence of an external nucleophile e.g. H,O or H2S which gives rise to products. In the gas phase the stability of the chloronium ion (14) is less than that of its bromonium counterpart (15). X --c/_r_\c-.-/\ (14) X = CI (15) X = Br The mechanism of the a-chymotrypsin-catalysed hydrolysis of amides and esters is believed to involve nucleophilic attack by the OH group of serine-195.However N-nitroso-amides from alanine and phenylalanine release carbo-cations at or near to the active site.18 These are then capable of alkylating nucleophilic groups such that the a-chymotrypsin activity is reduced by 100% and <9% for D-and ~-(16) 0 N=O II I Ph CH2-CH -C-N-CHzPh I NHCOCHMe2 (16) respectively at ratios of substrate to a-chymotrypsin of 40:1; the inhibition may be irreversible. 3 Carbo-cations Addition of (17) to SbF,-S02CIF-S02F at -130 “C gave rise to the first example of a bisected primary cyclopropylcarbinyl cation (18),formed by a mechanism given in Scheme 2.19 The 1,S-dimethylcyclo-octyl cation has been assigned a p-hydrido bridging structure (19) on the basis of a signal at very high field (8 -6.3) for the bridging l6 M.Hanack and W. Holwager J. Chem. SOC., Chem. Commun. 1981 713. ” G. Angelini and M. Speranza J. Am. Chem. Soc. 1981 103 3792. ’* E. H. White L. W. Jelinski I. R. Politzer B. R. Branchini and D. F. Roswell J. Am. Chem. SOC. 1981,103,4231. l9 L. R. Schmitz and T. S. Sorensen Tetrahedron Lett. 1981 22 1191. Reaction Mechanisms -Part (ii) Polar Reactions Scheme 2 proton a rather small coupling constant (lH-l3C)of 40 Hz and a S separation of ca. 2Hz on isotopic perturbation by substitution of a CD group.2o Analogous behaviour was noted for the cyclononyl homologue but not for the 1,4-dimethyl- cyclo-octyl cation.The same criteria indicate p bridging in 1,5- and 1,6-dimethylcyclododecyl cations.21 Molecular orbital calculations on acyclic prototypes e.g. CH3--H--CH3+ indicate a preferred CHC bonding angle of ca 180" the hybridisation of these carbons being intermediate between sp2 and sp3. The p-hydrogen is indicated to have a slight negative charge and to be located in a loose potential such that it is capable of movement (up down or sideways) at very little energetic cost. The bridging hydrogen is not electrophilic and cannot be removed as a proton.21 By means of both 'H and *Hn.m.r. spectroscopy a rapidly established equilibrium was determined22 such that K{ = [(21)]/[(20)]} is 4.5 at -130°C. This value is similar to primary isotope effects in rate studies; however in (20) and (21) the C-H(D) bonds are considerably elongated and they simulate a purported transi- tion state in a hydride-transfer reaction.The barrier to interchange of HTand p-H in (20) and (21) and in their alkylated analogues is independent of the nature of R. (20) (21) (22) (23) The positive charge lies preferentially on 13C in (22) rather than in (23) with which it is connected by hydride The equilibrium constant varies between 1.0014 and 1.0197 in the temperature range -135 to -62 "C as determined by a 13 C shift-difference method that is limited to molecules (or ions) that are undergoing 20 R. P. Kirchen K. Ranganayakulu B. P. Singh and T. S. Sorensen Can. J. Chem. 1981,59,2t73. 21 R.P.Kirchen K. Ranganayakulu A. Rauk B. P. Singh and T. S. Sorensen J. Am. Chem. SOC.,1981 103 588. 22 R. P. Kirchen N. Okazawa K. Ranganayakulu A. Rauk and T. S. Sorensen 1 Am. Chem. SOC. 1981,103,597. 23 M.Saunders M. R. Kates and G. E. Walker J. Am. Chem. SOC.,1981,103,4623. D. G.Morris degenerate rearrangement. The solution was referenced with the corresponding ion which was labelled with 13C at both potential positive carbon sites and which absorbed at higher field. A previous and surprising report claiming observation of the n.m.r. spectrum of the 2-methylbicyclo[3.2.2]nonatrienyl cation has been amended.24 Thus 2- methylenebicyclo[3.2.2]nona-3,6,8-trienein FS03H-S02CIF-S02F2 at -136 "C gave in two separate experiments only the 9-methyl-9-barbaryl cation (24); at -1 16 "C this rearranged in turn to the more stable cation (25).Scheme 3 Aryl-substituted Coates' ions (26; R = Ar) show symmetry in their 13C n.m.r. spectra i.e. C-2 -3; C-6 -7; and C-4 -5show single peaks attributed to a rapid bridge 'flipping' which interconverts (26a) and (26b) as shown in Scheme 3 and which contrary to the behaviour if R = H or Me is not frozen out at -110 "C. It is considered25 that the ions may become classical if they are substituted by a strongly electron-demanding aryl substituent. The 'H n.m.r. spectrum of the dication (27) with both CD3 groups in basal positions gave two singlets in the ratio ca. 1:3 at -50 0C.26 Between -60 and -10 "C the 90.52 MHz I3C n.m.r. spectrum of (27) showed a small (0.44 p.p,m.) deuterium-induced perturbation of the low-field absorption in accord with the attribution of a non-classical symmetrical bridged structure for (27) although the ion is formally outside the terms of reference of Saunders' original proposal.12+ (27) (28) (29) The same method involving isotopic perturbation of chemical shifts has been applied to the 9-barbaryl cation prepared from the precursor (28) that is labelled as shown.*' The 13C n.m.r. spectra are in accord with a series of structures for the barbaryl cation as exemplified by (29) which undergoes a rapid degenerate 24 M. J. Goldstein J. P. Dinnocenzo P. Ahlberg C. Engdahl L. A. Paquette and G. A. Olah J. Org. Chem. 1981,46,3751. 2s D. G. Farnum and T. P. Clausen Tetrahedron Lett.1981 22 549. 26 H. Hogeveen and E. M. G. A. van Kruchten J. Org. Chem. 1981,46,1350. 27 P. Ahlberg C. Engdahl and G. Jonsall J. Am. Chem. Soc. 1981,103 1583. Reaction Mechanisms -Part (ii) Polar Reactions rearrangement with a barrier of <4 kcal mol-'. Alternative cited structures are precluded. The I3C n.m.r. spectrum of the s-butyl cation (from the chloride and SbF,) has been recorded in the solid state by means of an apparatus shown diagrammatically.28 At temperatures <-85"C a sample that was enriched with I3C at C-3 showed scrambling over the four-carbon unit although too slowly to effect coalescence of the 13C signals at -60°C. The scrambling occurs via a protonated cyclopropane but takes place much more slowly than in solution.A break in the Hammett plot for the rate constants for ethanolysis of aryl- substituted mesylates (30) (as their logarithms) us u+has been construed in terms of stabilization of the a-keto carbo-cation by a resonance form such as (31);29 in these aberrant cases the rate constants are greater than expected on the basis of (++. (30) (31) The reaction of 3-phenylpropyne with liquid HCl gave (34) the proposed mechanism of whose formation (Scheme 4) involves the first reported 1,2-phenyl shift towards the double-bond of the first-formed vinyl cation (32); this leads to the allylic cation (33) which then gives (34) in unexceptional Ph Ph I + PhCH,-C=CH (32) (33) J (34) Scheme 4 A partially reversible initial slow protonation has been proposed3' in the acid- catalysed hydrolysis of methyl vinyl selenides which is associated with a kinetic solvent isotope effect kH30+/kD30+in the range 1.4-1.8.Reversibility is favoured by stabilization of the carbo-cation centre by a methylseleno-group and by the build-up of hemiselenoacetal during the hydrolysis (Scheme 5).To the better known ability of an a-SeR group to stabilize a carbanion is thus added the ability of a-SeR to render a carbo-cation more stable. In an enticingIy entitled paper Bunnett's group3' describe the generation of the 2-pyridyl cation (33 one of whose resonance forms (36) indicates that interception 28 P. C. Myhre and C. S. Yannoni J. Am. Chem. SOC.,1981,103,230. 29 X.Creary J. Am. Chem. SOC., 1981,103 2463.30 K.Vittinghoff and K. Griesbaum Tetrahedron Lett. 1981,22 1889. 31 L. Hevesi J.-L. Piquard and H. Wautier J. Am. Chem. Soc. 1981 103 870. 32 J. F.Bunnett and P. Singh J. Org. Chem. 1981 46,4567. D.G. Morris (slow) HY 0 cI lH23 3 Me Scheme 5 by means of anthracene may be feasible. In the event no evidence for aryne character could be detected; rather the products obtained were those of elec- trophilic aromatic substitution. 4 Carbanions By means of semi-empirical MNDO and ab initio-calculations the structures of a number of anions have been calc~lated.~~ For HCO the C=O bond length was calculated to be 1.254W,owhich is longer than in HCHO; the calculations indicate a long C-H bond (1.66 A) which is hence weak.The species has been calculated to be only slightly stable with respect to loss of CO. Greater stability has been ascribed to alkylated derivatives. In acetaldehyde however loss of a proton from the methyl group is estimated to be 28 kcal mol-’ more favourable than from the carbonyl-bonded proton. Ab initio calculations indicate that there is no necessary relationship between the electron-withdrawing or -releasing character of a substituent that contains a first- or second-row heteroatom and its ability to stabilize positive or negative charges via a u-effe~t.~~ A second-row heteroatom is calculated to stabilize both anionic and cationic substrates; the converse is true when the charge is delocalized. A number of cases offering experimental support are It has been calculated3’ that the interactions between occupied and unoccupied orbitals that produce homoaromaticity can only be realized when an empty p-orbital is juxtaposed geometrically and energetically with a filled T-or 0-orbital; such is the case in carbo-cations.In neutral molecules or anions which have been exten- sively investigated and to which extensive references are given the interactions induce overwhelming losses of bonding or increased four-electron repulsions. A similar conclusion that no homoaromatic stabilization exists for the bicyclo[3.2.l]octa-3,6-dien-2-yl anion has been proposed by Schleyer’s group,36 33 J. Chandresakar J. G. Andrade and P. von R. Schleyer J. Am. Chem. SOC.,1981,103,5612. J. R.Larson and N. D. Epiotis J. Am.Chem. SOC.,1981,103 410. 35 J. B.Grutmer and W. L. Jorgensen J. Am. Chem. SOC.,1981.103 1373. 36 E.Kaufmann H. Mayr J. Chandrasekhar and P. von R. Schleyer,J. Am. Chem. SOC.,1981,103,1375. Reaction Mechanisms -Part (ii) Polar Reactions who concluded that the observed stabilizing effect of the C-6-C-7 double-bond is brought about by an inductive effect. A very large primary kinetic isotope effect has been reported for the ionization of (4-nitrophenyl)[ l-3Hl]nitromethane. Internal return was found to be absent and the observed kH/kT is 45.8 at 25 "C; this translates to a value of 14 for the corresponding kH/kD,which is ca 3 times smaller than previously determined although still of such a magnitude as to indicate some proton t~nnelling.~~ N-Pivalylphenylalanine dimethylamide (37) underwent exchange with an excess of retention with Bu'OK in BU'OD such that the ratio of the bimolecular rate constants kJk (k for exchange; k for loss of optical activity) is 2.4 at 30°C.38 A prerequisite of the observed behaviour is a large group R since k = k for (38); this equality is also observed when a crown ether is added thereby indicating an important role for K' in determining the rate of stereochemical change.The pivaloyl oxygen in (37) is considered to complex with K' and it thereby lowers the transition state for protonation sufficiently for it to compete with conformational change. With still bulkier groups k = k, probably because steric retardation of protonation increases the lifetime of the enolate such that conformational equilibration occurs with attendant loss of any stereochemical preference for protonation.The anionic moiety in (39) experiences negligible bonding interaction with the counter-ion and accordingly is highly nu~leophilic.~~ Thus if (40) Me,CHCHO PhCH2-CH-CONMe2 0-I NHR (37) R = COCMe3 (38) R = H (39) and a catalytic amount of (39) are allowed to react at -78"C with excess of Me,SiF a good yield of the p-trimethylsiloxy-ketone (41) is obtained which consists entirely of the erythro-isomer under kinetic control. Less diastereoselection is prj-OSiMe,0 &!:::HMe R2 R' R3 (40) (41) (42) observed at higher temperatures and with other substrates. The origin of diastereoselectivity is different from that with conventional Lewis-acid-co-ordinated enolates in that an enolate that is derived from (39) shows a high negative charge in the now extended transition state (42) arising from (39) and RCHO.This has the effect of minimizing electrostatic interactions between oxygen atoms. An interesting set of results has been obtained from the reaction of m-ClC,H,CHCNM' with aromatic aldehydes in THF in the presence of H' and 37 A. J. Kresge and M. F. Powell J. Am. Chem. Soc. 1981 103 201. 38 R.D.Guthrie and E. C. Nicolas J. Am. Chem. SOC.,1981,103,4637. 39 R.Noyori T. Nishida and J. Sakata J. Am. Chem. SOC.,1981 103 2107. D. G.Morris added HMPA.40 When M is Li the reaction is faster in the absence of [2.1.1] cryptand since complexation of the carbonyl oxygen with lithium ion prevails over association of the cation with the substituted acetonitrile anion; the converse is observed when M is K40 The aldehyde p-NCC6H4CH0 reacts more slowly in the presence of Li' again on account of association of the Li' with the acetonitrile anion.The authors4' suggest that complexation with lithium ion brings the energy levels of the LUMO of the aldehyde very close. In a tight ion-pair the HOMO level of the nucleophile is lower in energy than in a loose ion-pair. The differences in frontier orbital between the HOMO of a tight ion-pair and the LUMO of the complexed aldehyde are greater than those of the HOMO of a loose ion-pair and the LUMO of a free aldehyde for p-NCC6H4CH0 whereas the opposite holds for p-MeOC6H4CH0.5 Reactivity of Carbonyl Groups Reactions of a series of carbanions with esters have been studied41 in the gas-phase via ion cyclotron resonance and products have been detected with mass/charge ratios that are consistent with there having occurred an addition-elimination- deprotonation sequence as indicated in Scheme 6. Reaction co-ordinate diagrams have been constructed in a number of specific cases and in contrast to solution reactions have the feature that the tetrahedral ion (43) is lower in energy than the reactants. (43) Scheme 6 The reaction of the benzoquinone imine (44)with excess phenol gives good yields of (46)in an acid-catalysed reaction that is mediated by the N-protonated derivative of (44),one resonance form of which is (45).42Polarization of a carbonyl group in the sense that is observed occurs in only a few examples and is made possible by aromatization of the protonated species (45).62Me 6 50" 0(44) (45) O+ .-QOH (46) 40 A. Loupy M. C. Roux-Schmitt and J. Seyden-Penne Tetrahedron Lett. 1981 22 1685. 41 J. E. Bartmess R. L. Hays and G. Caldwell J. Am. Chem. Soc. 1981 103,1339. 42 K. Shudo Y. Orihara T. Ohta and T. Okarnoto J. Am. Chem. SOC., 1981,103 943. Reaction Mechanisms -Part (ii) Polar Reactions 75 Ph Ph + \ c=o-Po~~-/ Me (47) (48) The long-sought monomeric metaphosphate ion PO3- has been generated; it reacts with the oxygen of acetophenone to give (47) which reacts with base to yield (48).43 Attack of aniline on (47) produces PhC(Me)=NPh in a reaction that is parallel with the enzyme-catalysed reactions of ATP which may be a source of Po,2-.The acid-catalysed bromination of 2,4,6-trimethylacetophenonein 50% aqueous acetic acid shows a first-order dependence of the rate on the concentration of this is in marked contrast to the behaviour of e.g. acetophenone. An inexplicably rapid rate of formation of an enol has been proposed for 2,4,6-trimethylacetophenone the slow step being the reaction of bromine with the enol to give (49); subsequent steps leading to the isolated mono-bromo-ketone are + Br-(49) rapid. In the condensation of 4-methylindan-1 -one (50)with 2-lithio-NN-diethyl-l- naphthamide a 30% yield of (51) is produced; repetition of the experiment with both hydrogens a to the ketone replaced with deuterium results in an essentially doubled4' yield of (51) since side-reactions of (50) which are mediated by enoliz- ation are now suppressed on account of an adverse isotope effect..(50) (51) On the basis that the rate of H,O'-catalysed ketonization of vinyl alcohol should be comparable with that of the known rate of hydrolysis of methyl vinyl ether a premise subsequently found to be inaccurate the generation of vinyl alcohol was achieved by methods which produce it at a faster rate than that of its ket~nization.~~ Several related precursors were employed exemplified by (52) from which an intermediate divinyl hemiorthoformate (53)was detected by 'H n.m.r. spectroscopy. An interesting method of inducing selectivity of reduction of carbonyl groups has been developed.47 Thus (EtO),SiH activated with KF reduces aldehydes 43 A.C. Satterthwait and F. H. Westheimer J. Am. Chem. Soc. 1981 103 1177. 44 A. G. Pinkus and R. Gopalan J. Chem. SOC.,Chem. Commun. 1981 1016. 45 S. A. Jacobs C. Cortez and R. G. Harvey J. Chem. SOC.,Chem. Commun. 1981 1215. 46 B. Capon D. S. Rycroft T. W. Watson and C. Zucco J. Am. Chem. SOC.,1981,103 1761. 47 J. Boyer R. J. P. Corriu R. Perz and C. Rei J. Chem. Soc. Chem. Commun. 1981 121. 76 D. G.Morris quantitatively and with 100% selectivity in the presence of ketones; in like vein (EtO),SiH or Me(EtO),SiH when activated with CsF reduces ketones selectively in the presence of esters. A wide variety of other functionality is tolerated.OSiMe I fiCHPh (53) R = M(D) Benzaldehyde does not react with (54) if held at 100 “C for 3 days. In the presence of a catalytic amount of KOBu‘ however an unusual substitution takes place even at low temperature to give (59,in high yield by means of a mechanism that is as yet ~ncertain.~~ 6 Reactivity of Esters Hydrolysis of (56) proceeds with expulsion of an ethoxy-group followed by hydra- tion to give (57) thereby providing the tetrahedral intermediate in the lactonization of (58) to (59);(57) may also revert to (58).The intermediate (57)has its maximum lifetime (ca 0.1 s) at pH 3.5. A very rapid breakdown of (57),at a rate probably close to the diffusion limit takes place in base. At pH C3.5,the breakdown of (57) is merely rapid in a hydronium-ion-catalysed rea~tion.~’ (56) R = Et (57) R = H The greater basicity of HO-in 80% Me,SO-H20 led to an investigation” of the hydrolysis of phenyl esters of p-nitrophenylacetic acid for which the rate constants for loss of a carbanion correlate quite well with u-with p = 4.4.For esters such as (60) and (61) a departure from linearity is noted and in such cases unusually free-radical intermediates were detected. MeSO2CHzSO20Ar (60) X = Me (62) (61) X = OMe 2-MeS02-C -S020Ar MeS02C=S02 MeS02CH=S02 F. Effenberger and W. Spiegler Angew. Chem. Znt. Ed. Engl. 1981 20,265. 49 R. A. McClelland and M. Alibahi Can. J. Chem. 1981 59 1169. T. J. Broxton and N. W. Duddy J. Org. Chem.1981 46 1187. Reaction Mechanisms -Part (ii) Polar Reactions 77 Base -cat a1 ysed hydrolysis of met hylsulp hon ylme t hanesulp hona t e ester (62) occurs via an Elcb mechanism involving the sulphene (63)as the product-forming intermediate that is produced by loss of OAr- from the conjugate base of (62).51 In more strongly basic media a second proton is lost to give the dianion (64) from which the expulsion of OAr- now leads to the sulphene anion (65) as the precursor of a sulphonic acid. The authors claim that (64) represents the first example of a 1,l-dicarbanion that exists in aqueous solution. 'Water-catalysed' hydrolysis of p-nitrophenyl dichloroacetate occurs in 10-'M- HC1 solution and in Bu'OH-H20 (containing 0.i mole fraction Bu'OH) in which solvents the solvent isotope effect kHzO/kDzO has values of 3 and 3.40 re~pectively.~~ The hydrolysis reaction is estimated to have an order of ca 5.7 with respect to water and the authors implicate between four and seven water molecules in the transition state for hydrolysis; however they note that in other cited work changes in the order for water do not always reflect the structure of the transition state.A transition state (66) is envisaged and extrapolation to catalytic sites in hydrophobic pockets of enzymes has been considered. '0-H H (66) (67) The 'H n.m.r. spectrum of the carbamate (67) in fluorosulphuric acid at -74 "C indicates protonation on The variation in 'H chemical shift as a function of concentration indicates that in dilute solution there is substantial 0-protonation.Values of pK(0) and pK(N) are -2.8 and -4.9 respectively. 7 Elimination and Addition Reactions An alternative method to the Hofmann elimination has been developed; this centres on the formation of an acridinium salt (68) from amine and pyrylium salt. After thermolysis of (68) at 150°C with triphenylpyridine olefin was isolated in high yield; small amounts of non-terminal olefin were also formed.54 Ph q-p ,c=o 51 S. Thea G. Guanti and A. Williams J. Chem. SOC.,Chem. Commun. 1981 535. 52 W. P. Huskey C. T. Warren and J. L. Hogg J. Org. Chem. 1981,46 59. 53 P. J. Battye J. F. Cassidy and R. B. Moodie J. Chem. SOC.,Chem. Commun. 1981 68. 54 A. R. Katritzky and A. M. El-Mowafy J. Chem. SOC., Chem.Commun. 1981 96. D. G.Morris Activated 0-aryl esters undergo base-catalysed hydrolysis via rate-determining loss of OAr from the conjugate base of the ester e.g. (69); hydrolysis of the corresponding S-aryl ester follows an analogous mechani~rn.~~ With the aid of isobasic plots it was shown that for identical values of pKa RO-is a better nucleofuge than RS-from an ester such as (65) by a factor of 80; this figure is enhanced to 5 x lo3 when these groups leave from the appropriate acetoacetate by the same Elcb mechanism. In a related there is shown to be no correlation between the leaving-group ability of a carbon nucleofuge and the pKa of its conjugate acid. The inverse solvent isotope effect on initial rates increases with increasing buffer concentration at constant buffer ratio in the base-induced (e.g.AcNHO-) elimina- tion of (70) and it attains a maximum value of 7.7.57This together with curvature (70) in a plot of log kobsdvs[AcNLO-] (L = H or D) has been postulated to establish an Elcb mechanism when transfer of H' is not entirely rate-controlling and in particular to distinguish it from an E2 mechanism occurring with concurrent isotope exchange; a freely solvated intermediate rather even than an ion-pair is required. Both enantiomers of (71) bind with approximately equal effectiveness to the active site of carboxypeptidase A (CPA) though only ( + )-(71) (of uncertain absolute configuration) is a substrate for enzyme-catalysed elimination to (72).58 The authors consider that the p-nitrophenyl group would be placed in a hydrophobic pocket and the carboxylate anion of (71) would interact with the positively charged side-chain of Arg-145 with the carbonyl oxygen of (71) co-ordinated to the zinc ion in the reactive site and to a hydrogen of the a-methylene group that is reasonably close to the y-carboxylate group of Glu-270.In tetrahydrofuran t-butyl alcohol or benzene the rate of elimination of (73) to form (75) was 35 times faster than that of (74).59In t-butyl alcohol with potassium H R' \c=c=c=c/ ~~ Me3C/ 'R2 Me3C-C~C-C=C-CMe3 (73) R' = C1,R2 = CMe3 (74) R' = CMe3,R2 = C1 (75) 55 K. T. Douglas and M. Alborz J. Chem. SOC.,Chem. Commun. 1981,551. 56 M. Varma and C. J. M. Stirling J. Chem.SOC.,Chem. Commun. 1981 553. '' J. R. Keeffe and W. P. Jencks J. Am. Chem. SOC.,1981,103,2457. N. T. Nashed and E. T. Kaiser J. Am. Chem. SOC.,1981,103,3611. 59 M. Schlosser C. Tarchini Tran Dinh An R. Ruzziconi and P. J. Bauer Angew. Chem. Znt. Ed. Engl. 1981,20,1041. Reaction Mechanisms -Part (ii) Polar Reactions 79 t-butoxide the formal concentration of base enters the rate equation with an exponent 0.8. This has been rationalized in terms of a trimeric entity as in (76) effecting elimination with the faster reacting isomer. Gas-phase eliminations of dialkyl ethers with p-hydrogens occur readily in preference to substitution with NH2- and OH- and have been investigated by means of a flowing afterglow technique.60 The most acidic &proton is abstracted with NH2- as expected for an Elcb mechanism; the direction of elimination is less predictable when the reagent is OH- with the stability of the resulting alkoxyl anion being important.With for example s-C4H90C2H5 two products are formed i.e. EtO-(84%)and S-C,H,O- (11'/0). Electrophilic additions to olefins e.g. bromination and oxymercuriation exhibit different patterns of reactivity with a common set of olefins. However when account is taken of steric effects in the transition state calculated from the charge-transfer spectra of electron donor-acceptor complexes a common mechanism of elec- trophilic addition is indicated for both reactions.61 8 Miscellaneous An experimental determination of the difference in enthalpy between the chair conformations of e.g.4-chloro-1,1-bis(trifluoromethyl)cyclohexanehas shown62 that the values obtained are not accounted for on the basis of the alternation of induced charge in saturated systems which derives from CND0/2 calculations. In a detailed analysis of linear free-energy relationships Sjostrom and Wold63 emphasize the local validity of linear free-energy relationships and caution against the combination of scales derived from different data sets. +/T%.. Me2N NMe2 I MeomoMe (77) Loss of tritium from (77) to an aqueous solvent has been studied in the presence of a basic catalyst.64 An oxygen-containing base gives a rate profile against pH which consists of two phases whereas much more gradual curvature is shown in 'O C. H. De Puy and V.M. Bierbaurn J. Am. Chem. Sac. 1981,103,5034. 61 S.Fukuzumi and J. K. Kochi J. Am. Chem. Sac. 1981,103,2783. '* R.D.Stolow P. W. Samal and T. W. Giants J. Am. Chem. Sac. 1981,103 197. 63 M.Sjostrorn and S. Wold Acta Chem. Scand. Ser. B 1981 35 537. 64 A.J. Kresge and M. F. Powell J. Am. Chem. Sac. 1981,103 973. D. G.Morris the slower transfer to nitrogen-containing bases on account of the lower elec- tronegativity and the poorer hydrogen-bond acceptor character of nitrogen. The authors thus suggest that N-DN transfers of protons are intrinsically slower than their N-+O counterparts. It has been shown that both the carbanion Ph3C- and the carbo-cation Ph3C+ can be converted into the radical of intermediate oxidation state (Ph3C*) when either isfllowed to react with di-t-butyl nitroxide; this is converted into Bu$NO- and Bu:N=O respectively.6s 65 H.Singh and J. M. Tedder J. Chem. SOC.,Chem. Commun. 1981,70.

ISSN:0069-3030    

DOI:10.1039/OC9817800065

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

4 Reaction Mechanisms Part (iii) Free-radical Reactions By R. A. JACKSON School of Chemistry and Molecular Sciences University of Sussex Brighton BN1 9QJ 1 General There has been a flurry of activity on magnetic effects on radical reactions. Three different groups photolysed dibenzyl ketone in solution .' Cage products (recovered dibenzyl ketone and an isomer benzyl p-tolyl ketone) were enriched in I3C whereas radical escape products (dibenzyl CO) were impoverished in I3C. The large magni- tude of the effect which is enhanced by low temperatures and medium viscosities suggests that nuclear magnetic moment and/or magnetic spin isotope effects are involved rather than a kinetic mass isotope effect. The yields of n-butyl t-butyl ether and octane from n-butyl-lithium and t-butyl peroxide were decreased when the reaction was carried out in a magnetic field,* and a magnetic effect was also observed in the photo-induced substitution reaction of 4-methylquinoline-2-carbonitrile in ethanol3 (Scheme 1).The heats of formation of simple aliphatic radicals are still not satisfactorily established. New values of AH,.,, for Et* = 117 f4 and But' = 39 f4 relative Me Me I I + H H EtOH + M~~HOH 1 Me Scheme 1 N. J. Turro Ming-Fea Chow Chao-Jen Chung and B. Kraeutler J. Am. Chem. SOC., 1981,103,3886; N. J. Turro D. R. Anderson Ming-Fea Chow Chao-Jen Chung and B. Kraeutler ibid. p. 3892; L. L. Sterna Report 1980 LBL-10594; Energy Res. Abstr. 1981,6 13 (Chem. Abstr. 1981,95 114 602); A. L. Buchanchenko V.F. Tarasov and V. I. Mal'tsev Zh. Fiz. Khim. 1981 55 1649. Yu. A. Kurskii Yu. N. Baryshnikov G. I. Vesnovskaya N. N. Kaloshina and Yu. A. Alexandrov Dokl. Akad. Nauk SSSR,1981,258,936. N. Hata and M. Hokawa Chem. Lett. 1981,507. 81 R. A. Jackson to Me- = 144 kJ mol-’ have been obtained by e.s.r. measurements of concentration in radical buffers of R* + R’I; the higher value for Et* reconciles conflicting data on the self-reaction of ethyl radical^.^ AHf,300 (But*) has been evaluated as 44 f 4kJ mol-’ from kinetic measurements in both directions of the reaction of hydrogen atoms with isob~tene,~ in reasonable agreement with the radical buffer results. The question of separation of polar and resonance effects of substituents on radical reactions continues to arouse interest.Afanas’ev favours a two-parameter equation for correlating radical reactivity of aromatic compounds.6 u;I values are derived for a substituent X from the effects of X on the spin density of CH,CHX but these values have to be applied in the negative sense for electron-releasing substituents and in the positive sense for electron-withdrawing groups. This Reporter7 has suggested the thermal decomposition of substituted dibenzyl mer- curials [equation (l)]as a model reaction in which the full stabilizing effect of the substituent on the benzyl radical formed should be available in the transition state. Polar effects are allowed for by decomposing meta-substituted compounds and a scale is defined for para-substituents by equation (2).(TO values correlate well DCH2-+ *Hg-CH X with literature data and an excellent correlation is found with Hammett polar substituent constants via the expression (lu*-ul)/n,where u*is u+for electron- releasing and u-for electron-withdrawing substituents and n = 1 for radicals conjugated with an unsaturated centre or n = 2 for substituents conjugated with a lone pair. In the latter case the stabilization for a radical should be approximately half that for a cation whereas in the former case stabilization should be approxi- mately the same in the radical the cation or the anion (see Figure 1).It should n = 1 electron in non-bonding n = 2 electron in anti-bonding orbital in radical orbital in radical Figure 1 Molecular orbital diagrams for the two types of substituted radicals A.L. Castelhano P. R. Marriott and D. Griller J. Am. Chem. SOC.,1981,103,4262. C. E. Canosa and R. M. Marshall Int. J. Chem. Kinet. 1981,13 303. I. B. Afanas’ev Int. J. Chem. Kinet. 1981 13 173. ’ S. Dincturk R. A. Jackson M. Townson H. AgirbaS N. C. Billingham and G. March J. Chem. SOC. Perkin Trans. 2 1981 1121; S. Dincturk and R. A. Jackson ibid. p. 1127. Reaction Mechanisms -Part (iii) Free-radical Reactions 83 be emphasized however that in many perhaps most free-radical reactions polar rather than radical-resonance interactions appear to be the dominating influence. 2 Structural Studies Two separate groups report UHF calculations on the t-butyl radical with the 4-31G basis set.8 The radical is calculated to be markedly non-planar with each Me-C* bond predicted to be 22.1" out of the plane defined by the radical centre and the other two methyl group carbon atoms.The inversion barrier is calculated to be 5.1 kJmol-'. The geometry of the radical centre in bicyclo[2.2,l]hept-2-y1 systems has been debated recently. Group IVB (Si Ge Sn) radical adducts to camphor and thiocam- phor are intense enough for the 13C satellites to be observed in natural ab~ndance.~ The results suggest non-planarity at the radical centre but comparison with non-cyclic analogues indicates that the non-planarity is due to the substituents rather than to bond-angle strain. Non-planar geometry is also indicated at the radical centre of 1-hydroxycyclohexyl radicals and substituted derivatives by variable temperature e.s.r.and '3C-labelling studies." The interconversion of the two geometrical isomers of the [1-2H]allyl radical made by the reaction of t-butoxyl radicals with deuteriated alkyl phosphites," has been studied in the temperature range 50-1 10"C [equation (3)].The rotational barrier is 65.7 f 4.2 kJ mol-' (log A = 13.5 f OS) suggesting a value for the ally1 delocalization energy of 58.6-60.7 kJ mol-'. Pent-2-en-4-ynyl radicals can exist in forms (1)and (2) where (1)is just the more stable methane-based stabilization energies for (1)and (2) were estimated to be 112 and 110 kJ mol-' respectively from the observed barriers to rotation r47.5 and 49.2 kJ mol-' for reactions (4) and -(4) respectively].'*" H I (1) (2) [Me3Si-SiMe3]t and [Me3Ge-GeMe3]t have been made by y-irradiation of solid solutions at 77 K.In contrast with [Me3C-CMe3]t [cf. Ann. Rep. Prog. Chem. Sect. B 1980 77 541 which showed a septet indicating a strong interaction with one hydrogen only of each methyl group the silicon and germanium analogues show M. N. Paddon-pow and K. N. Houk J. Am. Chem. SOC. 1981 103 5046; M. Yoshimine and J. Pacansky J. Chem. Phys. 1981,14,5168. A. Alberti M. Guerra and G. F. Pedulli J. Am. Chem. SOC.,1981 103 6604. R. V. Lloyd and J. G. Causey J. Chem. SOC.,Perkin Trans. 2 1981 1143; J. C. Micheau B. Despax N. Paillous A. Lattes A. Castellano J. P. Catteau and A. Lablache-Combier Nouu. J. Chim. 1981 5 257. H.-G. Korth H. Trill and R. Sustmann J. Am.Chem. SOC., 1981,103,4483. (a) C. Roberts and J. C. Walton J. Chem. SOC., Perkin Trans. 2 1981 553; (b) J. T. Wang and F. Williams J. Chem. SOC., Chem. Commun. 1981,666. 84 R. A.Jackson spectra that indicate that interaction occurs with all 18 hydrogen atoms suggesting that the unpaired electron is effectively localized in the M-M u-bonding orbital."' E.s.r. parameters for a range of transient tin-centred radicals have been reported.' In alkyltin radicals R3Sn- @-proton couplings of 3.0-3.1 G are noted; for radicals Ph,Me,-,Sn* only coupling to the methyl protons is observed. Line broadening takes place owing to the rapid exchange reaction of trialkyltin radicals with the corresponding hydride this is confirmed by the observation of line broaden- ing in the n.m.r.spectrum of Me3SnH during irradiation with t-butyl peroxide. A structure with partial phenyl bridging is inferred for the benzyldimethylgermyl radical on the basis of the small temperature-variable CH2 splittings compared with the CH3 proton ~p1ittings.l~ t-Butoxyl radicals abstract hydrogen from BH,- or BH3CN- in fluid solution to give the radical anions BH,' and BH,CN' respectively;" for BH3' lines due to 10 B as well as to "B can be observed. The relatively small "B coupling constants (19.9G and 14.3G respectively) confirm that both species are essentially planar. BH3' resembles silicon-centred radicals in some respects BH,' abstracts halogen readily from n-propyl halides (PrC1 PrBr PrI) and displaces alkyl radicals from alkyl isocyanides.The AlH3' radical prepared analogously,16 has a (27Al)= 154.2G which indicates a divergence from planarity similar to that found for SiH3'. The AlH,' radical anion abstracts halogen from Pr"-Hal (Hal = C1 Br or I) and adds to ethylene and benzene to give the corresponding substituted ethyl and cyclohexadienyl radicals. Flash photolysis of N-bromo-3,3-dimethylglutarimidein CCl gave a radical postdated to be the imidyl radical (3) its U.V. and rate of quenching by alkene or alkane is inconsistent with identification of the transient as 'CCl (Scheme 2). The 'precursor' may be an excited state of the precursor molecule or another electronic state of the imidyl radi~a1.l~ 'W0 (3) Scheme 2 Several amidyl radicals RNCOR have been studied by ex.and show a 7r structure in which the electron resides mainly in a N 2p orbital perpendicular to the R-N-C plane.18 Cyclic and acyclic oxyamidyls have been preparedlg and their 170enrichment e.s.r. studies show less delocalization onto the C=O oxygen atom in PhCONOEt compared with *CHCO(CH2$4. A reinvestigation of the structure of 1,3-dialkyltriazenyl radicals indicates that these radicals have a u structure with the electron occupying an anti-bonding three-centre l3 M. Lehnig and K. Doren J. Organomet. Chem. 1981 210 331. '* K. Mochida M. Kira and H. Sakurai Chem. Lett. 1981 645. Is J. R. M. Giles and B. P. Roberts J. Chem. Soc. Chem. Commun. 1981,360. l6 J. R. M. Giles and B. P. Roberts J. Chem. SOC.,Chem. Commun. 1981 1167. R. W. Yip Y.L. Chow and C. Beddard J. Chem. Soc. Chem. Commun. 1981,955. l8 R. Sutcliffe D. Griller J. Lessard and K. U. Ingold J Am. Chem. Soc. 1981 103 624. l9 A. R. Forrester and H. Irikawa J. Chem. Soc. Chem. Commun. 1981,253. Reaction Mechanisms -Part (iii) Free-radical Reactions molecular orbital with greatest spin density on the central nitrogen atom rather than the T structure with the spin density located mainly on the terminal nitrogen atoms as previously suggested.20 Irradiation of the powdered phosphorane at 77 K gave the phosphoranyl radical (4). The electron is equatorial and M-1 Berry pseudorotation takes place with the unpaired electron acting as the pivot. A single-crystal e.s.r. study shows that at 77 K the equatorial radical (5) is formed but on warming to 193 K isomerization to the axial form (6) takes place (Scheme 3).2' Phosphoranyl radicals OCH2CH20$(OR')SR have trigonal bipyramid structures with the SR group and (4) Scheme 3 one of the bridging groups in apical positions.Line-width effects show that apical- equatorial exchange takes place probably uia a (T* intermediate or transition state.22 Dialkyltriazenyl radicals add to trimethyl phosphite to give the cyclic radical (7a). Above 280 K exchange between (7a) and (7b) is fast enough to make N' and N3 magnetically equivalent (Scheme 4).23 ABut-NNN-Bu' +(Me0)3P OMeI .OMe + Bu'-N'-PP(I I OMe NZ-N~-B~~ Me0 ,OMe\ #'5 But-N' -P-OMe I I.N~-N~-BU' (7a) (7b) Scheme 4 A series of overcrowded galvinoxyl radicals has been made.For compounds such as t-butylgalvinoxyl (8 R = But) the bulky t-butyl group attached to the central carbon atom prevents coplanarity of the two rings. Thus (8a) and (8b) are in true equilibrium in contrast to galvinoxyl itself (8,R = H) where structures similar to (8a) and (8b) (but with the benzene rings coplanar) are mere contributions to the overall Photolysis of acetyl hypohalites (CH3C02X X = C1 or Br) in the presence of the corresponding halogen neopentane and dichloromethane gives substantial amounts of CHC1,X and neo-C5HI1X with a selectivity of about 12 in each case. 22 J. C. Brand and B. P! Roberts J. Chem. SOC.,Chem. Commun. 1981,748. " J. H. H. Hamerlinck P. H. H. Hermkens P. Schipper and H. M. Buck J. Chem. SOC.,Chem. Commun. 1981 358; J.H. H. Hamerlinck P. Schipper,.and H. M. Buck ibid. p. 1148. ''J. R. M. Giles and B. P. Roberts J. Chem. SOC.,Perkin Trans. 2 1981 1211. 23 J. C.Brand and B. P. Roberts J. Chem. SOC.,Chem. Commun. 1981 1107. 24 B. Kriste W. Harrer H. Kurreck K. Schubert H. Bauer and W. Gierke J. Am. Chem. Soc. 1981 103,6280. R. A. Jackson In the absence of halogen and with 1,l-dichloroethylene as a trap virtually no halogenation of the neopentane or dichloromethane takes place. It is suggested that two different types of acetoxy radicals may be involved one produced in the presence of halogens which can escape from the cage and abstract hydrogen atoms (probably the 2A27r or 2Bou state) the other produced in scavenged systems which undergoes decarboxylation too fast to take part in hydrogen-abstraction reactions (2A1cr 3 Formation Destruction and Radical Stability The formation of free radicals by a 'spontaneous' reaction between nitroso- compounds and other organic compounds such as alcohols or ethers has often been ascribed to molecule-assisted homolysis (MAH).However Chatgilialoglu and Ingold have shown that in several such reactions the radicals are formed by simple photolysis of the nitroso-compound by laboratory light and it is suggested that the role of light on other 'spontaneous' radical-forming reactions should be investi- gated.26 On the other hand a molecular beam of the reaction between fluorine and methane at 300-400 K indicates that the dominant initiation reaction is the MAH reaction (9 with log(A/l mol-'s-') = 9.3 f 0.3 and E = 47 f 8 kJ mol-'.Stannous chloride is reported to react reversibly with aqueous methylcobalamin by methyl-group abstraction to give CH3SnC12* [equation (6)].276 CH4 + F2 + CH3. + H-F + F* (5) SnC12 + CH3-B12 $ CH3SnC12.+ B12r (6) Very-low-pressure pyrolysis (VLPP) of ethylbenzene isopropylbenzene and t-butylbenzene takes place by p-C-C bond homolysis.28 Derived AH values are +166 kJ mol-' for PhCHMe- and +136 kJ mol-' for PhCMe2* in good agreement with toluene and aniline carrier experiments. VLPP of the appropriate compound R-CH3 gave the substituted propargyl radicals [Re= HCrCCHCH, 25 P. S. Skell and D. D. May J. Am. Chem. SOC.,1981,103,967. 26 C. Chatgilialoglu and K. U. Ingold J. Am. Chem. Soc.1981,103,4833. " (a) C.Seeger G. Rotzoll A. Lubbert and K.Schugerl In?.J. Chem. Kine?. 1981 13 39. (b)Y.-T. Fanchiang and J. M. Wood J. Am. Chem. SOC., 1981,103,5100. 28 D.A.Robaugh and S.E. Stein Int. J. Chem. Kinet. 1981,13,445. Reaction Mechanisms -Part (iii) Free-radical Reactions 87 CH3C=CCHCH3 and CH,CrCC(CH,),] and the 1-methylallenyl radical CH3C=C=CH2. Stabilization energies are 42 f 9 32 f 12 41 f 12 and 33 f 9 kJ mol-' indicating that methyl substituents on propargyl radicals have little influence on stability.29 A shock tube reinvestigation of the thermal decomposition of ethane has been carried out [equation (7)].30 An RRKM model is produced giving excellent agree- ment with other recent data with log k? = 17.2 -380.7 kJ/2.3RT and log k7( 1mol-' s-') = 10.44.CH3-CH3 + 2CH3. (7) U.V. irradiation of pentamethylcyclopentadiene gives the pentamethylcyclo- pentadienyl radical in an unusual reaction involving homolysis of the ring C-H bond.3' A nanosecond laser flash photolysis study of the photosensitized reaction of t-butyl peroxide by ketones or aromatic hydrocarbons such as benzophenone or benzralanthracene suggests that triplet energy transfer to the peroxide leads to a repulsive excited state that decomposes into two Bu'O- radicals.32 The t-butoxyl radical has been generated in aqueous solution by the reaction between Bu'OOH and Ti"' at very low pH the protonated form Bu'OHt is im~licated.~~ Twenty-one bridgehead peresters RC020Bu' have been thermolysed. A transi-tion state of type (9) is invoked and it is thought that the thermolysis constants are determined mainly by the polar effect in the transition state [equation (8)].34 s+ s-RCOOBU' -B [R--C-I-O--OBU']+ R.+ C02 + *OBu' (8) II II 0 0 Di-tin compounds of the type R3Sn-SnR3 (R = Ph cyclohexyl 1-adamantyl) do not dissociate into R3Sn* radicals at temperatures up to 230 "C but for the more hindered compounds R = 2,4,6-trialkylphenyl (alkyl = Me Et Pr') the dissoci- ation temperature can be as low as 20°C with a dissociation energy of only 36 kJ mol-' for the isopropyl omp pound.^' Cobalamins with an organic group attached to cobalt possessing a P-hydrogen atom decompose by p-elimination in the absence of such a hydrogen atom compounds such as benzyl- or neopentyl- cobalamin decompose spontaneously with Co -C bond homolysis.In the absence of air recombination takes place but in the presence of air the radicals are rapidly oxidized making these compounds highly air-sensitive in aqueous Several groups have carried out photochemistry in micelles :xanthone phenyl alkyl ketones dibenzyl ketone and benzyl phenylacetate have been in~estigated.~' 29 T. T. Nguyen and K. D. King J. Phys. Chem. 1981 85 3130; K. D. King and T. T. Nguyen Znt. J. Chem. Kinet. 1981 13 225. 'O G. B. Skinner D. Rogers and K. B. Patel Int. J. Chem. Kinet. 1981 13 481. 31 A. G. Davies and J. Lusztyk J. Chem. SOC.,Perkin Trans. 2 1981 692. 32 J. C. Scaiano and G. G. Wubbels J.Am. Chem. SOC.,1981,103,640. 33 B. C. Gilbert P. D. R. Marshall R.0. C. Norman N. Pineda and P. S. Williams J. Chem. SOC. Perkin Trans. 2 1981 1392. 34 C. Ruchardt V. Golzke and G. Range Chem. Ber. 1981,114 2769. 3s H.-U. Buschhaus W. P. Neumann and T. Apoussidis Liebigs Ann. Chem. 1981 1190. 36 G. N. Schrauzer and J. H. Grate J. Am. Chem. SOC.,1981,103 541. '' J. C. Scaiano and J. C. Selwyn Can. J. Chem. 1981,59 2368; N. J. Turro Min-Fea Chow Chao-Jen Chung Y. Tanimoto and G. C. Weed J. Am. Chem. SOC.,1981,103 4574; D. Avnir L. J. Johnston P. De Mayo and S. K. Wong J. Chem. SOC.,Chem. Commun. 1981,958. R. A. Jackson In view of the importance of t-butyl peroxide as a radical initiator the rate constant for the 'self-reaction' for the t-butoxyl radical is of considerable interest. The absence of an e.s.r.signal from the t-butoxyl radical makes 2kt difficult to determine directly but by setting up a competition between the self reaction and hydrogen abstraction a value of 2kt = (1.3 f0.5) x lo9M-* s-l at 293 K has been obtained in a flash photolysis-e.s.r. e~periment.~~ This value is close to the diffusion- control limit. The decay of radical (10) is not thought to be diffusion controlled there is an activation energy of 17 kJ mol-' and an intermediate is postulated on the route to the product pinacol (Scheme 5),39 Scheme 5 4 Radical Transfer Fluorination of organic compounds by molecular fluorine is a reaction that is difficult to control. Some of the difficulties are surmounted by fluorinating the hydrocarbon as an aerosol suspension in helium at low temperatures thereby allowing high yields of perfluorinated compounds to be made from the corresponding hydrocarbon with minimum disruption of the carbon ~keleton.~' High positional selectivity of chlorination of hexane at the 2-position is achieved by photolysis with [Pr;NMeC1]'C1O4- (the ratios of 1-:2-:3-chlorohexanes = 7.1 :76.6 :16.5).The enhanced reactivity at the 2-position is attributed to steric difficulty of approach by the bulky aminium radical to the 3-po~ition.~~ The decomposition of a steroidal peracid has been used to achieve a remote hydroxyla- tion (Scheme 6).42 There has been a continuing interest in determining absolute values of rates of radical-transfer reactions. Time-resolved e.s.r. measurements show that the decay of t-butyl radicals in methylcyclopentane solution is of second order but in the presence of chloroform a pseudo-first-order term is added from which the rate constant for the abstraction of a hydrogen or a chlorine atom from chloroform by the t-butyl radical is deduced.43 Laser-flash photolysis has been used to determine the rates of reaction of triethylsilyl radicals with alkyl halides (halogen abstraction) and with ketones (addition to the carbonyl oxygen atom).44 38 S.K. Wong Int. J. Chem. Kinet. 1981 13 433. 39 H. Krohn R. Leuschner and J. K. Dohrmann Ber. Bunsenges. Phys. Chem. 1981,85 139. 40 J. L. Adcock K. Horita and E. B. Renk J. Am. Chem. SOC.,1981,103,6937. 41 S.E.Fu1ler;J. R. Lindsay Smith R. 0.C. Norman and R. Higgens J. Chem. Soc.Perkin Truns. 2 1981,545. 42 J.-P. BCguC D. Lefort and T. D. Thac J. Chem. Soc. Chem. Commun. 1981,1086. 43 H. R.Deutsch and H. Fischer Int. J. Chem. Kinet. 1981,13,527. 44 C.Chatgilialoglu K. U. Ingold J. C. Scaiano and H. Woynar J. Am. Chem. Soc. 1981,103 3231. Reaction Mechanisms -Part (iii) Free-radical Reactions (12) + Scheme 6 Hydrogen abstraction by t-butoxyl radicals from the hydroxyl hydrogen of sub- stituted phenols indicates that positive charge is built up on the aromatic system in the transition state. The reaction is slower in polar solvents such as pyridine hydrogen bonding decreases the reactivity of the phenolic OH group.45 Abstraction of chlorine from several N-and O-heteroarylmethyl chlorides by triphenyltin radicals suggests a transition state with contributions both from the heteroarylmethyl radical and the anion.46 Stereoelectronic factors are important in abstraction by t-BuO' radicals from C-H bonds adjacent to oxygen or nitrogen the rate of abstraction is enhanced if the dihedral angle between the C-H bond and the N or 0 lone pair is Tunnelling appears to be important in the abstraction of hydrogen from toluene by bis(trifluoromethy1)nitroxide:the Arrhenius A factor is unusually low and the difference between the activation energies for deuterium compared with protium abstraction is unusually large.48Q Tunnelling is also invoked4*' to explain the difference in AV' observed in the rearrangement of 2,4,6-(Me3C)3C6H2* by transfer of a hydrogen atom from the t-butyl group to the ring (5.3f 1.7 cm3 mol-l) compared with the deuteriated compound 2,4,6-[(C2H3)3C]3C6H*.(-1.2 f 2.0 cm3 mOl-').Photolytic brominolysis of trans-1,2-diarylcyclopropanes involves S,2 attack by a bromine atom at a strained-ring carbon atom (Scheme 7).49The reaction rate is B. + ArAH 5 ArCHBrCH,CHBrAr + Br. H" "Ar Br Scheme 7 enhanced by electron-donating substituents on one or both rings. SH2 attack at saturated oxygen centres appears to be more difficult than at sulphur. However trichlorosilyl radicals react with cyclohexyl octyl ether to give a preponderance of '' P. K. Das M. V. Encinas S. Steenken and J. C. Scaiano J. Am. Chem. Soc. 1981,103,4162. 46 H.Soppe-Mbang and G. J. Gleicher J. Am. Chem. Soc. 1981,103,4100. 47 D. Griller J.A. Howard P.R. Marriott and J. C.Scaiano,J.Am.Chern. SOC.,1981,103,619;V. Malatesta and K. U. Ingold ibid. p. 609. '* (a)V. Malatesta and K. U. Ingold J. Am. Chem. SOC.,1981,103,3094; (6)P.R.Marriott and D. Griller ibid. p. 1521. 49 D. E. Applequist and R. D. Gdanski J. Org. Chem. 1981,46,2502. R. A. Jackson HSiCI, CI3Si’ + OC8Hl + Cl3SiOC8Hl7 + 0. -0 Scheme 8 cyclohexane in accord with Scheme 8 where the more stable cyclohexyl radical is displaced in preference to the octyl radi~al.~’ 5 Addition to Multiple Bonds and Homolytic Aromatic Substitution Alkoxyl alkyl and acyl radicals add to the P=N bond according to reaction (9) the 31P coupling constant of 519G for (13 X = Bu‘O) indicates a pyramidal geometry for the radical.’l Cyclization of the 5-hexen-1-yl radical (14 X = Y = CH,) gives virtually exclusively the less thermodynamically stable cyclopentylmethyl (Me3Si)*N -P=NSiMe3 + X .-+ (Me3Si),N -f’=NSiMe3 (9) I X (13) (14) (15) (16) radical (15 X = Y = CH,). Experiments have now been carried out with analogues containing a Me2Si group at position X or Y. For (14 X = Me,Si Y = CH,) the 5-ring product (15) still predominates but for 14 (X = CH, Y = Me2Si) the 6-ring product (16) dominates the product ratios indicating that it is a reduction in the rate of cyclization to the 5-ring that gives this res~lt.’~ The longer Si-C bond length may make the transition state leading to the 5-ring less favourable. 1-Alkenylmercuric halides react photolytically with organic disulphides to give the 1-alkenyl~ulphide.~~ Addition to the 1-position to give the intermediate radical (17) which may be stabilized by bridging is followed by loss of the mercurio- substituent [equation (lo)].The water soluble nitroso-compound (18 X = H or ,H) has been proposed as a spin trap for aqueous An advantageous simplification of the spectra is achieved in the deuteriated spin trap (18 X = ,H). A study of the kinetics of the reaction of hydroxyl radicals with benzene and toluene shows that below room temperature electrophilic addition to the aromatic nucleus predominates. Above 500 K reversal of the addition reaction reduces the RCHZCH-HgCl + PhS -B RCH-CH-SPh + RCH=CHSPh + .HgCI (10) *.. I *-HgCl (17) 50 R. A. Jackson F. Malek and N.Ozaslan J. Chem. SOC., Chem. Commun. 1981,956. ’* B.P.Roberts and K. Singh J. Chem. SOC.,Perkin Trans. 2 1981 866. 52 J. W.Wilt J. Am. Chem. SOC.,1981,103 5251. 53 G.A. Russell and J. Hershberger J. Am. Chem. SOC., 1980 102,7603. 54 H.Kaur K. H. W. Leung and M. J. Perkins J. Chem. SOC.,Chem. Commun. 1981 142. Reaction Mechanisms -Part (iii) Free-radical Reactions SO<Na+ Br NO (18) amount of aromatic substitution product and abstraction of ring or side-chain hydrogen atoms becomes competiti~e.~~ Competition between ring hydroxylation and side-chain attack has also been studied in the presence of 0,or NO to convert the intermediates into stable Thermal decomposition of Na&08 in aqueous solutions of benzene and nitrobenzene gives phenol biphenyl and 0-and p-nitrophenol.The nitrophenols are not formed in the absence of benzene. Steps (a) and (b) in Scheme 9 are known to be rapid. It is proposed that the hydroxycyclohexadienyl radical can dissociate [step (c)] to benzene and the hydroxy radical which in turn attacks the nitrobenzene to give the nitro phenol^.^' Scheme 9 Improved partial rate factors for homolytic aromatic arylation have been obtained by carrying out the arylation by benzoyl peroxide in the presence of C6H5N0 C6F5N0 or Cu" or FeIII ben~oate.'~ Oxidation of the initial u complex is almost quantitative thus eliminating uncertainties in partial rate factors caused by dimeriz- ation. For eight substituents partial rate factors for the meta-position were close to unity (0.85-1.35); at the para-position all were greater than 1and ranged from 1.1(F)to 4.0 (COPh).6 Fragmentation Arrhenius parameters for alkoxy radical fragmentations have been re-evaluated. For the important reaction (ll) values of log, A(s-') = 14.1 and E = 64.0 kJ mol-' have been ~ecommended.~~ Substituted 9-decalinoxyl radicals frag- ment to give both cyclohexyl and cyclodecyl derivatives which bond cleaves seems to depend on a number of factors including substituent stabilizing effects and the cyclic ketone ring strain.60 (CH3)3C-O. + (CH3)2C=O + CH3. (11) 55 F. P. Tully A. R. Ravishankara R. L. Thompson J. M. Nicovich R. C. Shah N. M. Kreutter and P. H. Wine J. Phys. Chem. 1981,85 2262. 56 R. A. Kenley J. E.Davenport and D. G. Hendry J. Phys. Chem. 1981,85,2740. 57 M. K. Eberhardt J.Am. Chem. SOC.,1981,103 3876. " R. Bolton B. N. Dailly K.Hirakubo K. H. Lee and G. H. Williams J. Chem. SOC.,Perkin Trans.2 1981,1109. '9 Kwang Yul Choo and S. W. Benson Int. J. Chem. Kinet. 1981,13,833. 6o T. L. Macdonald and D. E. O'Dell J. Org. Chem. 1981,46 1501. R. A.Jackson Scheme 10 Cyclobutylmethyl 1-cyclobutyl-1-methylethyl and cyclobut-2-enylmethyl radicals (19) undergo ring-opening reactions (Scheme 10) with log, A(s-')in the range 12.2-13.6 and E/kJ mol-' 42-58. Ring opening of (19) gave the E E-pentadienyl radical (21) presumably via the E 2-form (20). Independent experi- ments show that (20) can be converted into (21) but not vice versa from the rotation barrier a methane-based stabilization energy of 104 kJ mol-' was esti- mated.61 Glycidol derivatives (22 M = R or R3Si) react with t-butoxyl radicals as shown in Scheme ll(a).Ring opening appears to be followed by loss of [H'] and attack by ROO gives the observed acyl radical (24). Where M = acetyl an alternative reaction (b) takes place acyl transfer allows radical (25) to be observed.62 O=CH-CH=CHOM T7-I + c? 0 OM 0 OM M (22) (23) (25) Scheme 11 Tributyltin hydride causes elimination from vicinal dinitro-compounds or p-nitro- sulphones to give the corresponding alkene. The reaction is stereospecific (anti) in the latter but not in the former case. Scheme 12 involving radical anion intermedi- ates is po~tulated.~~ 7 Electron Transfer Both triphenylmethyl cations and anions undergo electron transfer with di-t- butylnitroxide to give triphenylmethyl radicals.If oxygen is admitted to the anion system the di-t-butylnitroxide is regenerated [equation (12)] but when the cation is the precursor the degradation reactions (13) and (14) take place.64 61 K. U. Ingold B. Maillard and J. C. Walton J. Chem. SOC.,Perkin Trans. 2 1981 970; A. G. Davies D. Griller K. U. Ingold D. A. Lindsay and J. C.' Walton ibid.,p. 633. 62 A. G. Davies J. A.-A. Hawari B. Muggleton and Man-Wing Tse J. Chem. SOC.,Perkin Trans. 2 1981 1132. 63 N. Ono H. Miyake R. Tamura I. Hamamoto and A. Kaji Chem. Lett. 1981 1139. 64 H. Singh and J. M. Tedder J. Chem. SOC.,Chem. Commun. 1981,70. Reaction Mechanisms -Part (iii) Free-radical Reactions 93 \ / Bu3SnH or Bu3Sn-+ / + Bu3SnHor Bu3Sn+ + C-C /I -I\ NO; X 'C-C / -+\C-C / +NO X X.+ Bu3SnH + X-H + Bu3Sn-Scheme 12 02 Ph3C-+ BU~NO -+ Bu~N-0-+ Ph3C.++ Ph3COO. B~;NO-+Ph3COO-+ Bu~NO. 02 Ph3C+ + Bu~NO+ Bu$=O + Ph3C. __* Ph3COO-B~;NO. -+ Ph3COO-+ Bu$ = 0 0 II Ph3COO- H-CH2-CMe2p7-But + Ph3COOH + CH2=CMe2 + Bu'NO (14) Nitro-compounds are reduced to alkanes by trialkyltin hydrides in the presence of radical initiators. Electron transfer followed by fragmentation and hydrogen transfer are postulated [reaction (15) compare scheme 12 for di-nitro corn-R~S~H + R. -R$n* + RNO2 -B R:Sn' + RNOS RH + R:Sn* (15) Radical cations can be used to catalyse the Diels-Alder reaction of neutral dienophiles;66 the dienophile is converted into the radical cation that then reacts more rapidly with the diene without loss of the suprafacial stereospecificity of the uncatalysed reaction.For example reaction (16) which uncatalysed gives a 30% yield at 200 "C after 20 h with 4 1 endo exo selectivity takes place in 70% yield at 0 "C in 15min in the presence of [(p-BrC6H4)3N'SbC15-] with 5 1 endo ex0 selectivity. (16) 65 D. D. Tanner E. V. Blackburn and G. E. Diaz J. Am. Chern. SOC.,1981,103,1557. 66 D.J. Bellville D. D. Wirth and N. L. Bauld J. Am. Chem. SOC.,1981,103 718. R. A. Jackson In principle Grignard addition to ketones can take place by a polar route or by single-electron transfer (SET)as shown in Scheme 13.Evidence for the participa- tion of the SET pathway both for primary and for tertiary Grignard reagents was obtained by the use of the unsaturated reagents RMgCl[R = CH2=CH(CHz)3CMez-or CHz=CH(CH2)zCMe2CH2-].In each case some cyclized products were isolated indicating that free R* radicals must have been involved in the reaction.67 ‘RMgX’ + ArzC=O1- Polar --+ ArzC=O %MgX [R + ArzC-OMgXl diffusion\:; reaction in cage ,Ar2C-OMgXIR R + ArzC-OMgX 1 11 RH ArzC-OMgX I Ar2C-OMgX Scheme 13 The SR,l reaction of the nitro-compound (26)with sodium benzenethiolate gives (27),with retention of configuration at high thiolate concentrations though at lower thiolate concentrations some inversion takes place. It is concluded that a pyramidal benzyl radical is formed and trapped (Scheme 14).68 BU,dr6H4N02 Bu‘&:6H4N02 H4N02 (27) Reagents i +e-; ii -NO;; iii PhS-; iv -e-Scheme 14 Russell and co-workers suggest that in the SRNlmechanism [equations (17)-(19)] steps (17) and (18)may merge so that R-is not formed as an intermediate.69 RX‘ -D R-+ X-(17) R*+ N- -D RNT (18) RNT+ RX -+ RXT+ RN (19) 67 E.C. Ashby and J. R. Bowers jun. J. Am. Chem. SOC.,1981,103,2242. 68 R. K. Norris and R. J. Srnyth-King J. Chem. SOC.,Chem. Commun. 1981,79. 69 G. A. Russell B. Mudryk and M. Jawdosiuk J. Am. Chem. SOC.,1981,103,4610. Reaction Mechanisms -Part (iii) Free-radical Reactions When enolate ions E = RC(0-)=CHR’ react with nitro-compounds XCMe2N02 the substitution product ECMe2N02 and the enolate dimer E-E are formed in ratios which do not depend on [XCMe,NO,] or [E-] but which depend strongly on the nature of X(X = C1 NO2 or p-MeC6H4S02).Decomposition of benzoyl peroxide with 4-fluoroanisole in KOAc-HOAc gives 4-methoxyphenyl acetate and benzoate in a maximum ratio of 10:1. Scheme 15 involving a radical cation intermediate is suggested. Initiation can take place with Nu* = PhC02* but in the presence of OAF Nu-= OAc-and the predominant product will be the acetate.” ArX + Nu-+ [ArXNu]. + ArN? + X-T I Scheme 15 Note Readers interested in a computer-readable version of the references in this Section for their own data systems are invited to contact the Reporter. ’O Chem. Commun. 1981 133. L. Eberson and L. Jonsson J. Chem.SOC.

ISSN:0069-3030    

DOI:10.1039/OC9817800081

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By D. W. KNIGHT Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 General Rearrangement reactions of carbenes and nitrenes have been summarized in a recent monograph.' A book describing various vacuum pyrolytic methods has also appeared2 which will be of interest to those working with thermally generated carbenes and nitrenes. 2 Arynes Several reports this year have emphasized the value of benzyne intermediates in cycloaddition reactions. Thus the anthranilic acid (1)can be converted into (2) in 86% yield via an intramolecular Diels-Alder reaction of the intermediate benzyne generated in the usual way.3 The reaction also works very well (90"h yield!) when the benzyne is generated from a 1,2-dibromobenzene and n-BuLi.The extra- ordinarily high yields obtained in these reactions serve to emphasize the advantages of many intramolecular cycloadditions in comparison to similar intermolecular procedures and suggest that such reactions with arynes as dienophiles will be of considerable value in synthesis. In this case the method has been used to prepare the natural 0-naphthoquinone Mansonone E. An apparently related reaction is Me Me I the conversion of amide (3) into (5) (in 25% yield) on treatment with n-BuLi; the aryne (4) seems to be a plausible intermediate.4 W. M. Jones 'Rearrangements of Carbenes and Nitrenes' in 'Rearrangements in Ground and Excited States' ed. P. DeMayo Academic Press New York 1980 Vol. 1. R. F. C. Brown 'Pyrolytic Methods in Organic Chemistry.Application of Flow and Flash Vacuum Pyrolytic Techniques'. Academic Press New York 1980. W. M. Best and D. Wege Tetrahedron Lett. 1981,22,4877. W. J. Houlihan Y. Uike and V. A. Parrino J. Org. Chem. 1981,46.4515. 97 D. W.Knight The use of 1,2,4,5-tetrabromobenzenesas dibenzyne equivalents has been repor- ted in full.’ The reaction which is almost certainly a stepwise process can be used to prepare compounds such as (6)by treatment of the halobenzene with n-BuLi and excess anthracene at room temperature. Analogous reactions can be performed using 2,3,6,7-tetrabromonaphthalenes. (6) Another full report concerns the preparation of anthraquinones in ‘one-pot’ by the addition of phthalide anions to arynes generated in situ from halobenzenes.6 In related studies the sulphonylphthalide anion (7)has been found to react regioselectively with benzyne (8) to give intermediate (9),which readily collapses to give anthraquinone (lo).’ Unfortunately the addition is not so regioselective with other benzynes.0 0 OMe + (7) QyJ \ \ OMe I Me0 I ‘S02Ph Me0 0 (9) H. Hart S. Shamouilian and Y. Takehira J. Org. Chem. 1981,46,4427; H. Hart and S. Shamouilian ibid. p. 4874. ‘ D. J. Dodsworth M.-P. Colcagno E. V. Ehrmann B. Devadas and P. G. Sammes J. Chem. Soc. Perkin Trans. 1 1981 2120. ’ R. A. Russell and R. N. Warrener J. Chem. Soc. Chem. Commun. 1981 108. Arynes Carbenes Nitrenes and Related Species 99 Convincing arguments have been presented' in full that pyrolysis (at ca.320 "C) of cis-1,2-dialky1-1,2-diethynylolefins involves the intermediacy of a 1,4-dehy- drobenzene as a discreet biradical most likely in the singlet state and with an estimated lifetime of between lo-' and s at 200 "C. Details of the synthesis and isolation of the di- and tetra-dehydrocyclo-octenes (11)and (12) have been published.' Although potentially anti-aromatic the com- pounds seem to be planar and are stable enough for some reactions to be carried out on them. The spectral characteristics and some chemistry of the acenaphthyne (13) have also been recorded." The intermediacy of 2,3-thiophyne and 2,3-benzothiophyne has been evoked to account for some of the products obtained by flow vacuum pyrolysis of the corres- ponding anhydrides in the presence of various dienes." 3 Nitrenes Photoelectronic spectra of methyl azide at 850 K show the main decomposition product to be methanimine and not methylnitrene in accord with the results of theoretical calculations.'2 The recently reported pyrolytic conversion of aryl azidoformates into benzoxazolones does not seem to involve a spiro-intermediate as a para-substituented azide (14) leads only to isomer (15).13If the two ortho-positions in (14) are blocked a cyclohexa-2,4-dienone is initially formed which subsequently dimerizes.Thermolysis or photolysis of phenyl azide in acetic acid gives azepin-2- one (presumably via l-azacyclohepta-1,2,4,6-tetraene)together with various sub- stituted anilines whose formation can most reasonably be rationalized by assuming the intermediacy of (16) an adduct of phenyl nitrene and acetic acid.I4 HN -0Ac D0&0+ 0 fJo>o -_* R ' N3 R' H (14) (15) (16) T.P. Lockhart P. B. Comita and R. G. Bergman J. Am. Chem. SOC., 1981,103,4082; T. P. Lockhart and R. G. Bergman ibid. p. 4091. H. N. C. Wong and F. Sondheimer Tetrahedron 1981 37 Supp. 1,99. lo 0.L. Chapman J. Gano P. R. West M. Regitz and G. Mass I. Am. Chem. Soc. 1981 103 7033. M. G. Reinecke J. G. Newsom and L.-J. Chen J. Am. Chem. SOC.,1981,103 2760. '* H. Bock R. Dammel and L. Horner Chem. Ber. 1981,114,220. l3 0. Meth-Cohn and S. Rhouati J. Chem. SOC.,Chem. Commun. 1981 241; see also R. N. Carde P. C. Hayes G. Jones and C. J. Cliff J. Chem.Soc. Perkin Trans. 1 1981 1132. 14 H. Takeuchi and K. Koyama J. Chem. SOC., Chem. Commun. 1981,202. 100 D. W.Knight An interesting case of what appears to be stereochemical control in the decompo- sition of a heteroaromatic nitrene has been rep~rted.'~ Thus 5-azido-2-fury1 ketones decompose rapidly at 20 "C to give the nitrile esters (18)presumably via the nitrenes (17; R = Pr' or Bu') whereas the corresponding aldehyde is much more stable. This can be explained by assuming that in the case of the ketones the presence of the bulky substituent R results in the ketone carbonyl being out of plane with the rest of the molecule and hence the contribution from the canonical form (20) which leads to (18) is greater than the alternative (19) which can contribute when the carbonyl group is in plane as in the case of the aldehyde.a& -NQCOR 0- (19) (20) A general route to nitrenes (22) involves the thermally induced fragmentation of hydroxylamine derivatives (21).l6 Kinetic data support a concerted mechanism for the reaction. Sulphenylnitrenes (22;R2 = SR) can be obtained by Pb(OAc) oxidation of 2,4-dinitrobenzenesulphenamides;a full description of this method and of the subsequent trapping of these nitrenes by electron-rich olefins has been given.l7 SiR13 / R~-N 'OSiR13 R2 = C02Et,ArC0 (21) ArS02,Me R2 # SiMe3 Pyrolysis of p-phenethylsulphonyl azides gives rise to the sulphonylnitrenes (23) and not P-phenethylnitrenes uia a Wolff -like rearrangement and SO2 elimination as could perhaps happen." Some unexpected products from (23) are the pyridines (24):a reasonable mechanism for their formation involves azepine intermediates.By contrast the sulphonylnitrenes (25; n = 3 or 4) preferentially undergo intramolecular insertion into the aryl ring to give the sultans (26) whereas nitrene (25; n = 5) gives mainly (27) by insertion into an alkyl C-H bond." A general Is P. J. Newcombe and R. K. Norris Terruhedron Lett. 1981 22,699. l6 Y.H. Chang F.-T. Chiu and G. Zon J. Org. Chem. 1981,46 342. R. S. Atkinson and B. D. Judkins J. Chem. Soc. Perkin Trans. 1. 1981 2615. See also W. Bludssus and R. Mews Chem. Ber. 1981,114,1539. '* R. A. Abramovitch W. D. Holcornb and S. Wake J. Am. Chem. SOC.,1981,103 1525. l9 R.A.Abramovitch S.B. Hendi and A. 0.Kress J. Chem. SOC.,Chem. Commun. 1981 1087. Arynes Carbenes Nitrenes and Related Species route has been developed for the preparation of aryloxysulphonyl azides ArOS02N3 which can serve as precursors to aryloxysulphonylnitrenes.20 Benzoylnitrene (28) can be obtained by photolysis of benzoyl azide but not by thermolysis as under such conditions the substrate undergoes a Curtius rearrange- ment.21 The stereoselectivity of the insertion reactions of (28) into C-H bonds suggests that it is formed in the singlet state. Ethoxycarbonylnitrene reacts with stable mesoionic compounds (29) by addition across the 1,3-dipole followed by a rearrangement to give the imidazolines (30).22 Cyanonitrene NC-N :,apparently in its triplet state has been generated from sodium cyanamide and t-butyl hypochlorite in methan01,*~ and the borylnitrene (Pri2N)2BN can be obtained by thermolysis (450485"C) of the corresponding a~ide.~~ There has been considerable interest in the chemistry of amino-nitrenes during the past year.Phthalimidonitrene (3 1)reacts with nitroso-compounds to give the diazene-1-oxides (32) in a reaction that can be reversed by photolysis or acid (31) (32) hydroly~is.~~ Subsequent nucleophilic attack on (32; R = R'*N) results in the forma-tion of amino-nitrenes (33) whose fate depends upon the attacking species. Oxida- tion of the quinazolones (34) by Pb(OAc) in chloroform leads to the expected 'O M. Hedayatullah and J. C. Hugueny Synth. Commun. 1981,11,643. *' M.Inagaki T.Shingaki andT. Nagai Chem. Lett. 1981 1419. 22 T.Sheradsky and D. Zbaida Tetrahedron Lett. 1981 22 1639. 23 M.G.K. Hutchins and D. Swern Tetrahedron Lett. 1981 22,4599. 24 W.Pieper D. Schmitz and P. Paetzold Chem. Ber. 1981,114,3801. *' L. Hoesch and B. Koppel Helv. Chim. Acta 1981,64,864;L. Hoesch ibid.,p. 890;see also C. Leuenberger L. Hoesch and A. S. Dreiding ibid. p. 1219. 102 D. W.Knight nitrene which can be trapped intermolecularly by olefins such as styrene but which can also undergo intramolecular trapping probably via an intermediate dipolar species (35) formed by overlap of thq empty nitrene orbital with the terminus of the olefinic side-chain.26 A record has been set by the recording of the "N n.m.r. spectrum of the diazene (36)at -90 "Cin dimethyl ether the nitrene nitrogen appears 917 p.p.m.downfield from 15NH3,which is the most highly deshielded nitrogen atom yet observed in a neutral organic compo~nd.~' The pyrrolidine analogue (37) has been found to be stable in solution at -78 "C for several days.28 Preliminary photochemical studies of this compound suggest that its So and S1states have differing geometries (theory predicts that the So state is planar while the S1is pyramidal) and that the S1-Tl gap is large again in agreement with theoretical predictions. The reactions of N-aminopyrroles with dimethylacetylene dicarboxylate have been found to have several interesting The initial reaction presumably involves formation of the Diels-Alder adducts (38) which fragment to give the phthalates (39) and amino-nitrenes (40).Another product isolated from the reaction mixture is dimethyl maleate which could arise from reduction of the starting acetylene by di-imides formed by isomerization of nitrenes (40).Finally the pyrazolines (41)are also formed conceivably by an ene reaction in which (40) behaves as a diazene followed by disrotatory ring closure (Scheme 1). 4 Carbenes STO-3G calculations have further confirmed the influence of substituents on the singlet-triplet energy gap in carbenes ?r-donors stabilize the singlet state more 26 R. S. Atkinson J. R. Malpass K. L. Skinner and K. L. Woodthorpe J. Chem. SOC., Chem. Commun. 1981 549; R. S. Atkinson J. R. Malpass and K. L. Woodthorpe ibid. p. 160. 27 P.B. Dervan M. E. Squillacote P. M. Lahti A. P. Sylwester and J. D. Roberts J. Am. Chern. Soc. 1981,103,1120. 28 P. G. Schultz and P. B. Dervan J. Am. Chem. Soc. 1981,103 1563. 29 A. G. Schultz M. Shen and R. Ravichandran Tetrahedron Lett. 1981 22 1767. Arynes Carbenes Nitrenes and Related Species (40) E E (E = C02Me) Scheme 1 than the triplet whereas the reverse is true with T-acceptor sub~tituents.~' Estimates of these singlet-triplet gaps can be made using either calculated T charges for the corresponding substituted benzenes or empirical ug constants. 6,6-Dimethylfulvene has been shown to be a useful standard in the assessment of carbene philicities. (Scheme 2).31Frontier molecule orbital calculations agree with the outcome of these experiments in that they predict that there will be a predominance of LUMO (CCl,)/HOMO (fulvene) interactions (CC12 is an electrophilic carbene) whereas the reverse is true for a nucleophilic carbene such as C(OMe)2.Nucleophilic or @/ {arnbiphilic:CR2 Styrenes t-@QI-< q& @C electro~hilic R R :cR Scheme 2 It is well known that the selectivity of addition of singlet carbenes to olefins increases with decreasing carbene reactivity. However recent frontier orbital calcu- lations suggest that this is only true for electrophilic carbenes whereas carbenes generally thought of as being nucleophilic [e.g. C(OMe)2 C(NMe,),] should show the reverse effect i.e. increasing selectivity with increasing rea~tivity.~~ In general alkyl- and dialkyl-carbenes cannot be used in synthesis owing to their propensity 30 P.H. Mueller N. G. Rondan K. N. Houk,J. F. Harrimn D. Hooper B. H. Willen and J. F. Liebman J. Am. Chem. SOC., 1981,103,5049. 31 R. A. Moss C. M. Young L. A. Perez and K. Krogh-Jespersen J. Am. Chem. SOC., 1981,103,2413; see also K. Steinbeck T. Schenke and J. Runsink Chem. Ber. 1981 114 1836. 32 W. W Schoeller Angew. Chem. Int. Ed. Engl. 1981 20 698; see also B. Giese and W.-B. Lee Chem..Ber. 1981 114 3306. 104 D. W.Knight for very rapid intramolecular rearrangement to give olefins. In an extension of previous work two closely related iron complexes have been which can effect overall transfer of an ethylidene group to olefins leading to methyl- cyclopropanes often in very high yields (Scheme 3).It may well turn out that higher OMe -[ r 1 Cp(CO)(L)Fe -CH / cp(cy$eT 'Me Me (L = COorPPh3) bMe SPh Cp(CO)*Fe -CH \ Reagents i Me,SiOSO,CF,-CH,CI,-)=( -78 OC; ii FS0,Me-CH,CI,-)=( ,25 "C Scheme 3 homologues of these complexes can also be used in this way. A more conventional method has been used to prepare dimethylcarbene (n-BuLi and 2,2-dibromopropane) which can be trapped in siru by olefins at low temperatures (-70 "C):yields of the gem-dimethylcyclopropanes are however usually fairly An exception to this general rule that alkyl carbenes are unstable with respect to rearrangement is adamantylidene. This species can be generated by photolysis of spiro(adamantane-2',3'-diazirine) and undergoes mainly cyclopropanation reac- tions with olefins together with some C-H insertion into the ~lefin.~~ Probably these reactions succeed because intramolecular rearrangement of the carbene would lead to highly strained adamantene.It is known that bicyclo[ l.l.O]butenes react with carbenes to give penta-1,4- dienes and the mechanism is usually thought to involve a diradical intermediate. Thus the 1,2,2-trimethyl derivative (42) would be expected to give diradical (43) by attack of the carbene at the less-hindered site (Scheme 4) (43) then can undergo two possible modes of cleavage (a) or (b). Pathway (a) seem most likely as this will lead to the more highly substituted diene (44); however only diene (45) is formed.37 This has lead to the proposal that the reaction does not involve a diradical intermediate but rather that it is a concerted process as shown in Scheme 4 pathway (c).Evidence has been found that suggests that the mechanism for the photochemical addition of alcohols to acylsilanes (46) leading to acetals (47) involves the TIstate of (46) but that this gives an intermediate most likely the siloxycarbene (48) before reaction with the alcohol R30H.38"Ry contrast the photochemical forma- 33 M. Brookhart J. R. Tucker and G. R. Husk J. Am. Chem. SOC., 1981 103,979. 34 K. A. M. Kremer P. Helquist and R. C. Kerber J. Am. Chem. Soc. 1981 103 1862. 3s P. Fischer and G. Schaefer Angew. Chem. Znt. Ed. Engl. 1981,20,863. 36 R. A. Moss and M. J. Chang Tetrahedron Lett. 1981 22 3749. 37 G.B. Mock and M. Jones jun. Tetrahedron Lett. 1981 22 3819. 38 (a)R. A. Bourque P. D. Davis and J. C. Dalton J. Am. Chem. Soc. 1981 103 697; (b)J. C. Dalton and R. A. Bourque ibid. p. 699. 105 Arynes Carbenes Nitrenes and Related Species (45) Scheme 4 tion of cyclopropanes (49) from (46) and electron-poor olefins does not involve the carbene (48) but rather both the S1 and TI states of the acylsilane which add directly to the ~lefin.~~~ This does not appear unreasonable as (48) should be an electrophilic carbene. n m Me,SiO R' OV0 svs (49) (50) (51) (52) Calculations indicate that initial cleavage of one C-0 bond in carbene (50) is a lower-energy pathway than a concerted process in the decomposition of (50) to ethylene and carbon dio~ide.~' This may be due to the fact that in a concerted mechanism the carbon dioxide will necessarily have a bent structure when initially formed and further suggests that when possible precursors to (50) e.g.carbonate tosylhydrazones decompose to give olefins the carbene (50)is not an intermediate. Semi-empirical CND0/2 calculations indicate that the carbene (51) from 1,3- dithiole is a stable species with respect to ring opening whereas the isomeric species (52)from 1,2-dithiole is not and will undergo cleavage to give thioacyl thi~ketene.~' This agrees well with experimental observations (51)can serve as a useful precursor to 1,1',3,3'-tetrathiafulvenesby dimerization whereas analoguous reactions with (52) fail. The nucleophilic carbenes (53) are formed when 2-tetrazobenzo[d]thiazolines prepared from the corresponding azidinium salts and LiN3 at -50 "C are warmed to ca.O°C.41 They can be trapped by various reagents such as methanol (to give the 2-methoxy-derivative) sulphur (to give the 2-thione) and diazonium salts 39 D. Feller E. R. Davidson and W. T. Borden J. Am. Chem. SOC., 1981 103,2558. 40 C. Th. Pederson J. Oddershede and J. R. Sabin J. Chem. SOC., Perkin Trans. 2 1981 1062; H. Behringer and E. Meinetsberger Liebigs Ann. Chem. 1981 1729. 41 H. Balli H. Griiner R. Maul and H. Schepp Helu. Chim. Acru 1981,64 648. 106 D. W.Knight N=N 0 OMe 0 OMe OMe (54) (55) (56) (57) (leading to 2-arylazo-thiazolinium salts). Reaction with electron-deficient olefins such as tetracyanoethylene yields the expected cyclopropane derivatives.In agreement with some recent calculations it appears that carbonyl yields can decompose to give carbene~.~~ Thus thermolysis at 80 "C of the oxadiazoline (54) gives similar amounts of the two carbenes (56) and (57) probably uia the ylide (55). Theory indicates that an electron-donating substituent (in this case OMe) is necessary for this fragmentation. Certainly more work is needed to confirm these ideas. A useful new method for the generation of carbenes from gem-dihalo- compounds involves passage of the compound through a heated (24-140 "C) evacuated tube packed with glass turnings coated with methyl-lithi~m.~~ The attraction of this procedure is that it couples the advantages of vacuum pyrolysis techniques (e.g.high dilution and hence suppression of intermolecular reactions) with mild conditions allowing the isolation of unstable products in high yields which is obviously of benefit in any study of intramolecular carbene rearrangements. Using this technique a temperature dependence has been observed in the rearrange- ment of carbene (58) to (59) and (60),which suggests that migration of the ethano bridge in (58) leading to (60) is slower than reaction at the olefin leading to (59) (Scheme 5). A similar dependence has also been found in the vinylcyclopropylidene + cyclopentadiene and vinylallene rearrangement with higher temperatures favouring the latter. Scheme 5 The cyclopropylidene thioethers [(61) generated using MeLi in ether from the corresponding gem- dibromide] undergo two types of intramolecular reaction,44 namely the well known cyclopropylidene + allene rearrangement and insertion into the C-H bond shown the extent of insertion following the usual order of 42 M.Bekhazi and J. Warkentin J. Am. Chem. Soc. 1981 103,2473. 43 U. H. Brinker and J. Ritzer J. Am. Chem. Soc. 1981 103 2116. 44 M. S. Baird J. Chem. Res. (S) 1981 352. Arynes Carbenes Nitrenes and Related Species reactivity i.e. 3’ 21 2y > 1’. By contrast the homologous thioethers (62) gives only allenes whereas (63) rearranges to the tetrahydrothiopens (65) presumably by way of ylide (64). f RS 73 (65) a; R = H b; R = CH2CHCH2 The vinylcyclopropylidene + cyclopentenylidene rearrangement is very well known (cf.ref.43) but the homologous vinylclobutylidene + cyclohexenylidene is not known Such a rearrangement has now been observed in the highly constrained system (66) (Scheme 6).45The intermediate (67) when generated in other ways gives (68) in very high yield which strongly suggests that the reaction proceeds as shown. However (68) is a minor product as the predominant pathway for decompo- sition of (66) involves the usual cyclobutylidene + methylenecyclopropane rear- rangement and vinyl migration. When cyclopropylphenylcarbene (69) is generated by photolysis of the corres- ponding diazo-compound in the presence of an olefin the main product is the cyclobutene (70) very little reaction occurs with the ~lefin.~~ However at lower temperatures (-95 + -130 “C)much more of the bicyclopropyl product is obtained at the expense of (70).The reasons for this are not clear but one explanation is that the formation of (70) involves an excited singlet state of carbene (69) which would be less populated at the lower temperatures. Further studies on the rearrangements of carbene (71) have confirmed that benzvalene (72) is not fo~med.~’ Furthermore MIND0/3 calculations predict that the conformation of (71) is not suitable for the expected intramolecular cyclo- propanation which would lead to (72) to occur. The isolated products are instead the isomeric benzvalene (73; 38%) arising via intramolecular [ 1,4]-addition of the carbene together with cyclopentadiene (74; 0.9’/0) from insertion into the methyl group and toluene (18%) from a [1,2]-carbon shift.45 U. H. Brinker and L. Konig J. Am. Chem. SOC.,1981,103,212. 46 R. A. Moss and W. P. Wetter Tetrahedron Lett. 1981 22 997. 47 U. Burger G. Gandillon and J. Mareda Helu. Chim.Acta 1981,64,844. 108 D. W. Knight (73) (74) (75) X = CH2 NH 0,or S The phenylcarbenes (79 generated by pyrolysis (500 "C) of the sodium salts of the appropriate aldehyde tosylhydrazones undergo intramolecular insertion into an adjacent C-H bond when X = CH or NH to give dihydroanthracenes and dihydroacridines re~pectively.~~ However when X = 0 or S cyclopropanation of the adjacent phenyl ring occurs leading to benzo[b]cyclohepta[d]furans and thio- phens. Overall yields for all these reactions are rather low possibly because of the high temperatures used.The absolute rates of decay of the syn- [e.g. (76)] and anti-[e.g. (77)]rotomers of 1-and 2-naphthylcarbene 9-anthrylcarbene and 2-pyridylcarbene have been measured in low-temperature mat rice^.^' Under such conditions decay is via pseudo-first order hydrogen abstraction from the matrix at a much faster rate than syn-anti-conversion suggesting that the barrier between the two forms must be greater than 4.5-6.3 kcal mol-'. In related four isomeric quinolylcar- benes have been generated photolytically in the solid state. E.p.r. spectra indicate H Y' the presence of two similar triplet states (i.e.rotomers) whose zero-field parameters are quite similar to naphthylcarbene showing unexpectedly that the nitrogen atom has little effect on the .rr-spin distribution.Cyclopentadienylidene (78) has been characterized in low-temperature matrices by both U.V. and i.r. spe~troscopy.~~ The carbene is generated by photolysis of the 5-diazocyclopentadiene via an excited A1state of the latter. Desilylation of the tropylium salt (79)with fluoride ions leads to cycloheptatrieny- lidene (go) which not surprisingly reacts as a nucleophilic carbene giving the expected cyclopropanes on reaction with electron-deficient olefins such as dimethyl 48 W. D. Crow and H. McNab Aust. J. Chem. 1981,34,1037. 49 V. P. Senthilnathan and M. S. Platz J. Am. Chem. SOC., 1981,103 5503. 50 R. S. Hutton H. D. Roth M. L. M. Schilling and J. W. Suggs J.Am. Chem. SOC., 1981,103,5147. 51 M. S. Baird I. R. Dunkin N. Hacker M. Poliakoff and J. J. Turner I. Am. Chem. SOC., 1981 103 5190. Arynes Carbenes Nitrenes and Related Species fumarate or 1,2-di~yanoethylene.~’ The nucleophilicity of (80) is also evident from its reactions with alcohols. In general carbenes react with saturated alcohols in three possible ways namely by direct insertion into a C-H bond by electrophilic (79) Pl .. (82) (83) attack on the alcohol oxygen followed by proton transfer or by protonation in the case of a nucleophilic carbene to give a carbonium ion. Good evidence has been obtained which suggests that (80) does indeed react via the latter pathway to give an intermediate tropylium cation and moreover that cyclopentadienylidene (78) reacts as an electrophile via the second pathway thereby giving rise to a cyclopentadienyl anion.53 Reaction between lithium cyclononatetraenide and p-nitrobenzenesulphonyl azide appears to lead to cyclononatetraenylidene (8 1) via the unstable diazo-derivative judging by the isolation of hydrocarbon (82) a rearrangement product which could reasonably arise from dimerization of triplet (8l),followed by electrocyclic ~earrangement.~~ The activation parameters for intersystem crossing of singlet to triplet fluoreny- lidene (83) have been measured directly in a~etonitrile.~~ The results agree with theoretical work and are consistent with the observation that singlet (83) reacts exothermally with olefins.The measurements were made by monitoring both the appearance of triplet (83) at 400nm or the decay of singlet (83) at 470nm and both give the same answer within experimental error.However it has been pointed outs6 that the disappearance of singlet (83) cannot be used.as a measure of the rate constant for intersystem crossing as this species also abstracts protons from the solvent to give a radical (A,, 500nm). This curious triplet-like behaviour of singlet (83) is also shown by its non-stereoselective addition to cis-~lefins~~ (cf.ref. 58). Cycloalkenylidenes with (4n + 2)~ electrons are known to be nucleophilic (cf. ref. 52,53). A study of one such carbene (84) shows that this species exhibits both typical singlet and triplet reactivity suggesting that the two states are in rapid eq~ilibriurn.~~ (Such an equilibration has previously been used to explain various ’’ R.W. Hoffmann M. Lotze M. Reiffen and K. Steinbach Leibigs Ann. Chem. 1981,581; E. E. Waali J. Am. Chem. SOC.,1981,103,3604. s3 W. Kirmse K. Loosen and H.-D. Sluma J. Am. Chem. SOC.,1981 103 5935; see also T. Harada and A. Oku ibid. p. 5965. 54 E. E. Waali and C. W. Wright J. Org. Chem. 1981,46,2201. ” J. J. Zupancic and G. B. Scltuster J. Am. Chem. SOC.,1981,103,944. 56 P. C. Wong D. Griller and J. C. Scaiano J. Am. Chem. SOC.,1981,103,5934. s7 J. J. Zupancic P. B. Grasse and G. B. Schuster J. Am. Chem. SOC.,1981 103 2423. 58 H. Durr and A. Hackenberger J. Chem. Res. (S) 1981 178. 110 D. W.Knight aspects of the chemistry of diphenylmethylene).The cyclopropanation reactions of (84) do indeed suggest that it is a nucleophilic species. calculation^,^^ in agreement with experimental evidence have shown that viny- lidene H2C=C is very unstable with respect to isomerization to acetylene uia rapid tunneling through a barrier of ca. 4 kcal mol-'. Two useful synthetic procedures involving isopropylidenecarbene (86)have been developed. The carbene is generated by treatment of the silyl vinyl triflate (85) with Bun4NF when this is carried out in the presence of an isocyanate vinyl carbamates (87) are formed probably by way of an intermediate ylide produced by electrophilic attack of (86) on the oxygen of the isocyanate.60 This procedure is significant in that little is known about the chemistry of vinyl carbamates.Carbene (86)also reacts with thiones to give divinyl sulphides (88),presumably by insertion into the S -H bond of the tautomeric enethiok61 The thione + enethiol equilibra- tion is sufficiently slow to allow the isolation of pure enethiols and hence if these were used as substrates the yields of (88)should be considerably higher than those (2540%) obtained by this method. Alkadienylidenecarbenes (89) insert into Group 4 hydrides (RL,MH; M = Si Ge or Sn) to give the novel cumulenes (90) as isolable compounds in 26-88% yields.62 Although alkylidene carbenes (9 1)with n = 2 [e.g. (86)] n = 4 [e.g. (89)l are known,62 in the 'odd-numbered' series only (91; n = 3) has been observed. Evidence has now been found that supports the intermediacy of dimethyltrienylidenecarbene (91; n = 5 R = Me) in the elimina- tion of the elements of hydrogen chloride from 5-chloro-5-methylhexa-1,3-diyne by potassium t-buto~ide.~~ A full report has appeared on the generation of difluorocarbene CF2 by reaction between (CF,),Cd.glyme (formed from (CF3)2Hg and Me2Cd in glyme) and an acyl halide.64 The other product of the reaction which can be carried out at temperatures of -78 "Cor lower is the acyl fluoride.In addition it is evident that 59 Y. Osamura H. F. Schaefer 111 S. K. Gray and W. H. Miller J. Am. Chem. SOC.,1981 103 1904. 60 P. J. Stang and G. H. Anderson J. Org. Chem. 1981,46,4585. " P. J. Stang and S. B. Christensen J. Org. Chem. 1981 46 823. 62 P. J. Stang and M. R. White J. Am.Chem. SOC.,1981,103 5429; see also P. J. Stang and M. Ladika ibid. p. 6437. 63 W. J. le Noble S. Basak and S. Srivastava J. Am. Chem. SOC.,1981,103,4638. 64 L. J. Krause and J. A. Morrison J. Am. Chem. Soc. 1981 103,2995. Arynes Carbenes Nitrenes and Related Species 111 R:C=C=C=C R:C=C=C=C MR; / \ R2C=(C)n-2=C H (89) (90) (91) the carbene is obtained in the singlet state by reason of its stereoselective cyclopro- panation reactions with olefins. Under phase-transfer conditions dichlorocarbene (from NaOH-CHCl,) adds to 3-alkenoic acids to give the dichlorocyclopropanes (92) generally in 55430% yield.65 Such reactions with alkenoic acids have not been previously reported. Steric limitations in the addition of CC12 and CBrF to olefins have been found for example these carbenes give no cyclopropanes with very hindered olefins such as 1,1,2-triphenylpropene and bifluorenylidene.66 The classical Reimer-Tiemann reaction of phenol results in a mixture of 2- and 4-hydroxybenzaldehydes in a ratio of ca.59 :41. If however small amounts of a-cyclodextrin are added to the reaction mixture then the ratio becomes 18:82; presumably this is because of complexation between the cyclodextrin and the phenolate anion thereby giving rise to steric shielding of the ortho An extraordinary feature of the reactions between dihalocarbenes and norbor- nadienes is that homo-[ 1,4]-adducts (93) are formed together with the expected em-[ 1,2]- (94) and endo-[1,2]-(95) adducts. [the latter two products are actually isolated as the rearranged dienes (96)].In previous studies Jefford and Huy found that the exo-[1,2] homo-[1,4] ratio [i.e. (94) :(93)] decreased when substituents of increasing electron-attracting ability were present at C-2 (i.e. more homo-[ 1,4] addition with less electron-rich norbornadienes). This was explained by assuming an increased nucleophilic character for CF in the homo-[ 1,4]-additions. However it has now been pointed out that this ratio decrease could also be due to a decrease in the rate of exo-[1,2]-addition and that substituent effects in these systems should be judged by comparing homo-[1,4] endo-[1,2] ratios6* The reaction of 7-CIP CI OzH 6 xx R f XX (95) 65 T. Fujita S. Watanabe K. Suga and K. Sugahara Synthesis 1981 1004.66 L. Anke D. Reinhard and P. Weyerstahl Liebigs Ann. Chem. 1981 591. 67 M. Komiyama and H. Hirai Bull. Chem. SOC.Jpn 1981 54 2053. 68 G. W. Klumpp and P. M. Kwantes Tetrahedron Lett. 1981 22 831. 112 D. W. Knight substituted norbornadienes with phase-transfer-generated CCl has been studied and it was found that the substituent has relatively little effect on the amounts of homo[l,4]- and endo-[1,2] adducts [(93) and (95)] formed but significantly affects exo-[1,2]-adduct (94) formation. The results are consistent with the known elec- trophilicity of CC12 it may be that homo-[ 1,4]-addition occurs in norbornadienes because unfavourable steric constraints tend to prevent the incoming carbene from approaching the olefin along the best reaction pathway for [1,2] addition.Monochlorocarbene reacts with enol ethers stereoselectively and in high yields to give the cyclopropanes [e.g. (97) from cis-ethylpropenylether] even though such olefins are only weakly nucleophilic presumably the reaction is assisted by com- plexation between the carbenoid species and the ether oxygen.69 The carbene is generated from CH2C12 and MeLi.LiBr and this results in the formation of the bromocyclopropanes corresponding to (97) as by-products. The scope of C-H insertion reactions of carbene (98) has been investigated; the species is generated by thermolysis of tetrachlorocyclopropene.70A complete experimental procedure has been presented for the generation of chloro(pheny1)carbene (99) by thermolysis (reflux in C6H6) of the corresponding diazirine together with its subsequent trapping EtO Me (97) (98) (99) by p-methylstyrene followed by base-induced elimination of HC1 to give 1,2- diphenyl-3-methylcyclopropene.71Carbenes such as (99) also insert into reactive C-H bonds such as the 1-C-H adamantane and the 2-C-H of 1,3-dio~olanes.~~ Yields of the adducts are in the range 10-44°/~ when the carbenes are generated from a,a-dihalotoluenes and potassium t-butoxide at 80°C in the presence of 18-crown-6.(Trifluoromethyl)chlorocarbene CF,CCl can be obtained by photolysis of the appropriate diazirine and undergoes standard cyclopropanation reactions with ole fin^.^^. These are stereoselective with cis-and trans-butene indicating that the carbene is formed in the singlet state and in addition it is evident that the trifluoromethyl group strongly destabilizes the species with respect to chloro(methyl)carbene MeCC1.The rhodium cluster complex Rn,(CO),, has been found to be a very efficient catalyst in the formation of cyclopropanecarboxylic acid esters from olefins and ethyl dia~oacetate.~~ When more conventional catalysts such as rhodium(I1)carboxy- lates are utilized such reactions are often inefficient unless a large excess of olefin is used. However better yields can be obtained without the need for such an excess 69 R. Barlet R. LeGoaller and C. Gey Can. J. Chem. 1981 59,621. 70 E. V. Dehmlow and Naser-ud-Din J. Chem. Res. (S) 1981 144. 71 A. Padwa M. J. Pulwer andT. J. Blacklock Org.Synth. 1981 60 53. 72 K. Steinbeck and J. Klein Angew. Chem. Znt. Ed. Engl. 1981 20 773. 73 R. A. Moss W. Guo D. Z. Denney K. N. Houk and N. G. Rondan J. Am. Chem. Soc. 1981,103 6164. 74 M. P. Doyle W. H. Tamblyn W. E. Buhro and R. L. Dorow Tetrahedron Lett. 1981 22 1783; M. P. Doyle W. H. Tamblyn and V. Bagheri J. Org. Chem. 1981,46 5094. Arynes Carbenes Nitrenes and Related Species if the diazoacetate is added slowly to the reaction mixt~re.~' Rh(OAc)2-catalysed decomposition of methyl a-diazoalkanoates leads to the corresponding cis-cw,@-unsaturated esters; the mechanism of this reaction is as yet unclear.76 In general a-oxocarbenes are not prone to react as 1,3-dipoles. The formation (Scheme 7) of 1,3-dioxole-4-carboxylates(100) from aldehydes and methyl a-diazoacetoacetate seems to represent an exception.77 The a-cyanimino carbene OQ-f *L C0,Me 1 =,Bx 0 C0,Me (100) C0,Me Scheme 7 (101) also appears to react in this manner with benzene giving (102) as the sole product and with acetonitrile leading to the [3 + 2]cycloadduct (103) (Scheme 8).78However alternative mechanisms can be drawn which involve initial cyclopro- pane or 1-azirine formation respectively followed by a vinylcyclopropane to cyclopentene type rearrangement.Scheme 8 In contrast to the usual outcome of a Buchner reaction the addition of diazoace-tate to monosubstituted benzenes when catalysed by Rh2(02CCF3) results in the formation of the kinetic non-conjugated cycloheptatrienes (104)in high yields 75 M.P. Doyle D. van Leusen and W. H. Tamblyn Synthesis 1981,787. 76 N.Ikota N. Takamura S.D. Young and B. Ganem Tetrahedron Lett. 1981,22,4163. 77 M.E.Alonso and A. W. Chitty Tetrahedron Lett. 1981 22 4181. D. Danion B. Arnold and M. Regitz Angew Chem. Int. Ed. Engl. 1981 20 113. For a review of a-iminocarbene chemistry see M. Regitz B. Arnold D. Danion H. Schubert and G. Fusser Bull. SOC.Chim. Belg. 1981,90,615. 114 D. W.Knight (270%) as a mixture of all three possible isomer^.'^ An example of a Buchner-type reaction involving thiophen is the formation of (106) in 11% yield from the diazopenicillanate (105) presumably by way of a spiro-cyclopropyl adduct." R' C02R (105) Solution photolysis of a-diazo-amides produces various compounds such as p-lactams and amides the latter by reaction with the solvent or by Wolff rearrange- ment.81 Although the precise nature of the products is obviously solvent and substituent dependent it appears that carbenoid intermediates only arise from the sym-E-conformation (107) whereas the sym-2-conformation (108) leads to p-lactams etc.via an excited singlet state of the diazo-amide. HO "QO NkNR1R2 H N-R2 Ph Ph,P-PPh2 PhP< 0. II II II PPh, 00 O II 0 (109) (110) (111) In an extension of previous work it has been found that photolysis of (109) gives the carbene (1lo) which rearranges to phosphene (111)by a [1,2]-phenyl migration before it can be trapped.82 5 Silylenes Further work has confirmed that silenes having a proton attached to the silicon [e.g.(1 12)] undergo rapid thermal isomerization to silylenes [e.g. (113)]. Further-more it has been found that the process can be reversed by photolysis in an argon matrix at 50 K and that under such conditions (112) only slowly reverts to (113) and instead dirneri~es.~~ In view of those observations it is not surprising that '9 A. J. Anciaux A. Demonceau A. F. Noels A. J. Hubert R. Warin and P. TeyssiC J. Org. Chem. 1981 46 873; A. Demonceau A. F. Noels A. J. Hubert and P. TeyssiC J. Chem. SOC.,Chem. Commun. 1981,688. L. Chan and S. A. Matlin Tetrahedron Lett. 1981 22 4025; see also E. Wenkert M. L. F. Bakuzis B. L. Buckwalter and P. D. Woodgate Synth. Commun. 1981 11 533. H. Tomioka M. Kondo and Y. Izawa J. Org. Chem.1981,46 1090. M. Regitz F. Bennyarto and H. Heydt Liebigs Ann. Chem. 1981 1044; see also M. Regitz A E. M. Tawfik and H. Heydt ibid. p. 1865; M. Regitz and H. Eckes Tetrahedron 1981 37 1039. T. J. Drahnak J. Michl and R. West J. Am. Chem. SOC.,1981,103 1845. Arynes Carbenes Nitrenes and Related Species thermolysis of the silacyclobutane (114) at 625 “Cleads only to products arising from (113) and not (l12).84Previous theoretical results have also indicated that silenes such as (112) are less stable or at best as stable as the corresponding silylenes (1 13). By contrast some new calculations suggest that the dimethylsilene (1 15) is more stable than the isomeric silylene (1 16) or the isomeric carbene (117)? EHMe Me2Si=CH2 MeSiCH2Me Me2HSiCH (115) (116) (1 17) In general silylenes appear to react with ethers in an electrophilic sense to give ylide intermediates.The isolation of methoxysilanes [e.g. (1 IS)] from the reactions of photochemically generated dimethysilylene with ally1 methyl ethers seems to confirm this (Scheme 9).86 Scheme 9 The silylsilylene (119) can be generated photochemically from (Me,Si),SiPh and adds to olefins stereo~electively.~~ For example reaction with trans-1-chloropropene leads to (120) whereas with cis- 1-chloropropene the cis-isomer (121) is formed. One explanation for this is that the reaction involves an intermedi- ate silacyclopropane which then undergoes rearrangement to give the observed products. I ,Si Me,Si 84 R. T. Conlin and D. L.Wood J. Am. Chem. Soc. 1981 103 1843; however see Y.Yoshioka and H. F. Schaefer 111 ibid. p. 7366. 85 M. Hanamura S. Nagase and K.Morokuma Tetrahedron Lett. 1981,22 1813; see also M. S. Gordon and R. D. Koob J. Am. Chem. Soc. 1981,103,2939; M. S.Gordon and J. A. Pople ibid. p. 2945. 86 D.Tzeng and W. P. Weber J. Org. Chem. 1981 46,693; see also W. Ando M. Ikeno and Y. Hamada J. Chem. SOC.,Chem. Commun. 1981,621. M. Ishikawa K.Nakagawa and M. Kumada J. Am. Chem. SOC.,1981,103,4170.

ISSN:0069-3030    

DOI:10.1039/OC9817800097

出版商:RSC    

年代:1981

数据来源: RSC

摘要:

6 Electro-organic Chemistry* By M. SAINSBURY School of Chemistry University of Bath Bath BA2 7AY 1 General and Mechanistic Aspects Following the example set last year this report beings with a survey of the more important recent contributions to mechanistic electrochemistry. A review has been published in which the scope mechanism and selectivity of certain electrochemical syntheses are discussed,' and a detailed analysis of the reductive cleavage of aryl halides has been undertaken that complements earlier work with related systems.' The first step after formation of the radical anion is scission of the carbon-halogen bond and the production of a neutral aryl radical. The radical then may undergo three concurrent reactions (a) hydrogen abstraction from solvent (b) electron transfer from the electrode and (c) electron transfer from the initial radical anion (Scheme 1).ArX + e $ ArXS -* Ar'+ X-Ar'+ SH -B ArH + S. Ar'+ e + Ar-Ar'+ ArXS + Ar-+ ArX X = halogen SH = solvent Scheme 1 The threefold competition has been studied with 9-anthryl- 1-naphthyl- and 4-cyanophenyl-chlorides -bromides and -iodides in dimethylsulphoxide and acetonitrile solutions and correlated with various parameters including stirring rate and cell design. In the case of 2-halonitrobenzenes at a mercury cathode in acidic 50% n-propanol-water cleavage of the carbon-halogen bond competes with reduc- tion of the nitro group when bromine is present but 2-fluoro- and 2-chloro- nitrobenzenes are reduced without dehalogenation to the corresponding anilines in good yield.3 A similar study has been undertaken with 2-hal0-5-nitrothiophenes.~ If other chlorinated heteroaromatic compounds such as 4-chloro-2,7,8-trimethylquinoline or 2-chloroquinoxaline are reduced in the presence of carbon dioxide a competition between dehalogenation and reductive carboxylation is set ' J.H. P. Utley Philos. Trans. R. SOC. London Ser. A 1981,302 (1468) 237. F. M'Halla J. Pinson and J. M. Saveant J. Am. Chem. Soc. 1980 102,4120. J. Marquez and D. Pletcher Electrochim. Acra 1981,26 1751. I. M. Sosonkin G. N. Strogov T. K. Ponomareva A. N. Domarev A. A. Glushkova and G. N. Freidlin Khim. Geterotsikf. Soedin. 1981,195(Chem. Abstr. 1981,94 147 380y). * Based on Chemical Abstracts during the period November 1980to November 1981.117 118 M. Sainsbury up and from cyclic voltammetric data and preparative scale experiments it has been estimated that when the rate constant for cleavage is <lo4 s-' carboxylation without dehalogenation is initiated.5 The electroreduction of bridgehead iodides such as iodoadamantane or iodocubane to the corresponding hydrocarbons at a mercury cathode is as expected a two-electron process,6 but the usual Haber scheme for the reduction of aromatic nitro compounds is disputed.' It is proposed that azoxybenzenes are formed by the dimerization of nitroso radical anions (Scheme 2); this conclusion backed by voltammetric evidence supports the view previously expressed by Fry.8 Some additional data concerning the reduction (and oxidation) of hydrazo and azo compounds have also been compiled.' ArN02 2e 8 ArNO % ArNO; iArfi=NAr 2H+ -Hz0 I -H2O 0-Scheme 2 It has been confirmed that the reduction of allyl- benzyl- cinnamyl- and polyenyl-phosphonium salts in aprotic solvent takes place by overall one-electron transfer to give the corresponding ylides.In cyclic voltammetric experiments peaks for the reduction of the ylides have been identified and it is proposed that the mechanism by which they are formed requires an initial two-electron transfer and scission to an allylic or benzylic carbanion that then abstracts a proton from a second molecule of the phosphonium salt (Scheme 3). However on the longer time-span of preparative electrolysis competitive cleavage of the phosphonium salts may occur giving radical intermediates which then dimerize." ~ R'CH2;R 2e RICH; R1CH26R R'CH=PR -PR3 -R'M~ Scheme 3 Stereochemical control in electrode reactions has been reviewed," as have the effects of inorganic and organic mediators.'2 A very interesting reductive coupling reaction of aryl carbonyl compounds has been ob~erved,'~ the course of which is directed by the nature of their complexes with p-cyclodextrin (p-CD).Aceto-phenone for example forms a 1:1 complex with the sugar wherein the aryl ring of the substrate is held within the apolar cavity of the p-CD torus. On reduction in dimethylformamide solution at -1.34 V a novel coupling reaction occurs that gives rise to two products (1)and (2) and it is proposed that in order to form these P.Fuchs U. Hess H. H. Hess and H. Lund Actu Chem. Scund. Ser. B 1981 B35 185. R. S. Abeywickrema and E. W. Della J. Org. Chem. 1981,46 2352. ' L. J. T. Janssen and E. Barendrecht Electrochim. Actu 1981 26 1831. A. J. Fry,'Synthetic Organic Electrochemistry' Harper and Row New York 1972 p. 225. G. A. Elenien N. Ismail J. Reiser and K.Wallenfels Liebigs Ann. Chem. 1981 1598. lo V. L. Pardini L. Roullier J. H. P. Utley and A. Weber J. Chem. Soc. Perkin Trans.2 1981 1520. T. Nonaka Kuguku Kogyo 1981,32,401 (Chem.Abstr. 1981,95,60 720f). l2 T. Shono and M. Yoshihiro Kuguku (Kyoto) 1981 36 635 (Chem. Abstr. 1981 95 186 176b 186 178d). l3 C. Z. Smith and J. H. P.Utley J. Chem. Soc. Chem. Commun. 1981,492. Electro-organic Chemistry PhCOMe (complexed?) 1 /\ *H (cavity?) (-Jp(==-J+o Me (I2-ro:OMe OH OH 4 COMe (1) (2) Scheme 4 compounds hydrogen abstraction occurs inside the host complex possibly as shown in Scheme 4.One-electron reduction of maleimide (M) in aqueous solution gives the dihy- drodimer (MH), but different reaction pathways occur for low M) and high (>lop4M) maleimide concentrations and it is concluded that substrate-substrate hydrogen bonding becomes important at When this concentration is exceeded the mechanism of reduction is consistent with the steps shown in Scheme 5.14 (i) 2M(aq) $ Ma * * M (as) (ii) M * -* M (as) + e $ M * -M7 (aq) (iii) 2(M * * MS) (aq) + M * * * MZ-* M (aq) (rate determining) (iv) M * -MZ-.-* M (aq) + 2H20 + M --(MH)2 --M (as) + 2HO- (v) Ma -. (MH),. . -M(aq) $ (MH)*(s) + 2M(aq) Scheme 5 The product distribution of the dimers (4),(3,and (6) formed by the reduction of the 2-cyclohexenone (3) is a function of the water content of the solvent. In l4 R. G. Barradas S. L. Gust and J. D. Porter Tetrahedron Lett. 1981 22,4579. 120 M. Sainsbury (51 (6) acetonitrile for example the presence of water favours (6) but reduces the relative amounts of the other two products (4) and (5).15 The mechanistic course of the electrodimerization of methyl cinnamate also differs in the presence or absence of water. A higher-order mechanism operates when water is excluded and the data obtained in this case are interpreted in terms of a radical-substrate coupling process.16 The mechanism of the anodic oxidation of dialkyl alicyclic and diaryl dithioacetals to disulphides has been studied.l7 For diaryl dithioacetals the reaction proceeds by C-S bond cleavage followed by dimerization whereas for the aliphatic compounds a dicationic intermediate is proposed which scavenges available nucleophiles. An analysis of the anodic behaviour of 10-methylphenothiazine (HPNMe) in the presence of nucleophiles (R-)suggests that attack at the aminomethyl function is initiated by iminium salt formation (by an ECE process) and then a simple cation-anion interaction whereas substitution in the carbocyclic -+ ring involves a radical (R') radical-cation (HPNMe) coupling. In the latter case the radical is generated by reaction of the nucleophile with the radical cation derived by initial electron transfer from the heterocyclic system (Scheme 6)." HP&=CH~ 5HPNCH~R t HPNMe 3HPNMe HPNMe = a:n Me Scheme 6 lS P.Tissot J. P. Surbeck F. 0.Guelacar and P. Margaretta Helu. Chim. Acta 1981,64,1570. l6 V.D.Parker Acta. Chem. Scand. Ser. B 1981,B35,149. J. Gourcy P. Martigny J. Simonet and G. Jeminet Tetrahedron 1981,37 1495. l8 G.Bidan and M. Geniks Nouu. J. Chim. 1981,5,117. Electro-organic Chemistry The cation radical (7) and ultimately the dication (8) are formed when oc dianisidine is oxidized in a thin-layer cuvette both species being detected by spectrophotometric methods.19 However the dication (10) is in equilibrium with the cation radical (9)generated from the anodic oxidation of 1,4-diaminobenzene.The rate of protonation and the kinetic stability of the cation radical have been studied by e.s.r. techniques.*’ L (9) The product distribution and the mechanistic course of the anodic oxidation of 1,2- 1,3- 1,4- 1,5- 1,6- 1,7- 2,3- 2,6- and 2,7-dimethoxynaphthalenesin MeOH-KOH have been studied by Swenton’s groupz1 as a preliminary to a synthesis of daunomycinonq and it has been demonstrated that the key step in the oxidation is the reaction of methoxyl radicals with the aromatic radical cation by a sequence deemed to be of the EEC,C type (see also reference 36). 2 Anodic Processes Carbon-carbon coupling reactions continue to attract the interest of organic chemists; thus the Kolbe synthesis of alkanes with multiple quaternary carbon atoms has been studied.These couplings proceeded normally although some rearrangements and elimination products were noted.22 The oxidation of p -oxocar-boxylate cyclic acetals (11) in methanol does not afford C-C coupled products; instead 2-methoxy-l,4-dioxacycloalkanesare formed possibly through a ring expansion of the intermediate cations (12) followed by neutralization with methoxide (Scheme 7).23 Liquid products from the electrolysis of sodium butanoate in methanol containing methylsodium carbonate at a graphite anode include propyl- and l-methylethyl- methylcarbonates whereas sodium 2,2-dimethylpropanoate at a platinum anode affords 1,l-dimethylethylmethylcarbonate.Typical secondary products derived from propene or 2-methylpropene are also formed.24 However the anodic oxidative decarboxylation of the endo,cis half ester (13) in methanol containing 0.1 mol. l9 M. Otto J. Stach R. Kirmse and G. Werner Z. Chem. 1981 21,296. 2o J. W. Albery R. G. Compton and I. S. Kerr J. Chem. SOC.,Perkin Trans. 2 1981 825. *’ M. G. Dolson and J. S. Swenton J. Am. Chem. SOC.,1981,103 2361; M. G. Dolson B. L. Chenard and J. S. Swenton ibid. p. 5263. 22 N. Rabjohn and G. W. Flasch jun.,J. Org. Chem. 1981,46,4082. 23 D. Lelandais C. Bacquet and J. Einhorn Tetrahedron 1981 37 3131. 24 R. Brettle M. A. Khan and J. D. Rowbottom J. Chem. SOC.,Perkin Trans. 1,1981 2927. 122 M. Sainsbury MeO' R2 Scheme 7 equiv.of sodium methoxide causes exclusive oxygen-assisted Wagner-Meerwein rearrangement to the hemiacetal (14).*' MeOCO. CO H (13) A review has been published concerning the electro-oxidative behaviour of pyrrole indole and carbazole as well as their substituted and partially reduced derivatives,26 and the preparations of pyridoxine maltol cyclotene and tropinone derivatives through the anodic oxidation of furans have been The complex mechanisms involved in aryl-aryl coupling reactions have again been demonstrated,28 and it is stated that aryl-aryl coupling of the bridged ether deriva- tive (15) of (&)-retidine affords the proethyrinadienone (16) and the mor-phinadienone ( 17).29 '' T. Imagawa S. Sugita T. Akiyama and M. Kawanisi Tetrahedron Lett.1981 22 2569. 26 J. M. Bobbitt C. L. Kulkarni and J. P. Willis Heterocycles 1981 15,495. " T. Shono and Y. Matsumura Kagaku (Kyoto) 1981,36,426 (Chem. Abstr. 1981,95,61 871t). '' B. Aalstad A. Ronlan and V. D. Parker Acta Chem. Scand. Ser. B 1981 B35 247. 29 M. Murase and S. Tobinaga Heterocycles 1981 15 1219. Electro-organic Chemistry 123 In related studies it has been observed that there is often a strong preference to form products resulting from six- rather than five-membered transition Thus the first of these two products is unusual and it is also surprising that it does not undergo a dienone-phenol rearrangement. In an extension of earlier studies it has now been shown that the electrochemical oxidation of 2-propenylphenols in methanol solution using an undivided cell yields a variety of products including astone- carpanone- arylpropanoid- and aus- trobailignan-like compounds (Scheme 8).31 Meoq HO I Me 3 Me + OMe OMe Scheme 8 Another example of C-C coupling is provided by the oxidation of NN-dimethyl- mesidine (18),which is assumed to give rise to a radical cation which deprotonates to a neutral radical.This may either then dimerize or be further oxidized to an iminium cation which is trapped by nucleophiles such as diethylphosphonate when these are present in the electrolysis medium (Scheme 9).32 Recent illustrations of carbon-nitrogen coupling reactions include the preparation of the dehydrotetramer (19)of acridine through oxidative electrolysis of the parent heterocycle at +1.45 V.Analysis of this product shows that it is in equilibrium with the N-acridylacridinium radical cation (20) and it is proposed that the formation of the tetramer from acridine takes place by an ECEC mechanistic pathway.33 30 M. Powell and M. Sainsbury Tetrahedron Lett. 1981,22 4751. 31 M. Iguchi A. Nishiyama H. Eto and S. Yamamura Chem. Lett. 1981,939. 32 G.Bidan M. Genies and R. Renaud Electrochim. Acta 1981,26,275. 33 K. Yasukouchi I. Taniguchi H. Yamaguchi and K. Arakawa J. Electroanal. Chem. Interfacial Electrochem. 1981,121,231. 124 M. Sainsbury .Me 1 Me Me J Me Scheme 9 2c10; (19) Several useful synthetic procedures also rely on the fact that methylene groups a to a nitrogen atom can be anodically oxidized and the product iminium species then reacted with a nucleophile.For example the lactone (22) an intermediate en route to alkaloids of the eburnamoine type is obtained by oxidation-cyclization of the substituted piperidineacetic acid (21).34Similarly a-methoxylated carbamates (24) are prepared by the oxidation of carbamates (23) in methanol Et &coZH -2e,-2Hi aTAo N I I C0,Me C0,Me (21) (22) R' / R'-N-CH,R' -2e -2H+ R2-N-cH __7 I I\ CO,R~ MeOH CO,R~OMe (23) (24) 34 K. Irie and Y. Ban Heterocycles 1981 15 201. 35 T. Shono Y. Matsumara and K. Tsubata Tetrahedron Lett. 1981,22,2411,3249; J. Am. Chem. Soc. 1981,103 1172. Electro-organic Chemistry 125 Interest in the methoxylation and acetoxylation of aromatic ring systems has been sustained throughout the year and several examples of quinone monoacetals (27) have been prepared by the oxidation of 2-(4-methoxyaryloxy)ethanols(25) in 1Oh methanolic potassium hydroxide followed by selective monohydrolysis of the initial products (26).36 Similarly anodic nuclear monoacetoxylation of some alkylated aromatic hydrocarbons ( p-xylene isodurene mesitylene and durene) can be achieved with high selectivity with respect to acetylation in the side-chain by carrying out the reaction in the presence of palladium on carbon in a non-divided cell.In this case any products resulting from benzylic oxidation are cleaved by hydrogen formed at the cathode thereby continuously regenerating the sub~trate.~' 2-Acetoxyfuran (28) is readily prepared by the anodic oxidation of furan in the presence of sodium acetate and acetic acid in a~etonitrile,~' and chromone deriva- tives (29) undergo electrohalomethoxylation when oxidized in aqueous methanol containing potassium chloride or bromide to give 2-methoxy-3-halogeno- chromanones (30) .39 ' I' '0 qR' R2 -Kt,"' 0 0' 0 (29) (X = ClorBr) (30) Enol ethers (31) are oxidized at a glassy carbon anode in acetonitrile solution to give dimers and oligomers but if nucleophiles are present the anode reactions are more selective.In the reaction shown in Scheme 10 for example the nature of the products (32) and (33) suggests that after the transfer of tvlo electrons from the substrate an epoxonium species may form which can ring-open in two ways (Scheme lo)."" A regiospecific generation of vicinal fluoroamides from alkenes occurs when the latter are oxidized in acetonitrile solution containing tetraethylammonium tetrafluoride (Et4N+H3F4-).Difluoro compounds aie formed as by-products. The 36 M. G. Dolson and J. S. Swenton J. Org. Chem. 1981,46 177. 37 L.Eberson and E. Oberrausch Acta Chem. Scand. Ser. B 1981,B35,193. 38 T.Shono Y. Matsumura and S. Yamame Tetrahedron Lett. 1981,22 3269. 39 M.Yamauchi S. Katayama Y. Nakashita and T.Watanabe Synthesis 1981 33. 40 M.A.Le Moing G. Le Guillanton and J. Simonet Electrochim. Acra 1981,26 139. 126 M. Sainsbury R3 R2 -2e R3 H-NC OR' Ho-NC -NC COR2 R' R)I1<Rz3R3COCOR2 NC OH OR'OH (33) Scheme 10 alkene reacts by direct one-electron transfer followed by combination of the cation radical with tetrafluoride ion; further oxidation leads to a carbonium species which then adds acetonitrile (Scheme 11).41 A similar reaction takes place with cyclo- alkenes but it is now claimed to be both regiospecific and stereo~elective.~~ Scheme 11 The anodic oxidation of some aldehyde hydrazones (34) in the presence of pyridine leads to s-triazolo[4,3-a]pyridinium salts (37) and here it is proposed that nitrilimines (35)and dihydropyridines (36)are intermediates in the reaction^.^^ Ar' -2e Ar'NHN=CHAr2 -2~+-Ar'%-N=6-Ar2 -(34) (35) Ar2 41 A.Bensadat G. Bodennec E. Laurent and R. Turdivel,Nouo. J. Chim. 1981 5 127. 42 In ref.41 p. 397. 43 I. TabakoviC and S. Crljenak Heterocycles 1981,16 699. 127 Elec tro -orgaItic Chemistry HN-NH -2e N=N R R (38) (39) A convenient preparation of 1,2,4-triazoline-3,5-diones (39) is achieved through the oxidation of urazoles (38);however in an undivided cell simultaneous cycloaddi- tion may take place if a suitable diene is added.44 a-Thiolated aldehydes (41)are valuable synthons and many routes to them have been developed; however a very attractive alternative approach requires the anodic oxidation of vinyl sulphides (40) in aqueous acetonitrile Mechanistically the reaction probably proceeds through the intermediacy of an episulphonium ion (cf.reference 40). PhCH=CHSR -2e* -2H+ p PhCH(SR)CHO H2O (40) (41) Isoprenoids are chlorinated electrochemically in high yields by an ene-type reaction,46 and the anodic fluorination of benz[a]anthracene in acetonitrile solution containing tetramethylammonium trifluoride (Me4N+H2F3-) gives a mixture of 7-and 12-monofluoro- and 7,12-difluoro-ben~[a]anthra~ene.~~ 3 Cathodic Processes The regiospecific addition of substituted ally1 halides to a,@-unsaturated esters has been reported.48 Ally1 chloride for example reacts with methyl crotonate to give 3,4,4-trimethyl-5-hexenoate(42) as a single product when the two substrates are reduced at a platinum cathode.Similarly a,P -unsaturated carbonyl compounds under aprotic conditions and in the presence of alkylating agents afford dimers through coupling at the y-position and alkylation at the oxygen atom.1,2,3-Triphenylpropenone (43),for example when reduced together with dimethylsul- phate gives a mixture of two products (44)and (45).49 Ph Ph Ph (i) 2e 2H+ Ph Ph (ii) Me2S04 (43) Me0 Ph Ph (44) (45) 44 H. Wamhoff and G. Kunz Angew. Chem. Int. Ed. Engl. 1981 20 797. A. Matsumoto K. Suda and C. Yijima J. Chem. SOC.,Chem. Commun. 1981 263. 46 S. Torii K. Uneyama T. Nakai and T. Yasuda Tetrahedron Lett. 1981,22 2291. 47 R. F. O’Malley H. A. Mariani D. R. Butler and D. M. Jerina J. Org. Chem. 1981 46 2816. 48 S. Satoh H. Suginome and M. Tokuda TetrahedronLett. 1981,22 1895. 49 T. Troll W. Elbe and G. W. Ollmann in ref. 48 p. 2961. 128 M. Sainsbury Vicinal di-oxalates (46) undergo fragmentation and elimination on cathodic reduction providing a useful preparative route to alkenes.When applied to mono- oxalates this technique affords a means of selectively cleaving a single hydroxyl function in a dihydroxylated substrate (Scheme 12).50 R' I:') ~O~OC0,Et H)==(:2 (whenX = ErOCOC02) + or XR (46) (whenX = OH) HO R2 Scheme 12 An efficient reduction of alkenes is achieved both in polar and non-polar media in cells divided by a cationic membrane with electrocatalytic metal electrodes deposited on either side (the so-called spe method).51 In more conventional equip- ment and in weakly acidic solution the reduction of ketones of the type PhCOCH(R1)S02R2occurs by C-S bond cleavage giving a-ethylenic ketones. However in more strongly acid solution pinacols are formed.52 1,2-Bis(alkylthio)- 1,2-diphenylethenes (48) are formed by the reduction of dithiobenzoate esters (47) followed by the addition of an alkyl iodide,53 and a convenient synthesis of 3-methyl- lH-phenalen-1-one (50) is provided by the cathodic reduction of 1,8-diacetyl-naphthalene (49).54 S-SR RI I -2PhCS2R -% 2Ph-C:I __* 2Ph-C:-%PhC(SR)=C(SR)Ph I I (47) SR SR (48) -Mao [HI \/ \/ (49) (50) 1,2-Diphenylthi-iren dioxide (51) when reduced electrochemically yields 1,2- diphenylethene and the sulphinate (52) resulting from scission of one or both carbon-sulphur bonds (Scheme 13).55 On the other hand a,a'-dibromodibenzyl- sulphide -sulphoxide and -sulphone give stilbene as a major product together with other products which suggests that in these reactions the crucial step is cyclization to a sulphur-containing three-membered ring (53) which then ring- opens with extrusion of the sulphur atom or one of its oxides (Scheme 14).56 50 D.W. Sopher and J. H. P. Utley J. Chem. SOC., Chem. Commun. 1981,134. Z. Ogurni K. Nishio and S. Yoshizawa Electrochim. Acta 1981 26 1779. 52 R.Kossai and J. Simonet in ref. 51,,p.189. 53 G.Adwidjaja L. Kistenbriigger and J. Voss J. Chem. Res. (S),1981 88. " B. M.Davis P. H. Gore K. A. K. Lott E. L. Short and H. G. Hakim J. Chem. SOC., Perkin Trans. 2,1981 58. 55 A. J. Fry K. Ankner and V. K. Handa J. Chem. SOC.,Chem. C&nmun. 1981 120. 56 A. J. Fry K. Ankner and V. K. Handa Tetrahedron Left. 1981,22 1791. Electro-organic Chemistry VPh dso2-=bH5 S Ph Ph SO,- (5 1) 02 /-SO2 i-.- PhCH=CHPh $ Phe=CPh H Ph Ph SO,-H (52) Scheme 13 Ph Ph Ph BryxyB' ycX\rBr 2e -Br-X -A +PhCH=CHPh -Br-Ph Ph Ph Ph Ph Ph Ph' (X=S SO or SO2) (53) Scheme 14 The rate constant for heterogenous electron transfer from a platinum electrode to aromatic disulphides is low but that from aromatic anion radicals is higher.This fact can be used in the 'assisted reduction' of disulphides thus the anion radicals are generated at potentials more negative than the normal standard reduction potential of the disulphide but less negative than the observed reduction potential (Scheme 15).57 A+e $ A7 A' + ArSSAr + A + [ArSSAr]' [ArSSAr]' + ArS' + ArS-A' + ArS' -+ A + ArS-Scheme 15 When but-2-yne is reduced at a platinum electrode in sulphuric acid solution the main products are butane and E-and 2-but-2-enes with an overall total current efficiency of -100%; however whereas the alkenes are formed by the simple addition of two hydrogen atoms the production of butane is more complex and several routes to the compound are possible.58 Electrochemical reductive techniques are often used in the synthesis or modification of heterocyclic systems; some examples are included below.Thus isoxazoles (56)can be synthesized by the electrochemical reduction of nitroethylenic ketones (54) in acidic media (pH -l),possibly through the intermediacy of a 1,3-diketone monoxime (55),59and a convenient synthesis of 1,4-benzothiazin-l,1-dioxides (58)involves the electrochemical reduction of the 2-nitrophenyl sulphones (57).60Similarly the controlled potential reduction of y-nitroketones (59) at a " J.Simonet M. Carriou and H. Lund Liebigs Ann. Chem. 1981,1665. H. Kita and H. Nakajima J. Chem. Soc. Faraduy Trans. 1 1981,77,2105. " C.Bellec P. Maitte J. Armand and C. Viele Can. J. Chem. 1981,59,527. 6o C.P.Maschmeier H. Tanneburg and M. Matschiner 2. Chem. 1981,21,219. 130 M. Sainsbury 1 OH (57) (58) (59) 2Hf 2e 1 mercury electrode in aqueous organic media allows the sequential formation of 1-pyrroline-1 -oxides (60) pyrrolines (61) and pyrrolidines (62).61 The electrochemical reduction of 3-acetyl-1 -benzylpyridinium chloride (63) in an aqueous buffered solution gives the corresponding 1,6-dih~dropyridine,~~ but the reduction of 4-aminopyrimidine (64) under similar conditions is accompanied by isomerization ring-opening and/or deamination of the primary reduction prod- ucts and secondary chemical Similarly the l-benzyloxy-l,2,3,4-tetra-hydroisoquinoline (65) useful as an intermediate en route to the alkaloid cularine (67) can be synthesized by the electrochemical reduction of the iminium salt (66) in the presence of the appropriate benzyl bromide (Scheme 16).64 In conclusion three important reviews have been published which are concerned with (a) the industrialization of electrochemical reaction^,^' (b) electro-organic syntheses,66 and (c) the electrochemistry of that interesting group -the viologen~.~' " M.Carriou R. Hazard M. Jubault and A. Tallec Tetrahedron Lett. 1981 22 3961. F. M. Moracci S. Tortorella B. Di Rienzo and I. Carelli Synth. Commun. 1981 11,329. 63 B. Czochralska and P. J. Elving Electrochim. Acta 1981 26 1755. 64 T. Shono T. Miyamoto M. Mizakarni and H. Hamaguchi Tetrahedron Lett. 1981 22 2385. 65 R. E. W. Jansson Trans. R. SOC. London Ser. A 1981 302 285. 66 K. Koester and H. Wendt Compr. Treatise Electrochem. 1981 2 251. 67 C. L. Bird and A. T. Kuhn Chem. Soc. Rev. 1981 10,49. Elec tro -organic Chemistry 131 (64) J H2NCH=N-CH=CHCH(OH)NHZ H Me0 W M e ] + Me0oBf Me0 w ! M e A I OCH,Ph x-OCH,Ph J OMe (65) Me0 QMe M e 0 7 Me steps *--PhCH20 1 Me0QBr Me0 OMe OMe (67) Scheme 16

ISSN:0069-3030    

DOI:10.1039/OC9817800117

出版商:RSC    

年代:1981

数据来源: RSC