ISSN:0069-3030    

DOI:10.1039/OC96966FX001

出版商:RSC    

年代:1969

数据来源: RSC

ISSN:0069-3030    

DOI:10.1039/OC96966BX003

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

2 Physical Methods Part (i) Mass Spectrometry By ROBIN T. APLlN The Dyson Perrins Laboratory Oxford University THISyear has seen a further increase in the number of papers on this topic in the chemical literature although a large proportion of these have been either purely descriptive of the spectra or speculative interpretations of the observed fragmen- tation patterns of various specific groups of compounds. An almost equal number of papers has been concerned with a more detailed treatment of the kinetic aspects of mass spectrometry. This Report will be mainly concerned with the latter group of papers. General Method of Interpretation.-The techniques used by organic chemists to infer positive ion structures and mechanisms for the production of fragment ions in the mass spectrometer have been critically reviewed and the pitfalls inherent in the consideration of energetics metastable characteristics and substituent effects (the kinetic approach) and the caution required for the interpretation of deuterium and other isotope (I3C ;"N) labelling results have been considered.' Ion energies and structure.Considerable attention has been given to the inter- pretation of substituent effects in the fragmentation of aromatic compounds. The assumption resulting from the striking correlations of Hammett 0 values with ion intensities that the structure of the molecular ion can be equated with the structure of the molecule prior to ionisation and this structure used in fragmentation mechanisms has been questioned on thermochemical considera- tions2 which show that the relationship is one between the molecular ionisation potential and o,not the fragmentation mechanism.Application of the quasi-equilibrium theory (QET) of mass spectra to ion intensity ratios in substituted acetophenones Y-C6H4COMe has shown3 that the ratio of the fragment ion [A+] to the molecular ion [M"] is given by equation (l),where f is the fraction of molecular ions with sufficient energy to fragment ' I. Howe D. H. Williams and R. G.Cooks Org. Mass Spectrometry 1969 2 137. T. W. Bentley R. A. W. Johnstone and D. W. Payling J. Amer. Chem. SOC.,1969,91 3978. M. S. Chin and A. G. Harrison Org. Mass Spectrometry 1969 2 1073. R.T.Aplin and K1/K is the fraction of fragmenting molecular ions which form [A'].For acetophenones f depends on the nature of the substituent and f'K,/K is also substituent dependent for both [MeCO]+ and FC6H,CO]+. In the more complex case of the mass spectra of acetanilides and phenyl acetates where competition between simple cleavage (MeCO' formation) and rearrangement (M-C,H,O) is occurring methoxy and methyl enhance and nitro inhibits rearrangement relative to simple cleavage when compared with acetophenone. It was also demonstrated that rearrangement of isomeric molecular ions to a common species does not OCCU~.~ For the loss of water from the molecular ion of o-methoxybenzaldehyde a general feature of the spectra of aromatic carbonyl compounds containing 0-or peri-methoxyl groups examination of the metastable ion characteristics at various eV values suggested that the M -H20 ion is equivalent to the molecular ion of ben~ofuran.~ Calculations of substituent effects on the ratio of the molecular ion to the total daughter ion abundances (essentially m/e 91/M+') in the spectra of substituted benzyl ethers (XC6H4- OCH2Ph) based on considerations of internal energy distribution ionisation and appearance potentials has shown good agreement with their 20 eV spectra,6 although m-OH -0Me and -NH2 afforded more abundant molecular ions than predicted perhaps due to the intervention of isolated electronic states in [M"] or to poor fits for the assumed energy distribution.X = OMe; Me; F; C1; Br; CF,; NOz Y = OMe; H Scheme 1 R. H. Shapiro and K. B.Turner Org. Mass Spectrometry 1969 2 579. J. H. Bowie and P. Y. White J. Chem. SOC. (B) 1969 89. R. S. Ward R. G. Cooks,and D. H. Williams J. Amer. Chem. SOC.,1969 91 2727. Physical Methoh Application of the combined techniques of wide-range electron energy kinetics7 and metastable ion relative abundances to the spectra of substituted benzyl phenyl ethers and toluenes have shown that their fragmentations can be fitted structurally into Scheme 1.8 A similar study of substituted ethylbenzenes showed that their fragmentation can also be accommodated structurally by Scheme 1 (Y = Me).' Inhibition of quinonoid resonance has a dramatic effect on the loss of NO' from substituted nitrobenzenes. The ratio of (1) to (2) is reduced by an order of magnitude in going from R = H to R = Br or Me when Y = (Me)2N;'o*11 no change is observed when Y = OH.% -DO + NO i A number of the features of the spectra of aliphatic compounds have been examined in similar detail. Competition between loss of small and large radicals has been examined in various systems. Interpretation of the spectra of ethylene acetals (3) which were considered to be anomalous'2 since the ratio of the [M+' -RL']ion abundance was greater than the [M" -R,'] fragment ion abundance at 70 and 15eV has been resolved by studying the competing meta- stable transition^.'^ This showed that in most cases [R 3 C,] R,' is lost with a lower activation energy than RL'.Ketones of the type RsCORLbehaved sim- ilarly.I3 A variable electron energy study of a-fission in aliphatic amines (4) and (5)showed that loss of the small substituent again represents the lower activation energy proce~s.'~ These results agree well with earlier results for phenyl-substi- tuted alkanes," Schiffs bases and ethers.I6 [M" -RL] [M" -R;] 'P.Brown J. Amer. Chem. SOC. 1968,90,4459. a P. Brown Org. Mass Spectrometry 1969 2 1085. P. Brown Org. Mass Spectrometry 1969 2 1317. lo M. M. Bursey J. Amer. Chem. SOC.,1969 91 1861. l1 M. M. Bursey and M. K. Hoffman J. Amer. Chem. SOC.,1969,91 5023. J. T. B. Marshall and D. H. Williams Tetrahedron 1967 23 321. I3 R. G. Cooks A. N. H. Yeo and D. H. Williams Org. Mass spectrometry 1969,2,985. l4 C. A. Brown A. M. Duffield and C. Djerassi Org. Mass Spectrometry 1969 2 625.l5 A. B. King J. Chem. Phys. 1965,42 3526. l6 W. Carpenter A. M. Duffield and C. Djerassi J. Amer. Chem. SOC. 1967,89,6167. R. T.Aplin In two elegant papers Djerassi has used an ion cycIotron resonance spectro- meter to investigate the structures of the ion C3H60+' produced in the single17 and double' McLafferty rearrangement. The results strongly support the enol structure (6) for both processes and preclude the participation of the ion (7) previously proposed' *' for the double rearrangement. (6) (7) Internal hydrogen rearrangement is a function of ion lifetime in aliphatic ketones. At 70eV single bond cleavage is faster than scrambling but rearrange- ment processes show some evidence of H-D scrambling prior to fragment formation.Ions of relatively long lifetime (ca. 5-lops or longer) show H-D scrambling for all fragmentations regardless of whether the process is a single- bond cleavage or rearrangement.' ' In isopropyl-n-butyl ether (8)H-H rearrange-ment occurs predominantly from the p-and y-positions before loss of C4HSor H,O from the M" -Me ion (9).20These results emphasise that care must be exercised in the interpretation of deuterium-labelling experiments particularly when rearrangement ions are implicated. Me \ +* +aPyh CH-OCH2CHzCH2Me -M; MeCH=O-CH2CH2CHzMe / Calculation of charge separation from the kinetic energy released on the decomposition of the doubly charged ions of 1,1,4,4-tetraphenylbutatriene(10) indicates that several structures are present ranging from 11.97 & 0-038 (1 1) for the dissociation of M+ to m/e 278 and m/e 78,to 8-24& 0.208,for the dissocia- + tion m/e 341" to m/e 313' + 28f.21 ' J.Diekman J. K. MacLeod C. Djerassi and J. D. Baldeschwieler J. Amer. Chem. SOC. 1969,91 2069. I' G.Eaden J. Diekman and C. Djerassi J. Amer. Chem. Soc. 1969,91 3986. F. W. McLafferty and W. T. Pike J. Amer. Chem. SOC.,1967 89 5953. l9 A. N. H. Yeo and D. H. Williams J. Amer. Chem. SOC. 1969,91,3582. 2o G. A. Smith and D. H. Williams J. Amer. Chem. SOC. 1969 91 5254. 21 T. A. Elwood M. K. Hoffmann P. F. Rogerson E. H. Palczewski M. M. Bursey and D. Rosenthal Org. Mass Spectrometry 1969 2 761. Physical Methods Fragmentation and Rearrangement Processes.-The studies of hydrogen random- isation in both aromatic and aliphatic molecules reported last year22 have been continued this year by the use of deuteriated and l3C-labe1led systems.All-truns- hepta-2,4-dien-6-yn-l-o1 (12) whose mass spectrum is virtually identical to that of benzyl alcohol similarly shows complete H-D randomisation in the formation of the C7H7+ ion.23 The substituted diphenylacetylene (13) showed no evidence of hydrogen randomisation in the formation of the M -1 ion but cu. 50% hydrogen scrambling between the two phenyl rings before the loss of acetylene fromethe M -1 ion.24 In stilbene hydrogen randomisation precedes the forma- tion of both the M” -H’ and M+’-Me’ species at 15 eV. The carbon atom involved in the M+‘ -Me‘ elimination originates randomly from the whole molecule.25 Loss of acetylene from the ions C8H9+ and C9H1 formed from + PhC2H4X and PhC3H6X (X = Br NOz) is preceded by virtually complete hydrogen randomisation.26 Examination of the ions equivalent to C3H5 in + the spectra of (14) and (15) between 9 and 15 eV confirms that not only does H-D CD3CHzCH=CH2 3CH3CH2CH=CH2 (14) (15) randomisation occur but loss of carbon identity occurs as well.27 Full details of the ’3C studies on aniline acetanilide and sulphanilide reported last year2’ have appeared.28 Deuterium and 13Clabelling of a series of isomeric l-phenyl- heptenes has shown extensive hydrogen and phenyl migration.29 Djerassi has shown that in the case of reciprocal hydrogen transfer ring size plays a crucial role in determining the ease of hydrogen migrati~n.~’ The relative proportions and ring sizes leading to the m/e 111 fragment in the spectrum of 2-(1’-octy1)- 22 J.M. Wilson Ann. Reports (B) 1968 65 7. 23 R. T. Aplin and S. Safe Canad. J. Chem. 1969,47 1599. 24 S. Safe Chem. Comm. 1969 534. 25 P. F. Donaghue P. Y.White J. H. Bowie B. D. Roney and H. J. Rodda Org. Muss Spectrometry 1969 2 1061. 26 N. M. M. Nibbering and Th. J. de Boer Org. Mass Spectrometry 1969 2 157. 2’ G. G. Meisch J. Y.Park and B. G. Giessner J. Amer. Chem. SOC. 1969,91 1555. 28 A. V. Robertson and C. Djerassi J. Amer. Chem. SOC.,1969 91 6992. 29 A. F. Gerrard and C. Djerassi J. Amer. Chem. SOC. 1969,91 6808. 30 G. Eadon and C. Djerassi J. Amer. Chem. SOC.,1969,91,2724. R. T.Aplin 30 % Scheme 2 cyclohexanone (16) are shown in Scheme 2.Loss of the carbons of the ester group in the esters (n 5) of trimellitic anhydride (17) occurs with transfer of three rather than two hydrogens back to the nucleus.31 -[Me(CH,). -3H] 0AO(CH,),Me (17) Following last year's report on the detailed 'mechanisms' of the fragmentation of chole~tane,~~ a similarly detailed account of the fragmentation of androstane has appeared.33 An investigation of the metastable ions produced in the first field-free region of a double focussing mass spectrometer during the fragmentation of cholestane has supported the complex origins revealed by deuterium labelling studies,32 for a number of the major fragments.34 31 S. Meyerson I. Puskas and E. K. Fields Chem.and Znd. 1969 1845. 32 L. Tokes G. Jones and C. Djerassi J. Amer. Chem. SOC. 1968,90 5465. 33 L. Tokes and C. Djerassi J. Amer. Chem. SOC. 1969,91 5017. 34 C. C. Fenselau and F. P. Abramson Org. Mass Spectrometry 1969 2 915. Physical Methods 11 Further large numbers of skeletal rearrangements continue to be reported. Alkyl and aryl isoxazoles lose CO + HCN as a one-step pro~ess.~’ Trimethyl- silyl groups in a series of hydroxytestosterone bis-trimethylsilyl ethers undergo 1,3-or 1,2-migrations to produce the ion (18).36 Nevertheless these derivatives + (Me@-O =CH-Si( Me)3 (18) continue to provide2’ useful structural and molecular weight information particularly when used for combined g.c.-m.s. analysis’ 7,3 and also show potential in the analysis of small peptides and unusual amino-acid~.~~ Skeletal rearrangements have been uncovered in a number of sulphur-containing com- pounds :p-(alky1thio)propionic acids and esters:’ thioglycollic acids and esters:’ and some phenyl vinyl s~lphones.~~ methyl vinyl s~lphoxide,~~ Pinacol-pinacalone rearrangement ions were observed in the spectra of phenyl-2-quinolyl-pinacol which exhibited 2-quinolyl migration and phenyI-8-quinolylpinaco1 which exhibited phenyl migrati~n?~ processes which parallel their solution chemistry.The fragmentation of benzotrifuroxan is of interest in that the succes- sive loss of five NO groups from the molecular ion is supported by metastable peaks some being lost as pairs.,’ The characteristic M -C,H4 fragment of ali- phatic aldehydes ( 2C,) is formed via a cyclobutanol intermediate (Scheme 3) in a manner analogous to the photochemical reaction path of these c~rnpounds.~~,~’ Scheme 3 35 J.H. Bowie R. K. M. R. Kallury and R. G. Cooks Austral. J. Chem. 1969,22,563. 36 J.-A. Gustafsson R. Ryhage J. Sjovall and R. M. Moriarty J.Amer. Chem. Soc. 1969 91 1234. 37 W. Vettar W. Walther M. Vecchi and M. Cereghetti Helv. Chim. Acta 1969 52 1. 38 D. C. DeJongh T. Radford J. D. Hribar S. Hanessian M. Bieber G. Dawson and C. C. Sweeley J. Amer. Chem. Soc. 1969 91 1728. 39 K. M. Baker hl. A. Shaw and D. H. Williams Chem. Comm. 1969 1108. 40S.-0. Lawesson L. Dalgaard J. 0. Madsen J. H. Bowie and D. B. Cobb Chem. Comm. 1969 218. 41 J. H. Bowie J. C3. Madsen S.-0.Lawesson and R. G. Cooks Org. Mass Spectrometry 1969 2 413. 42 R.G. Gillis J. L,. Occolowitz and J. F. Pisani Org. Mass Spectrometry 1969 2 425. 43 W. Weringa Telrahedron Letters 1969 273. “E. V. Brown and M.G. Frazer J. Heterocyclic Chem. 1969,6 567. 45 A. S. Bailey C. 4. W. Gutch J. M. Peach and W. A. Waters J. Chem. SOC.(B) 1969 681. 46 R. J. Liedtke and C. Djerassi J. Amer. Chem. SOC.,1969,91 6814. 47 C. C. Fenselau J. L. Young S. Meyerson W. R. Landis E. Selke and L. C. Leitch J. Amer. Chem. Soc. 1969 91 6847. 12 R.T.Aplin An analysis of primary fragmentation processes in a wide range of organic compounds has shown that the criterion that the positive charge remains on the fragment with the lowest ionisation potential as previously noted for alkanes is generally true.48 Meyerson has shown49 for the four stereoisomeric 1-methyl-decalins and the four 2-methyldecalins that the relative ion intensities of the [M+' -Me'] ion but not the parent ion [M"] parallels the order of relative stabilities of these molecules.The stereoisomeric effects observed in the frag- mentation of 3-and 4-substituted cyclohexanols have been rationalised.'" Application of Various Techniques-Field ionisation spectroscopy is being increasingly employed for various problems. The improved design of combined field ionisation (F.I.)electron impact (E.I.)sources has enabled high sensitivity (submicrogram) and high resolution (ca.20,000)field-ion spectra to be determined for a wide range of compound^'^ including carbohydratess3 and nucleotide~.'~ A modification of the general F.I.technique in which the sample is absorbed on the emitter from solution shows considerable promise for the examination of thermally labile compound^.^^ The F.I. technique also shows promise in the examination of pesticidal and other biologically significant compounds s6 where the more abundant F.I. molecular ion facilitates compound identification. F.I. mass spectrometry has also been used in an examination of the kinetics of fast unimolecular reactions with rates between 10-l1 and s~c.'~ The F.I. intensities of the metastable peaks due to the loss of water from the mollecular ions of a series of long-chain ketones (C to CI8)vary symmetrically with the number of degrees of vibrational freedom in the molecule suggesting that this loss is a localised specific process.58 Chemical ionisation (C.I.)mass spectrometry has during the last year moved into the structural elucidation field and its applicability to th,e study of sugars alkaloids free dipeptides and steroidal ketones has been illu~trated.'~ This technique was successfully employed in the determination of the structure of the antibiotic botryodiplodin (19) which proved unstable on electron impact examina- tion.60 48 H.E. Audier Org. Mass Spectrometry 1969 2 283. 49 S. Meyerson and A. W. Weitkamp Org. Mass Spectrometry 1969 2 603. M. M. Green R. J. Cook W. Rayle E. Walton and M. F. Gristic Chem. Comm. 1969 81. E. M. Chait T. W. Shannon W. 0. Perry G. E. Van Idear and F.W. McLafferty Internat. J. Mass Spectrometry Ion. Phys. 1969. 2 141. s2 P. Schulze B. R. Simoneit and A. L. Burlingame Internat. J. Mass Spectrometry Ion Phys. 1969 2 183. 53 H. Krone and N. D. Beckey Org. Mass Spectrometry 1969 2,427. "P. Brown G. R. Pettit and R. K. Robbins Org. Mass Spectrometry 1969 2 521. 55 H. D. Beckey Internat. J. Mass Spectrometry Ion Phys. 1969 2 503. s6J. N. Damico R. P. Barron and J. A. Sphon Internat. J. Mass Spectrometry Ion Phys. 1969 2 161. 57 H. D. Beckey H. Hey K. Lersen and G. Tenschert Internat. J. Muss Spectrometry Ion Phys. 1969 2 101. 58 M. Barber R. M. Elliot and T. R. Kemp Internat. J. Mass Spectrometry Ion Phys. 1969 2 157. 59 H. M. Fales G. W. A. Milne and M. L. Vestal J. Amer.. Chem. Sac. 1969 91 3682.6o G. P. Arsenault J. R. Althaus and P. U. Divekar Chenv Comm. 1969 1414. Physical Methods Meco&H (19)H The sequencing of peptides by mass spectrometry has received further attention during the year. A comparison of the volatility and abundance of sequence ions of the acetyl- trifluoracetyl- benzyloxycarbonyl- methoxycarbonyl- ethoxy- carbonyl- methylaminocarbonyl- phenylaminocarbonyl- phthaloyl- and stear- oyl- derivatives of glycyl-leucylphenylalanine methyl ester has shown that the acetyl derivative provides the best combination of volatility and abundance of sequence ions.61 The desulphurisation of sulphur-containing peptides con- siderably simplifies the sequence determination by mass spectrometry.62 Phenyl- thiohydantoins behave very well under combined g.c.-m.s.conditions6 and offer promise for sequencing in conjunction with the degradation procedure. Permethylation of the N-acetyl-peptide methyl esters derived from a chymotryptic digest of the protein silk fibroin of Bombyx mori facilitated their sequence determinati~n.~ A combination of high-resolution mass spectrometry and combined g.c.-m.s. analysis of the N-trifluoroacetyl-peptide methyl esters derived from a partial hydrolysis of antamanide the toxin of Amanita phalloides led to the determination of the sequence (20)of this cyclodecapeptide.66 Pro-Phe-Phe-Val-Pro I I Pro-Phe-Phe- Ala-Pro (20) Negative-ion spectra of thioglycollic acids afford abundant [M -1 ions and fragments involving hydrogen rea~angement.~~ Arylsulphinylamines also afford [M-] ions but fragment by simple cleavage.67 A detailed examination6* of the negative-ion spectra of simple nitroalkanes (C ,C2,and C,) over a range of electron energies (80-4.5 eV) agreed well with earlier 75 eV spectra.69 How- ever at low eV the spectra are reduced to the ions [NO2-3 and [O-].An attempt to obtain negative molecular ions from highly fluorinated compounds where no positive molecular ion was observed proved unsuccessful.70 Electron attachment 61 R.T. Aplin I. Eland and J. H. Jones Org. Mass Spectrometry 1969,2 795. 62A.A. Kiryushkin V. A. Gorlenko B. U. Rosinov Yu. A. Ovchinnikov and M. M. Shemyakin Experientia 1969 25 913. 63 B. W. MelvAs Acta Chem. Scand. 1969 23 1679. 64P.Emdan Acta Chem.Scand. 1950,4,283. 65 A. J. Geddes G. N. Graham H. R. Morris F. Lucas M. Barber and W. A. Wolsten-holme Biochem J. 1969 114,695. 66 A. Prox J. Schmid and H. Ottenheym Annalen 1969,722 179. "J. H. Bowie F. Duus S.-0.Lawesson F. C. U. Larsson and J. 0.Madsen Austral. J. Chem. 1969,22 153. 60 S. Tsuda A. Yokohata and M. Kawai Bull. Chem. SOC.Japan 1969,42,607. 69 R. T. Aplin H. Budzikiewicz and C. Djerassi J. Amer. Chem. SOC.,1965 87 3 180. 'O E. M. Chait U. B. Askew and C. B. Matthews Org. Mass Spectrometry 1969.2 1135. R. T.Aplin negative-ion spectra where [M + OH]-molecular ions are obtained,” has been usefully applied to the qualitative and quantitative analysis of solid petroleum hydrocarbon^.^' The general application of computers to mass spectrometry has been described7 and two notable papers on the acquisition and computer interpretation of mass spectral information were published.One described a relatively simple and inexpensive means of obtaining both high and low resolution spectra on punched tape74 for processing on any general purpose computer and the other described an approach to the interpretation of the low-resolution spectra of aliphatic ketones using artificial intelligen~e.~’ ” M. V. Ardenne K. Steinfelder and R. Tummler Z. phys. Chem. (Leipzig) 1962 220 105. l2 C. Kajdas and R. Tummler Org. Mass Spectrometry 1969 2 1049. ”J. R. Chapman Chem. in Britain 1969 5 563. l4 A. Carrick Internat. J. Mass Spectrometry Ion Phys. 1969 2 333. ”A. M. Duffield A. V.Robertson C. Djerassi B. G. Buchanan G. L. Sutherland E. Feigenbaum and J. Lederberg J. Amer. Chem. SOC.,1969 91 2977.

ISSN:0069-3030    

DOI:10.1039/OC9696600005

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Part (ii) Electron Spin Resonance Spectroscopy By COLIN THOMSON Department of Chemistry University of St. Andrews St. Andrews Fife Scotland SEVERAL reviews on e.s.r. have appeared during 1968-1969. General reviews are by Symons' and Mackor,* a discussion of substituent effects is given by Jan~en,~ applications in inorganic chemistry by Marov et.uZ.,~ and applications in photo- chemistry by Kholmog~rov.~ Reviews dealing with other applications include relaxation in radiation chemistry,6 radical ion chemi~try,~ polymers' and spin labels in polymer^,^ and photo-transfer and photosensitised reactions. The present review deals only with those papers which in the author's opinion are of particular interest to the e.s.r. spectroscopy of radicals in solution and does not deal with applications to the gas phase polymers radicals in solids and experimental techniques nor this year to studies of triplet states.Hyperfine Interactions and Spin Density Calculations.-Recent developments in the ab-initio calculation of electronic wave functions and applications of such calculations to free radicals are of considerable interest to the e.s.r. spectroscopist and this year has seen the first attempts to perform ab-initio calculations of hyperfine coupling constants in polyatomic free radicals. The first method used is the familiar open-shell S.C.F. method (R.H.F.) of Roothaan," which has been used to study the simplest n-radical CH, and the simplest a-radical vinyl.I2 Gaussian orbitals are used as basis functions but in view of their poor behaviour at the nucleus good agreement with experi- ment is not to be expected unless a very large basis set is used.Proton splittings in planar radicals must be calculated by the Unrestricted Hartree Fock (U.H.F.) ' M. C. R. Symons Ann. Rev. Phys. Chem. 1969 20 219. A. Mackor Chem. Weekblad. 1969 65. 13. E. G. Janzen Accounts Chem. Res. 1969 2 279. 1. N. Marov V. K. Belyaeva A. N. Ermakov and Yu. N. Dubrov Zhur. neorg. Khim.. 1969 14,2640. V. E. Kholmogorov Uspekhi. Khim. 1968,2 628. V.I. Muromtsev and I. G. Akhvlediani Khim. vysok. Energii. 1968 2,483. 'M. Szwarc Progr. Phys. Org. Chem. 1968 6 323. 'P. Yu. Butyagin A. M. Dubinskaya and V. A. Radstig Uspekhi. Khim. 1969,38 593. J. D. Ingham J. Macromol. Sci. 1969 C2 279.lo V. E. Kholmogorov Uspekhi. Khim. 1969 38 321. "C. C. J. Roothaan Rev. Mod. Phys. 1960,32,179. Ph. Millie and G. Berthier Internut. J. Quantum. Chem. Symposium Issue 1968 S2 567. C.Thomson method since the R.H.F. method does not give a non-zero spin density at the protons. Morukuma et. a1.13 have calculated the value of aH in CH3 using the U.H.F. approach and obtained a value of -35.6 Gauss using a small basis set. The more accurate calculations of Millie and Berthier however used the R.H.F. method and this work does not give hyperfine coupling constants for cH3. Morukuma et ~1.'~ also studied CHF2 CH2F and CF3 and these calculations are in agreement with the experimental evidence that CH3 is planar and that the radical becomes increasingly pyramidal along the series CH2F CHF ,CF3.For a-radicals on the other hand the R.H.F. method predicts finite spin density in the a-orbitals and for vinyl H2C-CH a(13C) and aH were calculated directly. The 3C coupling constants are in reasonable agreement with experiment but as expected the proton coupling constants are poor and in fact the order is not in agreement with experiment. However later work by these authors referred to in Ref. 12 using a perturbation method to calculate the a's should give better agreement with experiment. Extensions of this work to other radicals and the use of larger basis sets should be of considerable interest and such calculations are currently in progress. An alternative approach to this problem has been proposed and tested by Cook et ~1.'~ In this method a mixed basis set of Slater (S.T.O.) and Gaussian functions (G.T.O.) is used in which one-electron integrals are evaluated using the S.T.O.and a Gaussian expansion of the S.T.O. used to evaluate electron repulsion integrals. The radicals H2S; H,S+ CH3 NH3+ and NH were studied. In contrast to the S.C.F. results above the proton coupling constants are in good agreement with experiment but the 14N and 3C coupling constants are in poor agreement. The spin density in H2S7 was shown to be very sensitive to the inclusion of d-orbitals in the basis set. Although the above results are preliminary it seems clear that such calculations will prove useful in the future and may help to place semi-empirical methods such as INDO" on a surer footing.Turning to more conventional approaches the theory of hyperfine interactions in hydrocarbons has been re-examined but unlike the McConnell treatment including all overlap terms.I6 The results with applications to CH3 and CH2CH2* show that the neglect of overlap in earlier treatments leads to an artificial sensitivity of Q'd to the sigma bonding details. This work gives Q values in good agreement with experiment both for 'H and 13C splittings. Contributions from inner shells and interactions between a-bonds are shown tobe very important. A re-investigation of the coupling constants of 14N 13C etc. has appeared,17 based on U.H.F. theory which is similar to McLachlan's treatment.18 Spin polarisation parameters are derived for the CHC2 and NC fragments and for l3 K.Morukuma L. Pedersen and M. Karplus J. Chern. Phys. 1968,48,4801. l4 D. B. Cook A. Hinchcliffe and P. Palmieri Chem. Phys. Letters 1969 3 223. J. A. Pople D. L. Beveridge and P. A. Dobosh J. Amer. Chem. Soc. 1968,90,4201. l6 M. T. Melchior J. Chem. Phys. 1969 50 5 11. '' T. Yonezawa T. Kawamura and H. Kato J. Chem. Phys. 1969,50 3482. A. D. McLachlan Mol. Phys. 1960 3 233. Electron Spin Resonance Spectroscopy the C=O group. The agreement with experiment is not very good but this method of spin density calculation may be more useful for molecules containing hetero- atoms than the McLachlan treatment. Among the large number of spin density calculations of particular interest are a detailed study of the toluene anion and cation in which vibronic interaction is included," and detailed studies of two dimethyl substituted quinoxaline anions.20 The results show that experimentally aSH< a2H and this is in conflict with the results of U.H.F.21 and INDO" calculations which predict a5H > a2H.McLachlan calculations predict the correct order of spin densities but these are sensitive to the value of h adopted. An interesting analysis of the Giacometti-NordbPavan (G-N-P) and Colpa-Bolton (C-B) relations for correlating spin densities and hyperfine splittings in hydrocarbon ions has been carried out by Moss and Fraenkel.22 In contrast to the assertion of Ref. 23 the authors show that within the Huckel approximation the modified Q implies a dependence of Q on the radical not on the different positions within the same radical and thus the G-N-P relation23 does not arise from a nearest neighbour effect.Statistical analysis of the available data shows the C-B relation24925 to be superior and in fact it implicitly includes the G-N-P effects at least within the Huckel approximation. Further work with the U.H.F. meth~d~~v~~ has resulted in a method in which the spin delocalisation and spin polarisation contributions to the spin density distribution can be calculated separately. Rather surprisingly there have been few further applications of the INDO method but one application of interest is to the CF3 group splittings in (CF3)2CO; where aF = 34-94G,28and the calculated coupling constant is in quite good agreement with experiment.Experimental measurements are discussed in Ref. 131. I3C and "0 c-n spin polarisation constants have been investi- gated by LUZ.~~~~' For I7Othe most important contribution to a. comes from spin density on oxygen i.e. a relation of the type ao = Q& PO with Q& = 45 & 5G adequately accounts for the data on a large number of semiquinones ketyls and semidiones. Calculation of Q& using the general formalism of McLachlan3' et al. and approximate R.H.F. wave functions for l9 D. Purins and M. Karplus J. Chem. Phys. 1969 50,214. 'O J. A. Pederson and L. T. Muus Mol. Phys. 1969 16 589. 21 P. J. Black and C. A. McDowell Mol. Phys. 1967 12 233. 22 R. E. Moss and G. K. Fraenkel J. Chem. Phys. 1969,50 252. 23 G.Giacometti P. L. Nordio and M. V. Pavan Theor. Chim. Acta 1963 1,404. 24 J. R. Colpa and J. R. Bolton Mol. Phys. 1963 6 273. 25 J. R. Bolton J. Chem. Phys. 1965,43 309. 26 T. Yonezawa H. Nakatsuji T. Kawamura and H. Kato Chem. Phys. Letters 1968,2 454. "T. Yonezawa H. Nakatsuji T. Kawamura and H. Kato J. Chem. Phys. 1969,51,669. 28 K. Morukuma J. Amer. Chem. SOC.,1969,91 5412. 29 M. Broze and Z. Luz J. Chem. Phys. 1969,51 738. 30 M. Broze and Z. Luz J. Chem. Phys. 1969 51 749. 3! A. D. McLachlan H. H. Dearman and R. Lefebvre J. Chem. Phys. 1960,33 65. 18 c. 7’hornson H .HC03* are in good agreement with the semi-empirical Q& and show that the origin of a is due mainly to spin polarisation of the lone pair 2s electrons. For 3C analysis of available data using the Karplus-Fraenkel relation3’ gives Q& = 36-OG and Q“ = -24-36.The interest in fluorine hyperfine splittings continues and Ray33 has used the U.H.F. method to calculate spin densities in a variety of fluorine-containing anion radicals. In contrast to other workers a one parameter relation OF = QeFfrPc (2) with Q:ff = +54.276 fits the data quite well for anions. Extended Hiickel Theory (E.H.T.) has been applied to the nitroso benzene cation radical,34 following earlier work by the same a~thors,~’ with results in reasonable agree- ment with experiment. Some iminoxyl radicals derived from 1,3-dicarbonyl compounds were also studied by this method.36 Linewidths and Relaxation Theory.-A detailed review of lineshapes in solution has been given by Hudson and Luckhursf3’ and the alternating linewidth effect has also been reviewed.38 Further theoretical work on saturation effects by FreedY3’ on relaxation in gases,40 and on spin relaxation rnechani~rns~’,~~ has appeared.Most work has been concerned however with applications of existing line- width theories to particular systems. Heisenberg spin exchange has been studied in detail for the tetracyanoethy-lene (TCNE) anion and di-t-butyl nitroxide (DTBN) radical43 in both DME and THF. The former in DME undergoes strong exchange and the rate constant was measured. However in THF anomalous concentration-dependent linewidths were observed but DTBN showed similar spin exchange properties in both solvents. The theory of spin exchange developed in this paper and the effect on the saturation parameters were in good agreement with experiment.Similar saturation studies were also carried out on C6H6’ in THF-DME44 and on tropenyl in molten bitropenyl. The results show that the observed small relaxation times are attributable to effects involving the degenerate ground states of these radicals. 32 M. D. Newton and W. E. Palke J. Chem. Phys. 1966,45,2329. 33 N. K. Ray Chem. Phys. Letters 1969,3,261. 34 R. E. Cramer and R. S. Drago J. Chem. Phys. 1969,51,464. 35 R. E. Cramer and R. S. Drago J. Amer. Chem. SOC. 1968,90,4790. 36 S. Wold and C. Lagercrantz Acta Chem. Scand. 1969,23 1878. 37 A. Hudson and G. R. Luckhurst Chem. Rev. 1969,69 191. 38 P. D. Sullivan and J.R. Bolton Ado. Magn. Resonance 1969. 39 J. H. Freed J. Chem. Phys. 1969,50 2271. 40 T. J. Schaafsma and D. Kivelson J. Chem. Phys. 1968,49 5235. 41 R. E. D. McLung and D. Kivelson J. Chem. Phys. 1968,49 3380. 42 L. Burlamacchi Mol. Phys. 1969 16 369. 43 M. P. Eastman R.G. Kooser M. R.Das and J. H. Freed J. Chem. Phys. 1969 51 2690. 44 R. G. Kooser W. V. Volland and J. H. Freed J. Chem. Phys. 1969,50 5243. Electron Spin Resonance Spectroscopy A very detailed study of linewidths in aromatic hydrocarbon ions has appeared.45 The expected linewidth variations due to anisotropic g-tensor and electron-nuclear dipole interactions were not in quantitative agreement with experiment. It is shown that this discrepancy is not due to errors in the spin densities.Both anisotropic rotational motions and fluctuations in hyperfine splittings affect the calculated linewidths and thus may account for the discrepancies noted. The determination of the relative signs of hyperfine coupling constants from higher order effects has been discussed by Fe~senden.~~ This method is applicable when the hyperfine coupling constants are relatively large and involves determina- tion of the shifts in line positions due to higher order effects. Application to I3CF3 33SF4* PF4 FP02- CH3CH2 and (CH3),CH was given and the relative signs are in agreement with theory. Hydrocarbon Radicals.4f considerable significance this year are the reports of a number of convenient methods of preparing alkyl radicals in solution in non-aqueous systems.Krusic and Ko~hi~~ generated radicals k by photolysis of a static solution of the hydrocarbon RH in the presence of di-t-butyl peroxide (t-BuO) . Abstraction of H atoms occurs by the t-BuO radical t-BuO + RH -+t-BuOH + R (3) and the radicals R can be detected in high concentration preferably at tempera- tures just above the solidification point of the system. Unlike radiolytic methods radicals produced by C-C bond scission are not observed. A large variety of such radicals such as CH3 C2HS.,benzyl etc.,can be observed and the spectra are well resolved with linewidths -80 mG. Photolysis of propylene containing (t-BuO) gives allyl radicals and from a series of butenes the corresponding methyl allyl radicals are observed including the possible isomeric species.4s The coupling constants in the previously unobserved 2-methylallyl (1) were Me I a:H3= 3.19G ~1,3~.= 14.686 and ~1,3pH= 13-8G. Cyclopropane is a suitable solvent as it does not give any radical signals under these conditions but in later work by these authors4' methylcyclopropanes react to give spectra which are very temperature dependent. At -150" the cyclopropylcarbinyl radical (2) 45 B. G. Segal A. Reymond and G. K. Fraenkel J. Chem. Phys. 1969,51 1336. 46 R. W. Fessenden J. Magn. Resonancz 1969 1 277. 4' P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC.,1968,90 7155. 48 J. K. Kochi and P. J. Krusic J. Amer. Chem. SOC.,1968,90,7157. 49 J. K. Kochi P. J. Krusic and D. R. Eaton J. Amer. Chem. SOC.,1969,91 1877.C.Thomson is observed but between 0" and loo" a spectrum attributable to the allycarbinyl radical (3) is observed. Between -150" and 0" both species are present. These results show that (2)and (3) are discrete radicals but rapid rearrangement occurs (4) (2) from 2-3 at temperatures above -120". The single tertiary H coupling constant in (2)is much smaller than usually observed for a P-proton and the structure of the species is deduced to be (4). From other alkylcyclopropanes,50 hydrogen abstraction gives two isomeric radicals one of which predominates at higher temperatures. A wealth of informa- tion on these species is derived from this important work. Another method for producing specific alkyl radicals in solution is by photolysis of dialkyl peroxides at low temperature^.^ Photolysis in isopentane silane or propylene as solvents (CH3CH2CH2CH2C02)Z !% 2CH3CH2CH2cH2+ 2C02 (5) gave no evidence of secondary reactions with the solvent.In the case of the neopentyl radical well resolved hyperfine structure due to all nine y-protons can be observed (ayH= l.OG),and this was not resolved in the work of Fessenden and Schuler on this radical." Metastable species can readily be observed; i.e. the 5-hexenyl radical (5)was observed to rearrange to the cyclopentylmethyl radical (6). Other peroxidic derivatives such as esters will also give alkyl radicals. II Reaction of t-BuO with bicyclobutane gives the same signal as with cyclobutene and the spectrum was ascribed to the cyclobutenyl radical (7).52At lower temperatures a second signal is observed due to bicyclobutyl (8) which has a rather unusual structure with a = 15" which is also predicted by INDO calcula- tions.(8) is rather unstable giving (7) at higher temperatures. 50 J. K. Kochi P. J. Krusic and D. R. Eaton J. Amer. Chem. SOC.,1969 91 1879. 51 J. K. Kochi and P. J. Krusic J. Amer. Chem. Soc. 1969,91 3940. 52 P. J. Krusic J. P. Jessou and J. K. Kochi J. Amer. Chem. Soc. 1969 91 4566. Electron Spin Resonance Spectroscopy A similar method for producing alkyl radicals developed by Davies and Robertss3 is to photolyse (t-BuO) in the presence of organometallic compounds in which case alkyl radicals are displaced from the latter e.g. tri-n-butylborane gives n-butyl radicals.Hudson and Jacksons4 on the other hand showed that photolysis of (t-BuO) dissolved in trialkylsilanes in the presence of organic bromides RBr gives only radical R-,and the following reactions occur t-BuO + Et3SiH + t-BuOH + Et3Si Et3Si + RBr --+ Et,SiBr + k The Et,Si radicals react too quickly to be observed and this technique should be very useful since organic bromides are readily available. Similar results occur with chlorides RCl but radical yields are smaller. Among the many alkyl radicals studied C6FscH2 is of particular interest (see halogen section). The use of these methods should enable one to study hitherto inaccessible substituted alkyl radicals. CH3 and CD3 radicals can also be observed during photolysis in a flow system of aqueous CH3CH0 and H202,” and in this case the temperature dependence of uHand uD has been studied from -50 to -15 “C where it is found that as the temperature is raised u&zD approaches the value of 6.49 predicted theoretically.Various methyl substituted phenyl radicals have been observed during photolysis of iodotoluenes on silica gel.56 The methyl coupling constants are very small implying that hyperconjugation is not important in these a-radicals. The 13C splitting of phenyl was also observed and is in good agreement with that predicted by the INDO method.’ More information on the tropenyl radical reported last year has come from the anisotropic g and hyperfine tensors measured when the radical is generated in CI0H8 and in C10D8,s7 and in this work the first report of the temperature dependence of the anisotropic hyperfine tensor has appeared.A re-interpretation of the spectrum of the cyclohexadienyl radical in terms of the monohomocyclopentadiene system (9) has been given and justified in terms of McLachlan calculation^.^^ 53 A. G. Davies and B. P. Roberts Chem. Comm. 1969,699. 54 A. Hudson and R. A. Jackson Chem. Comm. 1969 1323. 55 J. M. Riveros and S. Shih J. Chem. Phys. 1969 50 3132. 56 S. Nagai S. Ohnishi and I. Nitta J. Phys. Chem. 1969 73 2438. 5’ W. V. Volland and G. Vincow J. Phys. Chem. 1969,73 1147. 58 C. Corvaja and G. Giacometti Theor. Chim. Acta 1969 14 352. C. Thomson H Hydrocarbon Ions.-Several papers have dealt with the interpretation of the spectrum obtained in the reduction of benzene in THF-DME.The spectrum observed at 0 "Cconsists of 15 lines and in 1967 was attributed to a non-degenerate state of the anion,59 but this interpretation was questioned by Wormington and Bolton,60 who showed that under high resolution there are too many lines for this to be C&6 but the species was not identified. However later work6' showed that the presence of this second signal was dependent on how the solvent used was dried. If LiAlH4 was used the 15 line spectrum appeared but only the C6H6; spectrum was observed if the solvent was dried over K-Na alloy. The influence of the solvent was independently confirmed by Malinowski et ~1.~~ However the orbital degeneracy can be removed if the anion is cooled from 77 to 4-2K for in this case the seven line spectrum changes to 5 lines indicating that the antisymmetric state is the ground state at 4.2K.63 The radical anion of o-xylylene has been prepared by a novel reaction.64 Benzocyclobutene (10) reacts at -80" with K-Na in THF or DME to give the anion of o-xylylene (11).The splitting constants are a2H = 7.626 alH = 5.766 I-1 -? H4KH1 ._H (9) H HH a3H = 5.236 and a4H = 1.456. The relatively large difference between a2and al is of interest. Electrolytic reduction of (12) in THF gives a 9 line spectrum from 8 equivalent protons uH = 7.656. This was assigned to the tetramethylene ethane (2,2'-diallyl) anion (14),65 with a spin distribution consistent with simple Huckel theory. The broad lines and the existence of two non-bonding orbitals indicate that this is a result of exchange with the dianion as occurs with cyclo- octatetraene.59 W. Kohnlein K. W. Boddeker and U. Schindewolf Angew. Chem. Internat. Edn. 1967 6,360. 6o P. Wormington and J. R. Bolton Angew. Chem. Internat. Edn. 1968 7 954. 61 K. W. Boddeker G. Lang and U. Schindewolf Angew. Chem. Internat. Edn. 1968 7 954. 62 G. L. Malinowski jun. and W. H. Bruning Angew. Chem. Internat. Edn. 1968 7 953. 63 M. T. Jones R. D. Rataiczak and I. M. Brown Chem. Phys. Letters 1969 2 493. 64 N. L. Bauld and F. Farr J. Amer. Chem. SOC.,1969 91 2788. 65 N. L. Bauld and G. R. Stevenson J. Amer. Chem. SOC.,1969,91 3675. Electron Spin Resonance Spectroscopy An interesting reaction occurs during reduction of biphenylene in methyl- tetrahydrofuran.66 With Na or K the biphenylene anion is produced but reduction with Li gives rise to the dibenzo[fg oplnaphthacene anion (15) and no biphenylene anion.However with more biphenylene and Li the biphenylene anion-Li’ pair can be detected. L Investigation of the anion and cation radicals of the same molecule is of interest both theoretically and experimentally and the 2,3-and 2,7-dimethyl- anthracene ions have been studied by Bard et aL6’ The observed couplings are in good agreement with those calculated using a hyperconjugative model for the methyl group interaction. Both anion and cation radicals are also derived from heptafulvene (16).68 In the cation the spin is delocalised over both rings but it is localised in a single ring in the anion a novel observation for a n-electron species.The cation radical of naphthalene has not been prepared but the cations of octamethyl- 172,3,4,5,8-hexamethyl-, and 1,2,3,4,6,7-hexamethyl-naphthalene have been prepared by the SbC1,-CH2C12 te~hnique.~’ The expected numbers of lines were observed and splitting constants obtained using computer simulation. Some evidence was found for methyl group migration but the secondary species were not identified. A very detailed study of ring and methyl splittings in methyl-substituted naphthalene anions has been carried using published data as well as new results on the anions of 1-methyl- and 2-methyl- and 1,4,5,8-tetramethyl- naphthalene anions. Both Huckel and McLachlan spin density calculations were carried out using both inductive and inductive-hyperconjugative models for the methyl group; however methyl splittings proved more difficult to correlate with theory.Several types of relation were derived between the splittings 66 I. B. Goldberg R. F. Borch and J. R. Bolton Chem. Comm. 1969 223. 67 J. A. Valenzuela and A. J. Bard J. Phys. Chem. 1969,73 779. 68 M. D. Sevilla S.H. Flajser G. Vincow and H. J. Dauben J. Amer. Chem. SOC.,1969 91 4139. 69 K. D. J. Root and M. T. Rogers J. Magn. Resonance 1969 1 568. ’O R. E. Moss N. A. Ashford R. G. Lawler and G. K. Fraenkel J. Chem. Phys. 1969 51 1765. C. Thomson which do not depend on detailed spin density calculations. It was found that in the relation a% = Q~PC (1 1) QFH3is related to QzHas the ratio Q$E3/Q$ = 0.80 k0.02 and therefore QEH x 22G.However other considerations indicate that the sigma-pi relations for either or both the ring proton and methyl group proton splittings are not suffi-ciently accurate for detailed predictions of splittings at each position in a radical. In an interesting use of e.s.r. Lawler and Tabit7' have evaluated the relative stabilities and electron affinities of a variety of alkylbenzene anion radicals. Radicals Containing Halogens-Further work on fluorine containing cation radicals has appeared. Fischer and Zimmermann7 have used the SbC15-CH2C12 technique to prepare the mono cation radicals from 4-fluorobiphenyl 4,4-difluorobiphenyl and 1,5-difluoronaphthalene.In the cases of 1-and 2-fluoro- naphthalenes 3,3'-difluorobiphenyl and 6-fluorochrysene the spectra are believed to be due to the dimer cation. In these radicals the a-fluorine splitting is large as observed in CloF8 f-,73 and for 4,4'-difluorobiphenyl the para fluorine splitting of 19.286 is to be contrasted with the value of 3-13G observed in the anion.74 In the latter work the anions of 33'- and 4,4-difluorobiphenyl were also prepared. Correlation with theory continues to be difficult however due to the difficulty of accurate calculation of the spin density on fluorine. A large number of fluorinated nitro compounds have been studied by Fischer and Zimmermann7 who generated the radicals by electrolytic reduction. All the spectra were completely interpreted and most give the mononegative ion the ratios aF:aHbeing about 2-3 in comparison with the unsubstituted species.This ratio is however not even approximately constant in (C6F&C and this has been ascribed to steric However synthesis of tris(2,6-difluoro-pheny1)methyl shows these are unlikely to be the cause of the discrepancy and other possibilities are Reduction of aryl fluorides using the liquid NH flow technique78 shows that these compounds undergo complicated reactions and only give the anion radical in certain cases as further reaction is common. Monoprotonated tetrafluorobenzosemiquinone is produced by photochemic- ally induced H-atom abstraction in a variety of solvents.79 In some solvents the radical readily dissociates and time-averaged e.s.r.spectra characteristic of proton-transfer equilibria were observed. Usually fluorine substitution in anion radicals causes little spin density change in the aromatic rings as evidenced by the 71 R. G. Lawler and C. T. Tabit J. Amer. Chem. SOC.,1969,91 5671. 72 P. H. H. Fischer and H. Zimmermann Tetrahedron Letters 1969 797. 73 C. Thomson and W. J. MacCulloch Tetrahedron Letters 1968 5899. 7* A. L. Allred and L. W. Bush Tetrahedron 1968 6883. 75 P. H. H. Fischer and H. Zimmermann Canad. J. Chem. 1968,46 3847. 76 J. Sinclair and D. Kivelson J. Amer. Chem. SOC.,1968 46 3847. 77 S. V. Kulkarni and C. Trapp J. Amer. Chem. SOC.,1969,91 191. 78 A. R. Buick T. J. Kemp G. T. Neal and T. J. Stone J. Chem. SOC., (A) 1969 666. 79 A. Hudson and J.W. Lewis J. Chem. SOC.(B) 1969 531. Electron Spin Resonance Spectroscopy proton splittings but the first exceptions to this have been found.” 2-Fluoro- 2,5-difluoro- 2,3,5,6-tetrafluoro- 3,6-difluoro- and 2,5-dioxy-semiquinone were prepared and 170 studies indicate that the values of a. are close to those in the unfluorinated ketones ;however large redistributions of spin density in the ring are indicated by the proton splittings. McLachlan calculations reflect these changes. Very large changes have previously only been observed in the cation radicals of fl~orohydronaphthalenes.~The radical C~FSCH~ has been prepared and discussed the~retically.~~ There have been more reports of chlorine hyperfine splittings. An alternative interpretation of the spectra obtained from C12 in SbF5 has appeared.81 Cooling the solution enables the anisotropic splittings of the second species to be measured and this is assigned to ClOF t.The authors also suggest that C1,O ’is the main species not C1 +,principally because the g-value is less than the free spin value. The g-factors of some aliphatic chlorine-containing radicals have been dis- cussed,82 and the simplest perchloroalkyl radical cC13 has been observed during photolysis of (t-BuO) in CCl, in the presence of trirnethyl~ilane.~~ The = = chlorine splittings are u(~’C~) 6.256 and u(~~CI) 5-20G. The radical CHC1 was also observed in CH2C12 with uH = 20.566 and u(~’C~) = 3.5G. Perchloro- triphenylmethyl (C6Cl,)& and perchlorodiphenylmethyl (c&15),Ccl have been observed both in solution and in liquid crystalsg4 where the anisotropic hyperfine splittings and 13C splittings were observed.Kohin8’ has used data on (5ClHCOOH and CClHCONH2 in conjunction with liquid studies to evaluate the tensor components for the ClCRR’ fragment. Radicals Containing B Si S Ge and P.-Some small boron-containing radicals have been prepared recently and BH3- and HBOT have been detected in irradiated potassium borohydride.86 BH3T is isoelectronic with CH3 and NH3 + and the coupling constants are a(”B) = +25G and aH= 16.5G. The boron coupling constant increases considerably on cooling as is also observed for the 13C coupling constant in CH3. B2H6T has also been prepared by reaction of B2H6 with electrons generated by photoionisation of sodium atoms in an argon matrix at 4.2 Kg7 Although trialkylboron compounds react with photochemically generated t-BuO radicals to give alkyl radicals by homolytic rupture of the alkyl-metal bond trialkoxyboranes react differently and give boron-containing radicals 88 as in equation 12 (CHSCH~O)~BOCH~CH~ + t-BuO -+ (CH3CH20)2BOCHCH3 + t-BuOH (12) W.E. Geiger jun. and W. M. Gulick jun. J. Amer. Chem. SOC., 1969 91,4657. R. S. Eachus T. P. Sleight and M. C. R. Symons Nature 1969,222,769. 82 K. Mobius K. Hoffmann and M. Plato 2.Naturforsch 1968 23a 1209. 83 A. Hudson and H. A. Hussain Mol. Phys. 1969 16 199. 84 H. R. Falle G. R. Luckhurst A. Horsfield and M. Ballester J. Chem. Phys. 1969 50 258. R. P. Kohin J.Chem. Phys. 1969 50 5356. 86 R. C. Catton M. C. R. Symons and H. W. Wardale J. Chem. SOC. (A) 1969 2619. ”P. H. Kasai and D. McLeod jun. J. Chem. Phys. 1969 51 1250. 88 P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC. 1969 91 3942. C. Thomson with resolvable and very temperature dependent ‘B splittings. Among the papers on sulphur-containing radicals we only mention the observation of the e.s.r. spectra of the 2- and 3-thenyl(l7 and 18) radicals using Hudson’s photolytic technique with methylthi~phen.’~ In these radicals the two methylene protons m. cfH2 s CH (17) (18) have slightly different coupling constants ;and this type of inequivalence has been previously observed in ally1 and its substituted analogues. 48p90 There have been several reports of silicon-containing radicals.Cyclic permethylpolysilanes [(Me),Si] where n = 5,6,7 can be reduced to the anion radicals.” Compounds with n = 5 and 6 give (MezSi),? and (Me,Si),? by electrolytic reduction. In contrast all three compounds give only (Me,Si) with alkali-metals in ethers. The N-trimethyl silyl derivatives of aniline and p-phenylenediamine also give the radical ions,92 as do the t-butyl and trimethyl silyl derivatives of di-imine.93 The latter are of interest as they are simple n-radicals (19-21) (CH 3)3C-N =N-C(CH3)3 (CH3)3C-N=N-Si(CH3)3 (19) (20) (CH3)3 Si-N =N-Si( CH3)3 (21) and the splitting constants are discussed in terms of the ability of the SiMe3 group to delocalise the n-electrons. Extension of the new preparative techniques described previously has resulted in two independent reports of Me3$i radicals in solution with = 6.28 or 6-42G.Krusic and Ko~hi~~ obtained a high enough concentration to detect the 29Si splitting (natural abundance 4.7%) of 181.14G. These authors also observed MeSiH, Me,SiH and SiH3 the latter for the first time in solution. The large value ofa(Si) indicates considerable 3s character of the unpaired electron MO i.e.a pyramidal structure. The magnitudes of aHare unusual and are prob- ably due to a change in sign as the geometry changes along the series SiH3 -+ Me3%. An additional radical observed was Me2SicHz when Me2Si-SiMe2 reacted with Me,cO. 89 A. Hudson H. A. Hussain and J. W. E. Lewis Mol. Phys. 1969 16 519. 90 R. W. Fessenden and R.H. Schuler J. Chem. Phys. 1963,39 2147. 91 E. Carberry R. West and G. E. Glass J. Amer. Chem. SOC.,1969 91 5446. 92 F. Gerson U. Krynitz and H. Bock Angew. Chem. 1969,81 786. 93 U. Krynitz F. Gerson N. Wiberg and M. Veith Angew. Chem. Internat. Edn. 1969 8 755. 94 P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC.,1969 91 3938. Electron Spin Resonance Spectroscopy With Hudson's technique,54 the authors also prepared (CH3)3Ge (CH3)3SiSi (CH,) and (CH3)3SiSiH(CH3).95 Reactions with the t-BuO radical thus provide a convenient route to radicals containing Si Ge Sn B and P. Hudson and Jackson96 have reported many such radicals. Reaction of tetra-alkyl derivatives of Group IVB elements with t-BuO has also been studied by Krusic and Kochi," and a variety of radicals of the type (22) have been prepared and their splitting constants discussed.Little new work has appeared on phosphorus-containing radicals but reaction of trialkylphosphorus compounds with t-BuO radicals gives only the alkyl radicals,98 with the exception of (CH3)3P which gives CH3 and also (CH3)3P0C(CH3)3 with ap = 6.8G. Most organophosphorus compounds studied did not give phosphorus-containing radicals. R R = Alkyl aryl IR R-Ik'-CAR' R' = H Me M = Group IVB (22) Radicals Containing Nitrogen.-Nitrogen Heterocyclic Zons. There is a continu- ing interest in phenothiazine and its derivatives and Bolton and Sullivan99 have used a new solvent system H2S04-CH,N02 to prepare the cation radical with much narrower lines (90 mG).The 10-methyl derivative was also observed together with the cation radical of phenoxazine. Analysis of the results for these and a variety of other radicals containing N S and 0 in conjunction with McLachlan calculations resulted in a consistent set of MO parameters for N S and 0 in these molecules. There have been two reports of radicals derived from s-tetrazine. Oxidation of 1,4-dihydro-2-tetrazinesby conc. I2 solutions in THF gives a new class of stable cation radicals"' (23) related to the verdazyls (24) and tetrazolinyl radicals (25) and also to the stable s-tetrazine radical anions (26). 95S. W. Bennet C. Eaborn A. Hudson A. Hussain and R. A. Jackson J. Organo-metallic Chem. 1969 16 36. 96 A. Hudson and R. A. Jackson J. Chem. Sac.(B) 1969 793. 97 P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC.,1969 91 6161. 98 J. K. Kochi and P. J. Krusic J. Amer. Chem. SOC.,1969 91 3944. 99 P. D. Sullivan and J. R. Bolton J. Magn. Resonance 1969 1 356. loo W. M. Tolles W. R. McBride and W. E. Thun J. Amer. Chem. SOC.,1969,91 2443. C. Thomson All have 7 pi electrons and only a small spin density on the ring carbon atoms. The ring nitrogen coupling constants are almost constant in these molecules. Gerson and Skorianetz"' have prepared the anion radicals of 3,6dimethyl-s- tetrazine and the cation radicals of its 1,4-dihydro-derivative. I H N-X (27) (28) Some short lived free radicals derived from pyrimidines in solution have been reported.'" Reduction by sodium in liquid NH3 has been applied to substituted pyridine~."~ Most give the radical anion but 2,5- 56- and 3,5-dicarboxylic acids give a species with a proton attached to N.Prior addition of an acid or EtOH also gives the neutral radical (27). p-Benzoquinone di-imines resemble quinones in their reactions and can be reduced to the anions (28).'04 Extensions of this work to the bisarylsulphonyl quinone di-imines have been reported. Oxidation of trimethylenediamine' O5 with aqueous CIOz gives the cation radical (29) in a flow system. Nitroxides and Iminoxyl Radicals. New evidence on the mechanism of the de- composition of N-nitroso-acetanilide in solution has appeared. The independent generation of the chain carrier nitroxide in Perkins' mechanism'06 has been achieved by Forrester,"' but doubt has been cast on this mechanism by the observation that this radical is absent in some solvents and a new radical the phenyl diazotate a-radical(30) has been observed in all these systems suggesting this species may be the chain carrier radical."* That this radical is a a-radical with uN = 30-5G uNz= 2.3G has been confirmed by CND0/2 all valence electron calculations.This radical is isoelectronic with the iminoxyl radical (3 1). lot F. Gerson and N. Skorianetz Helu. Chim. Acta 1969 52 169. Io2 C. Nicolau M. McMillan and R. 0.C. Norman Biochim. Biophys. Acta 1969,174,413. lo3 A. R. Buick T. J. Kemp G. T. Neal and T. J. Stone J. Chem. SOC.(A) 1969 1609. Io4E. P. Goodings J. Myatt I. Thomas and P. F. Todd J. Chem. SOC.(B) 1969 321.lo' L. A. Hull W. P. Giordano D. H. Rosenblatt G. T. Davis C. K. Mann and S. B. Milliken J. Phys. Chem. 1969 73 2147. lo6 G. R. Chalfont and M. J. Perkins J. Amer. Chem. SOC.,1967,89,3054. lo' A. R. Forrester Chem. and Znd. 1968 1483. lo' J. I. G. Cadogan R. M. Paton and C. Thomson Chem. Comm. 1969,614. Electron Spin Resonance Spectroscopy The 170hyperfine splitting has been observed in the iminoxyl radical generated from the reaction of NO2 with methyl ethyl ketone.lo9 uo For this radical (32) is 22.756 with the carbonyl group and the iminoxy function anti. It is suggested that the sign of the coupling constant is negative in this radical. Me \ C=N /-\o MeCO (32) Further trapping experiments using nitroso-compounds have been carried out and a variety of nitroxides so formed have been investigated and used in the elucidation of reaction mechanisms.In particular the method is used to trap the intermediate radicals which occur during oxidation of RCH2C02H and RCH2CH20H with lead tetra-acetate,"' and in some other reactions such as a-acetoxylation of ketones by this reagent. The main drawback of the method is the small secondary splittings observed since the main interaction is with the -NO group. The spin trapping technique using phenyl t-butyl nitrone (PBN)has been used to study the radicals produced during photolysis of m-and p-substituted nitro- benzenes in THF."' The results of these experiments confirm Cowley and Sutcliffe's interpretation of the radical from nitrobenzene as (33) via the tetra- hydrofuranyl radical since the spin adduct (34) was formed in the presence of PBN.The same technique used during photolysis of organo-lead -tin and -mercury compounds,''2 enables the nitrogen and /3-H coupling constants of the spin adduct (an a-substituted benzyl t-butyl nitroxide) to be used to identify the trapped radical and a large variety of such radicals were detected and discussed. (33) The thermal rearrangement of aa-diphenyl-N-benzylhydrylnitrone(35)' to benzophenone 0-benzhydryloxime proceeds (at least partially) via a homolytic dissociation at the C-N bond and evidence for this mechanism is furnished by the observation of an e.s.r. spectrum attributed to the diphenyliminoxy radical (36) in low concentration in the melt of the pure nitrone.In solution in carbitol '09 B. C. Gilbert and W. M.Gulick jun. J. Phys. Chem. 1969 73 2448. S. Forshult C. Lagercrantz and K. Torssel Acta. Chem. Scand. 1969 23 522. E. G. Janzen and J. L. Gerlock J. Amer. Chem. SOC.,1969,91 3 108. E. G. Janzen and B. J. Blackburn J. Amer. Chem. SOC.,1969,91,4481. 'IJJ. S. Vincent and E. J. Grubbs J. Amer. Chem. Soc. 1969,91 2022. C.Thomson however and also in the pure nitrone a second signal which was assigned to the nitroxide (37) was observed this being formed by radical addition to the nitrone double bond. (37) An interesting new observation is that AlC1 and some other Lewis acids will protonate the stable nitroxide 2,2,6,6-tetramethylpiperidine-N-oxyl in CHzClz to give (38).'14 An increase of uN from 15.87 to 214G is observed and uH = 3.3G.Photolysis of various indoles phenols disulphides and thiols in presence of nitroso-t-butane gives stable nitroxides.' ' Photolysis of indole and CCl results in the trapping of the CCl3 radical which is detected uiu the nitroxide t-Bu-N( O)-CC13. The chlorine splittings were observed for 5Cl and 7Cl. Nitro Compounds. There continue to be several studies of the alkali-metal nitrobenzenide systems particularly concerned with the structure of the possible ion pairs in various For details we refer the reader to the refer- ences cited. Radicals containing Oxygen.-A recent report of the detection of the t-butoxyl radical t-BuO in solution in irradiated freshly washed and distilled di-t-butyl peroxide with a g value of 2.004 & 0.004' l9 has been questioned by Symons.2o He concludes from the g-value and the fact that RO type radicals should have a g-tensor which ranges from -0-4(and thus very broad lines in solution) that the radical CH3C000 is being observed. This has the same g value as the F3COO0 radical observed by Fessenden.12' A recent reinvestigation of 1,4-dimethyl anthrasemiquinone and a new report of the 1,4,5,8-tetramethylanthrasemiquinoneradicals show that in both cases 114 B. F. Hoffman and T. B. Eames J. Amer. Chem. SOC.,1969,91 2169. 115 I. H. Leaver G. C. Ramsay and E. Suzuki,Austral. J. Chem. 1969 22 1891 1899. 116 J. M. Gross J. D. Barnes and G. N. Pillans J. Chem. SOC.(A) 1969 109. 117 J. M.Gross and J. D. Barnes J. Chem. SOC.(A) 1969,2437. lt8 R. F. Adams and N. M. Atherton Trans. Faraday SOC.,1969 65 649. l9 S. Weiner and G. S. Hammond J. Amer. Chem. Soc. 1969 91 2182. lZo M. C. R. Symons J. Amer. Chem. SOC.,1969 91 5924. lZ1 R. W. Fessenden J. Chem. Phys. 1968,48. 3725. Hectron Spin Resonmce Spectroscopy the methyl groups are freely rotating on the e.s.r. time scale.'22 This is in contrast to a previous report in which the methyl groups were believed to exhibit hindered rotation.' 23 Further studies of ion pairs between 1,2-type semiquinones and metal ions have appeared,' 24 and a detailed study of hydrogen-deuterium effects in hydroxysemiquinones by Gendell et ~1.'~'has shown that the ratio u&:u& varies from 7.2 to 7.8.A correlation of these changes with spin density calculations was made and it was noted that the proton splittings previously reported for 1,4-dihydroxyanthrasemiquinoneare in error. The work on 7Osubstitution in fluorinated semiquinones has been mentioned previously.80 Further work on ketyls has continued and Nakamura and Hirota have studied the eflect of solvation by i-PrOH of the fluorenone ketyl radical'26 and obtained information on the details of the solvation process structure of ion pairs and their rates of formation and dissociation. E.s.r. continues to be of considerable use in this study of solvent effects and str~cture.'~~ Tetracyclone (tetraphenyl- cyclopentadienone) ketyl has been prepared and its e.s.r. spectrum analysed using McLachlan spin density calculations.'28 The spectrum is consistent with a twist of -30" for the four phenyl rings with respect to the five-membered ring.A similar study of chloro and bromo derivatives of tetracyclone ketyl~'~~ has appeared with information on g-value shifts occurring on substitution. The 13C spin polarisation parameters Q& and Q& of 17-76 and -27.1G deduced by Das and Fraenkel do not fit the 13Cdata of the hexamethylacetone ketyl,I3' suggest- ing that the molecule deviates from planarity at the carbonyl group. The ketyl of hexafluoroacetone (CF3)2CO' shows a large fluorine splitting (see ref. 28) and the 13C splitting has been measured re~ent1y.I~' The latter indicates that the radical is planar and the large value of uF has been interpreted in terms of an interaction between the lowest antibonding carbonyl orbital and the lowest unoccupied antibonding orbital on the CF3 group.28 A new preparation of the tropone anion has appeared using electrolytic reduction in DMF.132 Alkali-metal reduction does not give this species. The experimental spin densities are in good agreement with McLachlan calculations. A recent report of CH3C0 has clarified previous confusion about this spe- cies.'33 It was produced in the rotating cryostat by reaction of CH,COCl and K. The isotropic coupling uEH3= 51G is small and the structure appears to resemble the forinyl radical but the large coupling constant in this radical is still not fully Iz2 R. H. Schlossel D. H. Geske and W. H. Gulick jun. J. Phys.Chem. 1969.73 71. R. M. Elofson K. F. Schulz B. E. Galbraith and R. Newton Canad. J. Chem. 1965 43 1553. 124 E. Warhurst and A. M. Wilde Trans. Faraday SOC. 1969 65 1413. '25 J. Gendell R. Miller and G. K. Fraenkel J. Amer. Chem. SOC.,1969 91,4369. 126 K. Nakamura and N. Hirota Chem. Phys. Letters 1969 3 134. lz7 K. Nakamura and N. Hirota Chem. Phys. Letters 1969 3 137. lz8 N. K. Ray P. T. Narasimhan and R. K. Gupta Indian J. Pure Appl. Phys. 1969,7 175. Iz9 W. Broser H. Kurrick P. Siegle and J. Reusch Z. Naturforsch 1969 24b 685. I3O E. T. Strom J. Phys. Chem. 1968 72 4715. W. R. Knolle and J. R. Bolton J. Amer. Chem. SOC.,1969,91 541 1. 13' Y. Ikegami and S. Seto Bull. Chem. SOC. Japan 1968,41 2225. 133 J. E. Bennett B. Mile and B. Ward Chem.Comm. 1969 13. C. Thomson understood and a calculation of the CH3C0 coupling constants by INDO would be of interest. Alternating linewidths are observed in the e.s.r. spectrum of CH20H,134 and at -130°C the inequivalence of the methylene protons is fully resolved. The temperature dependence of the CH30 and HO proton splittings in a variety of cation radicals been rnea~ured.'~~ The effect is greater for the a& splittings and a discussion of the physical reasons for this was given. Radicals in Flow Systems.-A very detailed study has been made of the Ti3+-H202oxidising system using flow cells which allow one or more reactants to be added shortly after the others have been mixed but before the solution enters the cavity.'36 The results indicate that free OH is the primary oxidising agent but in addition one-electron oxidation of the resultant organic radicals by both metal ions and H202 is important in determining the relative concentrations of the various radicals observed.A great deal of detailed information on the use of this system is contained in this paper. A new method using C02- or C02H has been used to effect one-electron reduction of many aliphatic halogenated compounds.' 37 Loss of the halogen as Hal- gives the alkyl radical in the case of iodo compounds but not with bromo or chloro compounds unless a substituent capable of stabilising the resultant radical is present. C02H is a less effective reducing agent but can be used at pH 1-2. Several chlorine-containing radicals such as CC12C02H CC12C02- and CC12CH(OH)2 were detected in some systems.This method should be most useful for generating alkyl radicals and complements those described in the hydrocarbon section. Attempts to observe phenyl a.nd substituted-phenyl radicals in solution have been described but these radicals were not observed.'38 However a large number of species derived from phenyl radical reactions were detected. For instance phenyl reacts with nitrite NO2-,to give C6H5N02' and with NO to give diphenylnitroxide. Another system used is Ti3+-NH20H which produces NH radicals.13' Reaction of aldehydes and ketones with this system gives radicals of the type R'R2CO(OH)NH0. These radicals also are produced when OH adds to oximes. Both OH and NH also add to anions derived from some nitroalkanes to give radicals of the type RCH(OH)NO T and RCH(NH2)N02 7.Another oxidant used in flow systems is nickel peroxide and using nitrosobenzene or 2-methyl-2- nitrosopropane as radical scavengers to form nitroxides Terabe and Konake' 40 have identified some of the radical intermediates in such oxidations.Among such radicals detected via the nitroxide were (C6H5),CCN and (39). The nitroxide 134 A. Hudson J. Chem. SOC.(A),1969 2513. 135 P. D. Sullivan J. Phys. Chem. 1969 73 2790. 136 R. 0.C. Norman and P. R. West J. Chem. SOC.(B) 1969 389. 37 A. L. J. Beckwith and R. 0.C. Norman J. Chem. SOC.(B) 1969 400. 13* A. L. J. Beckwith and R. 0.C. Norman J. Chem. SOC.(B) 1969 403. 13' D. J. Edge and R. 0.C. Norman J. Chem. SOC.(B),1969 182.140 S. Terabe and R. Konake J. Amer. Chem. SOC.,1969,91 5655. Electron Spin Resonance Spectroscopy (40)was obtained during oxidation of acetanilide with NiOz in presence of nitro-sobenzene again confirming Chalfont and Perkins’ interpretation of the radical observed in the N-nitrosoacetanilide decomposition. O6 (39)

ISSN:0069-3030    

DOI:10.1039/OC9696600015

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

2 Part (iii) Optical Rotatory Dispersion and Circular Dichroism By P. M. SCOPES Westfield College Hampstead London N.W.3 THEchiroptical techniques optical rotatory dispersion (0.r.d.) and circular dichroism (c.d.) provide information about molecular dissymmetry and in particular about the absolute configuration of chiral compounds. The maximum differential dichroic absorption (As)in the c.d. curve occurs at the same wavelength as the corresponding maximum in the absorption curve and should also corres- pond to the midpoint between the two extrema of the Cotton effect in the 0.r.d. curve. The electronic transitions responsible for individual c.d. bands have usually been assigned by comparison with known U.V. spectra but a significant new development is the use of c.d.as a sensitive chiral probe to detect transitions which are not revealed in unpolarised absorption spectra. For example the c.d. of dialkyldiazenes has been used to explore the relationship between the energy level scheme and the observed electronic transitions,’ and parallel studies of U.V. and c.d. spectra have been used to analyse the vibrational fine structure of the absorption bands of phenylalanine and its derivatives2 In a different way combinations of U.V. spectra and chiroptical methods have been applied to study the chirality induced in symmetrical absorbing molecules by complex formation with non-absorbing chiral molecules. Induced c.d. has been reported for solutions containing cyclo-dextrins and azo-dyes3 and for symmetrical ketones in the presence of chiral but non-absorbing tetrahydrofuran derivatives4 or of (-)-menthol.’ Work with thioketones and nitrites6 suggests that induced c.d.may be a fairly general phenomenon and that the possibility of hydrogen bonding is not a necessary pre-requisite.6 Cotton effects have been reported7 which correspond to the charge-transfer bands in the absorption spectrum of an N-substituted pyridinium ion. D. J. Severn and E. M. Kosower J. Amer. Chem. SOC.,1969,91 1710. J. Horwitz E. H. Strickland and C. Billups J. Amer. Chem. SOC.,1969 91 184. K. Sensse and F. Cramer Chem. Ber. 1969 102 509. A L. D. Hayward and R. N. Totty Chem. Comm. 1969,676. K. Noack Helv. Chim. Acta 1969 52 2501. E. Axelrod G. Barth and E. Bunnenberg Tetrahedron Letters 1969 5031.’A. J. de Gee J. W. Verhoeven I. P. Dirkx and Th. J. de Boer Tetrahedron 1969 25 3407. Optical Rotatory Dispersion and Circular Dichroism Inherently Dissymmetric Chromophores.-Weigang and Nugent8 have described a semi-empirical exciton theory that enables the absolute configuration of ring- substituted paracyclophanes to be determined from their observed c.d. The theory has been appliedg to several compounds including (1) for which the predicted absolute configuration thus assigned is in agreement with that already determined by other methods. Several groups of compounds have been investigated in which the chromo- phore is part of a helix. The styrene chromophore which is twisted in a manner comparable to cisoid dienes gives rise to Cotton effects" which are negative and positive for right-handed and left-handed helices respectively.(This 'helicity rule' is opposite to that for dienes; the present work" corrects previous state- ments' '). A conformationally rigid homo-conjugated styrene ( + )2-methylene-5 6-benznorbornene (2) investigated by Sandman and Mislow' has a strong positive Cotton effect centred at 224 nm attributed by the authors to mixing of the ethylenic and benzenoid transitions in the extended n-system. Homoconjugated 1,4-cisoid dienes in the dihydrocarquejol series also behave as single chromo- phores for which positive dichroism at 200 nm is associated with a right handed helix.13 Very large Cotton effects (A&-45 at 221 nm) have been recorded14 for 1,5-dienes related to jurineolide.This has been attributed to chiral overlap of the n-orbitals of two non-conjugated double bonds in the 10-membered ring despite their separation by two saturated carbon atoms. 0.r.d. and c.d. curves have also been recorded' for a large number of santonene derivatives including many types of inherently dissymmetric chromophore. 0. E. Weigang and M. J. Nugent J. Amer. Chem. SOC. 1969,91,4555. M. J. Nugent and 0.E. Weigang J. Amer. Chem. SOC. 1969 91,4556. P. Crabbe Chem. and Ind. 1969,917. P. Crabbe and W. Klyne Tetrahedron 1967 23 3449. l2 D. J. Sandman and K. Mislow J. Amer. Chem. SOC.,1969,91 645. l3 G. Snatzke A. F. Thomas and G. Ohloff Helv. Chim. Acta 1969 52 1253. l4 M. Suchy L. DolejS V. Herout F. Sorm G.Snatzke and J. Himmelreich Coll. Czech. Chem. Comm. 1969,34 229. l5 L. Bartlett P. M. Scopes T. B. H. McMurry and R. C. Mollan J. Chem. SOC. (C),1969 1088. P. M. Scopes Unusually large values of A&have been recorded for two quite distinct types of compound containing a -CO-N- group. Derivatives of (+@burnamenine which are unsaturated at C-16 [with either carbonyl (as 3) or an olefinic double bond] show A&values much larger than the analogous compounds saturated at C-16.16 The authors tentatively suggest that the planar indole nucleus and the C-16 double bond form a section of a helix and behave as a single chromophore. Compounds of type (4) which are isomers of diketopiperazines have a single chromophore -CO-N-N-CO- and also show abnormally large Cotton effects.l7 Regional Rules for Symmetrical Chromophores.-A number of new regional rules have been proposed which relate the sign of the Cotton effect of a sym- metrical chromophore to the asymmetry of its surroundings.Scott and Wrixon’ have proposed an octant rule for olefines (5a b) and DeAngelis and Wildman” a quadrant rule for the aryl chromophore in compounds with an asymmetric benzylic carbon atom. The rules have been tested on an appreciable number of compounds and are in agreement with the predictions from symmetry considera- tions Other regional rules have been proposed for dithiocarbamates and dithiourethanes” and for cyclic thiocarbonates.21 Front -I+ Rear l6 K. Blaha Z. Koblicova and J. Trojanek COILCzech.Chem. Comm. 1969 34,690. ”C. G. Overberger G. Montaudo J. Sebenda and R. A. Veneski J. Amer. Chem SOL. 1969 91 1256. A. I. Scott and A. D. Wrixon Chem. Comm. 1969 1182 1184. l9 G. G. DeAngelis and W. C. Wildman Tetrahedron 1969,25 5099. 2o H. Ripperger Tetrahedron 1969 25 725. A. H. Haines and C. S. P. Jenkins Chem. Comm. 1969 350. Optical Rotatory Dispersion and Circuiar Dichroism Detailed studies of benzoates have led to a Sector Rule applicable to the benzoates of cyclic secondary alcohols22 and also to a useful orr relation^^ be-tween the configuration of glycol dibenzoates and the sign of the Davydov effect at 230-220 nm. The precise relationship between molecular geometry and the observed sign ofa carboxyl Cotton effect is still not clear.For bridged ring lactones the sign depends essentially on the chirality of the bridged system24 and the same relation- ship has now been proposed for bridged lac tarn^.^^ The carboxy chromophore has also been studied in an extensive series of A'2-triterpene-28-carboxylic acids,26 in which the interaction between the carboxy group and the $olefinic bond may be significant. For ap-cyclopropyllactones the sign of the n-n* Cotton effect may be predicted by application of the rule for aP-cyclopropyl A theory for calculating the anisotropy factor in optically active ketones has been presented.2 Quite apart from theoretical studies or regional rules collections of data for related series of compounds can form a satisfactory basis for empirical correla- tions.Sulpho~ide,~~ sulphiny13' and nitryloxy (R -ON02)3 derivatives of steroids have been studied in detail. The last-mentioned show three Cotton effects at about 265-270,230 and 210 nm respectively and since the signs of these Cotton effects are characteristic of the position and configuration of the chromo- phore with respect to the steroid nucleus the nitryloxy group may be a useful 'chromophoric derivative' of hydroxy groups particularly as the c.d. bands can be detected even in the presence of saturated keto groups. Isothiocyanate~~~ and salicylidene compounds33 have both been studied as potential chromophoric derivatives of amines amino acids and amino alcohols and sugar osazonesJ4 and benzylphenylhydra~ones~ as derivatives of carbohydrates.Data have also been reported for various CYPunsaturated lactones in sesquiterpenes of the guaianolide and related types;36 and the large group of phthalide isoquinoline alkaloids has 22 N. Harada M. Ohashi and K. Nakanishi J. Amer. Chem. SOC.,1968 90 7349; N. Harada and K. Nakanishi ibid. 7351. 23 N. Harada and K. Nakanishi J. Amer. Chem. SOC.,1969 91 3989; N. Harada K. Nakanishi and S. T'atsuoka ibid. 5896. 24 J. P. Jennings W. Klyne and P. M. Scopes J. Chem. SOC., 1965,7229. 25 A. F. Beecham Tetrahedron Letters 1969 4897; cf. Tetrahedron Letters 1968 2355 3591. 26 J. D. Renwick P. M. Scopes and S. Huneck J. Chem. SOC.(0,1969 2544. "G. Snatzke and E. Otto Tetrahedron 1969 25 2041. 28 G. M. Robinson and 0.E. Weigang J. Amer. Chem. SOC.,1969,91 3709.29 D. N. Jones M. J. Green and R. D. Whitehouse J. Chem. SOC.(0,1969 1166; D. N. Jones and W. Higgins ibid. 2159. 30 D. N. Jones D. Mundy and R. D. Whitehouse J. Chem. SOC.(0,1969 1668. 31 G. Snatzke H. Laurent and R. Wiechert Tetrahedron 1969 25 761. 32 B. Halpern W. Patton and P. Crabbe J. Chem. SOC. (B) 1969 1143. 33 H. Ripperger K. Schreiber G. Snatzke and K. Ponsold Tetrahedron 1969 25 827. 34 L. Mester H. El. Khadem and G. Vass Tetrahedron Letters 1969 4135. 35 W. S. Chilton J. Org. Chem. 1968 33 4459. 36 T. G. Waddell W. Stocklin and T. A. Geissman Tetrahedron Letters 1969 1313. P. M. Scopes been reviewed in detail.37 Cotton effects have been observed in the 0.r.d. and c.d. curves of a saturated alkyl iodide,38 and in some iodo steroids.39 A report has appeared4' of the 0.r.d.of some amine derivatives in which the chirality is due entirely to the difference between hydrogen and deuterium. The observed rotations are much greater than any previously recorded for molecules of this type. Configurational Assignments.4.d. and 0.r.d. data may be used to provide information about either configuration or conformation in a given molecule. It is very rarely possible to determine both together from c.d. and 0.r.d. alone; for molecules in which neither configuration nor conformation is known additional evidence is needed from another source e.g. n.m.r. or X-ray data of relative configuration. The dilemma is illustrated by Beecham's work on dioscorine (6),41 which exists in two enantiomeric forms each with two possible conformations (C-13 pseudoaxial or pseudo-equatorial with respect to the unsaturated lactone ring).From models it is apparent that Cotton effects of the same sign would be predicted for one enantiomer with C-13 equatorial and for the other enantiomer with 3-13 axial. In this case X-ray measurements show C-13 equatorial and therefore the positive c.d. maximum indicates the absolute configuration (6). In another configuration/conformation problem n.m.r. evidence has been used with 0.r.d. data to allot a revised absolute configuration to the alkaloid mesem- brine (7).42 OMe NH2. CH * C02H 0-Me (7) The careful choice of good analogies is essential for empirical comparisons of configuration.This can be illustrated by the work of Mis10w~~ on alliin (8) ( +)-S-allyl-L-cysteine sulphoxide and related compounds. The authors show that the c.d. curve of alliin is nearly enantiomeric to that of dihydroalliin of the same 37 G. Snatzke G. Woilenberg J. Hrbek F. Santavy K. Blaha W. Klyne and R. J. Swan Tetrahedron 1969 25 5059. 38 P. A. Hart and M. P. Tripp Chem. Comm. 1969 174. j9 S. Sarel Y. Shalon and Y. Yanuka Tetrahedron Letters 1969 961. 40 W. Meister R. D. Guthrie J. L. Maxwell D. A. Jaeger and D. J. Cram J. Amer. Chem. SOC.,1969,91,4452. 41 A. F. Beecham H. H. Mills F. B. Wilson C. B. Page and A. R. Pinder Tetrahedron Letters 1969 3745. 42 P. W. Jeffs R. L. Hawks and D. S. Farrier J. Amer. Chem. SOC.,1969 91 3831. 43 P. D. Henson and K.Mislow Chem. Comm. 1969 413. 39 Optical Rotatory Dispersion and Circular Dichroism absolute configuration and is also enantiomeric to the diastereoisomer with the opposite configuration at sulphur but the same configuration at the carboxyl centre. The choice of appropriate analogies is further illustrated by work on pulvilloric 0.r.d. and c.d. data have also been used to allot the absolute configurations to (+)tricyclo[4,4,0,0]dec-4-ene[(+)-twistenel (9),45 [by applica- tion of the octant rule to (+)-twistan-4-one] to the alkaloid (R)(-)-multiflor-amir~e,~~ to helianth~idin~~ to (-)transdecahydr~quinoline,~~ and to e~hinulin.~~ 0.r.d. curves have been recorded in the U.V.for about forty carotenoid~’~ and by empirical comparison of curves absolute configurations have been allotted to about fifteen compounds.Conformational Studies.-Conflicting reports have appeared in the literature in the past regarding the origin of the c.d. bands in lactic acid. This has been reinvestigated by Djerassi and co-workers’ with the aid of low temperature measurements and they conclude that the main band at 210 nm and the smaller band at 240nm have a common origin in the u-n* transition of the carboxy group. At low temperature the 240nm band disappears and the existence of two bands at room temperature is attributed by the authors to different con- formational preferences. C.d. results have been used with other physical techniques to suggest a solution conformation for erythr~mycin~~ and 0.r.d.data to deduce the preferred con- formation of the seven membered ring in B-homo steroids (C-7-twist chair).53 Solvent Effects.-Much greater attention should be given to the possible existence of solvent effects than has generally been done in the past. A detailed study of aryla~osteroids’~ has shown further good examples where a change of solvent can reverse the sign of a Cotton effect. Compound (10) in mixtures of carbon 44 G. C. Barrett J. F. W. McOmie S. Nakajima and S. Tanenbaum J. Chem. SOC.(C) 1969 1068. 45 M. Tichy and J. Sicher Tetrahedron Letters 1969 4609. 46 A. Brossi J. O’Brien and S. Teitel Helu. Chim. Actu. 1969 52 678. 47 H. Ripperger and K. Schreiber Tetrahedron 1969 25 737. 48 R. S. Burden L. Crombie and D. A. Whiting J. Chem. SOC.(0,1969 693.49 E. Houghton and J. E. Saxton J. Chem. SOC.(C) 1969 1003. 50L.Bartlett W. Klyne W. P. Mose P. M. Scopes G. Galasko A. K. Mallams B. C. L. Weedon J. Szabolcs and G. Toth J. Chem. SOC.(C) 1969 2527. 51 G. Barth W. Voelter E. Bunnenberg and C. Djerassi Chem. Comm. 1969 355; cf. R. D. Anand and M. K. Hargreaves Chem. Comm. 1967,421. 52 L. A. Mitscher B. J. Slater T. J. Perun P. H. Jones and J. R. Martin Tetrahedron Letters 1969 4505. 53 L. Kohout and J. FajkoS Coll. Czech. Chem. Comm. 1969 34 2439. 54 J. Buckingham and R. D. Guthrie J. Chem. SOC.(0,1969 1939. P. M. Scopes tetrachloride and dioxan gave a series of curves with the Cotton effect changing from positive to negative; all these curves passed through a common point for which the authors used the name isorotutory point (analogous to isosbestic point in adsorption spectra).54a The solvent-dependent c.d.of N-thiobenzoyl-L-a-aminoacids has been used to study the conformation of small peptides in solution.s5 Macromolecules.-Attention is drawn here to a few leading reviews and papers only ; the applications of the chiroptical techniques to proteinss6 and to nucleic acidss7 have been reviewed in detail. An important paper by Goodman and his colleaguess8 describes in detail the 0.r.d. and c.d. of some small cyclic lactams as conformationally rigid models for the amide group in proteins and peptides. The 0.r.d. of some diastereoisomeric cyclo-hexapeptides has been rep~rted.’~ Magnetic Circular Dichroism.-Magnetic circular dichroism (m.c.d.) has not previously been detected for simple ketones but improved instrumentation has now enabled Djerassi’s school to obtain curves which are physically significant but as yet impossible to interpret.60 The m.c.d.Cotton effect corresponding to the normal n-n* transition is of very low intensity and in some cases is split into two maxima of opposite sign. The strength sign and relative intensities of these two bands are closely dependent on the structure of the compound con- cerned. 54a K. Garbett and R. D. Gillard J. Chem. Soc. (A) 1966,204. 55 G. C. Barrett J. Chem. SOC. (0.1969 1123; cf G. C. Barrett and A. R. Khokhar J. Chem. Soc. (C) 1969 1120. 56 ‘0ptical Rotatory Dispersion of Proteins and Other Macromolecules’ by B.Jirgensons. Volume 5 in the series ‘Molecular Biology Biochemistry and Biophysics’ published by Springer-Verlag 1969. ’’J. T. Yang and T. Samejima Progr. Nucleic Acid Res. 1969 9 224. 58 M. Goodman C. Toniolo and J. Falcetta J. Amer. Chem. Soc. 1969 91 1816. 59 K. Blaha I. FriE and J. Rudinger Coll. Czech. Chem. Comm. 1969 34 3497. 6o G. Barth E. Bunnenberg and C. Djerassi Chem. Comm. 1969 1246.

ISSN:0069-3030    

DOI:10.1039/OC9696600034

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

2 (Part iv) X-Ray Crystallography By GEORGE FERGUSON Department of Chemistry The University of Guelph Guelph Ontario Canada The evergrowing number of crystallographic papers (especially short notes and communications) and limitation on space have made it impossible to have complete coverage of all papers published in the period in question ;many have had to be reported only briefly or omitted. In general if molecular geometry details are in accord with ‘accepted’ values they have not been reported again here. One of the major crystallographic events of 1969 was the announcement that at last after more than a decade of intensive research by Professor Hodgkin and her co-workers at Oxford the crystal structure of insulin had been unravelled. An electron density map calculated at 2.8 A resolution showed many details of the arrangement of the atoms in rhombohedra1 insulin crystals.Carboxylic Acids Amino-acids and Small Molecules.-An accurate analysis of potassium oxalate monohydrate shows that the hydrogen bond between one of the oxalate oxygen atoms and the water molecule is 2.760(2) A long; the other oxygen atom does not participate in hydrogen bonding. The carboxy-groups are not symmetric the bond between the carbon and hydrogen-bonded oxygen atom being 1.260(2)A and the other C-0 bond being 1.247(2)A. The central C-C bond is long [1-574(2)A] and residual electron density (0~26eA-~) is found in the centre of this bond.2 The hydrogen oxalate ion in potassium hydrogen oxalate is non-planar each of the C -C02 groups in the ion is planar but the two planes are twisted about the C-C bond by 12-7(6)”.The C-0 distances are 1.297(9) 1.267(9) and 1.229(9) 1-218(9) A and the C-C distance is 1-543(10) A. The 0-C-0 angles are 123.1(6)0 and 127.7(6)”. The crystal structure is built up of infinite chains of hydrogen oxalate ions held by hydrogen bonds 2.534(8) A.3 In crystals of sodium 2-oxovalerate the anions are maximally extended and packed into infinite layers4 Form A of the soap potassium caprate Me(CH2)* COOK has the hydrocarbon chains packed in a crossed-chain structure with M. J. Adams T. L. Blundell E. J. Dodson G. G. Dodson M. Vijagan E. N. Baker M. M. Harding D. C. Hodgkin B. Rimmer and S. Sheat Nature 1969 224 491. * D. J. Hodgson and J. A. Ibers Acta Cryst.1969 B25 469. B. F.Pedersen Acta Chem. Scand. 1968 22 2953. S. C.Jain S. S. Tavale and A. B. Biswas Acta Cryst. 1969 B25 584. G. Ferguson an angle of tilt of 57-9(5)". Very large anisotropic thermal motions of the four carbon atoms nearest the methyl group ends of the chains are reported. These are the four atoms which alter their positions at 76°C in the transition to the C-phase.' Molecules of propyl stearate form sheets with the chains tilting 63" towards the end group planes. The alcohol chain forms a continuation of the main zig-zag chain but a twist occurs at the carboxy-group.6 The two C-CO-0 fragments of monochloroacetic acid anhydride ClCH2- CO-0-CO-CH2Cl are planar and the angle between them is43". The valency angle at the ether oxygen is unusually large (121").The molecular symmetry is approximately Cz in the ~rystal.~ The crystal structure of ammonium acetate diammine MeCO2NH4,2NH3 is built up from MeCOO- NH4+ and NH groups linked into a three-dimensional network by normal hydrogen bonds (N+-H-N 2.874 A N+-H-O-2-832A N-H-0-3.163 A) and weak bifurcated hydrogen bonds.* The reaction product of ethylene and N203 is identified as the trans-dimer of 2-nitronitrosoethane with dimerisation through nitroso nitrogen atoms about a centre of symmetry (NOzCHZCH2NO-) . The nitroso N-0 bond length 1.262(4)A is considerably shorter than earlier estimates of this distance in C-nitroso dimers. The N-N distance is 1-304(6) %1.' New improved data" have been used in a further refinement of the anhydrous citric acid structure.' Both carboxy and peptide group planes in L-alanyl-L-alanine hydrochloride have dihedral angles of 27".l2 The structure of L-threonyl-L-phenylalanine p-nitrobenzyl ester hydrobromide has been determined and the backbone and sidechain conformations examined in detail. All five protons available for the formation of hydrogen bonds are utilisd in forming a three-dimensional network stabilising the structure.' The conformation of valine molecules in DL-valine is similar to that found in its hydrochloride derivatives. A useful comparative account of the molecular features of the valine molecule as found here and in other structures is also given.I4 A structural feature in glycylglycine hydro- chloride is the occurrence of apparently bifurcated hydrogen bonds involving each of the three hydrogen atoms of the terminal nitrogen atom.The account of the structure includes a list of some structures which also have bifurcated hydrogen + + bonds. ''In crystals of spermidine phosphate trihydrate [NH3(CH2)3NH2(CH2)4- + NH,I2,3[HPO4l2-,6H20 the molecules take a normal extended zig-zag chain E. L. V. Lewis and T. R.Lomer Acta Cryst. 1969 B25 702. S. Aleby Acta Chem. Scand. 1968 22 3146. ' A. J. De Kok and C. Romers Rec. Trav. chim.,1969 88 625. I. Nahringbauer Acta Chem. Scand. 1968 22 298 1. F.P. Boer and J. W. Turley J. Amer. Chem. SOC.,1969 91 1371. lo J. Glusker J. A. Minkin and A. L. Patterson Acta Cryst. 1969 B25 1066. l1 C.E. Nordman A.S. Welden and A. L. Patterson Acta Cryst. 1960 13 418. l2 Y.Tokuma T. Ashida and M. Kakudo Acta Cryst. 1969 B25 1367. l3 M.Mallikarjunan S. Thyagaraja Rao K. Venkatesan and V. R. Sarma Acta Cryst. 1969 B25,220. l4 M. Mallikarjunan and S. T. Rao Acta Cryst. 1969 B25 296. R. Parthasarathy Acta Cryst. 1969 B25 509. X-Ray Crystallography conformation with the overall structure consisting of two kinds of band-the molecular spermidine band and the phosphate-water band. SubstitutedBenzene and Polycyclic Aromatic Compounds.-E.p .r. measurements on trimesic acid (benzene-l,3,5-tricarboxylicacid) indicate that it may be damaged by X-radiation to produce a relatively high concentration of stable long-lived free radicals.' The basic structural motif of one modification which has six molecules of the acid in the crystal asymmetric unit is a continuous two-dimensional network comprising large open rings formed by six molecules of trimesic acid bonded together through pairs of O-H.-O hydrogen bonds.The Aetworks are not planar and continually interpenetrate one another ; through each large ring of one network pass three parallel rings of other net- works.'' The structure of 3,5-dinitro-4-methylbenzoicacid consists of hydrogen- bonded dimers (O-H-.O 2.647(6)A) with the nitro groups twisted out of the ring plane by 47.5 and 38.9" respectively.'' The angle between the two phenyl rings of 2'-chlorobiphenyl-4-carboxylic acid is 48.9" while that between the phenyl ring and its associated carboxy group is 5-6" ;20 in the 2-chloro derivative the corresponding interplanar angles are 46-1 and 7-9" respectively.2' The positions of the atoms in 2,4,6-tribromoaniline deviate slightly from a planar conformation owing to intramolecular repulsion between the amino group and the adjacent bromine atoms.The amino group is apparently pyramidal.22 Structure determinations of benzylideneaniline and of two p-substituted deriva- tives have shown that the aniline ring is twisted out of the C-N=C-C plane by 40-55" a result that bears on the apparently anomalous U.V. spectrum of benzylideneaniline derivatives. 23 In p-phenylenediamine dihydrochloride the + + C-NH distance is 1-49 A and the molecules are held in the crystal by NH3-C1-hydrogen bonds (3.20 3.21 and 3-25 A).'.The effect of intramolecular over- crowding between the carboxy and amino group in 3,5-dichloroanthranilic acid is to cause the exocyclic C and N atoms to be displaced from the aromatic plane in opposite directions by 0-063 and 0-045A respectively. The carboxy-group plane also makes an angle of 6" with the phenyl plane.25 The nitramine group in N-(B/?j?-trifluoroethyl)-N,2,4,6-tetranitroaniline is rotated 86" out of the plane of the benzene ring and the 2- 4- and 6-nitro groups are rotated 19,12 and 39". A list of rotation angles for other nitro-aromatic compounds is given for com- parison. The shortest 0-F separation (3-00A) is between atoms with the longest C-F and N-0 bond distances.26 The side chain in anti-2,6-dimethyl- l6 Y. Huse and Y.Iitaka Acra Cryst.1969 B25 498. G. Liebling Ph.D. Thesis California Institute of Technology 1965. D. J. Duchamp and R. E. Marsh Acta Crysf. 1969 B25 5. l9 D. F. Grant and J. P. G. Richards Acta Cryst. 1969 B25 564. 2o H. H. Sutherland Acta Cryst. 1969 B25 171. 21 H. H. Sutherland and T. G. Hoy Acta Cryst. 1969 B25 1013. 22 A. T. Christensen and K. 0.Stromme Acra Cryst. 1969 B25 657. 23 H. B. Burgi and J. D. Dunitz Chem. Comm. 1969,472. 24 R. Chandrasekaran Acta Cryst. 1969 B25 369. 25 S. K. Arora and L. M. Pant Acta Cryst. 1969 B25 1045. 26 J. R. Holden and C. Dickinson J. Phys. Chem. 1969 73 1199. G. Ferguson 4-chloro-N-methylbenzaldoxime [Clc,H,(Me),CH=N( -+O)Me] is twisted 55" out of the plane of the benzene ring. A short intermolecular distance between oxygen and a methyl hydrogen (2.14A) is interpreted as a hydrogen bridge.27 It has been suggested that a-2-hydroxy- and a-2-amino-arylidene substituted 5-membered lactones and lactams are trans-isomers and once trans-cis iso-merisation has taken place the cis-isomer undergoes cyclisation immediately.To establish the geometry of the stable isomer the structure of a representative or-arylidenelactone or-(2-hydroxy-3,5-dibromobenzylidene)-y-butyrolactone, has been determined and the planar molecule is in the trans-configuration with the substituted phenyl moiety trans to the carbonyl group of the lactone.2 Molecules of salicylaldehyde azine [HO-C6H4-CH=N-]2 are centrosymmetric and nearly planar. An intramolecular hydrogen bond (2-645 A) binds the phenolic OH to the nearest nitrogen in the azine chain.29 An extensive hydrogen bond network (with 0-H-0 distances of 2-80A) binds molecules of ninhydrin in the solid state.30 The lactone groups in rneso-3,3'-bis-(p-chlorophenyl)bi-3-phthalidyl are planar and exhibit the expected asymmetry of the C-0 bonds.The two halves of the molecule are related by a centre of symmetry and in each half the phenyl ring is tilted with respect to the phthalidyl ring system by 125".31 Full details of the related 3-(p-bromophenyl)- phthalide have also been published.32 Although the molecule is planar the benzene ring in benzo[ 1,2 :4,5]dicyciobutene is distorted somewhat from the usual geometry.33 Crystals of 3,6-bisdiazocyclohexanetetraone(1) are best described as a bis- diazonium enolate.The centrosymmetric molecule is planar with N-N 1-107(10)A C-N 1.357(10)A and C-REN 179.9". Bonds C(l)-C(2)and C(3)-C(4) are comparable (1.437 1.433A) but C(2)-C(3) is much longer (1.541 A). The two independent C-0 distances are also similar (1-208,1-214 A).34 N 27 K. G. Jensen and B. Jerslev Actu Cryst. 1969 B25 916. 28 D. F. Koenig C. C. Chiu B. Krebs and R. Walter Actu Crysf. 1969 B25 121 1. 29 G. Arcovito M. Bonamica A. Domenicano and A. Vaciago J. Chem. SOC.(B) 1969 733. 30 R. C. Medrud Actu Cryst. 1969 B25 213. 31 V. Kalyani and M. Vijayan Actu Cryst. 1969 B25 252. 32 V. Kalyani and M. Vijayan Acta Cryst. 1969 B25 1281. 33 J. L. Lawrence and S. G. G. Macdonald Actu Cryst. 1969 B25 978. 34G.B.Ansell J.Chem. SOC. (B) 1969 729. X-Ray Crystallography The results of an investigation of ~5-diamino-3,6-dichloro-i,4-benzoquinone (2) are in keeping with the suggestion (based on spectroscopic and quantum mechanical considerations) that 2,5-diamino-l,4-benzoquinonesmay be con- sidered as 'coupled polymethines'. Bond C( 1)-C(2) in the centrosymmetric molecule is effectively single (1-522 A) while the others C(2)-c(3) and C(3)-C(4) are much shorter (1.383 1.409 A re~pectively).~~ 2,2'-Di(l,4naphthoquinone)is overcrowded and an 0(1)-H(3') separation of 2.56A is achieved by a 47.3" twist of the two naphthoquinone halves of the molecule about the C(2)-C(2') bond. The quinone ring deviates slightly from planarity with the benzene ring adopting a shallow chair form.36 Galvinoxyl (3),a stable phenoxyl radical has (3) 2-fold symmetry in its crystals.The angle at the methine carbon atom is-134" and the phenyl groups are twisted with respect to each other by 12" allowing two hydrogen atoms adjacent on the phenyl rings to be separated by 2.37.&. This twist is smaller than predicted on the basis of an analysis of e.s.r. hyperfine splitting. The radical C-0 distance is 1-27 8 and the t-butyl groups are in a fully staggered conformation even including the hydrogen atoms.37 The molecular structure of 1:2,5 :6-dibenzanthraquinone consists of two planar naphthalene portions with an angle of 14.1" between the planes.38 Quite a number of papers report bifurcated hydrogen bonds. In naphthoquinonic compounds they have been found when a hydroxy or amino group occupies the P-p~sition.~' The crystal structure of 6-methoxy-8-nitro-5( 1H)-quinolone contains a bifurcated hydrogen bond.40 There is a pronounced distortion of the anthracene skeleton in 1,8-dichloro- 9-methyl-anthracene C(9) being displaced 0.19 8 from the molecular plane.The methyl carbon atom is 0.80 A displaced on the same side as C(9) while the chlorine atom lies 0.33 A on the opposite side.41 Non-Aromatic Carbocyclic Molecules.-The existing data on simple cyclobutanes have been summarised :rings which are not centrosymmetrically substituted are puckered ;rings which are may be planar. However centrosymmetrically sub- stituted cyclobutanes can also be p~ckered.~' In cis-cyclobutane-1,3-dicarboxylic 35 S.Kulpe Acta Cryst. 1969 B25 141 1. 36 H. L. Ammon M. Sundaralingam and J. M. Stewart Acta Cryst. 1969 B25 336. 37 D.E. Williams Mol. Phys. 1969 16 145. 38 R. F. Entwistle J. Iball W. D. S. Motherwell and B. P. Thompson Acta Cryst. 1969 B25 770. J. Gaultier and C. Hauw Actu Cryst. 1969 B25 546,419. 40 M. Sax R. Desiderato and T. W.-Dakin Acta Cryst. 1969 B25 362. 41 R. J. Dellaca B. R. Penfold and W. T. Robinson Acta Cryst. 1969 B25,1589. T. N. Margulis Chem. Comm. 1969 215. G. Ferguson acid the cyclobutane ring is puckered with a dihedral angle of 149 3". The ring C-C bond lengths average 1.554(10) The cyclobutenone moiety in 4-chloro-2-methyl-3-phenylcyclobut-2-enone is slightly puckered. The angle between the planes through C(l) C(2) C(3) and C(l) C(3) C(4) is 4"; the chloro- substituent is in a pseudo-equatorial position.44 As part of a series of papers in the conformation of non-aromatic ring com- pounds boat/twist-boat pseudorotation in six-membered ring compounds has been considered in detail.45 In the crystalline state 2-fold symmetry is retained for ( )-trans-cyclohexane- 1,2-dicarboxylic acid as a space group requirement and In the carboxy groups are di-eq~atorial.~~ (+)-trans-cyclohexane-dicarboxylic acid the carboxy-groups are also diequatorial but geometry calculations indicate positional disorder between carbonyl and hydroxy oxygen atom^.^' The crystal structure of 4,4-diphenylcyclohexanonehas been determined so that it may serve as a model for a general discussion of the various distortions available to chair six-membered rings.A correlation is noted between quantitative X-ray data from the solid state and qualitative n.m.r. data from the liquid state.48 Crote- poxide a novel tumor-inhibiting cyclohexane diepoxide has structure (4) with the cyclohexane oxide ring displaying a half-chair conf~rmation.~~ Both cyclo- hexane rings in (5) have the same chair conformation with the methyl groups equatorial. The molecules are linked in pairs by intermolecular hydrogen bonds between adjacent oxime groups (0-H-N 2.87 A).'' (4) Molecular symmetry S4is required of (6)by its space group. C-C bond lengths are 1.33 and 1-48A. Ring carbon valency angles are 122" not significantly different from cyclo-octatetraene and its other derivatives.' The eight-membered ring in cyclo-octane-cis-1,2-dicarboxylic acids2 occurs in the same approximately mirror symmetric (boat-chair) conformation as was found previously in the trans-is~mer.~~ The conformation of the twelve-membered ring in 2,12-dibromo- 43 E.Adman and T. N. Margulis J. Phys. Chem. 1969 73 1480. 44 S. M. Krueger J. A. Kapeck J. E. Baldwin and I. C. Paul J. Chem. SOC.(B),1969,796. 45 H. R. Buys and H. J. Geise Tetrahedron Letters 1968 5619. "E. Benedetti P. Corradini C. Pedone and B. Post J. Amer. Chem. Soc. 1969 91 4072. 47 E. Benedetti P. Corradini and C. Pedone J. Amer. Chem. SOC.,1969,91 4075. 48 J. B. Lambert R. E. Carhart and P. W. R. Corfield J. Amer. Chem. Soc. 1969,91,3567.49 P. Coggon A. T. McPhail and G. A. Sim J. Chem. SOC.(B) 1969 534. D. Mootz and B. Berking Acta Cryst. 1969 B25,828. 51 G. Avitabile P. Ganis and V. Petraccone J. Phys. Chem.. 1969 73 2378. 52 H. B. Biirgi and J. D. Dunitz Hefu. Chim. Acta 1968 51 1514. 5J M. Dobler J. D. Dunitz and A. Mugnoli Heltl. Chim. Acta 1966 49 2492. X-Ray Crystallography 47 cyclododecanone corresponds approximately to the idealised model with 422 symmetry containing 8 synclinal and 4anti-periplanar partial conformation^.^^ The hydrocarbon cembrene (thumbergene) is confirmed as (7)by X-ray analysis.55 The structure is consistent with the hypothesis of its biosynthesis from geranyl- geraniol. Hydroxylated derivatives of cembrene have been found in tobacco leaves and cigarette smoke.The constitution and configuration of the compound produced on hydro- bromination of trans-pinocarveol has been determined and confirmed as 6-bromo- isofenchone (8). Despite the strain introduced by the keto group the molecular skeleton is remarkably symmetrical about the median plane. The dihedral angle in each of the five-membered rings is about 55" and the C-C-C angle at the bridgehead carbon is 97" close to what is expected in such a system.56 Laurinterol acetate (9) contains a trisubstituted bicyclo[3,l,0]cyclohexane system which (7) (8) (9) adopts an overall boat-like conf~rmation.~~ The conformation of the five- membered ring in the diterpenoid bitter principle enmein and in a number of other bicyclo[3,2,l]octane derivatives is closer to half-chair than to envelope form.58 Compound (10) is interesting because of the ease with which it undergoes the Cope rearrangement.Molecular geometry calculations indicate that in addition to the favourable activation entropy a state of internal strain facilitates the rearrangement. The strain is reflected in the length 1-59 A of the C(6)-C(7) bond which is broken during the rearrangement.59 The structure of pseudo- clovene-A a substituted tricyclo[6,3,1,0 '.5]dodecane is established as (1 1) by an analysis of a derivative of its diol. The cyclopentane ring adopts an envelope conformation and the two six-membered rings occur in distorted chair and boat c1 ''R 4 YY "J. Dehli and P. Groth Acta Chem. Scand. 1969 23,587. ''M.G.B. Drew D. H. Templeton and A. Zalkin Acta Cryst. 1969 B25,261. 56 P.P. Williams Acta Cryst. 1969 B25,409. 57 A. F. Cameron G. Ferguson and J. M. Robertson J. Chem. SOC.(B) 1969 692. 5x P. Coggon and G. A. Sim J. Chem. SOC.(B) 1969 413. ''I. R. Bellobono R. Destro C. M. Gramaccioli and M. Simonetta J. Chem. SOC.(B) 1969. 710. G. Ferguson conformations.60 Structure (12) is confirmed for the adduct formed by thermal reaction of 2-chlorotropone and cycloheptatriene and its strained molecular geometry discussed in detail.61 Systems Containing Hetero Atoms.-Alkylamine boranes are strongly reducing substances and of potential interest as radical scavengers and radioprotective agents. During synthesis of such substances it was found that aziridine and certain of its derivatives gave addition compounds with borane that behaved differently from normal secondary amine borane complexes and whose structure is as in (13) with C-R-B = 121-6” C-N = 1-450(6)& N-B = 1.558(6)A and C-C = 1.460(6)A.62 The unit cell of crystals of (14) contains two crystallo- graphically independent molecules one of which lies with its five-membered ring on a mirror plane.Molecules of the second type are also planar within experi- mental error. The N-0 distance in this nitroxide free radical is 1~33(2)A~~ in agreement with that found in caryophyllene ‘iodonitrosite’ [1-31(2) A].64 The conformation of 4-cyano-N-[2-(p-methoxyphenyl)ethyl]pyridiniumiodide is gauche about the central C-C bond with a dihedral angle of 68”.The crystal structure does not explain the intramolecular charge-transfer bands observed in the U.V. absorption spectr~m.~’ Crystals of the ‘primary acid modification’ of the reduced form of the diphosphopyridine nucleotide model 1-(2,6-dichloro-benzyl)-4,4dihydronicotinamidehave been the subject of a precise analysis which identifies the product as l-(~6-dichlorobenzy1)-6-hydroxy-1,4,5,6-tetrahydro-nicotinamide dihydrate. The nicotinamide residue is planar with the exception of C(5)and the hydroxy group ; the benzyl C(H,) occupies the same plane. The amide oxygen is cis to the ring double bond and the hydroxy group occupies a pseudo-axial position. 66 Preferred axial quaternisation of 1-benzyl-4-phenyl- piperidine with methyl iodide has been demonstrated by X-ray analysis of the resulting methobromide.67 A number of isoxazole derivatives have been examined e.g. N-methyl-4-phenylisoxazolin-5-0ne,~ 4-bromo-N-methyl-3-phenylisoxazolin-5-one and 3-phenylisoxazolin-5-one.70The bond lengths and angles reported are in accord with accepted values. In 3,4’-di-isoxazole bond lengths and angles are also normal and indicate no conjugation between the rings. The five-membered rings are not coplanar but are mutually inclined at an angle of 2-8°.71Structures for the 6o D. M. Hawley G. Ferguson T. F. W. McKillop and J. M. Robertson J. Chem. SOC. (B) 1969 599. 61 Y. Fukazawa S. It6 and Y. Iitaka Acta Cryst. 1969 B25 665. 62 H.Ringertz Acta Chem. Scand. 1969 23 137. 63 G. J. Kruger and J. C. A. Boeyens Proc.Nut. Acad. Sci.U.S.A. 1968 61 422. 64 G. Ferguson D. M. Hawley and J. M. Robertson J. Chem. Soc. (B) 1968 1255. b5 L. Dik-Edixhoven and C. H. Stam Rec. Trau. chim. 1969 88 577. ‘‘H. Hope Acta Cryst. 1969 B25 78. 67 R.Brettle D. R. Brown J. McKenna and R. Mason Chem. Comm. 1969 339. 68 C. Sabelli and P. F. Zanazzi Acta Cryst. 1969 B25 182. 69 C. Sabelli and P. F. Zanazzi Acta Cryst. 1969 B25 192. ‘O M. Cannas S. Biagini and G. Marongiu Acta Cryst. 1969 B25 1050. 71 S.Biagini M. Cannas and G. Marongiu Acta Cryst. 1969 B25 730. X-Ray Crystallography (13) (14) 2-0xide'~ and 5-oxide7 of 4-methyl-3-(p-bromophenyl)-1,2,5-oxadiazole have been reported; the former has the N-oxide (furoxan) structure and is only approximately planar the latter has the configuration of furazan-5-oxide and is approximately planar.Sydnones are the products of dehydration of N-nitroso- a-amino acids. From X-ray analyses of 4,4-dichloro-3,3'-ethylenebis-sydnone and 3,3'-ethylenebis+ydnone the unusual formulation (14) has been postulated as the best single formula representative of a ~ydnone.~~ 1,2,4-Triazole has been reinvestigated at -160 0C.75 The hydrogen atom not found in an earlier has been located and the compound can be formulated as 1-hydrogen-1,2,4- triazole. N-H-N and C-H-N hydrogen bonds (length 2.82 and 3.30A respectively) link the molecules to form infinite corrugated sheets. In crystals of thymine pairs of molecules related by a 2-ford screw-axis are connected by two N-H-O=C hydrogen bonds (2.810 and 2433681).77 The structure of thymine-thymine adduct 5-hydroxy-6,4(5'-methylpyrimid-2'-one)-dihydrothymine isolated from thymine irradiated with U.V.light in frozen aqueous solution and presumably formed through rearrangement of the initial photo product has been confirmed except for the possibility of a hydrogen atom on the 3' nitrogen rather than on the 1' nitrogen atom.78 Thymine photodimer E has a centre of symmetry and consists of two head-to-tail thymine rings trans to each other joined by a cyclobutane linkage across their $6 double bond. The cyclobutane bond lengths are 1-547(3) and 1.587(3) A and the angle between the planes of the thymine and (planar) cyclobutane rings is 1 13-7°.79 Bis-(dimethyl)- thymine photodimer C is confirmed as the $6 :5,6 syn stereoisomer.The cyclo- butane ring is puckered each atom lying 0.5 A out of the plane of the other three. The thymine nuclei are planar if C(6) is omitted from the plane of each residue ; the angle between these planes is 33"." The 6-mercaptopurine molecule (15) in crystals of 6-mercaptopurine mono- hydrate is not strictly planar ;the maximum deviation from planarity is displayed by N(q(0.029 A). The sulphur atom is attached in th%e form to the C(6) atom and the C-S bond length is 1~676(2)A with N(l)-C(6)-C(5) = 110.4". In the imidazole ring the hydrogen atom is bonded to N(7) rather than to N(9)." A 72 M. Calleri G. Ferraris and D. Viterbo Acta Crysr. 1969 B25 1126. 73 M.Calleri G. Ferraris and D. Viterbo Actu Cryst. 1969 B25 1133.74 H.Hope and W. E. Thiessen Acta Cryst. 1969 B25 1237. 7s P.Goldstein J. Ladell and G. Abowitz Acta Cryst. 1969 B25 135. '' H. Deuschle Ber. Bunsengesellschaft phys. Chem. 1965 69 550. 77 K. Ozeki N. Sakabe and J. Tanaka Acta Cryst. 1969 B25 1038. 78 I. L. Karle Shih Yi Wang and A. J. Varghese Science 1969 164 183. 79 N. Camerman and S. C. Nyburg Acta Cryst. 1969 B25 388. N. Camerman D. Weinblum and S. C. Nyburg J. Amer. Chem. SOC.,1969 91 982. E. Sletten J. Sletten and L. H. Jensen Acra Cryst. 1969 B25 1330. G.Ferguson second independent determination of the same crystal structure with more data yielded very small differences from that reported by Sletten et al. e.g.C-S = 1-679A N(l)-@]-C(5) = 110-2°.82 The tosyl ester of the alkaloid lupine cyclises when heated and the rearrangement product has been shown to be 1,5-endo-methylenequinolizidiniumtoluene-p-sulphonate (16).The two trans-fused six-membered rings of the quinolizidine are in distorted chair forms and the four-membered ring is puckered.83 Molecules of carbazole are planar and have normal bond lengths :in the crystals a crystallographic mirror plane is normal to the molecular plane.84 The nucleophilic addition product of 2-aminopteridine hydrobromide with ethanol is confirmed as (17). C(2) and the three nitrogen atoms bonded to it form a planar guanidinium system all four hydrogen atoms being used in a hydrogen bond network to the bromide ions ;the pyrazine ring is also planar.85 The carcinogenic compound tricycloquinazoline (18) is planar to within f0.05 A and has within experimental accuracy 3-fold symmetry.A feature of the geometry is the arrangement of short and long C-N bonds in the central rings [mean values 1.279(7) and 1.397(8) A respectively]. The shorter values seem to be shorter than is usually observed in heterocyclic rings.86 A neutron diffraction study has located the central hydrogen atoms of phthalo- cyanine; four peaks corresponding to four half-hydrogens were found in the central portion of the molecule.87 Trioxan has point group symmetry 3 but retains its maximum symmetry 3m with C-0 = 1-421(6)A 0-c-0 = 109-6(3)O and C-0-C = 110.4(3)". Packing in the crystal is controlled by C-H-*-O contacts of 3.321 3-454 and G. M. Brown Acta Cryst. 1969 B25 1338.83 C. S. Huber Acta Crysf.,1969 B25 1140. n4 B. S. Basak and B. N. Lahiri Indian J. Pure Appl. Phys. 1969 7,234. 85 T. J. Batterham and J. A. Wunderlich J. Chem. SOC.(B) 1969 489. 86 J. Iball and W. D. S. Motherwell Acta Crysf.,1969 B25 882. "B. F.Hoskins S. A. Mason and J. C. B. White Chem. Comm. 1969 554. X-Ray Crystallography 51 3-645A.88The nine-membered ring in trimeric acetone peroxide can be described in terms of a twisted boat-chair and the molecules have approximately 03 symmetry. The environment of the ring carbon atoms is asymmetric; the distortion may possibly be explained by intramolecular repulsions between hydrogen and oxygen atoms. The C-0 distance is 1.48A and C-0-0 107”.89 A crystalline material isolated from contaminated animal feed fat and capable of producing hydropericardium in chicks has been identified by X-ray analysis as (19); the molecules are almost planar.” One of the photolysis products found on irradiation of (20) has been determined and characterized as (21).The combined benzene and lactone rings of (21) are nearly planar and the CI C1 c1 OPhCl 0-(21) dihedral angles between this plane and the phenoxy group and cyclopropane ring are 64” and 70” respectively.” Complete details of the molecule geometry of strontium 3-deoxy-2-C-hydroxymethyl-~-erythro-pentoate have been pub- li~hed.’~Only four of the carbon atoms in the anion form a planar zig-zag chain. The molecular topography of fi-adenosine-2’-fi-uridine-5-phosphoric acid (A2’P5’U) shows no unexpected features and has been compared with related molecules; a detailed summary of the conformations of phosphate diesters is included.93 Each A2’PSU molecule is hydrogen bonded to four adjacent nucleotide molecules and nine water molecules ;close contacts are also found between furanose-ring oxygen atoms with nearby bases.The configuration of the sulphur atom in methanesulphinic acid MeS02H is pyramidal with an S-C distance of 1.786(11) and S-0 distances of 1-502(9) and 1.60419) A. Hydrogen bonds link the molecules into spirals about one of the crystallographic axes.94 Tris(ethylsulphonyl)methane (TESM) is not iso-morphous with tris(methylsulphony1)methane and is free from the disorder which gives rise to spectacular diffuse scattering in the latter.Principal bond lengths in TESM are C-S 1-834(4),S-0 (mean) 1.442(7),S-CH2 1-785(10) H2C-Me +-1.496(15)A.95An analysis of crystals of (Me)2S-C(CN)2 confirms the view that stabilisation of the carbanion is due mainly to the overlap of the carbanion electrons with the sulphur 3d-orbitals. The configuration around the central carbon atom is planar.96 88 V. Busetti A. Del Pra and M. Mammi Actu Cryst. 1969 B25 1191. 89 P. Groth Actu Chem. Scund. 1969 23 131 1. 90 J. S. Cantrell N. C. Webb and A. J. Mabis Actu Cryst. 1969 B25 150. 91 L. J. Guggenberger and R. A. Jacobson Actu Cryst. 1969 B25 888. y2 P. E. Werner R. Norrestam and 0. Ronnquist Actu Cryst. 1969 B25 714. y3 E. Shefter M. Barlow R. A. Sparks and K. N. Trueblood Actu Cryst. 1969 B25 895.94 K. Seff E. G. Heidner M. Meyers and K. N. Trueblood Actu Cryst. 1969 B25 350. 9s D. R. McGregor and J. C. Speakman Actu Cryst. 1969 B25 540. 96 A. T. Christensen and W. G. Witmore Actu Cryst. 1969 B25 73. 52 G.Ferguson The S-S distance 2.019(5) 8 in a-bis-(p-nitrophenyl) disulphide implies some n-character in this bond; the dihedral angle about the S-S fragment is 72". The values closely resemble those found in rhombic sulphur.97 2-(2-Pyridyl- methy1dithio)benzoic acid (22) an active anti-radiation drug does not exist as a zwitterion in the crystal. The molecular geometry can be compared to a clamp with the two rings nearly parallel to each other and the -CH2SS- linkage acting as a hinge. The C-SS-C bridge adopts the peroxide configuration with a S II dihedral angle of 99-1".98 The C-N group in (Me)&SSCN(Me) is planar; \ C the C-S-S-C group has a dihedral angle of 99.6" with C=S and S-S 1.65 and 2.00 A re~pectively.~~ Bond distances in (23) are said to indicate a certain amount of n-electron delocalisation although the exocyclic C=O (1-195 A)and C=S (1-608 A)bonds are close to the values generally accepted as double bond distances.The structure shows a short (2.90 A) C(sp2)-0(carbonyl) contact ; this is an example of a specific structure determining intermolecular interaction between the charge centres C6+ -0'-dipoles in polycarbonyl compounds.' O0 The oxygen of the nitroso group in (24) is in very close contact with the neigh- bouring sulphur atom (04= 2.034(5) A),1o10.35 A shorter than the shortest previously reported S-.O intramolecular distance.' O2 There is a close similarity between the molecular dimensions of the extended thiathiophthen system (25) and the corresponding thiathiophen.S-S distances are S(l)-S(2) 2.482 S(2)-S(3) 2.208 S(3)-S(4) 2.966 A. The fourth sulphur atom appears to have a 97 J. S. Ricci and I. Bernal J. Amer. Chem. SOC. 1969,91,4078. 98 J. Karle I. L. Karle and D. Mitchell Acta Cryst. 1969 B25,866. 99 D.J. Mitchell Actu Cryst. 1969 B25,998. loo B. Krebs and D. F. Koenig Acta Cryst. 1969 B25,1022. lo' P. L. Johnson and 1. C. Paul J. Amer. Chem. SOC.,1969,91 781. J. A. Kapecki J. E. Baldwin and 1. C. Paul J. Amer. Chem. SOC.,1968 90 5800. X-Ray Crystallography negligible influence on the bonding of the three sulphur ~ystem.'"~ Thieno- [3,4-d]thiepin is'a planar molecule with a disordered crystal structure similar to that of the hydrocarbon azulene.By contrast the 6,6-dioxide derivative shows non-planar geometry which resembles that found for other thiepin sulphones.' O4 The non-hydrogen atoms in thiosemicarbazide are nearly coplanar with the sulphur and hydrazinic NH2 groups in a trans-position with respect to the C-NH bond. Some distances are C-S 1.685(5) C-N(1) 1.313(6) C-N(2) 1.337(6) and N(2)-N(3) 1-399(6) In the crystalline state pairs of thioacetanilide S-oxide molecules related by a centre of symmetry are linked into dimers by N-H-.O (2.86A) bonds. In each molecule the atoms (except hydrogen) lie on two planes one containing the aniline moiety and the other the thioacetyl S-oxide group; the angle between these planes is 45O.lo6 The zwitterionic structure for the five-membered ring in 2-imino-5-phenylthiazolidin-4-one has been clearly demonstrated for the first time.'07 The benzene and dithiolic rings in 1,2-benzodithiol-3-one oxime are nearly coplanar and observed bond distances indicate no remarkable n-delocalisation [C-S = 1.74(1),S-S 2.075(3) A]nor is conjugation observed for the C=NOH group [C-N = 1-31(1),N-0 1.42(1) A] which does not lie exactly in the thiolinone plane.The molecules pack with a strong [2.74( 1)A] 0-H-.N system of hydrogen bonds.lo8 Molecular geometry calculations show that (26) adopts the principai resonance structure indicated by the formula but there is considerable resonance interaction.Strong N-H-0 [2-776(3)A] and N-H-N [3.002(3)A] hydrogen bonds link the molecules in the ~rystal.'~' X-Ray studies of (27) reveal that the molecules undergo a change in orientation when the temperature is lowered from 25 to 3 "C. In both forms the conformation of the eight-membered ring is approximately boat-chair Another feature of the structures is the existence of a transannular N-H-O=S hydrogen bridge with an N-0 separation of 2-65(1)A.'lo The eight-membered ring in (28) is also in a boat-chair conformation and the transannular S-S distance H Me I I co ca / \ c104 I Ph H Me (28) Io3 J. Sletten Chem. Comm. 1969 688. Io4T. D. Sakore R. H. Schlessinger and H. M. Sobell J. Amer. Chem.SOC.,1969 91 399s. 'O5 P. Domiano G. Fava Gasparri M. Nardelli and P. Sgarabotto Acta Cryst. 1969 B25 343. '06 0.H.Jarchow Acta Cryst. 1969 B25 267. lo' L. A. Plastas and J. M. Stewart Chem. Comm. 1969 81 1. lo' G. D. Andreetti L. Calvalca A. Manfredotti and A. Musatti Acta Cryst. 1969 B25 288. lo9 D.L.Smith Acta Cryst. 1969 B25 625. 'Io I. C. Paul and K. Tee Go J. Chem. SOC.(B) 1969 33. 54 G.Ferguson 3.13 A is considerably shorter than twice the van der Waals radius for sulphur. l1 * Three isomers of S6(NH2)2 have been studied. Form I has symmetry 2 and a crown conformation with the nitrogen atoms in 1,Cpositio~ Average dimensizs are S-S 2.048(6) N-S 1.724(10)& Sm 119-1(5) NSS 110.1(4) and SSS 107-3(2)". Form I1 has rn symmetry with a puckered ring of sulphur and nitrogen atoms (crown) with nitrogen in the 1,5-positions.Form I11 was inferred to be the 1,3-di-imide from chemical spectroscopic studies. ' The observed Se-Se bond length in [(Ph)*CHSeI2 is 2-285(5)A. The Se-C length 1-97(1)A identifies it as an aliphatic Se-C bond. The C-Se-Se-C dihedral angle is 82".' l3 In trans-selenophthen the two independent C-Se bonds have lengths 1-93(2) and 1-87(1) A."" Molecular Complexes.-In the 1:1 addition compound of iodoform-1,4- dioxan the dioxan molecules are on centres of symmetry and the iodoform molecules on symmetry planes. In the crystal there are endless chains of donor- acceptor molecules with 0-1 3.04(4)A about 0.50 A shorter than anticipated for van der Waals contacts.The linewidth of the 'H n.m.r. spectrum narrows from 14 gauss at -50 "C to 4.2 gauss at + 10 "C demonstrating that a reorientation has taken place in the solid. The X-ray analysis indicates that the movement may be described as a rotation of the dioxan molecules about an axis drawn between the two oxygen atoms.' l5 The crystal structure determination of tetrabromoethylene and of the 1:1 pyrazine adducts of tetrabromo- and tetraiodo-ethylene reveals that the packing of the acceptor molecules is left virtually unaltered after placing the less voluminous pyrazine molecules into the lattice of the tetrahalogenoethy- lenes. Both adducts which are not isomorphous contain infinite chains of alternating donor and acceptor molecules linked by nitrogen-halogen inter-actions.The N-ha1 distance is just shorter for the iodine [2.979(26) A] than the bromine [3.018( 15) A] system.' The potassium methoxide adduct of 4-methoxy- 5,7-dinitrobenzofurazan exhibits some novel ring distortions which can be attributed to the differences in electron-withdrawing power of the substituents. Distortions in the benzene ring provide a novel example of substitution effects; the benzofurazan ring appears to have a stronger electron-withdrawing effect than the two nitro groups.'17 The C-C(N2)-C angle in the benzene ring in the 1:1 complex of benzenediazonium chloride with acetic acid is 124-0(5)" ;the C-N and N-N triple bond lengths 1.425(6) and 1-098(7)A respectively. The N2+ group is co-ordinated to two C1- ions in a linear arrangement with distances of 3.75 A from the centre of the N2+ group to the C1- ions.Acetic acid molecules are linked by hydrogen bonds to chlorine atoms [O(H)..*Cl = 3-01A].118Each S. M. Johnson and I. C. Paul Tetrahedron Letters 1969 177. I J. C. Van de Grampel and A. Vos Acta Cryst. 1969 B25 61 1. H. T. Palmer and R. A. Palmer Acta Cryst. 1969 B25 1090. A. C.Villa M. Nardelli and C. Palmieri Acta Cryst. 1969 B25 1374. 115 T. Bjorvatten Acta Chem. Scand. 1969 23 1109. 116 T. Dahl and 0. Hassel Acta Chem. Scand. 1968 22 2851. G. G. Messmer and G. J. Palenik Chem. Comm. 1969 470. * C. Romming and T. Tjornhom Acra Chem. Scand. 1968 22 2934. X-Ray Crystallography 55 molecule in the phenothiazine-trinitrobenzene charge-transfer complex is required by space group symmetry to lie on a 2-fold axis but the phenothiazine molecules seem to be disordered.The thermal ellipsoid data are interpreted in favour of a dynamic disorder."g The crystal structure of the ionic 1 :1 complex of ditoluenechromium and tetracyanoquinodimethane (TCNQ-) consists of stacks of ditoluenechromium cations and stacks of TCNQ-anions with inter- planar spacings of 3.42 A.12o 9-Ethyl-8-bromoadenine and 9-ethyl-8-bromo- hypoxanthine form a planar complex in which the molecules are joined by two hydrogen bonds ;an N-H-N bond 2.94 A involving N(1)of hypoxanthine and N(7) of adenine and an N-He -bond between the amino group of adenine and a0 the carbonyl oxygen of hypoxanthine. The overall crystal structure is a sheet-like one in which adjacent base-pairs are related by a centre of symmetry and are con- nected by hydrogen bonds between adenine residues.' ' Cytosine and 5-flUOrO- uracil are also joined by two hydrogen bonds (N-H-* -N 2.83 8 and N-Ha * -0 3.03 A) to form a cyclic dimer ;the dimers are in turn joined across centres of symmetry by other hydrogen bonds (N-H. -02-95A) to form infinite sheets.'22 The antimony atoms in the 2 1 complex between antimony trichloride and naphthalene exist in 4-co-ordinate distorted sp3d trigonal bipyramidal environ- ment. The antimony atom is 3.2 A from the plane of the naphthalene ring and Sb-Cl distances are 2-347,2.348 and 2.367 A. 123 The structure of the complex PhCOOTl .4(SCN2H,) is characterised by (T1' -4 thiourea) co-ordination columns with rnrn symmetry and is similar to those with 4/rn symmetry observed previously in thiourea complexes of other thallous salts.'24 Natural Products.-Three chlorinated metabolites having a structural similarity to terrein have been isolated from fermentations of Spororrnia afinis Sac.Bomm and Rouss. X-Ray methods were used to determine the structure and absolute configuration as (29).' Laurencin (30) provides an example of strong inter- molecular -CH-..O hydrogen bonding in the solid state as revealed by i.r. absorption data. In the crystal the terminal ethynyl group of the side chain is involved in a bifurcated hydrogen bond (CH. -.O 3.29 and 3.22 A) with ether and acetate oxygen atoms of a neighbouring molecule.' 26 In 2-bromo-P-santonin bond lengths and valency angles do not differ from those found in 2-bromo-a- santonin but the C(13)- * C(6) and C(13).* C(8)intramolecular separations (3.17 and 3-07A) are distinctly smaller than those in the a-isomer (3.79 and 3.49A re~pectively).'~' That centre 3' in ( +)-cis-and (-)-trans-khellactone methyl 'I9 C.J. Fritchie jun. J. Chem. Sac. (A) 1969 1328. R. P. Shibaeva L. 0.Atovmyan and L. P. Rozenberg Chem. Comm. 1969 649. lZ1T. D. Sakore and H. M. Sobell J. Mol. Biol.,1969 43 77. D. Voet and A. Rich J. Amer. Chem. SOC.,1969 91 3069. R. Hulme and J. T. Szymanski Acta Cryst. 1969 B25,753. L. H. W. Verhoef and J. C. A. Boeyens Acta Cryst. 1969 B25 607. lZ5W. J. MaGahren J. H. van den Hende and L. A. Mitscher J. Amer. Chem. SOC.,1969 91 157.126 A. F. Cameron K. K. Cheung G. Ferguson and J. M. Robertson J. Chem. SOC.(B) 1969 559. '" P. Coggon and G. A. Sim J. Chem. SOC.(B) 1969 237. G.Ferguson ether is R-chiral not S as earlier assigned on the basis of the Freudenberg rule has been determined by the Horeau method and by anX-ray analysis of the bromo derivative (31).'28 0 MeO-Ei I CH,-CH II Me Me OH 'OMe The steroid derivative cholestan-3-spiro-2'-( 1,3-oxathiolan)-4-one has normal geometry but the A and oxathialone rings are both distorted owing to dipole interactions between oxygen atoms associated with these two rings. The oxathia- lone ring has an envelope conformation in which significant differences are observed between the two C-S (1.819,1.852A)and the two C-0 bonds (1.415 1-399A).29 22J3-Dibrom0-9P-ergost-4-en-3-one belongs to the 88,9/3,108 series with a cis B/C junction. This gives rise to an unusual twist-boat conformation of rings B and c which is explained in terms of torsional angles. The conformation of ring D is nearly a C(13) enve10pe.l~' The A-ring of 6P-bromoprogesterone is highly distorted because of the A4 double bond and the ketone oxygen O(3). Rings B and c are chair-shaped with ring D a distorted half chair.13' Precise details of the crystal structure of 16~-bromo-3~,17a-dihydroxy-5a-pregnan-l1 20-dione have been reported. There are two hydrogen bonds in the structure 3p OH. *ell keto oxygen 2.77 A and 17a OH..-3P OH (2.84 A). The packing of the molecule as influenced by the position of the heavy atom and side chain substituents is disc~ssed.'~~ An intramolecular hydrogen bond (0-H.* -0 2.649 A) occurs between the two 1,3-diaxial hydroxy-groups in c-caesalpin. 133 '*' H. Bernotatwulf A. Niggli L. Ulrich and H. Schmid Helu. Chirn. Ada 1969 52 1165. lZ9A.Cooper and D. A. Norton J. Org. Chern. 1968,33 3535. IJo B. Hesper H. J. Geise and C. Romers Rec. Trav. chirn. 1969 88 871. 13' E. M. Gopalakrishna A. Cooper and D. A. Norton Acta Cryst. 1969 B25,639. 13' J. M. Ohrt A. Cooper and D. A. Norton Acta Cryst. 1969 B25,41. 133 K.B. Birnbaum and G. Ferguson Acra Cryst. 1969 B25,720. X-Ray Crystallography The A-ring of 1l~,l2a-dibromo-3a,9-oxido-cholanic acid methyl ester is boat- shaped with the B-and c-rings chair-shaped.The mean plane of the atoms of the A-ring is almost perpendicular (92") to that of the B-ring and the mean planes of the B-,c-,and wrings are all nearly parallel to each other.'34 Differences in hydrogen bonding packing environment and intramolecular steric effects cause the two molecules in the asymmetric unit of oestriol to be non-identical. Steric hindrance between the c-ring equatorial hydrogen atom at C(1 l),and the hydrogen at C(l) produces twisting about the C(9)-C(10) bond in opposite directions in the two molecules causing the B-ring to take up a half-chair conformation in the first and a twist-boat conformation in the ~ec0nd.l~~ Ring c of the 17-bromobenzoate of 3-methoxy-8~-methyloestradiol has a normal chair form and ring D is represented Details of the crystal and molecular by a slightly deformed P-en~elope.'~~ structures of the steroid derivatives 8-azaoestrone hydrobromide,' photoiso-pyrocalciferyl rn-brornoben~oate,'~ a derivative of batrachotoxinin A a frog venom,'39 and of digit~xigenin'~'have been published.Cephalonic acid (32)is a congener of ophiobolins and is the fourth e~arnple'~' The of a C25 ter~en0id.l~~ side chain 2-methylhept-2-en-6-yl group is extended from C(14) of ring c in place of the side chain portion containing the C(14)-C(17) oxide bridge in ophiobolin A. 0 134 E. M. Gopalakrishna A. Cooper and D. A. Norton Acta Cryst. 1969 B25 143. lJ5 A.Cooper D. A. Norton and H. Hauptman Acta Cryst. 1969 B25 814. 13' Y.Tsukuda T.Sato M. Shiro and H. Koyama J. Chem. SOC.(B),1969 336. 13' R. Majeste and L. M. Trefonas J. Amer. Chem. SOC., 1969 91 1508. IJ8G.L. Hardgrove R. W. Duerst and L. D. Kispert J. Org. Chem. 1968,33 3293. IJ9 I. L. Karle and J. Karle Acta Cryst. 1969 B25 428. I. L.Karle and J. Karle Acta Cryst. 1969 B25. 434. 14' Ann. Reports (B) 1968 65 65. 14' A. Itai S. Nozoe S. Okuda and Y. Iitaka Acta Cryst. 1969 B25 872.

ISSN:0069-3030    

DOI:10.1039/OC9696600041

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

3 (Part i) Reaction Mechanisms By J. G. TILLETT Chemistry Department University of Essex Co fchester Acidity Functions and Molecular Basicity.-Boyd has given an up-to-date account of acidity functions and their application to mechanistic studies.' New acidity function data reported this year include values of Ho for HC104 in aqueous ethanolY2 Ho for HCl in ethanol3 and HMin solutions of sodium and potassium methoxides in methan01.~ The variation of the Ho acidity function for aqueous H2S04 with temperature has been ~tudied.~ The correlation of the composition of solutions of acids with activity and acidity functions has also been discussed.6 Reagan has used a series of substituted azulenes 1,l-diaryl- ethylenes and aromatic polyethers to obtain a new Hammett-type acidity function H .' The series used is thought to be free from specific solvation and to provide a better measure of carbon protonation than HR,.Comparison of the rates of proton exchange of the imidazolium ion in aqueous acid with the rates of relaxation subsequent to temperature jump shows that the acid dissociation of the ImH ion occurs in two distinct steps-ionization and dissociation' (Scheme 1). For a weak acid the rate constant for the reversal ImH' . OH (aq.) 6 Im . HO .(HO) . HOH (aq.) kii I I H LkH Im.HO(aq.)+ H3+0(aq.) I H Scheme 1 R. H. Boyd in Solute Solvent Interactions,' Edited by J. F. Coetzie and C. D. Ritchie Marcel Dekker. New York and London 1969. P. Vetesnik K. Kothschein J. Socha. and M. Veteia.Coll. Czech. Chern. Comm. 1969 34 1087. ' B. Nahlovsky and V. Chvalovsky Cull. Czc~h.Chenz. Comm. 1968,33. 3122. F. 'Terrier Bull. Soc. chim. France 1969 1894. C. D. Johnson. A. R. Katritzky and S. A. Shapiro J. Amer. Chem. Soc. 1969,91,6654. C. J. 0 Conner J. Chem. Educ. 1969. 46 687. ' M. T. Reagan J. Amer. Chem. Soc. 1969 91 5506. * E. K. Ralph and E. Grunwald. J. Amer. Chem. Soc. 1969. 91 2422. 60 J.G. Tillett of the ionization step (k21) is extremely high whilst the rate of dissociation is limited by the rate of diffusion. The ionized state (1)can be detected as a reactive intermediate because the N . HOH hydrogen bond is broken at a rate which is comparable to k23. Similar results were obtained for the hydrogen ion-catalysed proton exchange of imidazole in aqueous t-butyl alc~hol.~ The rates of proton transfer have also been determined for a number of indicator systems of high pK .' The observations were interpreted in terms of the hydrolysis reaction k, k,, OH-+ HIn" F HO-...HIn"-' * In"-' + H20 (1) k2 1 k, (2) (3) where the state (2) represents an 'encounter complex' in which the ionic species arc linked by the minimum number of water molecules prior to proton transfer.The observed rate constants were considerably less than those expected for diffusion-controlled reactions which k12 and kzl were thought to be. This was attributed to the existence of a finite chemical activation barrier arising from strong intramolecular hydrogen bonds. Under such conditions the process (2)+(3) becomes rate-determining (k23<< k21 and kobs = K12k23 where KI2= k12/k2').The Bronsted coefficient was found to be 1.0 in each case indicating that the transition state is very similar in structure to the products. The detritiation of 1,4-di~yano[l-~H]butene in water is a measurably slow base-catalysed reaction at 25 "C [equation (2)]. kl + H,C(CN) * CH=CH -CH,(CN) + B * H,C(CN). CH=CHCH(CN) + BH (2) k General base catalysis was observed with amine and phenolate bases (p = 0.98 and 0.94 respectively).' The primary hydrogen kinetic isotope effect kH/kD is 3.9. An approximate value of 21 was obtained for the pK of this cyanocarbon. Calculation of the rates of the reverse reaction (k,) of the carbanion with the conjugate acid species shows that these do in fact approach the diffusion limit.That they are not actually fully diffusion-controlled is thought to be indicated by the fact that the Bronsted slopes have not quite attained their limiting values (a= 0-05for phenols) and slight steric hindrance was observed. The slow process is considered to be a bimolecular proton abstraction. As with the indicator systems discussed above the bond between the base catalyst and the proton is almost completely formed in the transition state. Bordwell and his co-workers have studied base-initiated proton abstraction in the systems X .C6H4. CH2 -CH(Me)N02 and X. C6H4-CH(Me)N02.I2 [Equation (3).] In both cases a Bronsted coefficient larger than 1-0was obtained (a= 1.31 and 1.61respectively).The values of p for protonation of the conjugate 'E. K. Ralph and E. Grunwald J. Amer. Chem. SOC.,1969,91 2426. lo M. C. Rose and J. Stuchr J. Amer. Chem. SOC.,1968,90 7205. 'I E. A. Walters and F. A. Long J. Amer. Chem. SOC.,1969 91 3733. l2 F. G. Bordwell W. J. Boyle J. A. Hautala and K. C. Yee J. Amer. Chem. SOC.,1969 91. 4002. Reaction Mechanisms bases must be -0.31 and -0-61 respectively. Bordwell et a1. suggest that the apparent restriction of Bronsted coefficients to the range 0 to -1 arises because application of the Bronsted equation has usually been restricted to oxygen and k -R.CH(Me)NO2 + B S R.C(Me)N02 + BH' t 3) k-1 (R = X * C6H4CH2- X . C6H4-) nitrogen acids and bases for which the positions of equilibria are more sensitive to structural changes than are the rates (k and k-l).This is because in these acids and bases structural effects influence k and k-l in an opposite manner. For carbon acids however where extensive structural reorganisation often occurs on anion formation substituents may affect kl and k-in the same manner and Bronsted coefficients beyond the 0 to + 1 range may be observed. The corollary to this observation is that the use of the Bronsted coefficient as a measure of the position of the transition state along the reaction co-ordinate may have to be modified. The equilibrium constants for the reactions between N-benzyl-N-methyl- piperidinium halides and pyridine and NN-dimethylpiperidinium bromide and pyridine [equation (5)]have been determined to provide a measure of the carbon basicity of nitrogen.' Combination of the results with earlier equilibrium data [equation (4)] shows that whereas N-methylpiperidine is a stronger base than pyridine towards a proton by a factor of 2.5 x lo6,the ratio of carbon basicities of N-methylpiperidine compared with pyridine is much smaller and is 1.7 x lo2 and 1-4 x lo3 for the N-benzyl-N-methylpiperidinium and NN-dimethyl-piperidinium reactions respectively.(4) + + ox-0=c)ox-N N N Me/N' I I H Me H -1 I I R' 'Me R Me (R = Bz Me; X = C1 Br) l3 R.E. J. Hutchinson and D. S. Tarbell J. Org. Chem. 1969 34 66. J. G. Tillett Protonation studies on sulph~xides,'~*' substituted anilines,'6-1 unsaturated ketones,' azo-compounds,20 sulphonamides,2 and sulphonates" have been reported.Streitwieser and his co-workers have published further work on the acidities of alicyclic compound^'^-'^ and an attempt has been made to correlate C-H acidity with polarographic parameters.26 The protonation equilibria of substituted pyridines2' and aromatic hydrocarbons in excited states2 * have also been examined. Acid-Base Catalysis.-Carboxylic Esters Ethers Acetals and Related Compounds. Jencks has reviewed the mechanisms of catalysis in chemical and bio-organic sy~tems.'~ The hydrolysis of vinyl ethers is normally subject to general acid cata- lysis. The hydrolysis of 4-methoxybut-3-en-2-one, however is specifically acid- catalysed3' (Scheme 2). The solvent deuterium isotope effect (kD2O/kHz0 = 2.08) i Me0 .CH=CH. CO Me + H30 + H,O 1slow [Me- (f=CH.CHO OH +-+ Me. C.CHI. CHO II 10 OHIMe0 .CH .CH=C. Me IOH Scheme 2 and entropy of activation (AS* = -26.0 e.u.) are consistent with the suggested A-2 mechanism. Continuing earlier work Long and Huang have studied the decarboxylation of azulene-1-carboxylic acid.31 Below 0.01~ acid (rate K [H+]) I' G. Wada Bull. Chem. SOC.Japan 1969,42 890. l5 N. C. Marziano G. Cimino U. Romano and R. C. Passerini Tetrahedron Letters 1969 2833. I' P. B. Bolton and F. M. Hall J. Chem. SOC.(B),1969 259. I' S. Shanmuganathan and N. Vanajakshi J. Indian Chem. Soc. 1969,46 79. "S. Shanmuganathan and N. Vanajakshi Current Sci. 1969 38 63. l9 R. I. Zalewski and G. E. Dunn Canad. J. Chem. 1969,47 2263.2u M. A. Hoefnagel A. van Veen and 9. M. Wepster Rec. Trau. chim. 1969.88 562. z1 P. 0. I. Virtanen and K. Heinamaki Suomen Kem. 1969,42 B 142. l2 R. L. Reeves and R. S. Kaiser J. Phys. Chem. 1969 73,2279. ''A. Streitwieser W. R. Young and R. A. Caldwell J. Amer. Chem. Soc. 1969 91 527. l4 A. Streitwieser R. A. Caldwell and W. R. Young J. Amer. Chem. SOC.,1969 91 529. 25 A. Streitwieser and W. R. Young J. Amer. Chem. SOC.,1969 91 529. 26 K. P. Butin A. N. Kashin 1. P. Beletskaya and 0.A. Reutov J. Organometallic. Chem. 1969 16 27. 27 J. C. Doty J. L. R. Williams and P. J. Grisdale Cunad. J. Chem. 1969 47 2355. 28 S. F. Mason and B. E. Smith J. Chem. SOC. (A),1969 325. 29 W. P. Jencks 'Catalysis in Chemistry and Enzymology' McGraw-Hill New York 1969.'O L. R. Fedor and J. Maughlin J. Amer. Chem. Soc. 1969,91 3594. 31 H. H. Huang and F. A. Long J. Amer. Chem. Soc. 1969 91 2872. Reaction Mechanisms 63 no 13Cisotope effect was observed whereas at higher acidities (rate independent of [H']) kl2/kI3 = 1-06. These observations and the deuterium solvent effect are consistent with a rate-determining protonation at low acidities (k and bond-fission of the ampholyte (4) at higher acidities (k,) (Scheme 3). The decar- + k + AzCOOH + H30 AzHCOOH + H20 k2 + A + AzHCOOH + HIO AzHCOO-+ H,+O 4 (4) AzH+CO1-kAAzH + C02 Scheme 3 boxylative dehydrations of erythro- and thrro-P-anisyl-P-hydroxy-a-phenylpro-pionic acids proceed at different rates in dilute aqueous H2S04.32The rate of interconversion of the diastereoisomers is less than the rate of decarboxylation and both give trans-4-methoxystilbene.The data is interpreted in terms of decarboxylation of the zwitterion (9,analogous to (4)above followed by. rapid loss of carbon dioxide (Scheme 4). General acid catalysis has been observed in the dehydration of 1-(2,4,6-trimethoxyphenyl)-2-phenylethanol in dichloroacetic acid buffers.33 This is considered to provide further evidence that the reverse process and the hydration of other trans-stilbenes as well as the dehydration of t-aryl-2-phenylethanols proceed by direct rate-determining proton transfers. The hydration of fumaric 2-and 4-pyridine~arboxaldehydes,~~ and acrylic acid3' have also been studied. OH 'OH1 I I Ar-CH-CH .COOH Ar .CH-CH . COO-I I Arl Ar' Ar H \/ 5 Ar . kH-CH. COO-. + H20 ,c=c I H \Arl Ar' Scheme 4 The hydrolysis of simple alkyl acetals proceeds by an A-1 mechanism and an attempt to observe general acid catalysis in the hydrolysis of benzaldehyde diethyl acetal benzophenone diethyl ketal and methyl 2,5-anhydro-a-~-arabinofurano-'' D. S. Noyce and E. C. McGoran J. Org. Chem. 1969 34 2558. 33 G. M. Loudon and D. S. Noyce J. Amer. Chem. Soc. 1969,91 1433. 34 J. L. Bada and S. L. Miller J. Amer. Chem. Soc. 1969 91. 3948. 35 Y.Pocker and J. E. Meany J. Phys. Chem. 1969 73 1857. 36 S. K. Bhattacharyya and C. K. Das J. Amer. Chem. Soc. 1969,91 67 15 J. G. Tillett side was unsu~cessful.~~ Capon and Anderson3* and De Wolfe and his co- worker~~~ have discussed the factors which determine the mechanisms of hydro-lysis of acetals ketals and the related orthoesters.The former workers concluded that the hydrolysis of mixed alkyl aryl acetals of benzaldehyde should show general acid catalysis and have shown that this is indeed the case for the hydrolysis of benzaldehyde methyl phenyl acetal in acetate (and other) buffers.38 A mecha-nism involving concerted displacement of the PhCH- -0Me group (Scheme 5)was r 6+ 1 AcOH + Ph-CH,OMe -+ Ph.CH 'OPh I OPh ' d-H--OAC 1 Ph . CHO + MeOH Z'Ph. CHfOMe + Ph-OH + AcO-H ;O Scheme 5 considered preferable to that involving a slow proton transfer to oxygen followed by rapid breakdown of the conjugate acid.Carboxy-group catalysis was ob- served in the hydrolysis of 2-~arboxyphenyl-~-~-glucoside, 2-carboxyphenyl-a-~-glucoside 2-methoxymethoxybenzoic acid and 8-methyoxymethoxy-1-naph-thoic acid.40 The solvent deuterium isotope effect (kHzo/kDZo= 1.43) and the rate-acceleration of a nitro-substituent at position 4 for the hydrolysis of 2-methoxymethoxybenzoic acid are consistent with a mechanism involving general acid catalysis (Scheme 6). The possibility of the alternative intramolecular OH + CH,A OMe COY -H+JH,O C CH,O + MeOH Scheme 6 nucleophilic catalysis was eliminated by showing that methoxymethyl salicylate and benzo-1,3-dioxan-4-one are not present as intermediates. De Wolfe and his co-workers were unable to observe general acid catalysis of the hydrolysis of '' B.Capon and M. C. Smith J. Chem. SOC.(B) 1969 1031. 38 E. Anderson and B. Capon J. Chem. SOC.(B),1969 1033. 39 R.H. De Wolfe K. M. Invanetich and N. F. Perry J. Org. Chem. 1969 34 848. 40 B. Capon M. C. Smith E. Anderson R. H. Dahm and G. H. Sankey J. Chem.SOC.(B) 1969 1038. Reaction Mechanisms 65 benzophenone acetal in formate buffers,39 an observation in accordance with that of Capon and Smith in acetate buffers.37 However an extensive study of the hydrolysis of this acetal in chloroacetate and dichloroacetate buffers in aqueous dioxan did reveal slight general acid catalysis.39 The hydrolyses of most of the ketals studied were catalysed by dichloroaceticacid whereas that of benzophenone diethyl acetal is also catalysed by monochloroacetic acid.A study of the acid- catalysed hydrolysis of a series of 2-(substituted phenyl)-l,3-oxathiolanesshows that reduction in the basicity of a dioxolan ring by substitution of one oxygen atom by sulphur is not sufficient to produce a clear-cut change in mechanism from A-1 to The hydrolysis of orthoformates and the formation of acetals from orthoformates have also been in~estigated.~~.~~ The hydrolysis of iso- propylidene acetals of some 2-pentuloses and 2-hexul0ses,~~ and the sodium dodecyl sulphate-catalysed hydrolysis of p-substituted benzaldehyde diethyl a~etals~~ have also been reported. From a comparative study of the hydrolysis of 1-methoxycyclohexene and of the enolization of cyclohexanone it has been deduced that the acid-catalysed enolization involves a pre-equilibrium protonation of the carbonyl oxygen followed by a rate-determining proton transfer from carbon to the general base.46 A study of the rates of enolization of ketones possessing one or two tertiary amino- groups has not revealed any evidence of a concerted intramolecular general acid and general base-catalysed mechanism.47 The correlation of rate with ho,the deuterium solvent effect (k:120/ky20= 0-36) and the entropy of activation (ASS = +0.7 e.u.) for the hydrolyses of several methyl esters of pseudo-8-aroyl-1-naphthoatesare consistent with an A-1 mechanism for which two alternative formulations can be envisaged (Scheme 7).48 In dilute sulphuric acid the hydrolysis of a-acetoxy-p-styrene behaves like a simple ester.49 The deuterium solvent effect (kH2O/kD2O = 0.75) and correlation of rate with acidity are as expected for an A-2 mechanism.In concentrations of sulphuric acid greater than 50 % however the rate of hydrolysis increases rapidly with increasing acid concentration and equilibrium protonation on carbonyl oxygen gives way to rate-determining protonation on carbon (kH2O/kDz0 = 3.25 in 69 % H2S04)(Scheme 8). Whereas the hydrolysis of t-butyl acetate occurs mainly by the AA1lmechanism the main pathway for the hydrolysis of t-butyl formate involves acyl-oxygen bond fission.50 An I80-tracer study of the acid-catalysed hydrolysis of vinyl acetate has confirmed the AA,2 mechani~m.'~ The methyl esters of aliphatic 4' T. H. Fife and L.K. Jao J. Amer. Chem. SOC.,1969,91,4217. 42 M. Price J. Adams C. Lagenauer,and E. H. Cordes J. Org. Chem. 1969 34 22. 43 J. W. Scheeren J. E. W. van Melick and R. J. F. Nivard Chem. Comm. 1969 2175. 44 R. S. Tipson B. F. West and R. F. Brady Carbohydrate Res. 1969 10 181. 45 R. B. Dunlop G. A. Ghanim and E. H. Cordes J. Phys. Chem. 1969 73 1898. 4b G. E. Lienhard and T-C. Wang J. Amer. Chem. SOC.,1969 91 1146. 47 J. K. Coward and T. C. Bruice J. Amer. Chem. SOC.,1969 91 5339. 48 D. P. Weeks and G. W. Zuorick J. Amer. Chem. SOC.,1969 91 477. 49 D. S. Noyce and R. M. Pollak J. Amer. Chem. SOC.,1969,91 119. 50 R. A. Fredlein and I. Lauder Austral. J. Chem. 1969 22 19. '' E. K. Euranto and L. Hautomeini Acta. Chem. Scad. 1969 23 1288. J. G.Tillett Ar MeO-C COOH slow Products f /‘ Scheme 7 Me. CNo + Hf ‘low //O -+ Me.C \ \ 0-C-Ar II CH2 Me 0 0 // ,y I Me-C OH S Me.C OHL \+ I OH-C-Ar ‘O’.; Ar I I I Me (Ar = p-NOz.C,H,-) Scheme 8 Reuction Mechanisms 67 carboxylic esters have been shown to hydrolyse by the AAc2 mechanism in 77 ”/ H2S04 and with the exception of some halogen-containing esters by the AA,l mechanism in 95.9 % H2S04.52 Further evidence for this latter mechanism comes from a study of the hydrolysis of aliphatic and alicyclic esters in 95”0 H2S04 for which a p value of -3.6 was obtained and kinetic acceleration for sterically-hindered esters was ~bserved.’~ Unimolecular alkyl-oxygen bond- fission (AA,l)has been observed for the hydrolysis of methyl benzoates with strongly electron-withdrawing substituent~.~~ The kinetics of the acid-catalysed hydrolysis of a number of substituted carbarnates have been studied.” The kinetic deuterium solvent effect indicates specific hydrogen ion catalysis.The dependence of rate on acidity substituent effects and the reversal of the order of the catalytic eflectiveness of H2S04 and HCIO confirm that the mechanism of hydrolysis changes from AAc2to AA,l with increasing acidity. 2-Acetylphenyl mesitoate (6) hydrolyses in aqueous ethanol 130 times faster than the 4-i~orner.~’ Information about the nature of the keto-group participa- tion was obtained by studying the solvolysis in anhydrous methanol-methoxide solutions. Under such conditions the simultaneous disappearance of both the ester- and keto-carbonyl groups of (6)was observed with the formation of mesitoic acid and the dimethyl acetal of 2-hydroxyacetophenone (Scheme 9).The rates of hydrolysis of amides in dilute acid solution and the enthalpies of activation obtained have been correlated with various Taft equation^.^' The effect of N-methylation on intramolecular nucleophilic participation of the amide-group on ester hydrolysis has been examined.58 The introduction ofa methyl group reduces the rate of formation of the imide-intermediate (7) but greatly increases the rate of its decomposition (Scheme 10). This is considered to involve general acid- catalysed attack of hydroxide ion (8) rather than the kinetically-equivalent general base-catalysed attack of water.Bender and Killian have reported the first example of intramolecular bifunc- tional general acid-base catalysis by two hydroxy-groups in the hydrolysis of a siinple ester.’“ The pH-rate profile for the hydrolysis of methyl 2,6-dihydroxy- benzoate is a typical bell shape. Two kinetically-indistinguishable mechanisms are possible either a concerted general acid-general base catalysed hydrolysis of the monoionised ester (9),or a hydroxide ion-catalysed hydrolysis of the undissoci- ated ester assisted by bifunctional general acid catalysis by the two adjacent hydroxy-groups (10). 52 A C. Hopkinson J. Chem. SOC.(B) 1969 861. ’’ H. van Bekkum H. M. A. Buurmans B. M. Wepster and A. M. van Wijk Rrc. Trav. dim.1969. 88 301. j4 A. C. Hopkinson J. Chem. SOC.(B) 1969 203. ’j V. C. Armstrong and R. B. Moodie J. Chem. SOC.(B) 1969 934. 5b H. D. Burrows and R. M. Topping Chem. Comm. 1969 904. 5’ P. D. Bolton and G. L. Jackson Austral. J. Chem. 1969 22 521. s8 R. M. Topping and D. E. Tutt J. Chem. SOC.(B),1969 104. s9 F. L. Killian and M. L. Bender Tetrahedron Letters 1969. 1255. J. G.Tillett Me Me Mes -/Me*H anhyd 0Me-O MesC0,-+ Me-C OMe I I0 Scheme 9 0 Mes.C4 MesC/p MesC. \ MeNH '0 NMe I I Mes.C\ go NHMe 7 FyJ HO-. o=cfJ + 0 EtOH 0 p.0p.0 )$J)$J EtO/' OH Mes . C go\ + 8'0." NHMe Scheme 10 Reaction Mechanisms HO-//0 Bruice and Hegarty have reported the first example of an anionic nucleophilic displacement on a carboxy-anion6' [equation (6)] NUC-+ R.COO-+ HZO -+ RCONUC+ 2HO-(6) This remarkable reaction was observed in the cyclisation of N-(o-carboxyphenyl) urea in water to give 2,4-dihydroxyquinazoline (Scheme 11).Scheme 11 The importance of anhydride intermediates in neighbouring carboxy-group reactions continues to be studied. When salicyloylsalicylic-carboxy-'4C acid is recovered after 10 min. in water (pH 8 25") the isotope is equally distributed between the carbonyl groups61 (Scheme 12). The addition of sodium salicylate had no effect on the rate of equilibration or on the isotopic concentration and so the equilibration must occur intramolecularly via the anhydride intermediate (1 1).The reaction of salicyloylsalicylic acid in water is very slow compared to the rate of exchange. Analysis of the kinetic data for the reaction with hydrazine however shows that the products are formed by direct nucleophilic attack on the parent ''A. F. Hegarty and T. C. Bruice J. Amer. Chem. Soc. 1969 91 4925. 6' D. S. Kemp and T. D. Thibault J. Amer. Chem. Soc. 1969 91 7154. J. G. Tillett I X X 1 Scheme 12 compound and the anhydride plays no part in the reaction. This is attributed to the unlikelihood of cdrboxylates acting as nucleophiles in ester hydrolysis. This is likely to be the case in general but there may be specific instances where this is not so as discussed in the previous example.60 Fersht and Jencks have now reported the first direct observation by stop-flow spectrophotometry of the formation and disappearance of an acyl-pyridinium ion intermediate in the pyridine-catalysed hydrolysis of acetic anhydride in aqueous solution62 [equa- tion (711.Py + AczO Acpy + AcO-+ py + AcOH (7) Imidazole was found to catalyse the hydrolysis of N-methylphthalimide at pH values of 6.4and 7.2 but to inhibit the hydrolysis at pH 9.7.63 This was attributed to the reversible addition of imidazole to N-methylphthalimide to form a tetra- hedral intermediate which is relatively unreactive towards hydroxide ion (Scheme 13). The similarity between the second-order rate constants for the phosphate and imidazole-catalysed hydrolysis of N-methylphthalimide suggests that in this reaction imidazole acts as a general base catalyst.Such a mechanism has been confirmed for the hydrolysis of N-[2-(4-imida~olyl)ethyl]phthalirnide"~ (12). Bruice and Holmquist have obtained the first authenticated evidence for the Elcb mechanism of ester [equation (S)]. k,ZB SH S-+products k ,ZBH' ~ '' A. R. Fersht and W. P. Jencks J. Amer. Chem. SOC.,1969. 91 2125. O3 S. C. K. Su and J. A. Shafer J. Org. Chem. 1969 34 926. 64 S. C. K. Su and J. A. Shafer J. Org. Chem. 1969 34 291 1. 65 T. C. Bruice and B. Holmquist J. Amer. Chem. Soc. 1968,90 7138. 66 B. Holmquist and T. C. Bruice J. Amer. Chem. SOC.,1969 91 2993. Reaction Mechanisms 0 II 0;NMe fast \ II 0 0 II a)Me HNQNH II 0 Scheme 13 The pH-rate profiles for the hydrolysis of malonate esters possessing both enoliz- able hydrogens and good leaving groups (0-and p-nitrophenoxide) are unusual in the alkaline region and are characterised by accelerated rates and one or more plateaus.In the reaction with certain nucleophiles the rate becomes independent ofnucleophile at high nucleophile concentration. Rate accelerations of the order of lo4were observed only for esters containing or-hydrogens. The Elcb mechan- ism proposed involves general base-catalysed a-proton abstraction to form a resonance stabilised carbanion intermediate which decomposes to give a reactive keten (Scheme 14).The observations could not be accounted for on the basis of a tetrahedral intermediate. This study was also extended to examine the stabilising + effect of CN and MezS groups on a carbanionic intermediate.67 The rates of hydrolysis of o-nitrophenyl cyanoacetate and dimethylsulphonioacetatebecome independent of pH at high pH.The evidence suggests that the hydrolysis of the former compound and possibly also that of the latter proceeds via the spontaneous collapse of a carbanion to a reactive keten. The use of deuterium solvent isotope ''B. Holmquist and T. C. Bruice J. Amer. Chem. SOC.,1969 91 3003. J. G. Tillett 0 0 I1 II 0 .:.'.-II ,o _-'II EtO . C-CH-C-OAr EtO . C .CH2. C .OAr (Ar = 0-,or p-N02C6H40-) I Scheme 14 effects to distinguish between the Elcb mechanism of hydrolysis and that involv- ing the formation of a tetrahedral intermediate has been suggested.68 A study has been made of the effect of a-substituents on the rate of hydrolysis of o-nitrophenyl acetate esters in the pH range 1-12.53.69 The overall rate constant is the sum of the rates of the spontaneous general base-catalysed hydrolysis (kHzO) and the hydroxide ion-catalysed hydrolysis (koH-[HO-]).A plot of log kHzO versus log& for all the esters was found to be linear. The fact that esters containing formal positive charges also fall on the plot indicates that electrostatic effects on the nucleophilic displacement of o-nitrophenoxide by hydroxide ion is unimportant. When hydroxide ion is replaced however by other nucleophiles the positively charged esters exhibit abnormally rapid reactions with the anionic nucleophiles acetate phosphate and carbonate and slow reactions with various amines and glycine ethyl ester." These effects are attributed to electrostatic effects on the transition state.With the oxy-anions acetate HPOi- and CO',- the partial negative charge on the oxygen atom of the nucleophile can stabilise the transition state by interaction with the positive a-substituent (13). In a similar way the amine nucleophiles can destabilise the transition state (14). In the cases 06 -+ I XCH,. C .OR ~cH,.c.OR pjd + ?*-/'I\ R (13) (14) of imidazole and 2-aminopyridine whose rates correlate well with those of water for all esters the positive charge developed on nitrogen in the transition state can be delocalised so that electrostatic effects are relatively unimportant.Electrostatic effects have also been studied in the displacement reactions of benzoate esters with hydroxamate nu~leophiles.~' The first authenticated example of concurrent general acid and general base catalysis of esterification has been described.72 The rates of lactonization of some P. S. Tobias and F. J. Kezdy J. Amer. Chem. SOC.,1969 91 5172. 69 B. Holmquist and T. C. Bruice J. Amer. Chem. SOC.,1969 91 2982. '' B. Holmquist and T. C. Bruice J. Amer. Chem. SOC.,1969 91 2985. '' H. Kwart and H. Omura J. Org. Chem. 1969 34 318. l2 S. Milstein and L. A. Cohen J. Amer. Chem. SOC.,1969 91 4586. Reaction Mechanisms phenolic acids were found to have the kinetic form where ct is the fraction of undissociated acid at a given pH.Independent catalysis by both general acid and general base species was observed in a series of common buffers. The data obtained (including p values and Arrhenius parameters) could best be explained by mechanisms (15) and (16)in which rapid reversible formation of a tetrahedral intermediate is followed by its rate-limiting collapse to lactone. r* H H-A H-OH H A AqH-a Jo* C 0’ ‘CH, 0’”’ I X (15) The imidazole-catalysed hydrolysis of p-nitrophenyl N-benzyloxycarbonyl- glycinate involves nucleophilic catalysis by the base and the N-benzyloxycar- bonylaminoacetylimidazole intermediate has been detected spectrophoto-metri~ally.~ The hydrolysis of this ester is also catalysed by o-mercaptobenzoic acid. Nucleophilic attack by the dianionic species is considered to give the thiol ester (17) which can undergo intramolecular nucleophilic attack by the carboxy- late-anion to give the mixed anhydride (18)which rapidly hydrolyses (Scheme 15).0 II R . NH . CH,-C-OAr R . NH . CH,-C-S -I-ArO--02c (17) slow -s (R = C,H,CH,OCO-) Scheme 15 ” R. W. Hay and R. J. Tretheway Austral. J. Chem. 1969 22 109. J. G. Tillett The catalysis of the hydrolysis of p-nitrophenyl esters of amino acids by aromatic aldehydes and carbon dioxide has also been studied.74 The silver ion-catalysed hydrolysis of thiol esters is considered to involve a bimolecular mechanism75 (Scheme 16). The results for the mercuric ion-catalysed hydrolysis suggest a (R = H,p-OMe,p-NO,) Scheme 16 transition from a bimolecular to a unimolecular mechanism.The hydrolysis of the p-methoxy-ester (and possibly the unsubstituted ester) is envisaged as an AA,l process (Scheme 17). The metal ion-catalysed hydrolyses of the methyl esters of cy~teine~~ and the acid-catalysed hydrolyses of glycine~~**~~ and hi~tidine~~ Scheme 17 74 R. W. Hay and L. Main. Austral. J. Chem. 1969 22 155. 75 D. P N. Satchel1 and 1. M. Secemski Tetrahedron Letters 1969. 1991. 76 R. W. Hay and L. J. Porter J. Chem. SOC.(A) 1969 127. 7-R. W. Hay and P. J. Morris Chem. Comm. 1969 18. R. G. Lee D. A. Long and T. G. Truscott Trans. Faraduy SOC.,1969 65 503. -'R. G. Lee D. A Long. and T. G. Truscott Truns. Farirduy Soc. 1969. 65 820. Reaction Mechanisms 75 have also been studied.The hydrolysis of peptide linkages shows dramatic intra- molecular catalysis by a benzimidazolium group,80 1,2-dimethyl-5,7-dinitro-benzimidazolyl-4-alanylglycinehydrolyses 65,000 times more rapidly than 2,4-dinitrophenylalanylglycine. Bruice and Felton have reported the first example of intramolecular general base catalysis of an aminolysis reaction in water.81 The rate constants for the reactions of H,O OH- and a series of primary secondary and tertiary amines with 8-and 6-acetoxyquinoline were determined. A plot of log k for the reaction of the 8-isomer uersus that for the 6-isomer shows that the nucleophiles fall on two different lines. The bases falling on the line of slope 0.7 are those which are susceptible to general base catalysis whereas those falling on a line of slope 1.0 are not.In the reaction of water with 8-isomer the entropy of activation (TASt = -8.7 kcal mol- I) and deuterium solvent effect (kf1120/kF20= 2.35) are consistent with intramolecular general base catalysis. The effect of acyl substituents in the general base-catalysed aminolysis of esters has also been examined. 82 Phenyl salicylate reacts with n-butylamine considerably faster than phenyl o-methoxybenzoate in acetonitrile solution.83 The observed results are inter- preted in terms of an intermolecular general base-intramolecular general acid- catalysed mechanism. (Scheme 18). Consistent with this mechanism external H OPh I + __+ BuNH-C-OH + BuNH~ Bu Scheme 18 general acids do not catalyse the reaction.The aminolysis and amidinolysis of p-nitrophenyl acetate in chl~robenzene~~ and the aminolysis of phenyl acetate and disubstituted phenyl benzoates in diethyl ether8' have also been studied. The experimental data (including a value of a = 0) for the intramolecular aminolysis of S-acetylmercaptoethylamine above pH 2-3 is consistent with a simple proton transfer as the rate-determining step.86 The yield ofamine formed in the hydroly- sis of methyl [r-(et hyl thio)butylidene]phenylammoniuni tetrafl tiorohorate and *' K. t.Kirk and L. A. Cohen J. Org. Clirm. 1969. 34. 369. 8' T. C. Bruice and S. M. Felton. J. Amer. Chem. SOC.,1969 91 2799. 82 J. F. Kirsch and A. Kline. J. Amrr. Chem. Soc. 1969 91 1841. 8' F. M.Menger and J. H. Smith J. Amer. C'hem. Snc. 1969 91 5346. 84 H. Anderson C-W. Su and J. W. Watson J. Amer. Chcm. So(.. 1969,91,482. 85 D. P. N. Satchel1 and 1. I. Secemski J. Chem. SOC.(B) 1969. 130. 86 R. E. Barnett and W. P. Jencks. J. Amer. C'hem. Soc. 1969 91 2358. 76 J.G. Tillett methyl N-ethylthioacetimidate varies in a complex way which has been inter- preted in terms of two alternative mechani~rns.~~ The first involves three tetra- hedral intermediates (cationic neutral and anionic) in acid-base equilibrium and the second involves an anionic intermediate and two intermediates of zero net charge. Interconversion of the latter two species requires diffusion-controlled general acid-base catalysis. Further support for this latter mechanism comes from the observation that buffer catalysis of amine formation with a tetrahedral intermediate generated from ethyl thioacetimidate shows a Bronsted slope p of 0.54.The general base-catalysed hydrolysis of NN'-dimethyl-NN'-diphenylami-dinium salts has also been studied.88 The effect of bifunctional catalysts such as 2-pyridone on the aminolysis of 4-nitrophenyl acetate89 and benzyloxycarbonyl- L-phenylalanine-p-nitrophenylester" have also been examined. Strong nucleophiles such as diethylamine phenoxide. and 4-nitrophenoxide act as general base catalysts for the mutarotation of 2,3,4,6-tetramethyl-~- glucose.' Amine-phenol mixtures in organic solvents are considered to act as general base catalysts through the action of their conjugate acid-base ion pairs.Rony and his co-workers concluded that the evidence to date on the mutarotation of tetramethyl-D-glucose in benzene is consistent with the interpretation that there is not a concerted general acid-base reaction although the evidence does not rule out such a mechanism. The base-catalysed mutarotation of a-tetramethyl-D-glucose in mixed H,O-D,O solvents has been studied9 and weak stereospecifi- city in the mutarotation of this hexose catalysed by enantiomeric carboxylic acids has also been reported.93 Amidoximes are reported to be very reactive towards p-nitrophenyl acetate benzoyl fluoride and ethyl chl~roformate.~~ The results do not differentiate between intramolecular general base catalysis involving concomitant nucleo- philic attack of oxygen on the carbonyl group path (a),and hydrogen bonding to the carbonyl oxygen atom path (b),(Scheme 19).Other Reactions. The first preparation of an alicyclic sulphinic anhydride (19)has been re~orded.~' The hydrolysis of (19) is acid catalysed. The solvent isotope effect (kDzo/kHzo= 1.35) suggests an A-2 mechanism (Scheme 20). The function of the acid catalyst is to protonate the leaving group. The mechanism of the sulphide-catalysed disproportionation of aryl thiolsulphinates has also been in~estigated.~~ Recent work on diazo-compounds includes studies of substituent ''R. K. Chaturvidi and G. L. Schmir J. Amer. Chem. Soc. 1969 91 737. R. H. De Wolfe and M. W-L. Cheng J. Amer. Chem. SOC.,1969 91 2595. P. R. Rony J. Amer. Chem. Soc. 1969 91 6090.90 H. H. Huang A. N. H. Yeo and L. H. L. Chia J. Chem. Soc. (A),1969 836. " P. R. Rony W. E. McCormack and S. W. Winderley J. Amer. Chem. Soc. 1969 91 4244. '' N. Nakimizo Bull. Chem. SOC. Japan 1969,42 1071. 93 A. Kerzomard and M. Renard Tetrahedron Letters 1969 3041. y4 J. D. Aubort and R. F. Hudson Chem. Comm. 1969 1342. y5 J. L. Kice and K. Ikura J. Amer. Chem. Soc. 1968 90 7378. y6 J. L. Kice C. G. Venier G. B. Large and L. Heasley J. Amer. Chem. Soc. 1969 91 2028. Reaction Mechanisms 0-/OH + R2*COXC HZN<-?-/O / c-x R' /C=N RIACLN \\ H R2 / 11 Scheme 19 R.S-0-S.R II 80 + H,+O + R.S-0-S-RII I 0 OH t H20 (19) R .S-0-i. R + H20 '2 RS02+H2+ RS02H I1 I 0 OH (R = Bu') Scheme 20 effects on the acid-catalysed hydrolysis of arenesulphonyldiazomethanes97and of the acid-catalysed hydrolysis of o-diazoacetophenone and p-methoxy-o- diazoacetophenone in aqueous di~xan.~~ The acid-catalysed hydrolysis of 3-t- butylsydnone and 3-furfurylsydnone in contrast to that of 3-arylsydnones is thought to proceed by an A-1 me~hanism.~'A mechanism involving a double proton transfer has been suggested for the acid-catalysed hydrolysis of azobenzene 4-hydrogen sulphate'OO (Scheme 21) + Ph * N=N.C6H4.0S03- + H+ Ft Ph .NH=N. C6H4.OSO3- + + + Ph * NH=N * C6H4 * OS0,-+ H+ C Ph . NH=N- C6H4 OH. SO,-Ph . &H=N. C6H4. 6H. SO,-'9Ph . &H=N. C6H -OH + SO Scheme 21 Esters of Inorganic Oxy-acids. The hydrolyses of 2,4- and 2,6-dinitrophenyl phosphate differ from those of most simple phosphates in that the dianion rather 97 J.B. F. N. Engberts G. Zuidema B. Zwannenburg and J. Strating Rec. Trau. chim. 1969 88 641. 98 L. L. Leveson and C. W. Thomas J. Chern. Sac. (B) 1969 105I. 99 S.Aziz and J. G. Tillett Tetrahedron Letters 1969 2855. loo E. Buncel and W. M. J. Strachan Cunad. J. Chem. 1969,47 91 1. J. G. Tillett than the monoanion is the most reactive species. Oxygen-18 tracer experiments have established that the hydrolysis of this species in water proceeds with phos- phorus-xygen bond-fission.'O' Bunton and Farber have discussed possible causes of the rate maxima found for the acid-catalysed hydrolysis of weakly basic phosphate esters.lo2 The hydrolysis of bis-(2,4-dinitrophenyl)phosphate shows a peculiar anomaly in that whilst HClO shows a rate maximum HCI does not.'O3 The acid-catalysed hydrolyses of p-chloro- and p-bromo-phenyl ortho- phosphate^'^^ and 2- 3- and 4-pyridylmethyl phosphates"' have also been studied. The acid-catalysed hydrolysis of a-D-ribofuranose 1-phosphate is several hundred times faster than that of a-D-glucopyranose 1-phosphate and is considered to proceed by an A-1 rnechanism.'O6 An interesting result of this study is that apparently the order of effectiveness of added acids for the A-1 hydrolyses of acetals glucosides and glycosidic phosphates (HCI >,H2S04 or HC104) differs from those found for the A-1 hydrolyses of carboxylic esters and resembles that found for AAc2hydrolyses. An A-S,2 mechanism (Scheme 22) has Ph 0 \ II + 0 C=CH2 + H+ S (EtO),.P.O.CPh.Me II / (EtO) .P-0 JFI (20) (Et0),P02H + Me.C . Ph Scheme 22 been proposed for the hydrolysis of a-phenylvinyl diethyl phosphate (20).lo7 The hydrolysis of (20) also shows micellar catalysis. The action of hydrogen fluoride on a number of nucleotide and other phosphate esters has been com- pared with the normal acid-catalysed hydrolyses of such compounds. lo8 Reac-tion with 60 % hydrofluoric acid proceeds exclusively with phosphorus-oxygen bond-fission. The acid-catalysed hydrolyses of dialkyl methylphosphonates 'O9 and diethyl2-carboxyphenylphosphonate'lo have also been examined. Westheimer and his group have continued to provide further evidence of the significance of pseudorotation in the hydrolysis of phosphorus esters.The ethyl esters (21)and (22)of substituted bicyclic phosphinic acids undergo rapid hydroly- sis in acidic and basic solutions. ' ' The pH-rate profile for the hydrolysis of (21) and (22) shows that the rate is proportional to hydroxide-ion concentration at lo' C. A. Bunton and J. M. Hellyer J. Org. Chem. 1969 34 2798. Io2 C. A. Bunton and S. J. Farber J. Ore. Chem. 1969,34 3396. lo3 C. A. Bunton and S. J. Farber J. Org. Chem. 1969 34 767. Io4 M. M. Mhala M. D. Patwardhan and G. Kasturi Indian J. Chem.. 1969 7 145. '05Y. Murakami and M. Takagi J. Amer. Chem. SOC.,1969,91 5130. lob C. A. Bunton and E. Humeres J. Org. Chem. 1969 34 572. lo' C. A. Bunton and L. Robinson J. Amer. Chem. SOC.,1969,91 6072.lo* D. Lipkin B. E. Phillips and J. W. Abrell J. Org. Chem. 1969 34 1539. log J. I. G. Cadogan. D. Eastlick. F. Hampson and R. K. Mackie J. Chern. SOC.(B).1969 144. 'lo G. M. Blackburn and M. J. Brown J. Amer. Chem. SOC.,1969.91 525. ' R. Kiuger and F. H. Westheimer J. Amer. Chem. SOC.,1969 91 4143. Reaction Mechanisms EtO ,O P' O//P\OEt pH 8 independent of acidity near pH 5 and proportional to [Hf] around pH 2. The rate howc cr. then rises to a maximum at 0.2 M acid. It is suggested that the rate-limiting step for the overall reaction changes from the formation of a quin- quecovalent intermediate in dilute acid to pseudorotation in strong acid solutions and this latter phenomenon accounts for this rate maximum. Westheimer and his co-workers have also described a detailed examination of the pH rate profile for the hydrolysis of methyl ethylene phosphate."' The fraction of hydrolysis occurring with ring-opening path (a) and cleavage of the exocyclic methyl group path (h),was also determined (Scheme 23).In the pH region <4 the 0 ,o + MeOH O4 '\OH Scheme 23 percentage ofexocyclic cleavage falls to zero. It is considered that in this region of acidity as with the bicyclic phosphinates,' l1 the pseudorotation process itself becomes rate-limiting. It is assumed that whereas the rate of reaction of methyl ethylene phosphate with water to form a protonated hydrate intermediate (23) is acid-catalysed pseudorotation which takes place by interconversion of the deprotonated hydrate intermediates (24) and (25) occurs at a rate which is independent of acidity.On this basis the rate of ring cleavage via (23) would be expected to increase with increasing acidity since this does not require pseudo- rotation. The results are consistent with the view that since the rate of exocyclic cleavage is controlled by the slower of the processes [(a)attack of water at phos- phorus to form (23) and (b)pseudorotation as in equation (9)] as the acidity is I ' R. Kluger F. Covitz. E. Dennis and F. H. Westheimer J. Amer. Chem. Snc. 1969 91. 6066. J. G. Tillett ,;OH OH increased a point is reached where the rate of ring-opening exceeds the rate of pseudorotation and the yield of exocyclic products decreases. The reaction in the pH region 4 to 11 is assumed to take place through the monoanions (26)and (27).The anionic oxygen atom being more electron donat- ing than the electrically neutral oxygen atom of an OH or OMe group will take up an equatorial position. In strong base pseudorotation occurs between the diani- onic intermediates (28) and (29). Intermediate (29) will be more stable than (28) because it has both negative charges equatorial. This should cause rapid pseudo- rotation and hence facile exocyclic cleavage. A theoretical treatment of phos-phate esters113 is in accord with the main findings of Westheimer’s group and predicts that the intermediate (24)can undergo pseudorotation with a computed upper limit to the barrier of ca. 12-15 kcal mol- ’. -0 (28) (29) The concept of pseudorotation is further utilised in discussions of the hydrolyses of various phosphetanium’ 14-’ l6 and other phosphonium compounds.’ ”-’ 21 The base-catalysed hydrolyses of cis-and trans-1-ethoxy-3-methylthietanium ’’ D.B. Boyd J. Amer. Chem. Soc. 1969 91 1200. ‘I4 S. E. Cremer R. J. Chorvat and B. C. Trevidi Chem. Comm. 1969 769. I*’ J. R. Corfield J. R. Shutt and S. Trippett Chem. Comm. 1969 769. W. Hawes and S. Trippett J. Chem. SOC.(C) 1969 1466. I” D. W. Allen and I. T. Millar J. Chem. SOC.(C) 1969 252. ‘I8 D. W. Allen and I. T. Millar J. Chem. SOC.(0,1969 263. B. R. Ezzell and L. D. Freedman J. Org. Chem. 1969,34 1779. J. J. Brophy and M. J. Gallagher Austral. J. Chem. 1969 22 1385. J. J. Brophy and M. J. Gallagher Austral.J. Chem. 1969 22 1399. Reaction Mechanisms 81 hexachloroantimonates result in complete inversion of configuration suggesting that pseudorotation is much less ready in penta-co-ordinate sulphur compounds than in the phosphoranes.'22 The influence of leaving group abilitylZ3 and the effect of micellar catalysts on phosphorylation reactions' 24 have also been studied. Isotope Effects.-A comprehensive account of solvent isotope effects on equilibria and rates of reaction has been p~b1ished.l~~ The three most important factors which influence the magnitude of hydrogen isotope effects on proton transfer reactions are considered to be (a)the bending modes of the transition state (b)the stretching vibration of the transition state and (c)proton tunnelling.Bell has suggested that under certain conditions the contributions of proton tunnelling and of the symmetric stretching mode may cancel leaving factor (a)as the con- trolling factor in kinetic isotope effects.' 26 It has been argued however that this is not generally valid and only occurs under special circumstances. Since both factors (b)and (c)depend on the free energy change of the proton transfer step one factor tends to increase the magnitude of the isotope effect whilst the other tends to decrease it.'27 A further illustration of Bell's suggestion however has been found in the isotope effect for the transfer of a proton from hydrogen fluoride to ethylvinyl ether.12* The observed isotope effect (3.35 f0.05) is less than a quarter of the value estimated for uncompensated loss of the stretching vibration of hydrogen fluoride.The deuterium solvent effect and the Bronsted coefficient suggest a transition state (30)in which the proton lies midway between the water and ~ubstrate.'~' It was concluded that the low isotope effect could be directly ascribed to the absence of initial state bending vibrations in hydrogen fluoride leaving the effect of bending modes in the transition state uncompensated.12* 6+ 6+ [H20...H.-SIX (30) The first example of an inverse primary isotope effect for carbon (k"/k13 < 1) has been observed in the alcoholysis of 1-(p-methylpheny1)-1-bromoethane. 130 No apparent isotope effect could be observed for the Wolff rearrangement of ~arbonyl-'~Cand phenyl-1-' 4C labelled a-diazoacetophenone in t-butanol suggesting that the elimination of nitrogen and migration of the phenyl group occur in different steps13' (Scheme 24).122 R. Tang and K. Mislow J. Amer. Chem. SOC.,1969 91 5644. R. F. Hudson and R. Greenhalgh J. Chem. SOC. (B),1969 325. 124 C. A. Bunton L. Robinson and L. Sepulveda J. Amer. Chem. SOC.,1969 91 4813. P. M. Laughton and R. E. Robertson in 'Solute-Solvent Interactions' Marcel Dekker New York and London 1969. 1?6 R. P. Bell Trans. Faraday SOC.,1961 57 961. 12' D. B. Mathews Austral. J. Chem. 1969 22 463. 12' A. J. Kresge and Y. Chiang J. Amer. Chem. Soc. 1969 91 1025. lZy A. J. Kresge and Y. Chiang J. Chem. SOC.(B),1967 58. I3O J. Bron and J. B. Stothers Canad. J. Chem. 1969 47 2506.13' Y. Yukawa and T. Ibata Bull. Chem. SOC.Japun. 1969,42 802. J. G. Tillett [R . COC=N2-Ag&. Et3] -+ [R .COC-Agh3,] + N2 1 [R-C=C=O] + [AgNEt3]+ Scheme 24 Gold and Adsetts have studied the hydrochloric acid-catalysed tritium ex- change of me~ity1ene.l~~ Trimethoxybenzene (TMB) and mesitylene (M) are regarded as part of a series of substrates (S ... S,) of decreasing basicity which give rise to transition states for proton transfer which increasingly resemble the products [equations (10)+13)]. If the ratiof&/fs obeys the Setschenow equation. S + H,O' + [S,...H30lf(reactant-like T.S.) (10) it is suggested that equation (14) can be used to provide inforination about the resemblance of the transition state to the conjugate acid.The magnitude of A is a measure of the difference (A = 0 for S above) whilst a variable A indicates that the activity coefficient assumption is not valid. The hydrogen exchange of mesitylene correlates well with equation (14)with a value of A < 0.1. The rates of hydrogendeuterium exchange of 2-quinolonc at the 5- 6- and 8-positions are much more rapid whereas exchange at the 3-position IS CQ. 20 times slower than for the 4-q~inolone.'~~ It is suggested that the 2-hydroxy- group influences the benzene ring more than a 4-hydroxy-group because of the greater importance of p-quinoid structures of the type (3l) and because the electronic effect of the positive pole is more effectively reduced. Other studies I H '32 J. R. Adsetts and V. Gold J.Chem. SUC.(B),1969 950. ''I G. P. Bean A. R. Katritzky and A. Marzee Bull. Acad. polon. Sci. Ser. Sci. chim. 1968 16 453. Reaction Mechanisms reported of hydrogen-deuterium exchange include those of substituted phenyl- pyrylium salts,'34 methylisothiazoles,' 35 and tertiary amines.' 36 Katritzky and his co-workers have investigated the stereochemistry of base- catalysed hydrogen-deuterium exchange of a-sulphinyl protons in the conforma- tionally rigid cis-and trans-4-phenyltetrahydrothiopyran l-oxides (32) and (33).137Exchange was found to be stereospecific in water and [1-2H] methanol H H ph*s=o ph*i H H O (32) (33) but nonstereoselective in [1-'HI t-butyl alcohol and DMSO-methanol. The kinetics of exchange in [l-'H]methanol were interpreted in terms of two compet- ing pseudo-first-order reactions one for a-axial protons and one for the a-equatorial proton.The order of proton acidity adjacent to a sulphinyl group was found to be (u)trans to S=O and gauche to sulphur lone pair (b)gauche to S=O and to sulphur lone pair (c) gauche to S=O and trans to sulphur lone pair. Consistent with this another conformationally frozen sulphoxide 5,7-dihydro- 1,1l-dimethyldibenzo[c,e]thiepin6-oxide (34).shows a very slow rate of exchange for H' which is trans to the lone pair on sulphur,'38 and confirms earlier MO prediction^.'^^ The influence of steric interactions on isotope effects has been examined in the racemisation of ( -)[2,2'-'H2]-1 ,l'-binaphth~l.'~' Me Me (34) The significanceof the rule of the geometric mean in the treatment of thermo-dynamic and kinetic deuterium solvent isotope effect^'^' and the use of solvent 134 E.Gard. 1. Stanvio. A T. Balaban and F. Chiralen. RPU.Roumaine Chim. 1969 14 241. '35 J. A. White and R. C. Anderson J. Hcterocyclic Cliem. 1969 6. 199. 13' A. Kolbc TPtraiirdran Lelrcrs 1969. 1049. B. J. HutcIiin5on. K. K. Anderson and A. R. Katritzky. J. Amer. C/wtn.Soc. 1969. 91 3839. 13* R.R. Fraser and P. J. Schuber Chem. Comm. 1969 397. 139 A. Rank S. Wolfe and I. G. Csizmadia Canad. J. Chem. 1969 47 113. loo R. E. Carter and L. Dahlgren Acta. Chem. Scand. 1969. 23,504. 14' V. Gold. Trans. Fat-adccy SOC.,1969 65 2770. J. G. Tillett isotope effects as a mechanistic ~riterion'~~-'~~ have also been discussed.A comparison of solvent isotope effects with entropies of activation for the basic methanolysis of esters indicates the relative importance of changes in both internal vibrational frequencies and librational frequencies of solvent molecules in the origin of isotope effect^.'^' Secondary deuterium isotope effects in the Cope rearrangement,'46 the ionization of [2Hg]trimethylamine,'47 the trifluoro- acetolysis of isopropyltoluene-p-sulphonate the Bunte salt reaction of methyl halides,'49 and the formation of diazetidine from azodicarboxylates and vinyl ether^''^ have also been reported. Substituent Effects and Linear Free-energy Relationships.-There is a continuing interest in the relative importance of the direct and field effect components of the inductive effect.The differences in the calculated acidities (CND0/2 method) of 4-substituted bicyclo-octane carboxylic acids (equation 15) are satisfactorily accounted for in terms of a direct field effect alone." 'This method has also been used to calculate a a; scale of substituent n-delocalisation parameters.' 52 The good correlation of the rates of acetolysis of 4-substituted cyclohexyl methane- sulphonates with a field effect model again indicates the negligible influence of direct inductive effect^.''^ Changes in the differences in chemical shifts for P-protons in the 'H n.m.r. spectra of 4-substituted styrenes have also been ex- plained on the basis of an electric field effect although the a-proton shifts in this system could not be satisfactorily rati~nalised.'~~ A 'H n.m.r.determination of 0," constants and examination of direct polar effects in fluorobenzene derivatives have also been described.' ''The chemical shifts of the ring protons of mono- substituted mesitylenes and durenes and of the 10-protons of 9-substituted tripty- cenes and anthracenes show a similar dependence on solvent which increases with the dielectric constant of the rnedium.ls6 This cannot be attributed to a 14* B. D. Batts and V. Gold J. Chem. Sor. (A),1969 984. 143 W. J. Albery and M. H. Davies Trans. Faraday SOC.,1969 65 1066. 144 W.J. Albery and B. H. Robinson Trans. Faraday SOC.,1969,65 1623. 145 C. G. Mitton M. Gresner and R. L. Schowen J. Amer. Chem. SOC.,1969 81 2045. 14' K.Hunnski T. Strelkov S. Borice and D. E. Sunko Chem. Comm. 1969 693. 14' N. Northcott and R. E. Robertson J. Phys. Chem. 1969 73 1559. 148 A. Streitwieser and G. A. Dafforn Tetrahedron Letters 1969 1263. 14y G. E. Jackson and K. T. Leffek Canad. J. Chem. 1969,47 1537. lSo E. K. von Gustorf D. V. White J. Leitch and D. Hennenberg Tetrahedron Letters 1969 31 13. 15' R. B. Hermann J. Amer. Chem. SOC.,1969,91 3152. 152 R. T. C. Brownless and R. W. Taft J. Amer. Chem. SOC.,1968,90 6537. 153 D. S. Noyce B. N. Bastian P. T. S. Lan R. S. Monson and B. Weinstein J. Org.Chem. 1969 34 1247. 54 G. K. Hamer and W. F. Reynolds Canad. J. Chem. 1968,46 381 3. 155 G. P. Syrova V. F. Bystrov V. V. Orda and L. M. Yagupol'ski Zhur. obshchei Khim. 1969 39 1395. 156 K.Bowden J. G. Irving and M. J. Price Canad. J. Chem. 1968 46 3903. Reaction Mechanisms field effect through the medium but is considered to arise from hydrogen-bonding interactions between the C-H bond and the solvent. The substituent chemical shifts arise from a combination of substituent field resonance magnetic aniso- tropy and solvent effects. The difficulties of unravelling all these factors are emphasised in a study of the ‘H n.m.r. shifts of benzyl fluorides chlorides and bromides. ’’’ The determination of ortho-substituent constants (a;) has been reported for a system which through solvation effects is thought to be free from almost all proximity interacti~ns.”~ The relative chemicai shifts of OH in ortho-substituted phenols in DMSO are linearly related to those for para- substituted phenols.The strong intermolecular hydrogen bond between the phenolic OH and the solvent is apparently oriented away from a single ortho-substituent so that proximity effects are nominal. This is confirmed by the observation that as expected 2,6-disubstituted phenols do show anomalies due to steric effects. Attempts have also been made to correlate methyl [I3C]-H coupling constants and chemical shifts with Hammett a constants.’ 59 A comparison of Huckel and extended-Huckel treatments of the bonding in toluene has shown the necessity for including both hyperconjugative and induc- tive interactions between the ring and the methyl group.’60 Hyne and Greidanus have concluded from a ‘H n.m.r. study of intramolecular electronic effects in p-disubstituted-diphenyl sulphides sulphoxides and sulphones that there is a transmission mechanism across the sulphide bridge which is absent in sulphoxide sulphone and methylene bridges.16’ The relative magnitude of conjugative inductive and field effects of the three sulphurcontaining bridges has also been discussed.162 Stereospecific transmission across the sulphoxide bridge in a-methylbenzyl p-substituted phenyl sulphoxides has been observed. 163 The ionization and esterification with diazodiphenylmethane of a series of 9-substituted triptoic acids (35) has been studied.’64 The observed effects were found to be consistent with a field effect model. It seems likely that the bulky aromatic groups tend to exclude the solvent from the “wings” of the cavity x (35) 15’ T.Yokayama G. R. Wiley and S. I. Miller J. Org. Chem. 1969 34 1859. ”* M. T. Tribble and J. G. Traynham J. Amer. Chem. SOC.,1969,91 379. ’ 59 C. H. Yoder R. H. Tuck and R. E. Hess J. Amer. Chem. SOC.,1969,91 539. I6O D. Purins and M. Karplus J. Amer. Chem. SOC.,1968 90 6275. J. B. Hyne and J. W. Greidanus Canad. J. Chem. 1969,47,803. 16’ C. Y. Meyers Gazzetta 1969,99 1206. 16’ M. Nishio Chem. Comm. 1969 560. K. Bowden and D. C. Parkin Canad. J. Chem. 1969,47. 177. 86 J. G.Tillett transmitting the field effect. Similar results have been obtained for the esterifica- tion of 9-substituted 10-anthroic and 8-substituted 1-naphthoic acids.' 65 The reactions of diazodiphenylmethane with rnetu-and para-substituted phenylacetic acids and with ortho-substituted benzoic acids have been studied by Chapman and Shorter and their co-w~rkers.'~~,'~~ In the latter reaction the retarding influence of an ortho-methyl group which varies with the solvent is considered to arise from a polar effect combined with a steric effect on solvation.Charton has shown that Taft's E values are a linear function of the van der Waals radii and are independent of electrical effects.168 On the other hand E," values for ortho-substituents appear to be completely independent of steric bulk. He therefore concludes that the ortho effect for most substituents is a polar rather than a steric effect. This was confirmed by studies of the rates of acid-catalysed hydrolysis of methyl benzoates and acid-catalysed esterification of benzoic acids,16' and of the alkaline hydrolysis of methyl and ethyl benzoates and methanolysis of 1-menthyl benzoates.' 70 The Taft-Ingold equation has also been used by other workers to correlate the hydrolysis and ionization reactions of arylaliphatic carboxylic acids.Substituent effects at halogen oxygen sulphur nitrogen or phosphorus can all be correlated by substituent constants derived for compounds substituted at carbon.17**' 73 Arrhenius parameters for the hydrolysis of alkyl esters of naphthoic anthroic and phenanthroic acids have been ob- tai~~ed."~ Hammett constants have been used to correlate the ionization of vinylene compounds' 75 and 2-phenylben~otriazole~~~~ amino proton shifts and base strengths of substituted aniline~,'~~ and hydroxy and carbonyl i.r.fre-quencies. Electrophilic Aromatic Substitution.-Schofield and Moodie and their co-workers have reported further details of their studies on the nitration of reactive substrates. The nitration of mesitylene toluene and benzene show zeroth-order mixed-order and first-order kinetics in sulpholan. 79 First-order kinetics were obtained in aqueous sulpholan. In 7.5 % aqueous sulpholan and 15% aqueous nitromethane the rates of nitration of activated compounds reach a limiting value which is ascribed to the rate of encounter of nitronium ions and aromatic sub- strate. As a corollary it is suggested that the high substrate selectivities reported "j K. t3owden and D. C. Parkin Canad.J. Chem. 1969,41 185. Ihh N. B. Chapman J. R. Lee and J. Shorter J. Chem. SUC. (B) 1969 769. A. Buckley N. B. Chapman and J. Shorter J. Chem. SOC.(B),1969 195. '" M. Charton. J. Amer. Chem. SOC.,1969 91 615. lh9 M. Charton J. Amer. Chem. SUC. 1969 91 619. I7O M. Charton J. Amer. Chrm. SOC.,1969 91 624. 'I K. Bowden and R. C. Young Canad. J. Chem. 1969,47,2775. 172 M. Charton J. Org. Chem. 1969 34 1877. M. Charton J. Org. Chein. 1969 34 1882. 174 J. F. Corbett A. Feinstein P. H. Gore and E. C. Vignes J. Chrm. SOC.(B) 1969 974. M. Charton and B. I. Charton J. Org. Chem. 1969 34 72. 176 1. CepEinasky and J. Majer Cull. Czech. Chem. Comm.. 1969 34 72. 177 B. M. Lynch Tetrahedron Letters 1969 1357. N. Mori Y.Asano and Y. Tsuzuki Bull. Chem.SOC.Japan 1969,42 482. 179 J. G. Hoggett R. B. Moodie and K. Schofield J. Chem. SOC.(B),1969 I. Reaction Mechanisms by other workers for the nitration of anthanthrene and related compounds in acetic anhydride are anomalous and refer to nitration via nitrosation. The nitra- tions of NN-dimethylaniline N-oxide and of benzamide in 81-93 % H2S04have been shown to proceed through their conjugate acids.'80 Examination of the protonation equilibrium for acetophenone indicates that in the nitration of this substrate and of benzoic acid in H,SO (<90 %) the free base is the active species. The Exeter group has also published some preliminary results on the anomalous nitration behaviour of a number of reactive compounds in acetic anhydride.' 81 At least two mechanisms of nitration are considered to operate under the condi- tions used.Studies of the nitration of 2,4,6-trinitroaniline,' 82 2-nitro-1,4-di-t-butylbenzene,' 83 1-nitroanthraquinene '84 2,4,5-tri-isopropylacetanilide7'85 di-met hylnap ht halene ' 1,3,5-tri-phenyl benzene derivatives ' ' and substituted indoles' 88 have been published. The nitration of triptycene (36) gives mainly 2-nitrotriptycene and 2,6-and 2,7-dinitrotriptycenes.' 89 (Scheme 25). The pre- Scheme25 dominant nitration p to the bridge is considered to arise from an inductive effect. Transannular resonance and polarisation effects between the aromatic rings in this system appear to be absent. The orienting effect of positive poles changes from predominantly para-directing for the nitration of the benzyltrimethylphosphonium ion to mainly Is' R.B. Moodie J. R. Pention. and K. Schofield J. Chern. SOC. (B).1969 578. J. G.Hoggett R. B. Moodie and K. Schofield Chem. Comm. 1969 605. Zh. E. Grabooskaya and M. I. Vinnik Zhur.3~. Khim. 1969,43 901. A. J. Hoefnagel J. H. A. J. Nunnink A. van Veen P. E. Verkade. and B. M. Wepster Rec. Truo. chim. 1969 88 386. M. Adarncek L. Dole2alova and M. RemeS Chem. prfimysl 1969 19 255. A. J. Neale K. M. Davies and J. Ellis Tetrahedron 1969 25 1423. '86 A. Davies and K. D. Warren J. Chem. Soc. (B),1969 873. ''-G. P. Sharmin I. E. Moisak and E. E. Cryazin Zhur. org. Khim. 1969 5 1080. 88 P. NiSanjan and B. Aleksiev J.prakt. Chem. 1969 31 130. B. H. Klanderrnan and W. C. Perkins J.Org. Chem. 1969 34 630. J.G. Tillett meta-directing (50.7 %) for the benzyltriphenylphosphonium ion. From a cor- relation of electrophilic substitution data with the extended Hammett equation Charton has concluded that ortho-substituents have no significant steric effect in either halogenation or nitration (see also earlier discussion of this point) and that ortho:praratios depend only on the resonance effect of a sub~tituent.'~' The mechanism of halogenation of hexamethylbenzene by molecular chlorine and bromine has been formulated as in Scheme 26.192 The change in (X = Me or H) Products Scheme 26 emphasis on going from chlorination to bromination was rationalised in terms of (a)an increase in k-1 and (b)a decrease in the rate constant for decomposition to side-chain products k2.The rate-limiting step changes from formation of the Wheland intermediate to its decomposition to side-chain products. This can CHPhz Ph-CH.OH (37) (38) '')O T. A. Modio Bull. Acad. polon. Sci. Ser. Sci. chim. 1968 11 585. j9' M. Charton J. Org. Chem. 1969 34 278. E. Bacciocchi M. Casula H. Illuminati and L. Mandolin; Tetrahedron Letters 1969 1275. Reaction Mechanisms result in an interesting competition between dealkylation and side-chain halo- genati~n.'~~ Chlorination of (37) and (38) gave 93 and 97 % side-chain chlorina- tion. Thus despite a good leaving group the rearrangement of chlorine from nucleus to side-chain which occurs in a fast step is still predominant. On the other hand bromination leads to a quantitative yield of bromopentamethylben- zene (Scheme 27).In this case the decomposition of the benzenonium ion into side-chain products is completely overshadowed by the more ready dealkylation. Me Me ' Me Me Me Me CHPh2 Me Me@Me + Ph,CHBr Me Me Br Scheme 27 An indirect method has been used to estimate the partial rate factor metu to a nitro group (fZo2) for chlorination in acetic acid.'94 The chlorination of t- butylbenzene in 75 %acetic acid with chlorine acetate in the presence ofperchloric acid has also been studied.195 Both neutral and protonated chlorinating species take part in the reaction. The bromination of the phenyltrimethylammonium ion by hypochlorous acid in aqueous sulphuric acid gives 66 % meta- 33 para- and 2 % ~rtho-substitution.'~~ The figures for the corresponding arsenic positive pole are 90 % metu- 10% ortho-,and 3 % para-substitution.The results for nitrogen positive poles are interpreted in terms of an electrostatic model where part of the charge is transferred to the aromatic ring in the transition state but enough re- mains in the electrophile to lead to some electrostatic interaction opposing ortho- substitution. The substituent effects of other positive poles is consistent with the superposition of conjugative effects on an electrostatic model. 19' E. Bacciocchi G. Corrado and G. Illuminati Tetrahedron Letters 1969. 1279. 1940. M. H. el Dusouqui M. Hassan and B. Ibrahim J. Chem. SOC.(B) 1969 589. 195 M. Hassan and G.Yousif J. Chem. SOC.(B),1969 591. 196 A. Gastaminza J. K. Ridd and F. Roy J. Chem. SOC.(B) 1969 684. J. G. Tillett The chlorination of triphenylene (39) by molecular chlorine in acetic acid gives 1-and 2-chlorotriphenylene in the ratio 73:27 suggesting that at the 1-position steric hindrance is very small for a reagent of this size.lg7 A small amount of addition was found to accompany substitution. De la Mare and his co-workers have published details of a further study of the chlorination of phenanthrene.' 98 (39) The earlier assumption that cis-9-acetoxy-l0-chloro-9,l0-dihydrophenanthrene is one of the products has now been confirmed. The effect of added electrolytes and of change of solvent on the rate of chlorination and on the relative propor- tions of substitution and addition was determined.Comparison of these effects with phenanthrene with those for naphthalene led to the conclusion that the transition state leading to cis-addition of chlorine has considerable ionic character and is best formulated as (40). A detailed study of the chlorination and bromina- (40) tion of monosubstituted acenaphthalene has been re~orted.'~' The heats of formation in SbF3-HS03F at -62"for alkyl benzenonium ions fall in the order methyl > ethyl > isopropyl > t-butyl the effects being many times larger than those previously observed for series of this type.200 If as seems likely solvent and temperature effects are small and enthalpy changes give a direct measure of potential energy differences then the data are consistent with hyperconjugation being an important contributor to the observed Baker-Nathan order.Schubert and Gurka have concluded that whereas the formation of the benzenonium ion is not rate-controlling in the molecular bromination of neopentylbenzene t- butylbenzene and toluene in 78.3,86-1 and 93.3 %CF,COOH and for benzene in 93.3 CF3COOH the reversal of this step is more important in the bromination of benzene in more aqueous CF3C02H.201 The value offpMe reached a maximum in 93.3 % CF3C02H of 42,400 the highest activating effect so far observed for a 19' R. Bolton P. B. D. de la Mare and L. Main J. Chem. SOC.(B),1969 170. 19* P. B. D. de la Mare A. Singh. E. A. Johnson R. Koenigsberger J. S. Lomas V. S. del Olmo and A.M. Sexton J. Chem. Soc. (B).1969. 717. 199 P. R. Constantine L. W. Deady and R. D. Topsom J. Org. Chem. 1969 34 I1 13. E. M. Arnett and J. W. Larsen J. Amer. Chem. SOC.,1969 91. 1438. W. M. Schubert and D. F. Gurka J. Amer. Chcrn. SOC.,1969,91 1443. Reaction Mechanisms para-methyl group. No ortlzo-bromination of t-butylbenzene or meta-bromina- tion of any alkylbenzenes could be detected. Between 78.3 and 93.3 % CF3C02H the ratio kp-t-Bu/kp-Meincreased from 1.06 to 1.40 and kp-neopentyl/kp-Me from 0-13 to0.21. These observations areconsidered to be consistent with the Schubert- Sweeney hypothesis of greater steric hindrance to stabilisation of a polar transition state by specific non-nucleophilic solvation than of the non-polar ground state although alternative explanations are not excluded.In a related study Stock arid Himoe have reported on the chlorination of benzene toluene and t-butyl- benzene in carboxylic acid solvents.202 They concluded that whilst ground-state solvation was an important factor in determining the activating effects of alkyl groups such effects are also consistent with the differential contributions of C-H and C-C hyperconjugation to the Baker-Nathan effect. Reich and Cram have published fuller details of the electrophilic substitution of monosubstituted [2,2]paracyclophane~.~~~-~~~ The orientation data obtained was not consistent with a ground state electrostatic model based on transannular resonance interactions but could be correlated in terms of a rate-limiting and product-determining step in which the proton being substituted is transferred from a o-complex to the most basic carbon or substituent in the originally- substituted ring.The geometry of [2,2]paracyclophane is very favourable to proton transfers between rings and the aromatic nuclei are considered to be of base strength at least comparable to that of e.g. CH2C12. Furthermore the proximity of the rings hinders approach by external bases. Substituents that carry basic oxygen such as carbomethoxy direct entering substituents pseudo- gem by accepting the leaving group (Scheme 28). The polysubstitution of a-complex 1slow Scheme 28 lo' A. Himoe and L. M. Stock J. Amer. Chem. SOC.,1969 91 1452. '03 H. J. Reich and D. J. Cram J. Amer.Chem. SOC.,1969,91 3505. '04 H. J. Reich and D. J. Cram J. Amer. Chem. SOC., 1969,91 3517. '05 H. J. Reich and D. J. Cram J. Amer. Chem. SOC.,1969,91 3527. '06 H. J. Reich and D. J. Cram J. Amer. Chem. SOC., 1969 91 3534. J. G. Tillett [2,2]paracyclophanes was also examined. Caille and Corriu have repeated Olah and Kuhn's earlier work on the aluminium chloride-catalysed chlorination and bromination of aromatic substrates in nitrobenzene and nitromethane using a more accurate method of relative rate deterrninati~n.~" The rates parallel a-complex stability and it is not necessary to invoke a rate-determining step involving n-complex formation to explain the results. In order to provide further information about the relative reactivities towards electrophilic reagents of the a-and /?-positions in reduced benzocycloalkenes Eaborn and his co-workers have determined the relative rates of protodesilylation of o-xylene benzocyclobutene indan and tetralin208 (Scheme 29).For 1,2-Scheme 29 [Values of 103k (min-') for the cleavage of the aryltrimethylsilanes are shown against the appropriate positions of the parent hydrocarbons208] dihydrobenzocyclobutene the reactivity of the /?-position is the same as that of the corresponding position of o-xylene whereas the reactivity of the a-position is much reduced as is that of indan. The authors suggest that the relative reactivities of the a-and #%positions are determined by a combination of strain and carbon- hybridisation effects. As expected the inductive effect of a substituent plays a more important part than the resonance effect for ortho-substituents than for para-substituents in protodesilylation reactions.209 Protodetrimethylsilylation has been used to provide the first comprehensive relative order of deactivation by strongly electron-withdrawing substituents for electrophilic substitution and is + the same for both meta-and para-substituents NO > Me,N > S03H > CF3 > p-C02H > C02Me > C1.210 The substituent effects of positive poles on protodetrimethylsilylation are similar to those observed for nitration196 and are consistent with a strong inductive electron withdrawal and (p+d) bonding + in the case of $Me and to a lesser extent for ASM~,.~'' Protodetrimethyl- stannylation experiments show that the substituent effect of an ethynyl group in electrophilic substitution is consistent with that of a -I + A4 A study of the bromodestannylation of substituted benzene derivatives has also been reported.21 207 S.Y. Caille and R. J. P. Corriu. Tetrahedron 1969 91. 2005. A. R. Bassindale C. Eaborn and D. R. M. Walton J. Chem. SOC.(B) 1969 12. '09 C. Eaborn D. R. M. Walton and D. J. Young J. Chem. SOC.(B) 1969 15. 'lo C. Eaborn and P. M. Jackson J. Chem. SOC.(B) 1969 21. 211 C. Eaborn and J. F. R. Jaggard J. Chem. SOC.(B) 1969 892. "'C. Eaborn A. R. Thompson and D. R. M. Walton J. Chem. SOC.(B) 1969 859. P. Alexis and J. Nasielski J. Chim. phys. 1969 66 95. Reaction Mechanisms Cerfontain and his co-workers have published further data on the sulphonation of aromatic The sulphonation of 0-and rn-xylene in aqueous H2S04 is thought to proceed by two alternative mechanisms (a)and (b)shown in Schemes (30)and (31).At low concentrations of H2S04( <80 %) mechanism(a) provides the predominant pathway. In more concentrated acid (but <96 %) step (1) in mechanism (b) is rate-determining and the reaction proceeds via (3) S03H + ArH + H3S04 r.d.s. Ar+/ + H20 \H S03H so3-/ +/ Ar+ + HS04-S Ar + HzS04 \ \ H H Scheme 30 Scheme 31 and (4). As the sulphuric acid concentration is further increased however step (4) becomes partly rate-limiting and finally step (2) is rate-determining. Nucleophilic Aromatic Substitution.-A comprehensive account of nucleophilic aromatic substitution2' and a review on this topic218 were published during 1969.Meisenheimer complexes continue to attract interest. Their structures have '14 A. W. Kaandorp and H.Cerfontain Rec. Traa. chim.,1969 88 725. 'I5 A. J. Prinienand H. Cerfontain Rec. Trav. chim. 1969 88 833. 'I6 C. W. F. Kort and H. Cerfontain Rec. Trav. chim. 1969 88 860. '' J. Miller 'Nucleophilic Aromatic Substitution' Elsevier New York and Amsterdam 1969. 2'8 F. Pietra Quart. Rev. 1969 23 504. J. G. Tillett been deduced from 'H n.m.r. visible absorption and i.r. data often obtained under quite different conditions. To confirm that such spectroscopic data refer to the same species in all cases Norris has examined the 1,3,5trinitrobenzene- cyanide ion complex by such methods under very similar conditions.219 His observations were consistent with the o-complex (41) no evidence could be deduced for the existence of n-complexes (42) radical anion (43) or arylcarbanion (44)intermediates.Fendler and his co-workers have measured the rate constants for the formation and decomposition of the Meisenheimer complexes (49447) which were identified spectroscopically in situ and by isolation.220 The relative 02NQN02/ 02NQ--N02 NO NO2 (43) (44) (X= HO MeO EtO SO3- CN CH . CO. CH, CH3NH and C5H,,N). order of stability of the complexes is (47) > (45) > (46) the replacement of a para-nitro-group by a cyano-grovp has a much more serious effect on the stability of the complex than the corresponding replacement of an o-nitro-group.The rates of both the forward and reverse reactions for complex formation show a greater dependence on entropy rather than enthalpy of activation. In the in situ genera-tion of (45) and (46) from the reaction of methoxide ion with 2-cyano-4,6-dinitro- anisole and 4-cyano-2,6-dinitroanisole, in [2H,]DMS0 initially the formation of the 1,3-adducts (48) and (49) was observed. The 'H n.m.r. signal of these dis- appeared rapidly as conversion to the thermodynamically more stable 1,l-complexes occurred. The interaction of picric acid with aqueous sodium sulphite A. R. Norris J. Org. Chem. 1969 34 1486. 220 J. H. Fendler E. J. Fendler and C. E. Griffin J. org. Chem.. 1969 34,689. Reaction Mechanisnzs and sodium hydroxide has been studied.22 * In concentrated sodium sulphite solutions a 2 :1 sulphite :picric acid adduct is formed which is considered to have the unusual penta-anionic structure (50).At low sulphite concentrations a 1:1 adduct is formed (51). The visible spectrum ofa solution ofpicric acid and sodium hydroxide is very similar to that of (50)and suggests the diadduct structure (52). OMe OMe 0,No; 02N002.-H 0,N OMe CN OMe The addition of the dimsyl ion in DMSO to 1,3,5-trinitrobenzene leads to the formation of (53).222 An earlier report of the preparation of this complex is now thought to refer to (54) to which (53) is easily converted in the presence of traces of water. Illuminati and his co-workers have examined the effect of an aza-group in a heterocyclic ring on Meisenheimer complex f~rmation.~~~.~~~ The reaction of sodium methoxide with 3,5-dinitro-4-methoxypyridine in methanol and with 3,5-dinitro-2-methoxypyridinein methanol and in DMSO-methanol mixtures ‘’I M.R. Crampton and M. El-Ghariani J. Chem. SOC.(B) 1969 330. ’’C. A. Fyfe M. 1. Foreman and R. Foster Terruhedron Lerters 1969 1521. 223 P. Bemporad G. Illuminati and F. Stegal J. Amer. Chem. SOC.,1969 91 6742. z24 C. Abbolito. C. lavarone G. Illuminati F. Stegal and A. Vazzoler J. Amer. Chem. Soc. 1969 91 6746. J. G. Tillett leads to the formation of (55)and (56)respectively. The use of a semi-empirical theoretical procedure utilising bond-dissociation solvation and ionization energies to calculate the reactivity of amines towards neutral aromatic substrates in protic solvents has been des~ribed.”~ Athermo-chemical approach to the reaction of sodium thiophenoxide with 24-dinitro- fluorobenzene led to the conclusion that step (b)is rate-determining226 (Scheme 32).This is not in accord however with the conclusions of a study based on leaving-group ability.227 Since cleavage of the carbon-fluorine bond is known to be acid catalysed it is concluded since such an effect is absent in the above y (57) SPh Scheme 32 reaction that the rate-determining step is indeed the formation of (57) rather than its decomposition.228 The electrolytic and micellar effects on the reaction of 2,4-dinitrofluorobenzene with hydroxide ion are very similar to those observed with 2,4-dinitrochloro- benzene.229 Cationic micelles such as cetyltrimethylammonium bromide accelerate the reaction anionic micelles retard it and neutral micelles have almost no effect.Electrolytic effects on this system can be separated into effects on the activity coefficient of the substrate and on the relative activity coefficients of the hydroxide ion and the transition state. The relative reactivities of methoxide ion and methanol towards 4-chlorobenzyne have been determined from the variation 225 J. Miller Austral. J. Chem. 1969 22 921. 226 D. L. Hill K. C. Ho and J. Miller J. Chem. SOC.(B) 1966 299. 227 J. F. Bunnett and W. D. Merritt J. Amer. Chem. SOC.,1957. 79. 5967. 228 J. F. Bunnett and N. S. Nudelman J. Org. Chern. 1969 34 2038.229 C. A. Bunton and L. Robinson J. Org. Chem. 1969 34 780. Reaction Mechanisms 97 of the pura:metu ratio for chloroanisole with [NaOMe].230 Methoxide ion is estimated to be 157times as reactive as methanol towards the carbon in 4-chloro- benzene metu to chlorine and 70 times as reactive towards the position para to chlorine. The influence of steric and solvent on nucleophilic substitution has been further examined. The reactions of nucleophiles with l-cyan0-2-nitrophenazine,~~~ and 4-substituted 2-nitro- 3,4-dinitrothio~hen,~~~ fluorobenzenesz3’ have also been studied. 230 J. F. Bunnett and C. Pyun J. Org. Chem. 1969,34 2035. 23 ’ L. Di Nunno S. Florio and P. E. Todesco Boll. sci. Fac. Chim. ind. Bologna 1969,27 75. 232 G.Bartoli A. Lotrofa and P. E. Todesco Boll. sci. Fac. Chim. ind. Bologna 1969,27,79. 233 P. E. Todesco and G. Bartoli Boll. sci. Fac. Chim. ind. Bologna 1969 27 63. 234 S. M. Shein and A. V. Evstifeev Zhur. org. Khim. 1969 5 919. 235 S. Pietra G. Casiraghi and F. Rolla Gazretta 1969 99 665. 236 C. Dell’erba D. Spinelli and G. Leandri Cazzetta 1969 99 535. 237 J. F. Bunnett T. Kato and N. S. Nudelman J. Org. Chem. 1969 34 785.

ISSN:0069-3030    

DOI:10.1039/OC9696600059

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

3 (Part ii) Reaction Mechanisms By N. S. ISAACS Chemistry Departrnent The University Reading Stable Carbonium Ions.-The discovery of 'superacid' media mixtures of antimony pentafluoride with fluorosulphonic acid SO,CIF or HF has made pos- sible the study of an increasing spate of stable carbonium ions. The 13Cn.m.r. spectra (by the INDOR technique) of a large number of simple tertiary alkyl cations and oxocarbonium ions have been recorded,' and the established linear dependence of '3C-chemical shift with charge density applied to determine charge densities in these ions.2 Many expected trends are shown e.g. +CH(OH)2< fC(OH)3 (central carbon charge density) but Ph2+CH > Ph3+C (owing to non-planarity); scrambling of all carbons in the isopropyl cation is evident presumably proceeding via protonated cyclopropane ; the ions (1)-(3) are in rapid tautomeric equilibrium Me H Me Me H Me Me Me Me \+ I/ -\I +/ \+ / 'CH -CH,Me c-c c-c 2-YMe 7 /\ 4\ Me Me Me Me Me Me (1) (2) (3) The three isomeric t-hexyl cations (4a b c) can be observed in eq~ilibrium,~ the extent of which is somewhat solvent dependent while the t-heptyl cation (3, is deduced to undergo a rapid 1,3-hydride shift (or two 1,2-shifts) by the low- temperature broadening of the proton resonance as the process is slowed :4 Me I Me Me Me C Me Me Me \ / -\ /+\ / -v-r-CH2 CH2 CH2 /CH-c+ \ Me 'Me Me G.A. Olah and P. v. R. Schleyer 'Carbonium Ions' Interscience New York 1968. G. A. Olah and A. M. White J. Amer.Chem. SOC. 1969 91 5801. D. M. Brouwer Rec. Trav. chim. 1969 88 9. D. M. Brouwer and J. A. Van Doorn Rec. Trao. chim. 1969 88 573. Reaction Mechanisms A similar study has yielded activation parameters (EA = 11.4,log A = 13.2) for the isopropyl-n-propyl cation is~merisation~ and the isomeric interchange of the 2-butyl cation,6 which has been shown to be stable at -110°C The ionisation of isomeric 2,3-dimethylchlorocyclopropanes,(6a b c) in super- acid solution yields stereoisomeric 1,3-dimethylalkyl cations (7a b c) each with a distinct n.m.r. ~pectrum.~ The stereospecific ring-opening is thus shown to be concerted with ionisation and follows a disrotatory mode as predicted by the +Je Me Me (6c) (7c) Woodward-Hoffmann Hydride-ion transfer (e.g.from methylcyclo- pentane) to simple alkyl cations produced by ionisation of the halides gives the corresponding paraffins lo e.g. (8) while molecular hydrogen may also serve as the source of hydride ion:" Bu'+SbF,-+ Hz G Bu'-H + HSbF (8) the reverse reaction has also been demonstrated.' Hexamethyldewarbenzene M. Saunders and E. L. Hagen J. Amer. Chem. SOC. 1968 90 6881. 'M. Saunders E. L. Hagen and J. Rosenfeld J. Amer. Chem. SOC.,1968 90 6882. ' P. V. R. Schleyer T. M. Su M. Saunders. and J. C. Rosenfeld J. Amer. Chem. SOC. 1969 91 5174. * R. R. Woodward and R. Hoffmann Angew. Chem. Internat. Edn. 1969. D. T. Clark and G. Smale Tetrahedron 1969 13. lo G. M. Kramer J. Amer. Chem. SOC.,1969,91 4819. H. Hogeveen and C. J.Gaasbeek Rec. Trao. chim. 1969 88 719. lZ H. Hogeveen C. J. Gaasbeek and A. F. Bickell Rec. Trau. chim. 1969 88 703. 100 N. S. Isaacs (9) and hexamethylprismane (lo) are protonated in superacid media and form ultimately the hexamethylbenzenonium ion (1 l),by way of the isomeric bicyclo- [2,l,l]hexyl cations (12) and (13) (which are distinguishable16) and (14).13-" Sorensen and co-workers' have remarked on the greater stability of five-mem-bered allylic cations as against the six-membered analogues e.g.(1 5),(16) stable in moderately strong acids. Protonated cyclohexenones e.g. (17) likewise undergo (17) ring contraction,' although the corresponding protonated dienones are stable. Both 13C and 'H resonance spectra for the norbornyl cation in superacid media 'H.Hogeveen and H. C. Volger Rec. Trav. chim. 1968 87 1042. l4 L. A. Paquette G. R. Krow J. M. Bollinger and G. A. Olah J. Amer. Chem. SOC. 1968,90 7147. H. Hogeveen C. J. Gaasbeek and H. C. Volger Rec. Trav. chim. 1969 88 379. l6 H. Hogeveen and H. C. Volger Rec. Trav. chim. 1968,87 1047. T. S. Sorensen J. Amer. Chem. SOC.,1969,91 6398; P. M. Campbell N. W. K. Chin K. Dengan 1. J. Miller and T. S. Sorensen J. Amer. Chem. SOC.,1969 91 6404. '' H. Hogeveen Rec. Trav. chim. 1968 87 1295 1303. Reaction Mechanisms have been rep~rted'~ and concur with previous evidence from Raman spec- troscopy that this species is an edge-protonated nortricyclene (18). The 2-phenyl and 2,7-diphenylnorbornyI cations however show n.m.r.spectra which are consistent with the classical formulation2' e.g. (19) while 2-alkylnorbornyl carbonium ions (20) seem to show less advanced a-delocalisation than the parent ion.2' Protonation of the barrelene derivative (21) leads to a carbonium ion for which the nonclassical structure (22) has been advocated.22 2.94 Me Me 1.10 SbF HF A (2.38) H3.83 Me Me 1'99 5*7;21 2.38(2-94)~ An example of a stable oxycarbonyl carbonium ion23 is the red species (23) in which no doubt most of the charge resides on sulphur (24). Internal reflection spectroscopy has been of use in observing electronic spectra24of the methylbenzenonium ion (A,, 330 nm). Absorption at higher wavelengths is ascribed to products resulting from trms-alkylation. Bicyclo- [3,1,0]hexyl cations [e.g.(25)] may be observed by n.m.r.to undergo a five-fold degenerate rearrangement by a suprafacial 1,4-~hift,~ the C-6 methyl groups G. A. Olah and A. M. White J. Amer. Chem. SOC. 1969 91 3954 3956. 20 D. G. Farnum and G. Mehta J. Amer. Chem. SOC. 1969.91 3256. 'I G. A. Olah J. R. DeMember C. Y. Lui and A. M. White J. Amer. Chem. SOC.,1969 91 3958. 22 H. Hogeveen and C. J. Gaasbeek Rec. Trav. chim. 1969,88 367. 23 G. P. Nilles and R. D. Schuetz Tetrahedron Letters 1969 4313. 24 G. L. Roberts Appl. Spectroscopy 1969 23 165. 25 R. F. Childs and S. Winstein J. Amer. Chem. SOC. 1968 90 7146. R. F. Childs M. Sakai and S. Winstein J. Amer. Chem. SOC.,1968 90,7144. 102 N. S. Isaacs maintaining their identity as they migrate around the ring (25)-(26).At higher temperatures (25) isomerises to the benzenonium ion (27). Ionisation of 4-anisyl-1-chlorobutane gives the benzylic cation (28) presumably by a series of 1,Zhydride shifts rather than a bridged species (29).26 For a OMe OMe OMe number of di- and tri-arylcarbonium ions the stabilities (measured by the equilib- rium constant Ar3COH + Ph3C+ eAr3C+ + Ph3COH) have been corre-lated with the reduction potentials and hence the energies of the lowest filled molecule orbitals.27 Di-and tri-arylcarbonium ions result from the reaction of the halide or a 1,l-diarylethylene with SbC15 or SnC1,,28v29 e.g. Ar3C-Cl + SnCl -* Ar,C+SnC15-(P-M~OC~H~)~C=CH~ + SbC15 --* (p-MeOC,H,) + CCH2ClSbCI,-The very stable3' ferrocenyl carbonium ions (30)are not apparently distorted as in (31) since close similarities in stabilities and spectra are found between (32) and the bridged analogue (33).3 The stabilities of ions (30)increase in the order (30) (31) (32) z6 B.G. Ramsey and J. Cook Tetrahedron Letters 1969 535. 27 M. Feldman and W. F. Flythe J. Amer. Chem. SOC.,1969 91,4577. 28 K. M. Harmon L.-L. Hesse L. P. Klemann C. W. Kocher S. V. McKinley and A. E. Young Inorg. Chem. 1969,8 1054. 29 W. Bracke W. J. Cheng J. M. Pearson and M. Szwarc J. Amer. Chem. SOC.,1969 91 203. 'O E. A. Hill and R. Wiesner J. Amer. Chem. SOC.,1969 91 509. J. Feinberg and M. Rosenblum J. Amer. Chem. SOC.,1969 91 4324. Reaction Mechanisms 103 R = H < Me < Ph as expected. New work on aromatic cations includes identification of the tetramethylcyclobutenium dication (35),32and the octa- methyldihydropentalene dication (36).33 The equivalence of all the ring carbon atoms of the triphenylcyclopropenium cation has been demonstrated by the ring-opening of (37) to (38) in which the 14C label is equally located on carbons 1 2 and 3.34 Singlet 6.27 (36) (35) (a) SbF5 HFS03 SO2:-70 to -60 "C (37) (38) A wide variety of new heteroatom-substituted carbonium ions and related species have been reported mainly formed in SbF5 solutions.Protonated form- aldehyde (39) is characterised by very large geminal and trans-coupling con- stant~~~ while protonated thiocarboxylic acids exhibit the conformational equilibrium (40)(Z = S)36previously recorded for protonated carboxylic acids (40)(Z = 0)37 and for which E = 15.3 kcal mol- '.However dimethoxymethyl carbonium ion exists solely in the trans-form (41).38 The equilibria which occur H H \/ Y-,H \-Y-z Z Z F=? / / H2 H H (39) (a) (b) (4 J1.2 = 22.2 HZ (40) 52.3 = 21.1 51.3 = 8.7 R = H,Me(Z = S,a:b:c = 60:30:10%) 32 G. A. Olah J. M. Bollinger and A. M. White J. Amer. Chem. SOC., 1969 91 3667. 33 J. M. Bollinger and G. A. Olah J. Amer. Chem. SOC.,1969 91 3380. 34 I. A. D'Yakonov R. R. Kostikov and A. P. Molchanov Zhur. org. Khirn. 1969. 5 175. 35 A. M. White and G. A. Olah J. Amer. Chem. SOC.,1969 91 2943. 36 G. A. Olah A. T. Ku and A. M. White J. Org. Chem. 1969,34 1827. 37 H. Hogeveen Rec. Trav. chim. 1968 87 1313.38 Ch. H. V. Dusseau S. E. Schaafsma H. Steinberg and Th. J. De Boer Tetrahedron Letters 1969 467. 104 N. S. Isaacs Me \ 0 Me-C'+ k. Me O/ in superacid media and the subsequent fate of carbonium ions are often very complex. The former have been examined by 19Fresonance measurements which show evidence for the existence of H2S03Ff,HSbF, HSbzF,,,HS20,F and HS,O,F in mixtures of SbF and HS0,F (with or without SO,).39 Proto-nated ketones which have tertiary p-carbon atoms undergo fission at room temperature or above to give the appropriate tertiary carbonium ion,40 an example of carbon as a leaving group :e.g.(42). Protonated y-alkenones,however Me Me Me Me Me \ / I I I Me*;/-c,n,c< +c + - 'cb CH2 OH k =4.4 x ~o-~sec-~/ \ C -Me at 104 "C Me Me CH2 \OH Me (42) tend to cyclise to five-membered ring oxocarbonium ions (43).4 Multiply protonated imides [e.g.(44) (45)]have been observed42 and also protonated nitro compounds (46) which dissociate to carbonium ions. The crystal structure of acetyl hexafluoroantimonate (47) has been published.43 Q+ OH OH OH R-N +/ $0 . OH OH (45) 39 A Commeyras and G. A. Olah J. Amer. Chem. SOC.,1969 91 2929. 40 D. M. Brouwer and J. A. Van Doorn Tetrahedron Letters 1969 1353. 41 D. M. Brouwer Rec. Trav. chim. 1969 88 530. 42 G. A. Olah and T. E. Kiovsky J. Amer. Chem. SOC.,1968 90 6461 ; G. A. Olah and R. H. Schlosberg ibid. 1968 90 6464. 43 F. P. Boer J. Amer. Chem. Soc. 1968 90 6706. Reaction Mechanisms Solvolytic Reactions.-Solvolytic studies of esters of the norbornane series still command a great deal of attention the controversial matter being the extent to which ionisation is assisted by rearside n-or a-electrons e.g.(48)-+(48a) (49)-+ (49a). The subject has been reviewed by Win~tein,~~ a proponent of the non-classical representations (48a) (49a) of such ions which seems now to be well established for the 7-norbornenyl cation. Further support for the structure of this species comes from a comparison of the rates of acetolysis of the series (50),(51) (52). It is argued45 that the effect of the methyl groups is cumulative with the rate enhancement roughly equal for each additional methyl group. 1 13.3 148 Stabilisation of a non-classical cation (53) in this way is understandable whereas a classical ion (54) could derive stabilisation from only one such group introduc- tion of a second being expected to have little effect on the ratio of solvolysis.A rather large secondary deuterium isotope effect (13 %) for solvolysis of (55) (X = H) and (55) (X = D) indicates a considerable change in hybridisation of 44 S. Winstein Quart. Rev. 1969 23 141. 45 P. G. Gassman and D. S. Patton J. Amer. Chem. SOC., 1969,91 2160. 106 N. S. Isaacs C-7 during ionisation of the toluene-p-sulphonate (OTos) group and seems to fit the non-classical formulation (48a) better than a structure (56) in which there is formally no such change.46 The enormous solvolytic rate enhancement (a factor of 10") between 7- norborriyl(57) (X = H) and anti-7-norbornenyl esters (48) is reduced by a factor of 3 by satisfying the electronic demands of the developing cationic centre by a 7-anisyl group (57) (X =P-M~OC,H,).~~ The same 7-norbornenyl cation (48a) observable by n.m.r.in superacid medium is obtained by ionisation of either 7-norbornenyl esters or the tricyclic isomers (58a) ;the latter compounds solvolyse 10' times faster than even the anti-7-norbornenyl OS02Ar ma) In the norbornyl series the structure of the intermediate is variously depicted as the non-classical ion (48a) the alternating classical tricyclic species (56) or the edge-protonated cyclopropane derivative (58). Apparently the same ion [formu- lated as (49a)l is obtained from the reaction of 2-(A3-cyclopenteny1)ethyltosylate (59) and thi~cyanate.,~ 46 R.Eliason M. Tomic S. Borcic and D. E. Sunko Chem. Comm. 1968 1490. 47 P. G. Gassman and A. F. Fentiman jun. J. Amer. Chem. Soc. 1969 91 1545; (a)J. Lhomme A. Diaz and S. Winstein J. Amer. Chem. Soc. 1969 91 1548. 48 L. A. Spurlock and W. G. Cox J. Amer. Chem. Soc. 1969 91 2961. Reaction Mechanisms The solvolysis of exo-2-norbornyl esters shows a considerable secondary deuterium isotope effect from C-6 (60) (either cxo-or end~-~H) whereas the corresponding endo-esters show a negligible effect.49 If an effect of this magni- tudz (10 %) is held to denote a hybridisation change at the carbon atom to which the deuterium is bound then it follows that o-participation occurs for the ex0 but not the endo ester in agreement with earlier kinetic interpretations.Indeed a significant (2%) y-isotope effect is found in the related endo-2-methyl com- pound (61) normally considered to solvolyse without assistance but which may indicate a proportion of involvement by the 1-6 bond.” /3-Isotope effects amounting to values between 1.16-1.31 have been obtained for both exo- and endo-isomers of types (60),(61) (deuteriated at C-3).51The complete separation = kH/kD = 1-02 b.X = D of electronic and steric factors in such compounds is always problematical and the choice of appropriate models can emphasise the role of either. Thus Brown and co-workers claim with justification the operation of steric hindrance in the follow- ing system.Abnormally high rates of solvolysis for exo-(62) as compared with endo-(62a) isomers are rep~rted.~*,~~ Little o-participation would be expected in these tertiary esters the difference in rate being ascribed to relief and increase of steric compression respectively upon ionisation. On the other hand abnormally low exo :endoratios are found in (63)presumably because of depression of the exo Me Me Me gk ..-49 H. L. Goering and K. Humski J. Amer. Chem. Soc. 1969,91 4594. 50 H. L. Goering and K. Humski J. Amer. Chem. SOC.,1969 91,4595. 51 J. M. Jerkunica S. Borcic and D. E. Sunko Chem. Comm. 1968 1488. 52 S. Ikegami D. L. V. Jagt and H. C. Brown J. Amer. Chem. Soc. 1968,90 7124. z3 H. C. Brown and S. Ikegaini J. Amer. Chem. SOC.,1968 90 7122.108 N. S. Isaacs rate through interactions between the leaving group and the syn-7-Me group. The solvolyses of the endo- and exo-isomers of (64)show striking difference^.^^ The products from the former retain the tritium label in the 2-endo position to the extent of 96% while the 2x0-ester leads to products in which the label is distributed mainly over C-2 C-5 (-6),and C-7. This is nicely accommodated by a non-classical cationic intermediate but the classical ion would be expected to randomise positions 2 and 3. A pronounced solvolytic rate enhancement of anti- (but not syn-) 7-tosyloxy- norbornan-2-one (65)compared to 7-norbornyl tosylate has been re~orded.~ The authors rationalise this in terms of rearside participation of n-electrons in the enol form.It is thus significant that non-enolisable ketones (66)do not show this effect and lead to ring-opened products.56 No such enhancement is apparent in either 7- or 5-keto-2-norbornyl ester solvolyses;57 in fact the exo:endo rate ratios are much smaller than those of the parent 2-norbornyl compounds which may be interpreted as the lack of assistance in ionisation of the keto-esters. kel 1 2 x 10-5 0.2 (AcOH) Little stabilisation of the (presumed) allylic intermediate from (67)by the endo- cyclic n-bonds is e~perienced,~ * as judged from the modest rate enhancement 54 C. C. Lee B. Hahn and L. K. M. Lam Tetrahedron Letters 1969 3049. 55 P. G. Gassman and J. M. Hornback Tetrahedron Letters 1969 1325; P. G. Gassman J. L. Marshall and J.M. Hornback J. Amer. Chem. SOC.,1969,91 581 1. 56 P. G. Gassman and J. M. Hornback J. Amer. Chem. SOC.,1969,91 5817. 5’ J. C. Greever and D. E. Gwynn Tetrahedron Letters 1969 813. 58 G. D. Sargent J. A. Hall M. J. Harrison W. H. Demisch and M. A. Schwartz J. Amer. Chem. SOC.,1969,91 2379. Reaction Mechanisms compared with (68). But 7c-participation does seem to be a function of ring puckering of the 5-ring in the bicyclo-(x 2,l) series," e.g. (69). yo2 ,s' LX X = toluene-p-sulphonyl (49) krel 1 2 x lo8 10" 5 x 1014 Solvolytic ratesof the four isomeric 7-chloro-2-norbornyl toluene-p-sulphonates were examined with the expectation that the inductive effect of the chlorine would be felt more by a non-classical ion bearing positive charge at C-1 than by a classical ion in which the positive centre is two bonds removed from chlorine.60 The exo:endo rate ratio would thus be expected to be lower than in the parent compound if the ex0 solvolyses by an assisted pathway and the endo by an unassisted one.In fact a syn-chlorine does not greatly affect this ratio although anti-chlorine reduces it by a factor of four. The system is probably quite compli- cated other workers for instance have shown that anti-chloro (70)is isomerised to syn-chloro (71) during solvolysis,61 presumably by ion-pair return and a 24 hydride shift. Further light on the nature of the 2-norbornyl cation is shed by a 220 MHz n.m.r. examination of the products of addition of DCl to norbornene6* to give exo-2-norbornyl chloride with the isotopic distribution indicated (72).c J!iOTos H;6 -B TosO& H H OTos-OTos-(71) 59 B. A. Hess jun. J. Amer. Chem. SOC. 1969 91 5657; S. Masamune S. Takada N. Nakatsuka R. Vukov and E. N. Cain J. Amer. Chem. SOC.,1969,91,4322. 6o P. G. Gassman J. L. Marshall J. G. Macmillan and J. M. Hornback J. Amer. Chem. SOC.,1969 91 4282. 6' P. G. Gassman and J. M. Hornback J. Amer. Chem. SOC. 1969 91 4280; H. L. Goering and M. J. Degani J. Amer. Chem. SOC.,1969 91 4506. 62 J. M. Brown and M. C. McIvor Chem. Comm. 1969 238. 110 N. S. Isaacs Clearly the intermediate from protonation of the hydrocarbon is not entirely symmetrical. The norbornene products can be largely accommodated by the bridged ion (ignoring the stereochemistry) by attack of C1- at a or b to give the 3-and 7-deuteriochlorides respectively and similarly by the classical representa- tion where the rearrangement must occur at a rate competitive with attachment of chloride.If as seems likely the initial product of protonation is the edge- protonated cyclopropane (58) it must revert to (56) or (48a) rapidly otherwise one would expect deuterium at C-6. The bis( toluene-p-sulphonate) (73) has been claimed,63 on kinetic evidence to solvolyse simultaneously giving rise to an intermediate with the bishomocyclo- butenium structure. The analogous benzo-compound does not appear to behave similarly.64 (73) The question of n-assistance in the ionisation of 2-exo-benzonorbornyl esters (74)has also been examined in detail in the past year previous results had indi- ~ated~~ high exolendo ratios (e.g.62,000 for X = Bros) and conjugative inter- actions for the 'homopara' position 6. In these as in other cases of mechanistic dichotomy the solvolyses are profitably viewed according to Winstein,66 as occurring by two competing mechanisms the total rate (k,)being partitioned into a n-assisted component via the bridged ion (75) available only to the exo ester (k,) and a direct (S,2) displacement by solvent (ks). The stereochemistry of these two processes is retention and inversion respectively. The electronic availability of the aryl group is dramatically demonstrated by the rates of acetolysis of 6-and 7-substituted (74) (krel:6-H,l; 6-OMe 178; b3 J.B. Lambert and A. G. Holcomb J. Amer. Chem. Soc. 1969.91 1572. "H. Tanida and T. Tsuchima Tetrahedron Letters 1969 3647. 65 N. S. Isaacs Ann. Reports (B) 1968 65 103. ''J. P. Dirlam A. Diaz S. Winstein W. P. Giddings and G. C. Hanson Tetrahedron Letters 1969 3133. Reaction Mechanisms 7-OMe 0.72; 6-N02 giving a correlation with of p = -3.26.67 The soIvoIysis of optically active (74) leads to optically inactive exo product,6Ha result compatible with either a symmetric intermediate (75) or a 1,Zhydride shift in the secondary cation (76). However the exo-but not the endo-esters show a considerable degree of ion-pair return (from the 4-fold difference in polarimetric to titrimetric rates of the former).69 Taken together these results are more consistently accommodated by the bridged ion.and ($YOAc (76) 2-Alkyl (or aryl) substituents should reduce the demand for n-assistance to ionisation and indeed for the series (77) the value of p drops to -1.9. This is still large and the correlation with 0' rather than 0 indicates there is still a considerable contribution (50-100 7;)from kA,as do the high em :endorate ratios [I 1,000for(77),(Z = OMe)]." Thisattenuation ofexo :endoratio withincreasing Z A Y (77) '' H. Tanida H. Ishitobi T. Irie and T. Tsushima J. Amer. Chem. Soc. 1969 91 4512. 68 J. P. Dirlam A. Diaz and S. Winstein Tetrahedron Letters 1969 3133. 69 J. P. Dirlam and S. Winstein J. Amer. Chem. Soc. 1969 91 5907. 'O J. P. Dirlam and S. Winstein J. Amer. Chem. SOC.,1969 91 5905.112 N. S. Isaacs size and donor power of the 2-end0 substituent (H > Me > Ph) probably reflects a combination of steric and electronic factors. It is reported by Brown7 and co-workers that the ex0 :endo rate ratios of the 2-aryl-6-methoxy series (78 Y = OMe) are sensitive to substituents in the aryl group the rates decreasing as donor substituents are added as one would predict for decreasing n:participa-tion. In the series (78 Y = H) this ratio is almost insensitive to substituents Q though the absolute rates vary7' over a range of 10'. This observation was interpreted as arguing against n-participation and is apparently at variance with other interpretations. exo:X = OCO-C6H4.N02;Z= Z endo:X = ;Z = OCO * CbH4. NO2 (78) The solvolytic rate ratio of syn- and anti-(79) (kSyn/kanti= 4-4 x lo4) points up the superior ability of the ethylenic bond to the benzo ring in stabilising the carbonium i~n.'~,'~ Both solvolyses lead to retained products and hence are presumably assisted processes.Low exo :endo ratios are reported7' for the compounds (80) in which the benzo rings are too far removed for effective participation. syn:X = H,Y = Br exo X = OTos Y = H anti:X = Br Y = H endo:X = H Y = OTos (79) (80) A high level of interest has been maintained in the nature of the intermediates produced in the solvolyses of 2-phenethyl esters (81). The controversy centres around the representation of these species as either phenonium ions (82)7" or rapidly equilibrating pairs of classical carbonium ions (S3)7 while the observed equivalence of the c1 and p carbon atoms under certain conditions can be accom- modated by either theory.The evidence of the non-classical representation as an intermediate rather than a transition state rests on several lines of evidence 71 H. C. Brown and K. Liu J. Amer. Chem. SOC.,1969,91 5909. 72 H. C. Brown S. Ikegami and K. Liu J. Amer. Chem. SOC.,1969,91 5909 591 1. 73 J. W. Wilt and P. J. Chenier J. Amer. Chem. SOC.,1968,90 7366. 74 S. J. Cristol and G. W. Nachtigall J. Amer. Chem. SOC.,1968 90 7132 7133. 75 R. Baker and T. J. Mason Chem. Comm. 1969 120. 76 D. J. Cram J. Amer. Chem. SOC.,1964 86 3767. 77 H. C. Brown K. J. Morgan and F. J. Chloupek J. Amet. Chem. SOC.,1965 87 2137. Reaction Mechanisms or \ +/ -\+ c-c./ y (83) 1 Products Cram78 and Win~tein~~ among others have observed a high degree of stereo- specificity (retention) in the reaction indicative of some form of rearside protection of the reaction site. Kinetic data have been held to support phenyl participation as will be exemplified below and evidence which suggests the transient existence of (82) has come from the direct observation (n.m.r.) of the ethylene p-anisonium ion (84)80in HS0,F-SbF and the isolation of the spiro alcohol (85) which is perhaps less easily envisaged as formed by process b than process a.81 In this connection recent observations of stable carbonium ions include the diastereoisomeric pair (86a) and (86b) which retain their configura- tion in HS03F.82 Ionisation of the halides (87) and (88) in SbF5 leads to stable carbonium ions whose n.m.r.spectra indicate the phenonium ion structure (89) OMe H foH-H2C-CH2 6-c c-C-I I /I /L OH OH ''D. J. Cram J. Amer. Chem. SOC.,1949 71 3863; 1952 74 2129. '' S. Winstein and K. C. Schreiber J. Amer. Chem. SOC.,1952 74 2165. 'O G. A. Olah M. B. Comisarow E. Naman-Worth and B. Ramsay J. Amer. Chem. Soc. 1967 89 71 I 5259. L. Eberson J. P. Petrovich R. Baird D. Dyckes and S. Winstein J. Amer. Chem. SOC.,1965 87 3504. 82 D. Chamot and W. H. Pirkle J. Amer. Chem. SOC.,1969 91 1569. 114 N. S. Isaacs for X = H a mixture of (89) and (90) the classical open ion for X = Me and (90) for X = OMe,83 a rather curious observation in view of kinetic evidence which indicates the propensity of the anisyl group especially towards participa- tion.4-p-Anisyl-chlorobutane (91) also ionises to an open ion (92) rather than to (93).84 X X Me c,> Me or Me \ c1 + OMe OMe A dissenting view against bridged ions has been expressed by Brown and co- worker~~~ who interpret kinetic data in terms of the classical ions (83). The main lines of their arguments criticise the necessity of invoking benzenonium ions in the solvolyses of compounds which are more highly substituted at the p carbon than the a since these should have a propensity for aryl migration. Furthermore in this reaction the usual criterion for neighbouring group participation the existence of a substantial rate enhancement fails.For instance a comparison of the solvolytic rates of 2-phenethyl tosylate with ethyl toluene-p-sulphonate as model compound in ethanol acetic acid and formic acid at 75" yields relative rates [k (phenethyl)/k (ethyl)] of 0.24 0-37 and 2-1 re~pectively.~~ While the rate depressing effects of /?-phenyl are evident such small accelerations are judged to be incommensurate with full phenyl participation although some weaker inter- action between phenyl and reaction site is conceded in order to explain the stereochemical results. A widely accepted view today of the mechanisms of these solvolyses is that the measured rates k, are a sum of two competing processes a solvent-assisted component k, and a phenyl-assisted one k,;86 [the relationship is k = Fk + k, where F is the fraction of the assisted pathway which leads to products 83 G.A. Olah M. B. Comisarow and C. J. Kim J. Amer. Chem. SOC.,1969,91 1458. B. G. Ramsey and J. Cook Tetrahedron Letters 1969 535. 85 S. Winstein and H. Marshall J. Amer. Chem. SOC.,1952 74. 1120. 86 S. Winstein C. R. Hindegren H. Marshall and L. L. Ingraham J. Amer. Clrem. SOC.,1953 75 147. Reaction Mechanisms (1 -F)leading to ion-pair return87]. Within the range of solvents usually used nucleophilicity and ionising power vary widely and in opposite direction the former (especially in ethanol acetic acid) favouring k and the latter (especially in formic acid trifluorethanol and trifluoracetic acid) favouring kA.88 Rate enhancement should show up in values of kA,which are predominant in the most limiting (ionising) solvents.It is therefore notable that Nordlander and Dead- man89 have published a value of k (phenethyl)/k (ethyl) of 3040 for the above system in unbuffered trifluoroacetic acid and an enhancement of 564 for 3-phenyl- 2-propyl toluene-p-sulphonate over the 2-propyl analogue in the same solvent.90 Deuterium labelling studies also showed that after one half-life the product contained completely scrambled positions at C and C while unreacted material showed barely any rearrangement. Less scrambling (15 %) was evidentg1 in the acetolysis of 2-phenethyl [1-14C]- toluene-p-sulphonate but this reaction and those of the p-chloro p-methyl and p-methoxy analogues could be dissected into k and kAcomponents X 107k 107kA PhCH2.‘‘CH,0T~~ C1 1.97 0-39 H 2-07 2.39 Me 2.0 19.0 OMe 3.2 228 The effects indicate that the effect of +M substituents is to increase greatly the assisted component whereas the solvent-promoted rate remains almost un-affected. It was also found that log kAfor the above reactions was linearly related to log k for the solvolysis of neophyl toluene-p-sulphonates for which there is considerable evidence that the reaction is largely assisted. A new approach to the problem has been presented by Schleyer et dg2 who have measured the acetolysis rates of 3-aryl-2-butyl toluene-p-sulphonates = p-C,H,CH,aryl. X Me’ OS02C,H4Me (94) ”A. Streitwieser and W. D. Schaefter J. Amer. Chem.SOC.,1951 79 6233; A. F. Diaz I. Lazdins and S. Winstein J. Amer. Chem. SOC.,1968 90 6546. J. A. Thompson and D. J. Cram J. Amer. Chem. SOC.,1969,91 1778. 89 J. E. Nordlander and W. G. Deadman J. Amer. Chem. SOC.,1968,90 1590. 90 J. E. Nordlander and W. J. Kelly J. Amer. Chem. SOC.,1969 91 996. 91 M. G. Jones and J. L. Coke J. Amer. Chem. SOC.,1969,91,4284. 92 C. J. Lancelot and P. v. R. Schleyer J. Amer. Chem. SOC.,1969 91 4291 4296 4297; C. J. Lancelot J. J. Harper and P. v. R. Schleyer ibid.,1969 91 4294. 116 N. S. Isaacs A linear free energy plot against Hammett a-values was obtained for com- pounds (94 X = p-NO, p-C1 m-C1 rn-CF3) but an increasingly positive devia- tion was obtained for (94 X = H p-Me p-OMe). The rates of this latter group were then partitioned into k and k components the former being computed by a linear fit to the Hammett equation defined by the first group of substituents.The proportion of phenyl-assisted reaction found in this way increases from 36 % (7 X = H) to 67 % (X = Me) and 91 % (X = OMe) for acetolysis at 100"C. The values for formolysis are somewhat higher. A very similar approach has been made by Kim and Brown93 who studied solvolysis of the series threo-(94 Ar = p-C6H4Br) and noted a correlation of log k with the electrophilic substi- tuent constant a'. A striking feature of this study is the observation of a steady increase in the proportions of substitution to elimination products (ca. 100% substitution for X = OMe 13 % for X = NOz) with decreasing a' and a con- comitant increase in retained stereochemistry.It seems likely that elimination would occur with greater facility for a classical ion (95)than for a bridged ion (96) on account of the closer juxtaposition of positive centre and proton in the former B:,H B (95) (96) and that this result agrees with an increasing proportion of phenyl-assisted path- ways with the more nucleophilic substituent groups. The same authors found a similar range of stereochemical integrity in the acetolysis of the series (97). The deviation from linearity of (97 X = H Me OMe) in the plot of log k against CT is much less than for the system (94) an inconsistency which the authors claim to weaken the case for bridged intermediates. X OS0,Ar (97) A relatively small proportion of aryl assistance has been argued for 2-phenethyl solvolysis on the grounds that the (aryl-l-14C) isotope effect kI2/kl4 is very small (1.002).By contrast the value of this ratio for p-anisylethyl is 1.028 indicating a greater change in hybridisation at this position in the latter case.94 93 C. J. Kim and H. C. Brown J. Amer. Chem. SOC.,1969,91,4286,4287,4289. 94 Y. Yukawa T. Ando K. Token M. Kawada and S.-G. Kim Tetrahedron Letters 1969 2367. Reaction Mechanisms 117 By labelling the a-and /?-positions it has been shown that the extent of reten-tion of configuration (kd component) is 2.00 times the extent of label rearrange- ment in the solvolysis of 2-phenethyl tosylates and brosylates (98).95 This is the result required for a bridged phenonium ion mechanism the theoretical value of the ratio for the alternative classical carbonium ion route being non-integral and estimated to be within the limits 1-04-2-26.Thus although the latter is not ruled Ph €4 Ph H \ ;>D \ 14#*jH .c-c .c -c out such an integral value would be obtained only very fortuitously under the variety of conditions used. Further evidence in favour of phenonium ion inter- mediates in the 2-phenylpropyl series comes from the observation by Diaz and Win~tein~~ that the rates of the two components of a solvolytic reaction k and kA should correlate in a variety of solvents respectively with the rates of solvolysis of the corresponding 2-propyl and neophyl esters which represent model compounds which are totally unassisted and totally assisted by aryl groups.Using ethanol acetic formic and trifluoroacetic acids they found a good linear correlation between these parameters and the values of k and kAfor 2-phenyl- propyl toluene-p-sulphonate (99) obtained by partitioning k in the ratio of products with inverted and retained stereochemistry respectively. It is also OAc Me / k k,_ \ -d.jH 1 + Q ..-i”-.c‘. ,c-\* Me ,c-c. H-i \ H H H‘i \“Me H OAc H interesting to note that while kAincreases in the order C2H50H< CH3C02H< HC02H < CF3C02H,the values of k show solvent dependence in the nucleo- philicity order CH3C02H < C2H50H< CF3C02H < HC02Hindicating fun- damental differences in the mode of solvent participation in the two processes.Activation parameters have been reported97 for the individual kAand k compo-nents of the acetolysis of 2-phenethyl [a-14C]toluene-p-sulphonate(partitioned according to the proportion of rearrangement of the label). Entropies of activa- tion -14.6 and -21.5 cal mol- respectively indicate a smaller loss of freedom in the transition state for the phenyl-assisted than for the solvent-assisted process. ’’ R. J. Jablonski and E. I. Snyder J. Amer. Chem. SOC. 1969 91 4445. 9b A. F. Diar. and S. Winstein J. Amer. Chem. SOC.,1969 91 4300. 9’ J. L. Coke F. E. McFarlane M. C. Mourning and M. G.Jones J. Amer. Chem. Soc. 1969.91 1 154. 118 N. S. Isaacs This may be rationalised in terms of the bridged intermediate (82) which would lose only rotational freedom of the aryl group and moderate entropy of solvation owing to the delocalised charge.An alternative explanation in terms of the equilibrating cations (83) suggests that ASt.is less negative the more rapid the equilibrium since there will be increasingly insufficient time for solvent reorganisa- tion. It seems possible that there might be little but a semantic difference between such a rapidly equilibrating pair of classical ions and a bridged ion. Acceleration of ionisation due to a py-unsaturated group (homoallylic partici- pation) is well documented and other related systems have come under review. Even greater acceleration is achieved when the unsaturated group is extended e.g. (100)-(103)98and allenic group^^^-'^' are found to give some assistance to ionisation (104)as do By acetylenic groups (105) which solvolyses in formic acid six times faster than the saturated analog~e.'~~*'~~ An unexpected species CH5+,seems to be well established as a transient ion formed by protonation of methane in superacid medi~m."~ Paraffins appear to undergo isotope exchange hydrogen abstraction proteolytic cleavage and polycondensation leading frequently to a complex mixture of product^,"^ containing stable tertiary butyl tertiary hexyl etc.cations. Neopentane and hexamethylethane are cleaved by the strong acid.lo6 Two theoretical studies of Me,C HSbF, -+ (Me,C+H) -+ Me,C+SbF,-+ CH Me,C-CMe (Me,C+H-CMe,) -+Me,C+SbF,-+ Me,CH 98 N. A. Clinton and C. P. Lillya Tetrahedron Letters 1968 5693."M. Santelli and M. Bertrand Tetruhedron Letters 1969 3699; 251 I 2515. loo R. S. Bly and S. U. Koock J. Amer. Chem. SOC. 1969,91 3292 3299. lol T. L. Jacobs and R. S. Macomber J. Amer. Chem. Soc. 1969,91,4824. P. E. Peterson and R. J. Kamat J. Amer. Chem. SOC. 1969 91,4521. Io3 J. W. Wilson J. Amer. Chem. SOC.,1969 91 3238. Io4 H. Hogeveen J. Lukas and C. F. Roobeek Chem. Comm. 1969 920. G. A. Olah G. Klopman and R. H. Schlosberg J. Amer. Chem. Sor. 1969 91 3261. '06 H. Hogeveen and A. F. Bickel Rec. Trao. chim. 1969 88 371. Reaction Mechanisms CH5+ agree that the most favourable geometry is that with C symmetry (106) rather than for instance D3,, or Cq,.107*108 cs (106) Cyclopropane ring participation is apparently more effective in assisting the departure of the 7-anti toluene-p-sulphonate group than a double bond in the 7-norbornenyl series (107) but only when it is in the endo po~ition.’~’ With the krel 1 104 10l2 10 109 leaving group one bond further removed as in (108) no participation by the three-membered ring occurs.’’ The requirement for cyclopropyl group stabilisa- tion of a carbonium ion is the bisected geometry (109)the ‘parallel’ conformation (1 lo) as would form from the adamantane derivative (1 1 l) being unfavourable. (108) This is reflected in the destabilisation of the cation with respect to the gem-dimethyl analogue (112).’’’,’’* The existence of protonated cyclopropanes has gained substance the subject has been reviewed by Collins l1 who concludes that species of the structure (113) are well-established as intermediates.The migration of the 14C-label in but-1-ene (1 14)is best accommodated by interme- diates (a b) (isobutene is not involved)’ l4 and the ring opening of (115) by HBr lo’s.Ehrenson Chem. Phys. Letters 1969 3 585. lo* A. Gamba G. Morosi and M. Simonetta Chem. Phys. Letters 1969,3 20. ‘09 J. S. Haywood-Farmer and R. E. Pincock J. Amer. Chem. SOC.,1969,91 3020. ‘la R. Muneyuki T. Yano and H. Tanida J. Amer. Chem. Soc. 1969,91 2408. J. C. Martin and B. R. Ree J. Amer. Chem. SOC.,1969 91 5882. ‘I2 P. v. R. Schleyer and V. Buss J. Amer. Chem. SOC.,1969,91 5880. C. J. Collins Chem. Rev. 1969 69 543. ‘I4D. M. Brouwer Rec. Trav. chim.,1968 87 1435. 120 N. S. Isaacs OTos OTos OTos krel 1.6 x 10-3 1 0-6 (acetolysis) CH,Br ,5’-j$ __* may be similarly viewed.’ ’’ Protonated cyclopropane alkylates benzene to give a mixture of n- and iso-propyl benzenes.’ In the cyclobutane series a 70-fold rate difference between cis and trans isomers of (1 16)in solvolysis may be due to a favourable transannular interaction in the latter case.”7 Despite their relatively low. stability vinyl cations have been implicated of late with increasing frequency. Their occurrence has been reviewed by Rappoport,’ who considered elimination-addition and dissociative mechanisms for their for mat ion. ‘I3 J. B. Hendrickson and R. K. Boeckman jun. J. Amer. Chern. SOC.,1969 91 3269. N. C. Deno D. LaVietes J. Mockus and P. C. Scholl J.Arner. Chern. SOC.,1968 90 6457. K. B. Wilberg and G. L. Nelson Tetrahedron Letters 1969,4388. ‘I8 Z. Rappoport Adv. Phys. Org. Chern. 1969 7 1. 6' equation correlations using Reaction Mechanisms Several groups have reported ionisations of triaryl vinyl halides and sul- phonates as partaking of the characteristics of S,l reactions. Thus rates are suppressed by added leaving group anion' with which radioactive exchange also occurs12o and the rates are very sensitive to substituents on the cc-aryl group (p = -3.6) and to a lesser extent the P-aryl group (p = -1.5) the Hammett and 6 respectively.' 21 The simplest vinyl cation prepared 2-methylvinyl (117) seems to be produced by solvolysis in 80% ethanol of the triflate (trifluoromethanesulphonate) (118).122 The P-deuterium isotope effect 1.2 was inferred to be a secondary rather than primary effect thus ruling out an elimination-addition mechanism.Nonetheless the solvolyses of vinyl compounds are very slow e.g. (1 18H120).'23Cyclopropyl as well as aryl groups can stabilise vinyl cations'24 (which is predicted theoretically' 25) as also can conjugating double bonds. Further evidence for the transient existence of vinyl cations might be sought in trapping experiments with olefins. According to Woodward8 the cycloaddition to form a cyclobutyl cation is allowed by orbital symmetry considerations. This aspect of vinyl cation chemistry has been reviewed.' 26 Ph /40Tos dTos -OTos J7 OTos (1 18) (1 19) (120) krel 174 21 0-01 8 10,000 (as.MeOH) The identification of the assisted component (k,) of a solvolytic reaction (which is usually the focus of interest in such studies) clearly depends on suppressing the solvent-initiated reaction k, which may be achieved by reducing the nucleo- philic power of the solvent. It is well established now that the 'limiting' nature of the common protic solvents is in the order EtOH < H20 < AcOH < HC02H < CF3CH20H < CF3C02H. Even more ionising and less nucleo- L. L. Miller and D. A. Kaufman J. Amer. Chem. Soc. 1968 90 7282. lZo G. Modena and U. Tonellato Chem. Comm. 1968 1676. ''' Z. Rappoport and G. Gal J. Amer. Chem. SOC. 1969 91 5246; G. Modena U. Tonellato and F. Naso Chem. Comm. 1968 1363. P. J. S. Fong and R. Summerville J.Amer. Chem. SOC.,1969 91 4600. P. E. Peterson and J. M. Indelicato J. Amer. Chem. SOC.,1969 91 6194. C. A. Grob and R. Spaar Tetrahedron Letters 1969 1439. H. Fischer K. Hummel and M. Hanack Tetrahedron Letters 1969 2169. K. Griesbaum Angew. Chem. Internat. Edn. 1969 933. 122 N. S. Isaacs philic solvents are H2S04 and HS03F probably the most limiting to date which have been introduced in solvolytic studies of simple alkyl toluene-p-sulphonates. Solvolytic rates in H2S04 of the '8-methylated' series of toluene-p-sulphonates increase steeply with sub~titution,~~' and even more so in FS03H.12' This Me Et Pr" Bu' neo-Pe 105K(30")(HZSO4) 0.06 1.55 32.1 450 >3000 Kre~(HFS03) 1 120 3-104 5*105 1 Oh Krel (CFJCOZH) 1 13 93 3060 5970 reactivity order is the opposite to that experienced in SN2reactions and presum- ably reflects the release of steric strain and methyl group assistance to ionisation.From the deviation of a plot of logk against the Taft aliphatic substituent constant o* the ratio kA/k was estimated. For the solvolysis of n-propyl toluene-p-sulphonate the value of 100k& increases from 0402(EtOH) 16.8(AcOH) 76.3(HC02H) to 87.4(CF3COOH).129 The values for isobutyl toluene-p-sulphonate are proportionally higher. The solvolysis of ethyl toluene- p-sulphonate in HS03F apparently occurs by an ionisation route (kd) amounting to some 25% of the total rate as judged by the a-and P-deuterium isotope effect~.'~' The more limiting the solvent (i.e. the greater that kA contributes to the total rate) the larger are the secondary isotope effects.131 Cyclodec-5-enyl esters (121) are very labile compared to the saturated analogues (lo4:1) owing to transannular interactions as shown in the products including that from ion-pair recombination.32 A transannular hydride shift occurs in the destabilised carbonium ion (122) leading to 9,lO elimination with a primary isotope effect of 3.0.'33 OCOAr v Ho -I ArCO The possibility of non-chair conformations being of significance in solvolyses of cyclohexyl arenesulphonates has been current for several years and remains a focus of some controversy. A study of P-deuterium isotope effects by Shiner and Jewett 134 and careful quantitative studies of solvolytic products from 12' P.C. Myhre and K. S. Brown J. Amer. Chem. SOC.,1969,91 5639. 12* A. Diaz I. L. Reich and S. Winstein J. Amer. Chem. SOC.,1969 91 5637. 129 I. L. Reich A. Diaz and S. Winstein J. Amer. Chem. SOC.,1969 91 5635. I3O P. C. Myhre and E. Evans J. Amer. Chem. SOC.,1969 91 5641. 131 A. Streitwieser jun. and G. A. Dafforn Tetrahedron Letters 1969 1263. 132 H. L. Goering and R. F. Myers J. Amer. Chem. SOC.,1969 91 3386. 133 D. E. Homing and J. M. Muchowski Canad. J. Chem. 1968,46 3665. '34 V. J. Shiner and S. J. Jewett J. Amer. Chem. SOC.,1965 87 1383. 123 React ion Mechanisms I HO C02Me C0,Me H C02Me sulphonates of the 4-t-butylcyclohexanols1 35 and 4-t-butyl-2-methylcyclohexa- nol~'~' have been used to argue the importance of 'twist-boat' conformers (123) in solvolyses in the trans-4-t-butylcyclohexyl series and suggest also that rate acceleration by hydrogen participation (124) occurs in the cis-methyl series.The postulate has been further examined by measurement of solvolysis rates of 4,4-dimethylcyclohexyl toluene-p-sulphonate,' 37 it being argued that neither axial nor equatorial 4-methyl groups should markedly affect reactivities in a chair conformation but should cause steric compression in a twist form (125). %H 2H Me H A-(125) ( 1 26) (127) In fact no such retardation was experienced. The highly strained cis,cis,cis-perhydro-9b-phenolenol (126) has been synthesized :'38 its p-nitrobenzoate solvolyses at a rate 4 x lo6times that of the cis,cis,truns-isomer (127) and attri- buted to strain in the former system.Solvolysis of isotopically labelled homo- adamantyl arenesulphonates shows that the intermediate homoadamantyl cation (128) undergoes seven-fold degenerate rearrangement. '35 N. C. G. Campbell D. M. Muir R. R. Hill J. H. Parish R. M. Southam and M. C. Whiting J. Chem. SOC.(B) 1968 355. 13' M. Pankova J. Sicher M. Tichy and M. C. Whiting J. Chem. SOC.(B) 1968 362; M. Tichy J. Hapala and J. Sicher Tetrahedron Letters 1969 3739. J. E. Nordlander J. M. Blank and S. P. Jindal Tetrahedron Letters 1969 3477. H. C. Brown and W. C. Dickason J. Amer. Chem. SOC.,1969,91 1226. 13' P. v. R. Schleyer E. Funke and S.H. Liggero J. Amer. Chem. SOC.,1969 91 3965. 124 N. S. Isaacs OS0,Ar etc. Carbonyl Reactions.-The El cb mechanism of ester hydrolysis (129,130)might be predicted to occur if at all in esters with a fairly acidic ct-proton and at high pH.Bruice and co-worker~'~~ suggest at least a component of this hydrolytic mechan- ism for substituted o-nitrophenyl acetates (X = CN C02Et) since additional rate terms come into the kinetic analysis at high pH zeroth-order in [OH-]. A similar study of rate-pH profiles for some malonate esters (131 R' = R2 = H R' = H R2 = Me) and dimethylsulphonium acetates (132)indicates anomalies quite distinct from the profile exhibited by esters undergoing the more usual B,,2 reaction and the inference is made that the Elcb mechanism is here also important.14' Co-ordinated assistance to the hydrolysis of z6-dihydroxy- benzoates (133)by the two phenolic groups is inferred from the pH-rate profile which has a symmetrical maximum at the pH at which the maximum concentra- tion of monophenolate exists.'42 An estimate of the carbonyl stretching fre- quency during ester hydrolysis 1300 f 200 cm-' which conforms to that expected if a tetrahedral intermediate is being formed has been made by measur- ing the l60/'*O carbonyl isotope effect on ester hydrolysi~.'~~ An extremely polar transition state must be indicated in the iodine exchange of benzoyl iodide with molecular iodine; a solvent acceleration of 100 from hexane to dichloro- ethane and a Hammett p-factor of -7 is reported.'44 140 T.C. Bruice and B. Holmquist J. Amer. Chem. SOC.,1968,90 71 36. 141 B. Holmquist and T.C. Bruice J. Amer. Chem. Soc. 1969,91,2993 3003. 142 F. L. Killian and M. L. Bender Tetrahedron Letters 1969 1255. 143 C. G. Mitton and R. L. Schowen Tetrahedron Letters 1968 5803. 144 D. W. Hamilton and R. D. Nays J. Amer. Chem. SOC.,1969,91 1740. Reaction Mechanisms 125 Nucleophilic Displacement Reactions.-Isotope effects are being increasingly utilised in the investigation of a variety of nucleophilic displacement reactions ; theoretical predictions of secondary deuterium isotope effects have been pub- lished by Willi,'45 and include a value of 1.125 for the Me1 + I- exchange reaction which compares favourably with the experimental value 1.10. As in all such calculations the results are only as good as the assumed force constants.The values of kdkDfor displacements in aqueous ethanol by thiosulphate ion of methyl- and [2H3]methyl bromide iodide methanesulphonate and toluene- p-sulphonate are 1-03 1.05 1.1 1 and 1.12 re~pectively~~~-rather small values which may be compared with the value for isopropyl chloride (Cl- exchange) of 1.12-1.14 of 1-12 for acetolysis and 1.22 for trifluor~acetolysis.'~~ It appears that the value of the isotope effect is reduced by nucleophilic participation by solvent and could prove a useful probe. Rather surprisingly perhaps secondary 8-deuterium isotope effects on SN2-reactions are also quite considerable e.g. kH,/kDfor isopropyl [2H,]toluene- p-sulp hona te (trifluoroace to1 ysis) is 2- 1 2. ' Solvent isotope effects are also potentially valuable sources of information on details ofchanges in solvent structure with activation.Several studies of hydroly- sis in D20-H20 mixtures have been The rate ratio k,JkHof hydrolysis in water (kH)or a mixture with n atom % deuterium (k,) can be related by the Gross equation k ll'.s.(l-n + n+) -_-kH llR(l -n + n+) where 4 is a fractionation factor and the products are taken over all hydrogen atoms in reagent (R) and transition states (t.s.). Divergence from this equation by the hydrolysis of acetic anhydride and nitramide can be corrected by the introduction of a further parameter a which has the physical significance of the degree of proton transfer in the transition state. In the above reactions values of cc = 0.5 give a good fit. The careful rate studies of Robertson and co-workers show that systematic differences in AHSand ASs for hydrolysis in H20and D20exist for systems using SN1 and SN2 routes.'52 Another technique which despite complexity of equipment is becoming more widely used is the variation of rate with pressure and the measurement of activation parameters at constant volume which may be more fundamental than 14' A.V. Willi Z. phys. Chem. (Frankfurt) 1969 66 317. 146 G. E. Jackson and K. T. Leffek Canad. J. Chem. 1969,47 1537. 14' H. Strecker and H. Elias Chem. Ber. 1969 102 1270. 14' A. Streitweiser jun. and G. A. Dafforn Tetrahedron Letters 1969 1263. 149 B. D. Batts and V. Gold J. Chem. SOC.(A) 1969 984. 150 P. Salomaa Acta Chem. Scand. 1969 23 713. l5' W. J. Albery and B.H. Robinson Trans. Faraday SOC., 1969 65 1623. lS2 L. Treindl R. E. Robertson and S. E. Sugamori Canad. J. Chem. 1969,47 3397. 126 N. S. Isaacs those at constant pressure.' 53,154 Pressure dependence on rates of hydrolysis of methyl and isopropyl bromides,' of methyl iodide with pyridine ' and hydrolyses of some ally1 chlorides,' 57 have been reported. The relationships on the whole are complex and as yet not fully explained but the negative activation volumes obtained in the chloride hydrolyses have been interpreted as direct evidence of a transition state more polar than the initial state causing electro- striction of surrounding solvent. Displacements from vinylic carbon are usually difficult however (134) and (135) are unusually reactive and their reactions with thiophenoxide may occur by direct displacement,'58 as judged by the retention of stereochemistry in the product and the large negative entropy of activation (ASf = -20 em).The reaction with methoxide apparently occurs by elimination-addition as the intermediate acetylene can be is01ated.l~~ The substitution of either cis-or trans-(136) by methoxide leads to the same (trans)vinyl ether (137) probably by way of the carbanion (138)160 and similar considerations may control the predominant formation of trans-substitution products from either cis or trans bromocrotonic esters.'61 By a kinetic argument it has been concluded that both elimination and substitution products in the reaction of a-phenyl ethyl bromide with ethoxide ion arise from a common intermediate the ion pair.p-Me0 ' C6H4 p-Me0.C H H \ /y 64\ / H /c=c\H H./c=c\ Y Y = C1 Br 153 E. Whalley Ado. Phys. Org. Chem. 1964 2 93. 154 D. L. Gay and E. Whalley J. Phys. Chem. 1968,72,4145. 155 B. T. Baliga and E. Whalley J. Phys. Chem. 1969 73 654. 15' H. Heydtmann and D. Buttner Z. phys. Chem. (Frankfurt) 1969 63 39. 15' A. B. Lateef and J. B. Hyne Canad. J. Chem. 1969,47 1369. 15' D. Landini F. Montanari G. Modena and F. Naso J. Chem. SOC.(B) 1969 243. 159 D. Landini F. Montanari G. Modena and F. Naso J. Chem. SOC.(B) 1969 245. 160 G. Marchese F. Naso and G. Modena J. Chem. SOC.(B) 1969 290. Ibl J.-C. Chalchat F. Theron and R. Vessiere Compt. rend. 1968 267 Capital 1864. Reaction Mechanisms 127 Acid-catalysed electrophilic substitutions at saturated carbon have been reviewed;'62 the acid cleavage of (139) apparently partakes of both a uni-molecular and a bimolecular component' 63 while the base-catalysed cleavage of (140),which can be represented as an SE2reaction yields products with 61 % net retention of c~nfiguration.'~~ YH,HgCl Me PoOR; g -TCOOR H Me Ph Me Ph \OR Me Ph (139) Neighbouring-group Participation.-Halogen-participation is implicated in the rearrangement occurring to halogenoethers (141) when treated with trimethyl- oxonium salts in trifluoroacetic acid.'65 The 13C n.m.r.spectra of the P-bromo- cumyl cations stable in superacid solution are consistent with the structure (142) in which there is perhaps weak interaction between the carbonium carbon and bromine but not a symmetric bridged ion.'66 A stable bromonium ion (143) may be obtained by bromination of adamantylidineadamantane,' 67 and a n = 0,2 (141) + Me20 1CF,COOH CH -ha1 F C.COO Br (143) (142) number of simple halonium ions R-hal+-R characterised by n.m.r.in SnF,-SO at low temperatures.16* Assisted solvolysis in compounds (144a b c) 16' J. M. Williams jun. and M. M. Kreevoy Adv. Phys. Org. Chem. 1968 6 63. D. Dodd C. K. Ingold and M. D. Johnson J. Chem. SOC.(B) 1969 1071 1076. 164 T. D. Hoffman and D. J. Cram J. Amer. Chem. SOC.,1969,91 1000 1009. 165 P. E. Peterson and F. J. Slama J. Amer. Chem. SOC.,1969 91 6516. '66 G. A. Olah C. L. Jewell and A. M. White J. Amer. Chem.SOC.,1969 91 3961. I67 J. Strating J. H. Wieringa and H. Wynberg Chem. Comm. 1969 907. 16' G. A. Olah and J. R. DeMember J. Amer. Chem. SOC..1969,91 21 13. 128 N. S. Isaacs is indi~ated'~' by the rate enhancements compared with ally1 chloride 30 100 and 3000 respectively and the isolation of3-acetoxyoxetan (145)among acetolysis prod~cts'~'of epichlorohydrin (144b). Methoxyl participation is shown to be responsible for the solvolytic rate ratio of 113 for cis-(146) compared to the trans-isomer (147).' OAc -.+ AcOH_ [?I wCH2C*0 Z Z (144) (145) (a. Z = CH2) (b. Z = 0) (c. z = S) OSOzAr (147) The rearranged compound (146a) accounts for 75 % of the products and very similar results are found for the saturated ana10gue.l'~ The hydrolyses of a-bromophenylacetic acid salts (148) which from the retention of configuration of analogous compounds would be expected to occur via an a-lactone (149) never- theless show a Hammett p-factor of -2-66 which is typical for unassisted benzylic Ib9 H.Morita and S. Oae Tetrahedron Letters 1969 1347. 170 H. G. Richey and D. V. Kinsman Tetrahedron Letters 1969 2505. 17' J. R. Hazen and D. S. Tarbell Tetrahedron Letters 1968 5927. 172 J. R. Hazen Tetrahedron Letters 1969 1897. Reaction Mechanisms halide hydrolysis.' 73 Presumably the formation of the transition state still requires electron donation from the phenyl ring. The acetolysis of (150) is very fast compared with the rate for the para-isomer evidently due to assistance from neighbouring sulphur (k150/k151 = 220).This may be contrasted with the ortho- and para-methoxybenzyl chlorides which solvolyse at rates in the ratio 0.0017 :1 re~pectively.'~~ Stabilisation of the intermediate carbonium ion by hydroxyl participation occurs in bromine addition to (152 n = 3) for which the rate is a maximum compared to analogous n = 2 n = 4."' Methyl group participation in the trifluoroacetolysis by the ionisation route (kd) of neopentyl toluene-p-sulphonate (153) is suggested as being responsible for the high rate of this reaction (150-fold acceleration) relative to that of ethyl toluene-p-sulphon- ate.' 76 In the solvent-promoted ethanolysis (k,) the relative rates are more than reversed (1/1780). An example of carbonyl participation in ester hydrolysis is suggested for cyclohexanone-2-acetic ester (1 54) and cyclopentanone-2-acetic ester which hydrolyse 60 and 200 times more rapidly than cyclohexylacetic and cyclopentyl- acetic esters respectively.' An essentially electrostatic catalysis is invoked to explain the rate and high positive entropy of activation in the hydrolysis of (1 54a). The temperature coefficient of the enthalpy of a reaction AC," is charac- teristically distinct for SN1 and SN2 processes it has been shown that reactions partaking of neighbouring group participation behave as SN1reactions by this lT3 K. C. Kemp and D. Metzger J. Org. Chem. 1968,33,4165. 174 M. Hajo T. Ichi Y. Tamaru and Z. Yashida J. Amer. Chem. Soc. 1969,91 5170. 175 D. L. H.Williams E. Bienvenue-Goetz and J. E. Dubois J. Chern. SOC. (B) 1969 517 522. '16 W. G. Dauben and J. L. Chitwood J. Arner. Chern. Soc. 1968,90 6876. K. C. Kemp and M. L. Meth Chem. Comm. 1969 1260. 130 N. S. Isaacs 0 // \ OMe (154a) criterion which probably depends in the main on solvent ordering in the transi- tion state."' Carbanions and Enolization.-Carbon acidity has been reviewed by Shatenshtein l7 and Streitweiser's measurements of cyclohexylamide-catalysed proton exchange rates extended to some very weak acids."' The relative ex- change rates of benzene 1,4-di-t-butylbenzene and cyclohexane are 1:10-:10-'. The acidities of cycloalkanes fall sharply from cyclopropane through successively larger ring analogues levelling off at cyclo-octane to a constant value while these exchange rates correlate linearly with l3C-H coupling constants reflecting the ground state polarities of the C-H bonds.a-Proton exchange in nitrocyclo- alkanes also decreases with increasing ring size' '' but curiously the maximum exchange rate is reached with nitrocyclobutane nitrocyclopropane being very slow indeed perhaps on account of conjugation between the nitro group and the ring which reduces the 'pull' of the former. A large (> 8) primary kinetic isotope effect was also reported. The stabilisation of nitroalkane anions R CNO ,as in the order R = Me > Et > Pr" while the acidity of the bridgehead protons of trjptycene appears to be normal for sp3 carbon.'82 a-Proton exchange appears to be much more rapid for alkali-metal salts of carboxylic acids than other deriva- tives.' Pent-3-ynoic acid (155) is shown to equilibrate with two double bond isomers (156)and (157) by way of the dianions (158) and (159) the stability being shown by the relative Gibbs function for each.'84 Similar prototropic shifts (in the equilibration of 3-phenylpropyne phenylallene and the 3-phenylpropenes) M.J. Blandamer H. S. Golinkin and R. E. Robertson J. Amer. Chem. SOC.,1969 91 2678. 17' A. I. Shatenshtein and I. 0.Shapiro Uspekhi Khim 1968 37 1946. lE0 A. Streitwieser jun. and W. R. Young J. Amer. Chem. Soc. 1969 91 527 529. lS1 H. Shechter et al. J. Amer. Chrm. SOC.,1969 91 2797. A. Streitwieser jun. and G. R. Ziegler J. Amer. Chem. SOC.,1969 91 5081.lg3 P. Be'onger J. G. Atkinson and R. S. Stuart Chem. Comm. 1969 1067. la4 R. J. Bushby and G. H. Whitham J. Chem. SOC.(B) 1969 67. Reaction Mechanisms 131 MeC=C-CH,COO MeCH=CH=CHCOO MeCH,C=C-COO (155) (156) 4 3 (157) AG 0% @2\\ __ __ ___-MeC=C-CHCOO MeCH-C=C-COO (kcal mole-') (158) (159) 18 22 occur in dimethyl sulphoxide by an intramolecular mechanism there being no exchange with the solvent.'85 Vinyl hydrogen exchange has been reported,'86 the rate being promoted by a-or P-chlorine or bromine but not fluorine for example trichloroethylene exchanges nearly 500 times faster than trifluoroethylene. A new method for demonstrating the existence of transient carbanions has been rep~rted'~'.' 88 and utilises one-electron oxidation e.g.by nitrobenzene ; the resulting radical then undergoes dimerisation or other typical reactions. A quantum mechanical analysis of carbon acidity utilises the total change in n-and a-electronic energy on ionisation as a theoretical parameter to correlate with pK va1~es.I~~ The relative exchange rates of methylene and a-methyl protons in 2-butanone are found to be solvent dependent the ratio KCHz/KCH3 rising with increasing acidity of the medium'g0 while from consideration of the rates of tritium exchange in 2-ethoxycarbonylcyclopentanone,it is concluded that considerable quantum- mechanical tunnelling in the proton exchange occurs,' 'especially when fluoride ion is used as the basic catalyst. On the basis of a correlation between acid- catalysed enolisation and hydrolysis of enol esters it is concluded that the transition state for enolisation contains a substantially intact 0-H bond.' 92 An example of an ester which hydrolyses via a carbanion is claimed for 5-nitro- coumaranone (160); the rate is independent of H30+and OH-concentrations over a wide range of pH values and an intermediate identified as the carbanion is claimed to show a detectable absorbance at the beginning of the reaction its concentration being pH-dependent.The solvent deuterium isotope effect (KH20/KDz0= 1.7) is not compatible with a normal B,,2 mechani~rn.'~~ The mechanism of the Favorskii reaction (162) has been the subject of some uncertainty as to whether the formation of the cyclopropane occurred in a con- certed step or via the carbanion or enolate.B~rdwell'~~ has examined the IH5 J. Klyne and S. Brenner Chem. Comm. 1969 1020. lB6D. Daloze H. G. Viehe and G. Chiurdoglu Tetrahedron Letters 1969 3925. '13' G. A. Russell and S. A. Weiner Chem. and Ind. 1969 659. R. D. Guthree J. Amer. Chem. SOC.,1969 91 6201. lH9 D. H. Lo and M. A. Whitehead J. Chem. SOC.(A) 1969 15 13. lYo W. H. Sachs and C. Rappe Acta. Chem. Scand. 1968,22,2031. 19' J. R. Jones Trans. Faraday SOC.,1969 65 2430. 192 G. E. Lienhard and T. C. Wong J. Amer. Chem. Sac. 1969 91 1146. Iy3 P. S. Tobias and F. J. Rezdy J. Amer. Chem. SOC.,1969 91 5171. F. G. Bordwell and R. G. Scamehorn J. Amer. Chem. Soc. 1968 90 6751 ; F. G. Bordwell R. G. Scamehorn and W. R. Springer ibid. 1969 91 2087.132 N. S. Isaacs 0 (163) 1 /0-T 0 II I -:cNC\c-c1 c4c\c-cc1 /\ /\ /\ (162) electronic substituent effects on the rearrangement of (163)and finds a high degree of carbanion character in the transition state. Deuterium exchange prior to chloride ion departure occurs and a positive salt effect (though no common ion effect) appears to rule out the concerted reaction. Stable delocalised carbanions whose identities have recently been recorded include the stereoisomeric allylic anions (164a,b)’95 and structural isomers (165) and (166).’96A new non-classical NH,,NH2-H H PhCH,CH=CHMe -Ph(+Me 1 phy$H 0°C tt =30min H H (164a) (164b) Bz- 7 215 “C anion [complementing the bicyclo[3,2 lloctadienyl anion (167)] is the bicyclo- [3,2,2]nonatrienyl anion,’ 97 whose four proton resonances show splitting in accordance with the structure (168).The ‘anti-Huckel’ structure (169) has also been claimed in which n-overlap between all three branches delocalises the charge.’98 Some interesting electrocyclic reactions of conjugated carbonions have recently been reported for example the cyclisation of the heptatrienyl L95 H. Kloosterziel and J. A. A. Van Drunen Rec. Trau. chim. 1968 87 1025. 196 D. H. Hunter and R. W. Mair Cunad. J. Chem. 1969 47 2361. 19’ J. B. Grutzner and S. Winstein J. Amer. Chem. SOC.,1968 90 6562. 198 F. W. Staley and D. W. Reichard J. Amer. Chem. SOC. 1969,91 3998. Reaction Mechanisms -'b H (4-76 @ NajK 'I--'' H(6*95) (7-71) -I_ ---L---(167) H (5.02)~ (168) anion (170) to the cycloheptadienyl anion (1 7 1) :'99 and the pentadienyl-cyclopentyl rearrangement of (172) BuLi 35 "C + -+ by the predicted disrotatory process provides an entry into the cis-bicyclo[3,3,0] octane series2'' An analogous rearrangement occurs with the diaza anion (1 73) similarly stereospecific.20 The cyclobutadienocyclopentadienideanion H Ph Ph Ph I x \/ C N ,*r-% NAN, N NIC\N -p PhLi N(-)N II II 1:-i I -H-x PhCH CHPh PhCH CHPh Ph Ph Ph Ph (173) (174) has been postulated as an intermediate in the dehydrochlorination of (174a) R.B. Bates W. H. Davies D. A. McCombs and D. E. Potter J. Amer. Chem. Soc. 1969,91,4608. 2oo R. B. Bates and D. A. McCombs Tetrahedron Letters 1969 977.201 D. H.Hunter and S. K. Sim J. Amer. Chem. Soc. 1969,91 6702. 134 N. S. Isaacs since proton exchange occurs at the ring junction.202 A cyclohexyl anion (175) protonates almost exclusively from the equatorial side presumably less hindered to the approach of the acid :203 CN CN CH(C0ZEt)z CH(C0z Et)2 Molecular orbital calculations predict the most stable conformation of a sul- phony1 carbanion as having the lone pair cis to the oxygen atoms.z04 Polar Addition Reactions.-A review of the stereochemistry of electrophilic addition to olefins and acetylenes has been publishedzo5 and also an account of the special chemistry of additions to strained olefins.206 That the addition of bromine to an olefin occurs via a carbonium ion is emphasised by the additions to (176),(177) in which rates are correlated with o-values of 2 while for (178) in which alone the aryl group can become coplanar with the olefinic double bond the rates correlate with the electrophilic substituent constant Bromine addition rates to p-substituted stilbenes and styrenes usually correlate with but 4-nitro-4-substituted stilbenes are not correlated well with either o or o+.209 4-Nitro-4'-methoxystilbeneapparently adds bromine at a rate anomalously high considering the substituents which would be expected to contribute their effects additively.This may reflect some complication of mechanism as for example has been revealed by a detailed kinetic study of the bromination of cyclohexene in which a 2 1 adduct of olefin and bromine has been inferred as an interme- diate.2'0 @-Unsaturated sulphones are brominated at rates slower than the py '02 R.Breslow W. Washburn and R. G. Bergman J. Amer. Chem. Soc. 1969 91 196. 203 R. A. Abramovitch M. M. Rogic S. S. Singer and N. Venkateswaran J. Amer. Chem. Soc. 1969 91 1571. '04 S. Wolfe A. Rauk and I. G. Csizmadia J. Amer. Chem. SOC.,1969 91 1567. 205 R. C. Fahey Topics Stereochem. 1968 3 237. 206 T. G. Traylor Accounts Chem. Res. 1969 2 152. '07 J. E. Dubois and A. F. Hegarty J. Chem. SOC.(B) 1969 638. '08 J. H. Rolston and K. Yates J. Amer. Chem. Soc. 1969 91 1483. 209 G. Heublein and E. Schutz Z. Chem. 1969 9 147. 2'o G. B. Sergeev T. V. Pokholok and Chen. Ton-Kha Kinetika i Kutuliz 1969,10,47 51. Reaction Mechanisms isomers2" which is attributed by the authors to conjugation with the sulphenyl group although steric factors may play a part.The kinetic form is reminiscent of that for addition of HCl to olefins in nitromethane for which rate = k[olefin] [HC1I2 and presumably reflects the need for a second HCl molecule to assist in pulling off chloride ion from the first (179) in the absence of suitable solvent participation.2 The gas-phase addition of HI to 1,l-difluoroethylene yields mainly 1,l-difluoro-l-iodoethane l3 with very little of the other i~omer.~ This is interpreted as supporting an ion-pair mechanism rather than the point- dipole model The reaction of chlorine with 1,l-dichloroethylene yields the substitution product as well as addition It is inferred that a common intermediate or transition state (180) is partitioned in two directions c1 \ 1 1 ,/CH-cC13 CH2 = CCI 1H' The addition of DCl to norbornene yields exo-2 norbornyl chloride with deuterium at 3-exo and 7-synpositions (181).215The primary kinetic isotope for the addition of HF (DF) to ethyl vinyl ether is only 3.35 despite the fact that 211 A.Kasloa M. Paleeek and M. Prochazka Coll. Czech. Chem. Comm. 1969 34 1826 1826. l2 Y. Pocker and K. D. Stevens J. Amer. Chem. SOC.,1969,91,4199,4205. 213 T. S. Carlton A. B. Harker W. K. Natale R. E. Needham R. L. Christensen and J. L. Ellenson J. Amer. Chem. Soc. 1969,91 555. 214 A. I. Subbotin G. A. Korchagina and I. V. Bodrikov Zhur. org. Khim. 1968 4 2078.215 J. M. Brown and M. C. McIvor Chem. Comm. 1969,238. 136 N. S. Isaacs there must be almost complete rupture of the H-F bond in the transition state. It is postulated that the 'normal' value of kH/kD z 7 for such a reaction is only realised when the X-H bond which is broken loses bending as well as stretching modes of A theoretical investigation of bromine addition rates to substituted stilbenes finds a reasonable correlation of rates with localisation energies i.e. the dif- ferences in Huckel n-energy between (182) and (183).217The effect of solvents (182) (183) on the iodination (equilibrium) of cyclohexene has been examined. Non-polar solvents favour formation of the di-iodocyclohexane.2 trans-Addition of iodine azide to phenyl acetylenes appears to occur and it is inferred219 that the reaction proceeds via an intermediate bridged ion (184) rather than a vinyl cation.Electrophilic additions to cyclo-octatetraene may occur by way of homotropylium ions (185).220A large compilation of rate data comprising the kinetics of addition of I -SCN ArSCl and NOCl to 66 olefins has'been pub- lished.221 Olefin cis-trans isomerisation prior to addition occurs with the but-2-enes and HBr,22'a the reaction being light-initiated but not suppressed by radical scavengers. It is inferred that the addition occurs viu or parallel to the formation of an olefin-bromine atom complex and leads ultimately to 85 % trans adduct. CNF I-N3 Ph\ Ph\ /N3 Ph .C-C-R /c=c\ I R 0 '16 A. J. Kresge and Y.Chiang J. Amer. Chem. SOC., 1969,91 1025. G. Heublein and P. Hallpap Z. Chem. 1969 9 149. 'IBC.-S. Chao W.-T. Wu and C.-H. Wang Chemistry (Quart. Chinese Chem. SOC. Formosa) 196'7 59. 219 A. Hassner R. J. Isbister and A. Friederang Tetrahedron Letters 1969 2939. 220 L. A. Paquette J. R. Malpass and T. J. Barton J. Amer. Chem. SOC.,1969 91 4714. G. Collin et al. J. prakr. Chem. 1969 311 238 (a) D. J. Posto G. R. Meyer and S. Kang J. Amer. Chem. Soc. 1969,91 2163. Reaction Mechanisms 137 Elimination Reactions.-More examples of the El cb elimination mechanism are coming to light i.e. reactions in which rapid reversible proton loss precedes loss of the P-leaving group. In general this mechanism occurs in systems contain- ing an acid-strengthening group a-to the proton and a relatively poor leaving group.Thus Stirling and co-workers222 have reported Elcb mechanisms in a range of P-substituted phenethyl ethers e.g. NaOH Slow MeSO-CH2-CH20Ph MeSO-CH-CH2-OPh +MeSO-CH=CH2 (186) The highly electrophilic olefins (186) are rarely isolated as they rapidly add solvent. For these reactions the P-isotope effect is typically 0.7-0.8 (compare ca. 5 for a concerted reaction) but is due to the greater base strength of OD-than OH-. By varying the P-substituent the rates are found to vary in such a way that a good correlation with the substituent constants oR,is obtained indicating that resonance stabilisation of a carbanion is an important component of activation. This can be accomplished by vacant &orbitals but not a quaternary ammonium group hence the lO"-fold difference in rates of elimination between (187) and (188).Ph,P+ -CHZCH20Ph Me3"-CH2CH20Ph Similar mechanisms are suggested for eliminations of P-methoxy ketones (189) on the basis of specific base catalysis effects and a-hydrogen exchange (though this must occur by en~lisation),'~~ for 9-fluorenylmethanol (190),224 on the basis of rapid prior isotope exchange of the 9-proton and 1,2-dibromoethylene which is accompanied by a negligible isotope effect.22 Elimination of P-phenethyl- sulphoxides (191) display a Hammett p-value of +4.4,indicating strong facilita- tion of the reaction by electron withdrawal but there is a P-deuterium isotope 02- MeO-CH2CH2COMe .-MeO-CH,-CHCOMe MeOH + CH,=CHCOMe OH + OH-222 J.Crosby and C. J. M. Stirling J. Amer. Chem. SOC.,1968 90 6869. 223 L. R. Fedor J. Amer. Chem. SOC.,1969,91,908 913. 224 R. A. More O'Ferrall and S. Slae Chem. Comm. 1969,486. 225 W. K. Kwok W. G. Lee and S. I. Miller J. Amer. Chem. SOC.,1969,91 468. 138 N. S. Isaacs effect of 2.7 and no prior isotopic hydrogen exchange. It is concluded that the transition state has strong carbanionic character but is somewhere between that for pure E2 and Elcb mechanisms.226 The elimination of sulphoxyl groups can lead to cyclopropanes as well as ole fin^.^^',^^^ For instance in the series (192) (R = H Me Et Pri Bu') the proportion of cyclopropane increases in that order presumably due to steric inhibition of the 1,2-elimination.Deuterium exchange R R I I CH -+ CH -+ /\ / Ph-CH2 CH2SOMe PhCH2 \CHSOMe -( 192) indicates that ionisation takes place prior to departure of the sulphoxyl group. An interesting example of epimeric differences in reactivity is in the pyrolytic syn-1,2 elimination of the steroid sulphinate @)-ester (193) which gives the A6-olefin smoothly whereas under the same conditions the (S)-epimer (194) will not react.229 The reason is apparently due to the inability of sulphinate oxygen to approach the 7a-proton because of blocking by the 19-methyl group. The pyrolysis of neophyl esters (195) is accompanied by a rearrangement,230 as if the 8-carbon were developing carbonium ion characteristics. The b-isotope effect 226 R. Baker and M.J. Spillett J. Chem. SOC.(B) 1969 481. 227 R. Baker and M. J. Spillett J. Chem. SOC.(B) 1969 880. 228 R. Baker and M. J. Spillett J. Chem. SOC.(B),1969 581. 229 D. N. Jones and W. Higgins Chem. Comm. 1968 1685. 230 H. Kwart and H. G. Ling Chem. Comm. 1969 302. Reaction Mechanisms 1-5 is probably a primary effect and the transistion state (196) is suggested Ar Me I \ Further work has appeared supporting the ‘synanti’ mechanism of elimination of quaternary ammonium salts i.e. the formation of trans-olefin by syn-elimina- tion and cis-olefin by anti-elimination H \ / OH-__+ c=c /c-c\ /\ R3+N \ / OH-/ --+ c=c R3+N/c-c\H / The elimination of specifically cis-p-deuteriotetramethylcyclodecyltrimethyl-ammonium hydroxide (197) leads to both cis and trans cyclodecenes.The trans-olefin has lost all the deuterium label and its formation involves a primary isotope effect of ca. 3 while the cis-olefin retains all the label and its formation involves a negligible isotope effect.23 No mention of transannular reactions was made as has been found to contribute 12-13% to eliminations of cyclodecyl toluene-p-sulphonates. Similar conclusions have been reached in the acyclic series by considering the amount of deuterium loss from threo (198) and erythro (199) hexyltrimethylammonium hydroxide by syn anti and synanti mechan- isms. With n-butoxide as the base the observed values coincide with those predicted for anti-elimination but with isopentoxide the values coincide with the synanti me~hanism.~~~,~~~ Theoretical estimates of P-deuterium isotope effects on elimination span the range 2-5-53 according to the force constants 231 J.Zavada M. Svoboda and J. Sicher COIL Czech. Chem. Comm. 1968,33,4027. 232 V. Prelog E. Troxler and H. H. Westen Helv. Chim. Acta 1968 51 1678. 233 D. M. Froemsdorf H. R. Pinnick jun. and S. Meyerson Chem. Comm. 1968 1600. 234 D. S. Bailey and W. H. Saunders jun. Chem. Comm. 1968 1598. 140 N. S. Isaacs &Me3 Et Et *@ D$c D H (198) (199) Values recorded include 3-81 & 0-2 for the pyridine-catalysed elimina- tion of t-butyl and 3.2-6.7 for the anti-elimination of some trimethyl- ammonium ions from which it was deduced that the extent of /?-hydrogen breaking in the transition state decreases in the order 3-pentyl > cyclopentyl > cyclo-he~y1.~~~ Elimination mechanisms by an E, (i.e.with some co- ordination between base and or-carbon in the transition state) have been criticised by Eck and B~nnett~~~ who observe that the chloride-ion catalysed elimination of HBr from (200) is faster than that from t-butyl bromide. Chloride ion is expected to be C-basic rather than H-ba~ic~~' and hence the reaction should be of the E, type but clearly rearside co-ordination of the base would be inhibited by the neopentyl arrangement of (200). The results still seem to allow a transition Br state in which chloride co-ordinates to /?-carbon and P-H from the front side (201) although this then has no formal similarity with the SN2process with which the E 2c mechanism is postulated to merge.Trans-elimination is greatly favoured for the production of alkyne from cis and trans (202) with methoxide in dimethyl- sulphoxide the cis isomer more readily forming the allene (203).241 Pr" Pr" \ /Br \ /Pr" /c=c\ -+ Pr"-CEC-Pr" t /c=c\ --+ Pr"C=C=C-Et H Pr" H Br trans-(202) cis-(202) (203) krel 6-6 x lo4 1 1500 235 A. M. Katz and W. H. Saunders jun. J. Amer. Chem. Soc. 1969 91 4469. 236 D. N. Kevill and J. E. Dorsey J. Org. Chem. 1969 34 1985. 237 W. H. Saunders jun. and T. A. Ashe J. Amer. Chem. SOC.,1969,91,4473. 238 N. S. Isaacs Ann. Reports (B) 1968 137. 239 E. Eck and J. F. Bunnett J. Amer. Chem. Soc. 1969 91 3099. 240 D. Cook,A. J. Parker and M. Ruane Tetrahedron Letters 1968 5715.241 S. W. Staley and R. F. Doherty Chem. Comm. 1969 288. Reaction Mechanisms Molecular Orbital Applications.-With the availability of sophisticated self- consistent field molecular-orbital programmes it has become possible to carry out calculations on stabilities and reactivities of saturated compounds and ions as well as on n-electrons. Calculations of alkanes and alkyl cations by the CNDO method (Complete Neglect of Differential Overlap) predict the experimental orders of stability and for the cations favour a planar geometry.242 Homoallylic participation is predicted as being at a maximum for the geometry (204). Cyclo- propylmethyl cations are indicated as bearing considerable positive charge on the ring carbons243 and an examination of the vinyl and protonated cyclopropane ions indicates that structures (205) and (206) are favoured.244 Varying the geometry of the cyclopropyl cation and anion from the localised to the delocalised structures results in a gain and loss of energy respectively245 in accordance with the aromatic and antiaromatic nature of these species (208).Calculated excess charge densities on the carbons of substituted benzenes have been compared with resonance components oR,of Hammett substituent constants.246 Pyramidal geometry is theoretically favoured by carbanions as well as amine~.~~~ r\+ AHCalc= -49.57 kcal mol -AH = +143-16kcal mol-' A frontier-orbital approach has been used to correlate SN2and E2 reactivities ester hydrolysis and ketone halogenation using as reactivity index the population of the lowest unoccupied MO in extended Hiickel calculations.248~249 A trans-p elimination geometry is shown to be preferred.Calculations have been extended to some non-classical ions ;a delocalised norbornenyl cation is but 242 N. S. Isaacs Tetrahedron 1969 3555. 243 M. S. Tremper and D. D. Shillady J. Amer. Chem. Soc. 1969 91 6341. 244 R. Sustmann J. E. Williams M. J. S. Dewar L. C. Allen and P. v. R. Schleyer J. Amer. Chem. SOC.,1969,91 5350. 14' D. T. Clark Chem. Comm. 1969,637. 246 R. T. C. Brownlee and R. W. Taft J. Amer. Chem. SOC.,1968,90 6537. 247 T. P. Lewis Tetrahedron 1969 25 41 17. 248 K. Fukui H. Hao and H. Fujimoto Bull. Chem. SOC.Japan 1969 42 348. 249 H. Hao H. Fujimoto and K.Fukui Bull. Chem. SOC.Japan 1969,42 1256. H. 0.Ohorodnyk and D. P. Santry J. Amer. Chem. SOC.,1969,91,4711. 142 N. S.Isaacs the geometry at C-7 is variously interpreted as planar251 and pyramidal.252 The edge-protonated tricyclene structure (58) appears to be most stable for the norbornyl cation in agreement with the stable species in superacid ”‘ J. E. Williams jun. R. Sustmann L. C. Allen and P. v. R. Schleyer J. Amer. Chem. SOC.,1969 91 1037. 2s2 T. Tsuchima and H. Tanida J. Chem. SOC.Japan 1969,90 650. 253 G. Klopman J. Amer. Chem. SOC.,1969 91 89.

ISSN:0069-3030    

DOI:10.1039/OC9696600098

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

(Part iii) Reaction Mechanisms 3 By B. G. ODELL University Chemical laboratory Lensfield Road Cambridge CB2 IEW Orbital Symmetry Correlations.-'Since every elementary step in any chemical reaction is a concerted process correlative ideas must be applicable to all reac- tions'.' The major event this year in the interpretation of the mechanisms of concerted reactions if not in physical organic chemistry as a whole is the publica- tion of 'The Conservation of Orbital Symmetry' by Woodward and Hoffmann.' It is extensive and contains much previously unpublished material. In addition to explaining in lucid and colourful detail the principle that orbital symmetry is conserved in concerted reactions a principle that is so fundamental that there are no violations,' the new term 'pericyclic reaction' is introduced to describe reac- tions in which all first-order changes in bonding relationships take place in con- cert on a closed curve.In previous studies Woodward and Hoffmann have pointed out that electrocyclic processes can be treated as concerted intramolecular cycloadditions. It is now stated' that all pericyclic reactions may be looked on as concerted cycloadditions and must obey the selection rule :a ground state peri- cyclic change is symmetry allowed when the total number of [4q + 21 and [4rIa components is odd. For example in a cycloaddition reaction where there is an interaction between a 47c system and a 27c system both acting suprafacially [,4 + ,2J there is a single pertinent component namely [,2,] and so the well- known Diels-Alder reaction is symmetry allowed.A n-electron system acts suprafacially when both bonds are formed to the same face of the n-bond (1)and antarafacially when bonds are formed to opposite faces (2).' o-Bonds may also R. B. Woodward and R. Hoffmann Angew. Chem. Internat. Edn. 1Y69 8 781. 144 B. G. Ode11 act as components in a pericyclic process. A a-bond is involved in a suprafacial sense if configuration is retained (3) or inverted (4) at both of its termini in the course of the reaction it is involved in an antarafacial sense if configuration is retained at one terminus and inverted at the other (5). In the case where q = 0 the component may be an occupied orbital and where r = 0 it is an empty orbital. These are classified o components and may themselves act suprafacially (6) or antarafacially (7) e.g.disrotatory cyclopropyl cation to ally1 cation conversion can be considered as [,2 + ,O,] (8) or [,2 + ,O,] (9) and as such is allowed.' When attempting to apply the above rule the geometry of each individual case must be examined on its merits to see whether it can be realised in practice. When reacting components are joined by non-participating bonds antibonding inter- actions may be generated which may render an otherwise allowed process sym- metry forbidden e.g. in the prismane to benzene conversion' which has been studied experimentally by Oth.2 Odd-electron systems generally conform to the pattern for the system con- taining one more electron. ' There is in general corresponding to every ground- state reaction a related excited-state process for which the rule is reversed.' Correlations in excited-state reactions have been studied further by van der Lugt and Oo~terhof,~ and Kearns4 has presented an analysis of the cycloaddition reactions of singlet oxygen.Bryce-Srnith5 notes that thermal reactions of benzene are in agreement with Woodward-Hoffmann concepts but that there are apparent differences for excited-state processes. A review by Miller6 includes a study of the effect of orbital symmetry on stereoselectivity in organic reactions. Bauer7 has pointed out that vibrationally excited fragments are to be expected in concerted gas phase eliminations. A review' of unimolecular thermal reactions discusses the mechanisms of many types of' pericyclic and non-concerted processes in relation to kinetic parameters and puts in a plea to authors to present rate data on which Arrhenius parameters and activation enthalpies and entropies are based.For the purposes of this survey the classification of all pericyclic reactions as cycloadditions' will not be followed in all cases but this terminology will be used to clarify individual points. J. F. M. Oth Rec. Trav. chim. 1968 87 1185. W. T. A. M. van der Lugt and L. J. Oosterhof J. Amer. Chem. SOC.,1969 91 6042. D. R. Kearns J. Amer. Chem. SOC. 1969 91 6554. D. Bryce-Smith Chem. Comm. 1969 806. S. I. Miller Ado. Phys. Org. Chem. 1968 6 185. ' S. H. Bauer J. Amer. Chem. SOC.,1969 91 3688. H. M. Frey and R.Walsh Chem. Rev. 1969,69 103. Reaction Mechanisms Cycloadditions.-Saltie19 has demonstrated the stereospecificity of the photo- chemical [2 + 21 cycloaddition in a study of the irradiation of tricyclododecenes. The trans-isomer (10) gives only the trans-cyclododecenye (1 1) while the cis-compound (12) gives the cis olefin (13). In the latter case some 1,l-bicyclohexenyl 1 1 is also produced arising from the disrotatory opening of the cyclobutene rings. Paquette" has found that the [4,4,2]propellapentaene (14) is thermally stable despite the energy gained by fragmentation to naphthalene and acetylene. This fragmentation is allowed in the excited state and occurs readily on irradiation. The photochemical dimerisation of the but-2-enes has been shown" to be stereospecific in the liquid state.The electronically excited olefins (either free or as excimers) retain their geometrical configuration from photon absorption to the completion of the dimerisation. The interesting [,2 + ,,2,] photochemical process has been studied by Schaffner.I2 Photolysis of the specifically deuteriated steroidal enone (15) pro-ceeds with retention of configuration at C-l and inversion at C-10 which is in- consistent with fragmentation of the C-1-C-10 bond. In this particular system antarafacial participation of the double bond is necessitated by geometrical considerations. OAc OAc J. Saltiel and L.-S. Ng Lim J. Amer. Chem. SOC. 1969 91 5404. lo L. A. Paquette and J. C. Philips J. Amer. Chem. SOC.,1969,91 3973; Chem.Comm. 1969 680. " H. Yamazaki and R. J. Cvetanovic J. Amer. Chem. SOC.,1969 91 520. l2 D. Bellus D. R. Kearns and K. Schaffner Helv. Chim. Acta 1969 52 971. 146 B.G. Odell Much attention has been given to the [2 + 21 cycloaddition of two n-bonds and its reversal. Salem' has shown by a study of molecular vibrations that the decomposition of cyclobutane in the coplanar mode should proceed more favourably in two steps while the concerted process involves puckering in the B1,mode combined with twisting (Alu + Eg). FreyI4 has made the relevant observation that 1,1,3,3-tetramethylcyclobutanehas a rate of pyrolysis about one quarter that of 1,l-dimethylcyclobutane which suggests a 1,3-interaction in the transition state. The kinetic parameters also favour twisting.Frey also reports' on the thermal decomposition of trans-6,7-dimethyl- and cis-exo-6,7-dimethyl- bicyclo[3,2,0]heptane. The trans-compound (16) decomposes with ca. 75 % retention of configuration while the cis-isomer (17) yields equal amounts of the but-2-enes. Here a two-step process is preferred.' The concensus of opinion at present seems to be weighted in favour of a biradical mechanism for the fragmentation of simple cyclobutanes. It follows from an analysis of the crossed approach of two ethylenes in the symmetry-allowed [,2 + ,2,] process that a twisting of the participating olefins will be favourable. The further twisting which is generated along the reaction co-ordinate will be of opposite chirality in the two components.This leads to the prediction that optically active forms of olefins such as trans-cyclo-octene deriva- tives will dimerise less readily than the racemic substances.' The dimerisation of cis-trans-cyclo-octa-1,5-diene (both active and racemic) has been reported in a lecture by LeitichI6 and further studies in this field are awaited. Several reports of the cycloaddition of benzyne to olefins have appeared and a lack of stereospecificity has been found.17 This has led to the suggestion of a two-step pathway in the [2 + 21 process in contrast to the [4+ 21 cycloaddition of benzyne to trans-trans-hexa-2,4-dienewhich is stereospecific' leading to (18). These results imply a symmetric ground state for benzyne. The reaction of dimethyl azodicarboxylate with enol ethers which leads to diazetidines (19) has been studied.18 Substitution of protium by deuterium in the vinyl methylene group of ethyl vinyl ether accelerates product formation by a factor of 1.21 per deuteron while deuterium at the other vinyl position slows the l3 L.Salem and J. S. Wright J. Amer. Chem. SOC., 1969 91 5947. l4 A. T. Cocks and H. M. Frey J. Chem. SOC.(A) 1969 1671. l5 A. T. Cocks H. M. Frey and I. D. R. Stevens Chem. Comm. 1969,458. l6 J. Leitich Lecture at Hamburg reported in Angew. Chem. Znternat. Edn. 1969 8 909. M. Jones jun. and R. H. Levin J. Amer. Chem. SOC.,1969 91 641 1 ;P. G. Gassman H. P. Benecke and T. J. Murphy Tetrahedron Letters 1969 1649; R. W. Atkin and C. W. Rees Chem. Comm. 1969 152. E. K. v.Gustorf D. V. White J. Leitich and D. Henneberg Tetrahedron Letters 1969 31 13. Reaction Mechanisms Me rate to 0.89 of the protium value. When an sp2 hybridised carbon is converted to sp3 in the rate-determining step an inverse secondary deuterium isotope effect is normally ob~erved.'~ Here it is argued that only one centre is involved in an sp2 -+sp3 change in the transition state and a two-step mechanism is proposed supported by the trapping of an intermediate dipolar species. This [2 + 21 cycloaddition shows small rate dependence on solvent polarity a low enthalpy of activation and a large negative entropy of activation which here do not allow an unambiguous distinction between concerted and non-concerted processes. The related decomposition of (20) yields 94 % of norbornadiene and 6 % of quadri- cyclane.20 The symmetry allowed [2 + 2 + 21 process competes poorly if at all with what appears from its activation parameters to be a stepwise process.(Quadricyclane formation itself may be stepwise). A review of chemiluminescence from the concerted decomposition of per- oxides' ' and reports on dioxetan decomposition related to that described last year22 have appeared. K~pecky~~ has shown that decomposition of 3,3,4-trimethyl-1,2-dioxetane (21) at 60"C in benzene leads to luminescence and White24 has made use of the presumed excited-state carbonyl compound pro- duced. Products identical with those previously obtained photochemically were generated when substrates such as dienones were heated in the presence of (21) in the absence of light.The deuteriated bicyclopentane (22) adds maleic anhydride2 'below the flap of the envelope' to give (23). With maleonitrile and fumaronitrile the reaction is not stereospecific and leads to various products derived from the biradical(24). (23) 0 l9 See J. Sauer Angew. Chem. Internat. Edn. 1967 6 16. 2o N. Rieber J. Alberts J. A. Lipsky and D. M. Lemal J. Amer. Chem. SOC. 1969 91 5668. '' M. M. Rauhut Accounts Chem. Res. 1969 2 80. 22 A. Ledwith Ann. Reports (B) 1968 65 143. 20 K. R. Kopecky and C. Mumford Canad. J. Chem. 1969 45 709. L4 E. H. White J. Wiecko and D. F. Roswell J. Amer. Chem. SOC. 1969 91 5194. 25 P. G. Gassman K. T. Mansfield and T. J. Murphy J. Amer. Chem. SOC.,1969 91 1684.148 B. G. Ode11 Huisgen26 has presented an extensive account including kinetics,26e of the cycloaddition of diphenylketen to olefins vinyl ethers enamines and dienes. A one-step process involving a [,2 + .2,] cycloaddition where the keten is the antarafacial component is favoured in the case of olefins and enol ethers.’’26d Baldwin,” studying the reaction of diphenylketen with styrene found that there was an inverse secondary deuterium isotope effect (kdkD = 0.91 per deuteron) when the vinyl methylene hydrogens were replaced by deuterium and a normal effect (kdkD = 1.23)for substitution of the other vinyl hydrogen. This is inter- preted as meaning that both vinyl carbons are involved in bonding in the rate- determining step and that the centre which shows the normal isotope effect is being twisted out of conjugation with the adjacent carbon.Woodward and Hoffmann’ show the parallel between vinyl cations and ketens and interpret the surprising reaction28 of but-2-yne with chlorine in the presence of aluminium trichloride to give the dichlorotetramethylcyclobutene (25) as a [,2 + ,2,] reaction between a chlorovinyl cation and the acetylene. Me Me Me Me Me Me I I C+ #r.l c1-, II +i-)-y C C C1 /\ I Me Me Me Me Me C1 Me (25) The reaction between dimethylketen and N-isobutenylpyrrolidine gives the aminocyclobutanone (26) by two competing pathway^;^' one is a concerted 26 (a) R. Huisgen and L. A. Feiler Chem. Ber. 1969 102 3391 ; (b) R. Huisgen L. A. Feiler and P. Otto ibid.1969 102 3405; (c) L. A. Feiler and R. Huisgen ibid. 1969 102,3428; (d)R. Huisgen and P. Otto ibid. 1969,102,3475; (e)R. Huisgen L. A. Feiler and P. Otto ibid. 1969 102 3444. 27 J. E. Baldwin and J. A. Kapecki J. Amer. Chem. SOC.,1969,91 3106; note the conclu sions of ref. 20. 28 See R. Criegee and A. Moschel Chem. Ber. 1959 92 2181. 29 R. Huisgen and P. Otto J. Amer. Chem. SOC.,1969 91 5922. React ion Mechanisms cycloaddition and the other goes via a dipolar intermediate (27) which can also react with a further molecule of the keten. A striking endo-specificity of the [2 + 21-cycloaddition of aldoketens to cyclopentadiene has also been claimed this year.30 The mechanism of the thermal dimerisation of allenes to give dimethylene- cyclobutanes has been in question and has prompted new studies.31 Dimerisa- tion3la of optically active cyclonona-1,2-diene gives rise mainly to (28) while that of the racemic substance gives mainly (29).The combination of enantiomers is faster than the combination of molecules of like chirality. These results lead to the interpretation that the process is either [,2 + ,2,] or that there is addition to give a perpendicular 2,2’-bi-allylene diradical (30) followed by stereospecific closure which is probably conrotatory. The related reaction isomerisation of methylallene dimers occurs with conrotatory opening of the cyclobutane ring.32 Diels-Alder reactions between pentachlorocyclopentadiene and dienophiles yield predominantly anti-7-chloronorbornenes accounted for in terms of dipole- dipole or similar interaction^.^^ The retro-Diels-Alder elimination of nitriles in the reactions of isoxazoles with dimethyl acetylenedicarboxylate have been reported.34 Photolysis or pyrolysis of (31)leads to the loss of the (CO),bridge.35 In the mass spectrometer a metastable transition is observed suggesting the loss of carbon monoxide dimer in one step.The geometrically favourable ene reaction of (32)36 has a half-life of ca 6 hr at 45 “Cas compared with typical conditions reported in an excellent review of the reaction.37 The dimerisation of cyclopropene to 3-cyclopropylcyclopropene38 30 W. T. Brady E. F. Hoff R. Roe jun. and F. H. Parry jun. J. Amer. Chem. SOC. 1969 91 5679. 31 (a) W. R. Moore R. D. Bach and T.M. Ozretich J. Amer. Chem. SOC. 1969 91 5918; (b) E. V. Dehmlow Tetrahedron Letters 1969 4283; (c) T. L. Jacobs J. R. McClenon and 0. J. Muscio jun. J. Amer. Chem. SOC.,1969 91 6038. ”J. J. Gajewski and C. N. Smith J. Amer. Chem. SOC.,1969 91 5900. 33 K. L. Williamson Y.-F. Li Hsu R. Lacko and C. H. Youn J. Amer. Chem. SOC. 1969 91 6129. ’’ R. Grigg R. Hayes and J. L. Jackson Chem. Comm. 1969 1167. 35 J. Strating B. Zwanenburg A Wagenaar and A. C. Udding Tetrahedron Letters 1969 125. 36 J. M. Brown J. Chem. SOC.(B) 1969 868. 37 H. M. R. Hoffmann Angew. Chem..Znternat. Edn. 1969 8 556. 38 P. A. Dowd and A. Gold Tetrahedron Letters 1969 85. 150 B.G. Ode11 0 and the pyrolysis of ally1 formate to propylene and carbon dioxide3’ represent another example of this process and its reverse respectively.The ene mechanism for the reaction of singlet oxygen with olefins to give allylic hydroperoxides has been questi~ned.~,~~ The retro-homo-Diels-Alder reaction has been studied by Berson and In the case of the oxidation of 3,4-diazabicyclo[4,1,O]heptanese.g.(33) they found a large preference for a transition state (34) in which the departing nitrogen mole- cule and the cyclopropane,ring have a trans orientation. A similar reaction has been studied by Allred42 who found that (35) gave cyclo-octa-1,5-diene losing nitrogen lo5 times faster than 2,3-diazabicyclo[2,2,2]oct-2-ene.However it is not clear whether this is a concerted [2+ 2 + 21 process. The rate of the cycloaddition of fumaroyl chloride to cycloheptatriene has been compared with the addition of this dienophile to various model systems.43 The favoured conclusion is that this is a [2+ 2 + 21process (36) and does not proceed via a norcaradiene intermediate as had been frequently suggested.44 Woodward and Houk4’ have reached a similar conclusion.Two other interesting [2+ 2 + 21 cycloadditions have been reported. Maleic anhydride adds to basketene (37) to give (38)46and 1,3-di-t-butylcyclobutadiene 39 J. M. Vernon and D. J. Waddington Chem. Comm. 1969 623. ‘O W. Fenical D. R. Kearns and P. Radlick J. Amer. Chem. SOC.,1969 91 3396. 41 J. A. Berson and S. S. Olin J. Amer. Chem. SOC.,1969 91 777. 42 E. L. Allred and J. C. Hinshaw Chem. Comm. 1969 1021. 43 T. Tsuji S. Teratake and H.Tanida Bull. Chem. SOC.,Japan 1969 42 2033. “See ti. Maier Angew. Chem. Internat. Edn. 1967 6 402. 45 K. N. Iiouk Ph.D. Thesis Harvard Univ. 1968. ’‘E. Le Goff and S. Oka J. Amer. Chem. SOC.,1969 91 5665. Reaction Mechanisms ph@ phjg Ph Ph (39) is trapped by tetracyanoethylene to give (39).4’ Trapping experiments suggest that 1,2-diphenylcyclobu tadiene exhibits valence tautomerism (40).47 Most 1,3-dipoles are isoelectronic with ozone or with nitrous oxide and con- tain a three orbital system occupied by four electrons similar to that found in the allyl anion. Their [,4 + .2,] reactions which are 1,3-dipolar cycloadditions are symmetry allowed.’ Huisgen4*” has reported on the kinetics of nitrone addition to unsaturated compounds and favours a one-step many-centre mechanism ; pyridine-N-oxides have also been studied as 1,3-dipole~.~~~ In another series of papers49 Huisgen elaborates on the conrotatory opening of aziridines to azo- methine ylides and their cycloaddition to various olefins.Other related reports on azomethine ylides have appeared.” A new preparation of nitrilimines by oxidation of aldehyde arylhydrazones has been reported5 and 1,3-dimesityl- nitrilimine has been shown to exist in solution for some time and its pyridine adduct to di~sociate.’~ Water and some dipolar aprotic solvents have an appre- ciable effect on the rates of some 1,3-dipolar cycloadditions exemplified by the fact that addition of diazomethane to trans-stilbene proceeds at a preparatively useful rate in aqueous dioxan while there is no reaction in dry ether.Kadaba attributes this phenomenon to solvation of the transition state of a concerted proce~s.’~ A possible first example of the allowed cycloaddition of an allyl anion to a C-C double bond has been observeds4 but it may here be a two-step process. 47 P. Reeves J. Henery and R. Pettit J. Amer. Chem. SOC.,1969 91 5888; P. Reeves T. Devon and R. Pettit ibid. 1969 91 5890. 48 (a) R. Huisgen H. Seidl and I. Briining Chem. Ber. 1969 102 1102; (b)R. Huisgen H. Seidl and J. Wulff ibid. 1969 102 915. 49 R. Huisgen W. Scheer and H. Mader Angew. Chem. Internat. Edn. 1969 8 602; R. Huisgen W. Scheer H. Mader and E. Brunn ibid. 1969 8 604; R. Huisgen and H. Mader ibid. 1969 8 604. 50 F.Texier and R. Carrie Tetrahedron Letters 1969 823; F. Texier J. Jaz and R. Carrie Compt. rend. 1969 269 C 646; J. A. Deyrup J. Org. Chem. 1969 34 2724. 51 W. A. F. Gladstone J. B. Aylward and R. 0. C. Norman J. Chem. SOC.(C) 1969 2587. 52 C. Grundmann and K. Flory Annafen 1969 721 91. 53 P. K. Kadaba Tetrahedron 1969 25 3053. 54 A. T. Nielson R. C. Weiss and D. W. Moore Chem. Comm. 1969 1281. 152 B. G. Odell Chlorosulphonyl isocyanate C1-SO2-NCO probably reacts with olehs to give a dipolar intermediate which may close to form a variety of products. Cla~ss~~ has studied the kinetics of its reaction with simple olefins and Paq~ette~~ proposes a homotropylium cation intermediate in its reaction with cyclo- octatetraene. Stereospecificity of the 174-dipolar cycloaddition of (41) to olefins has been given5’ as evidence against a two-step mechanism.The conversion of the [6 + 41 carboethoxyazepin dimer to the [6 + 61 dimer a symmetry-forbidden process probably involves a diradical intermediate.58 N-Carboalkoxyazepines give Diels-Alder adducts5’ at the 4,Sdouble bond [e.g.(42)] the preference for this bond most remote from nitrogen does not depend on the electronic excess or deficiency of the diene component.59b [6 + 21-Cyclo-adducts of a methano-fidecene,60 tropone,6 ’and a sesquifulvalene6* have been reported the latter compound also yielding62 a symmetry allowed [,14 + .2,] adduct (43). Et02C*N&ph. Ph H’ H Me NC CN CN CN @ \ QJC’ c1 Ph c1 1,ZQuinones give rise to many surprising thermal and photochemical cyclo- adducts.Their thermal reactions have been reviewed63 and thermal [4 + 41-cycloadducts e.g. (44)reported64 for the first time. This process being thermally forbidden a dipolar intermediate is most likely. 55 K. Clauss Annalen 1969 722 110. 56 L. A. Paquette J. R. Malpass and T. J. Barton J. Amer. Chem. SOC.,1969 91 4714. 5’ R. R. Schmidt Angew Chem. Internat. Edn. 1969 8 602. 5* L. A. Paquette J. H. Barrett and D. E. Kuhla J. Amer. Chem. SOC.,1969 91 3616. 59 (a)J. R. Wiseman and B. P. Chong Tetrahedron Letters 1969 1619; (6)L. A. Paquette D. E. Kuhla,.J. H. Barrett and L. M. Leichter J. Org. Chem. 1969 34 2888; (c) C. M. D. Beels’ and B. G. Odell unpublished work. 6o H. Prinzbach L. Knothe and A.Dieffenbacher Tetrahedron Letters 1969 2093. 61 T. Miwa M. Kato and T. Tamano Tetrahedron Letters 1969 1761. 62 H. Prinzbach and H. Knofel Angew. Chem. Internat. Edn. 1969 8 881. 63 W. M. Horspool Quart. Rev. 1969 23 204. 64 W. Friedrichsen Tetrahedron Letters 1969 4425. Reaction Mechanisms Evidence for an intermediate has been presented6' in a metal-catalysed [,2 + .2,] cycloaddition reaction showing that in one case at least a concerted pathway is not followed. Electrocyclic Reactions.-The full Woodward-Hoffmann predictions on the disrotatory opening of cyclopropyl halides cannot be directly tested by solvolysis studies because one of the asymmetric centres is lost on capturing the allyl cation. Schleyer66 reports that in SbF,-SO,ClF at -100°C the isomeric dimethyl- cyclopropyl chlorides give allyl cations in complete accord with predictions (see Chap.3 Part (ii) p. 99). There can be no free cyclopropyl cation intermediate. Solvolytic ring opening of N-chlor~aziridines~~~~ is suggested to be an analogue of this process. cis-Bicyclo[4,2,0]oct-7-ene has been shown6' to open in the normal conrotatory mode to give cis,trans-cyclo-octadiene;as previously found the cis,&-diene is produced at high temperature. Sorenson7' has demonstrated the preponderance of allowed conrotatory closure of methylated pentadienyl cations to give cyclopentyl cations. The disrotatory 6n electrocyclisation of pentadienyl anions and their heteroatom analogues has been observed for the first time this year71 as has the opening of hetero-cyclopentenyl anions.72 Cyclo-octadienyl anion (45) is converted at 35 "C into cis-bicyclo[3,3,0]octenyl anion (46)7 la and amarine (48)is formed on cyclisation of (47)' lb In the 8nseries heptatrienyl anion has been observed7 to close to cycloheptadienyl anion which in contrast to (49 does not cyclise further.The related opening of l-oxa- cycloheptadienyl anion has also been obser~ed.~ 65 T. J. Katz and S. A. Cerefice J. Amer. Chem. SOC.,1969 91 6519. 66 P. von R. Schleyer T. M. Su M. Saunders and J. C. Rosenfeld J. Amer. Chem. SOC. 1969 91 5174. 67 P. G. Gassman and D. K. Dygos J. Amer. Chem. SOC. 1969 91 1543. 68 D. C. Horwell and C. W. Rees Chem. Comm. 1969 1428. 69 J. S. McConaghy jun. and J. J. Bloomfield Tetrahedron Letters 1969 3719 3723.70 P. H. Campbell N. W. K. Chiu K. Deugau I. J. Miller and T. S. Scxensen. J. Amer. Chem. Soc 1969,91 6404. 71 (a) R. B. Bates and D. A. McCombs Tetrahedron Letters 1969 977; (b)D. H. Hunter and S. K. Sim J. Amer. Cheni. SOC.,1969 91 6202. 12 (a) H. Kloosterziel J. A. A. van Drunen and P. Galama Chem. Comm. 1969 885; (6) U. Schollkopf F. Gerhardt and R. Schroder Angew. Chem. Znternat. Edn. 1969 8 672. 73 R. B. Bates W. H. Deines D. A. McCombs and D. E. Potter J. Amer. Chem. SOC. 1969 91 4608; H. Kloosterziel and J. A. A. van Drunen Rec. Trav. chim. 1969 88 1084. 154 B. G. Ode11 The kinetics of cyclisation of deca-2,4,6,8-tetraenes the conrotatory specificity of which had been previously reported have appeared.74 The reversibility of this process enabled the authors to show that the conrotatory closure is favoured by a factor of at least 11 kcal mol- over the forbidden disrotatory.Trans,cis,cis trans-diphenyloctatetraene cyclisation7 is less simple giving rise to eight products. The chemistry of medium sized polyenes and their analogues (especially nine- membered rings) has received much attention and provides many verifications of orbital symmetry conservation. Space permits only two examples all-cis-~xonin~~ cyclises readily to give cis-8,9-dihydrobenzofuran; all-cis-cyclo-decapentaene gives cis-9,lO-dihydronaphthalenewhile (cis),,trans-cyclodecapen-taene gives the trans-isomer. Sigmatropic Rearrangements.-The thermal rearrangement of some bicyclo- [2,l,l]hex-2-ene derivatives to bicyclo[3,l,0]hexenes has been shown to proceed with inversion of configuration at the migrating centre,78 e.g.(49)-(50) and kinetic measurements on the parent system" support a concerted [1,3]-sigma- tropic shift (a [,2 + ,2,] process) which is the formal reverse of the vinyl cyclo- propane rearrangement. The related thermal racemisation of thujene (5 1)" and the rearrangement (52)-+(53)have been observed.*' In these two cases inversion is precluded geometrically and two-step pathways are preferred. Chemically induced dynamic nuclear polarisation (CIDNP) has become an important probe for the detection of radical processes in sigmatropic rearrange- ments. By this technique the [1,3]-shift which leads to the aromatisation of (54) (52) (53) 74 R.Huisgen A. Dahmen and H. Huber Tetrahedron Lerters 1969 1461 ; A. Dahmen and R. Huisgen ibid. 1969 1465. 75 E. N. Marvel1 and J. Seubert Tetrahedron Letters 1969 1333. 76 A. G. Anastassiou and R. P. Cellura Chem. Comm. 1969 1521; J. M. Holovka R. R. Grabbe P. D. Gardner C. B. Strow M. L. Hill and T. V. Van Auken Chem. Comm. 1969 1522. 77 S. Masamune and R. T. Seidner Chem. Comm. 1969 542. 78 (a) S. Masamune S. Takada N. Nakatsuka R. Vukov and E. N. Cain J. Amer. Chem. SOC. 1969 91 4322; (6) W. R. Roth and A. Friedrich Tetrahedron Letters 1969 2607. 79 H. M. Frey R. G. Hopkins H. E. O'Neal and F. T. Bond. Chem. Comm. 1969 1069. Lecture by W. von E. Doering at Salt Lake City quoted by H. Tanida and Y.Hata J. Amer. Chem. SOC.,1969 91 6775. JA Meinwald and D. Schmidt J. Amer. Chem. SOC.,1969 91 5877; J. Meinwald and H. Tsurata ibid. 1969 91 5877; H. E. Zimmerman D. J. Robbins and J. Schantl ibid. 1969 91 5878. Reaction Mechanisms has been shown82 to involve homolytic dissociation. In the general process shown in the Scheme the [1,2]-rearrangement of ylides is thermally allowed with inversion of configuration of the migrating centre ; the [3,2]-process can occur suprafacially. Radical pathways are also possible with different stereochemical consequences. The well-known Stevens rearrangement is an example which has been studied exten~ively.'~ The rearrangement of sulphur ylides (where Y = CH,R and X = SR') is instructive. Compound (55) rearranges at low tempera- x = OR,NR?] SR etc.Y-x Scheme ture to give 99 % of (56) while at 65 "C many products are formed. These can arise via a homolytic process which only competes with the allowed [3,2]-rearrangement at higher temperatures. 84 The thermal isomerisation of the deuteriated biallyl (57)leads" to an equili- brium (Kzooo= 1-2). In related works6 the kinetic secondary deuterium isotope J. E. Baldwin and J. E. Brown J. Amer. Chem. SOC.,1969 91 3647. 83 U. Schollkopf U. Ludwig G. Ostermann and M. Patsch Tetrahedron Letters 1969 3415; See D. G. Morris Chem. Comm. 1969 1345 for many further references. 84 J. E. Baldwin and R. E. Hackler J. Amer. Chem. SOC.,1969 91 3646. R. Malojcic K. Humski S. Borcic and D. E. Sunko Tetrahedron Letters 1969 2003.K. Humski T. Strelkov S. Borcic and D. E. Sunko Chem. Comm. 1969 693. 156 B. G. Odelf effect in the Cope rearrangement of a substituted biallyl has been studied. The preferred conformation of the transition state of the aromatic Claisen rearrange- ment has been shown to be chair-likes7 while in another study assuming a chair transition state for the aliphatic Claisen and Cope rearrangements conforma- tional analysis has been used to predict cis:transratios in the products.ss This has led to stereoselective syntheses of trisubstituted 01efins.'~ A potential antarafacial [1,5]-hydrogen migration favoured by the twisted geometry of the system has been suggestedg0 for the photochemical rearrange- ment of (58). Migratory aptitudes in the 1,5-shift have been studied for arylated indenes." The order of preference for migration is H > Ph > Me which differs R= C0,Me from that found in cationic and radical rearrangements.Rearrangement of the azepin (59) may be a concerted [1,5]-nitrogen shift.92 Tropone dimethyl acetal (60)rearranges by a [1,7]-methoxy shift which may be concerted using the oxygen G. Frater A. Habich H.-J. Hansen and H. Schmid Hefu. Chim. Acta 1969 52 335. C. L. Perrin and D. J. Faulkner Tetrahedron Letters 1969 2783. 89 D. J. Faulkner and M. R. Petersen Tetrahedron Letters 1969 3243; N. Wakabayashi R. M. Waters and J. P. Church ibid. 1969 3253. 90 E. F. Kiefer and J. Y. Fukunaga Tetrahedron Letters 1969 993; E. F. Kiefer and C. H. Tanna J. Amer. Chem.SOC.,1969 91 4478. 91 L. L. Miller R. Greisinger and R. F. Boyer J. Amer. Chem. Soc. 1969 91 1578. 92 L. A. Paquette D. E. Kuhla and J. H. Barrett J. Urg. Chem. 1969 34 2879. React ion Mechanisms lone pair orbitals.93" The ready [1,5]-sigmatropic shift of hydrogen to sp hybri-dised carbon (61) +(62) has been studied.93b Examples of [1,7]-sigmatropic hydrogen shifts in o-butadienyl phenols have been observed,94 e.g. (63) +(64). An outstanding example of the power of orbital symmetry predictions is found in the synthesis95 of a corrin derivative. The key step is a light-induced [1,16]- hydrogen transfer leading to an antarafacial cyclisation. Cheletropic Processes.-These are defined as reactions in which two a-bonds which terminate at a single atom are made or broken in concert,' and are exem- plified by the reversible 1,4-addition of sulphur dioxide to butadiene.Chele- tropic processes have been analysed in detail' as linear where an electron pair in the group being added or eliminated is participating in a suprafacial manner (i.e. as an [,2J component) or nonlinear (i.e. L2,]) processes. Thermolyses of 2,7-dihydrothiepine dioxides give trienes and SOz.96 The cis-2,7-dimethyl iso- mer (65) gives trans,cis,cis-octatriene while the trans-isomer gives the trans,& trans-triene. The stereospecificity is greater than 97 %. The reverse of this linear cheletropic process would be classified [,2 + .6,]. Grigg34 discusses the cheletropic loss of inorganic fragments following thermal electrocyclic reactions and has provided an example of sulphur extrusion in the synthesis of a macro- cycle.97 The reaction of SO (probably 'Z-) with trienes to give thiepin deriva- tives has been studiedg8 as has work on the elimination of sulphides from inter- mediates containing tetravalent sulphur.99 The reaction of thietanonium salt (66) with butyl-lithium gives cis-l,2-dimethylcyclopropanewith a high degree of stereo~pecificity.~'~ This may be a concerted conrotatory fragmentation or may involve conrotatory closure of an intermediate trimethylene diradical.93 (a)R. W. Hoffman K. R. Eicken H. J. Luthardt and B. Dittrich Tetrahedron Lerters 1969 3789; (6) L. Skattebol Tetrahedron 1969 25 4933. "R. Hug H.-J. Hansen and H. Schmid Chimia (Switz.).1969 23 108; E.E. Schweizer D. M. Crouse and D. L. Dalrymple Chem. Comm. 1969 354. 9s Y. Yamada D. Miljkovic P. Wehrli B. Golding P. Loliger R. Keese K. Muller and A. Eschenmoser Angew. Chem. Znternat. Edn. 1969 8 343; A. Eschenmoser Pure Appl. Chem. 1969 20 1. 96 W. L. Mock J. Amer. Chem. SOC.,1969 91 5682. 97 M. J. Broadhurst R. Grigg and A. W. Johnson Chem. Comm. 1969 23. 98 R. M. Dodson and J. P. Nelson Chem. Comm. 1969 1159. 99 (a)D. C. Owsley G. K. Helmkamp and S. N. Spurlock J. Amer. Chem. SOC.,1969,91 3606; (6)B. M. Trost W. L. Schinski and I. B. Mantz ibid. 1969 91,4320; (c) B. M. Trost and S. Ziman Chem. Comm. 1969 181. 158 B. G. Ode11 Carbenes-Concise textbooks loo on the chemistry of carbenes have appeared and reviews by Moss"' and Bethelllo2 cover many aspects of their reactions.Decomposition of diphenyldiazomethane in water gives diphenylmethanol and benzophenoneazine. Invariance of the product proportions when H20 is replaced by D20,coupled with a large tritium isotope effect on the formation of diphenylmethanol denies a carbonium ion mechanism and suggests that ylide (67) is formed by attack of water on the carbene.'03 Ando et al. report on the reactions of dicarbomethoxycarbene with ally1 sulphides' and halides. 04' Addition to the double bond and insertion products are found and ylides [e.g. A H Ph H (67) (68) (68)] which undergo sigmatropic rearrangement have been proposed as inter- mediates to account for the allylic inversion observed. Dichlorocarbene formed by high-vacuum pyrolysis of CHCl or CCl, was introduced into olefin mixtures to deduce relative reactivities of olefins towards it.'05 These reactivities correspond to those determined using organo- metallic systems for the generation of the carbene.The implication is that free dichlorocarbene is the cyclopropane-forming reagent in these latter systems. In reply to criticism of his conclusions Seebachlo6 has given strong kinetic evidence for the production of free bis(pheny1thio)methylene in solution during the decomposition of (PhS),CLi. Carbene-forming decompositions of organometallic compounds have received increasing attention. Kinetic studies' O7 suggest that thermal decomposition of (MeO),Si. CH(OMe) gives free methoxycarbene; it can be trapped stereo- specifically by olefins.Seyferth'" has extended his organomercurial route to carbenes ; carbomethoxychlorocarbene and difluorocarbene are now available in high yield. The latter can also be obtained from organotin compounds."' loo T. L. Gilchrist and C. W. Rees 'Carbenes Nitrenes and Arynes,' Nelson London 1969; W. Kirmse 'Carbene Carbenoide und Carbenanaloge,' Verlag Chemie GmbH Weinheim/Bergstr. 1969. R. A. Moss Chem. Eng. News. 1969 June 16th p. 60 June 30th p. 50. lo* D. Bethell Ado. Phys. Org. Chem. 1969 7 153. Io3 D. Bethell A. R. Newall G. Stevens and D. Whittaker J. Chem. SOC.(E) 1969 749. (a) W. Ando K. Nakayama K. Ichibori and T. Migata J. Amer. Chem. SOC.,1969 91 5164; (b)W. Ando S. Kondo and T. Migata ibid. 1969 91 6516. lo5 P.S. Skell and M. S. Cholod J. Amer. Chem. SOC.,1969 91 6035. lo6 D. Seebach and A. K. Beck J. Amer. Chem. SOC.,1969 91 1540. lo' W. H. Atwell D. R. Weyenberg and J. G. Uhlmann J. Amer. Chem. SOC.,1969 91 2025. D. Seyferth D. C. Mueller and R. L. Lambert jun. J. Amer. Chem. SOC.,1969 91 1562; D. Seyferth S. P. Hopper and K. V. Darragh ibid. 1969 91 6536; D. Seyferth and F. M. Armbrecht jun. ibid. 1969 91 2616. Reaction Mechanisms Bromocarbomethoxycarbene has been shown to have a singlet ground state.' O9 The reaction of dichlorocarbene with optically active 2-phenylbutane gives an inactive tertiary insertion product while with truns-cyclopentane-1,2-diol diacetate methylene gives inversion of configuration ;' both results are com- patible with ion-pair intermediates.A new reaction of triplet methylene which is formally a radical displacement reaction has been observed.' '' 3(CH2)+ (Me),C -+ C,H,' + t-C,Hd Dichlorocarbene reacts with azoles in the gas phase to give high yields of ring-expanded products ;l' with imidazole the C=C is attacked in preference to C=N. These results are in agreement with the work of Rees on similar reactions in so1ution.lt3 Thermal decomposition of methylchlorodiazirine in the gas phase has been studied.' l4 Formation of a free carbene intermediate which rearranges to vinyl chloride is preferred to a mechanism where hydrogen migration is concerted with nitrogen loss. The formal insertion of a carbene into a C-C single bond (69) has been observed.' ' CIDNP has been used to study photolysis or thermolysis of diphenyl- diazomethane in toluene.' N.m.r.signals of l71,2-triphenylethane were ob- served some being inverted. This is evidence for a triplet state intermediate the nuclear polarisation occurring in the product-forming step. Moser' l7 has suggested a carbene-metal-olefin complex in the (trialkyl phosphite) copper(1) chloride catalysed decomposition of ethyl diazoacetate in the presence of olefins. Carbenes and carbenoids have previously been postu- lated as intermediates in the Clemmensen reduction. It has now been reported' ' that benzaldehyde in the presence of zinc and boron trifluoride gives a species which can be trapped by cyclohexene to give 7-phenylnorcarane. Woodward and lo9 U. Schollkopf and M.Reetz Tetrahedron Letters 1969 1541. 'lo ' V. Franzen and R. Edens Annalen 1969 729,33. H. M. Frey and R. Walsh Chem. Comm. 1969 159. F. S. Baker R. E. Busby M. Iqbal J. Parrick and C. J. G. Shaw Chem. and Ind. 1969 1344; R. E. Busby M. Iqbal J. Parrick and C. J G. Shaw Chem. Comm. 1969 1344. Ii3 See R. L. Jones and C. W. Rees J. Chem. SOC.(C) 1969 2255 for further references. M. R. Bridge H. M. Frey and M. T. H. Liu J. Chem. SOC.(A) 1969 91. 'I5 R. A. Moss and J. R. Whittle Chem. Comm. 1969 341. 'I6 G. L. Closs and L. E. Closs J. Amer. Chem. SOC.,1969 91,4549. l7 W. R. Moser J. Amer. Chem. SOC.,1969 91,1 135 1 141. 'I8 I. Elphimoff-Felkin and P. Sarda Chem. Comm. 1969 1065. 160 B. G. Ode11 Hoffmann have noted that the stereospecific combination of singlet carbenes with olefins is a non-linear cheletropic process.' Nitrenes.-Lwowski's' l9 textbook on nitrenes and a review'20 on azide reactions including many references to nitrenes have been published. 4-Cyanophenylnitrene generated in the presence of dimethylamine gives 1,1-dimethyl-2-(4-cyanophenyl)-hydrazinein addition to 4-cyanoaniline the triplet derived abstraction product. This behaviour contrasts with the usual ring expansion of aryl nitrenes to give 3H-azepins ;it is the first example of a singlet aryl nitrene undergoing an intermolecular insertion reaction.' 21 In contrast with previous reports terminal vinyl azides have been shown to give azirines on pho- tolysis or thermolysis.' From a detailed study of the reaction of carbethoxynitrene with 3-methyl- hexane it has been concluded'23 that only the singlet inserts into C-H bonds to a reasonable extent; the triplet does so much more slowly if at all.Pre-dominant if not complete retention of configuration has been found in the intramolecular insertion of an acylnitrene into a tertiary C-H bond.'24 Mesity- lenesulphonyl azide' 2s undergoes a Curtius-type rearrangement in dodecane 2,4,6-trimethylaniline being isolated. Huisgen'26 has reported on the decomposition of pyridyl- and pyrimidyl- azides and has found nitrene derived products. Benzonitrile and the nitrene from 2-azidopyridine give a 1,3-dipolar cycloadduct (70)in addition to the 2:1 adduct l9 W. Lwowski 'Nitrenes,' Interscience New York 1969. lZo G. L'Abbe Chem.Rev. 1969 69 345. lZ1 R. A. Odum and A. M. Aaronson J. Amer. Chem. SOC.,1969 81 5680. lZ2 K. Isomura M. Okada and H. Taniguchi Tetrahedron Letters 1969 4073. lz3 J. M. Simson and W. Lwowski J. Amer. Chem. SOC.,1969 91 5107. 12' S. Yamada and S. Terashima Chem. Comm. 1969 5 11. R. A. Abramovitch and W. D. Holcomb Chem. Comm. 1969 1298. lZ6 R. Huisgen K. v. Fraunburg and H. J. Sturn Tetrahedron Letters 1969 2589; R. Huisgen and K. v. Fraunberg ibid. 1969 2595. Reaction Mechanisms (71) derived from an intermediate nitrilimine.'26 Wentrup' 27 reports that the nitrogen atoms of the nitrene (72)are scrambled probably by way ofa symmetrical intermediate (73). Azide decomposition is catalysed by di-iron nonacarbonyl and probably involves nitrenes or nitrene complexes,128 but the product mixtures are more complex than those obtained in thermal reactions of the azides.Aminonitrenes have been studied extensively. They are formed by deoxygena- tion of nitro~oamines'~~ and by oxidation of 1,l-disubstituted hydrazines' 30 and have been observed to fragment' 29 and to add to olefins (stereospecifically)' 30 and to acetylene^.'^' The latter give rise to 2H-azirines probably by rearrange- ment of antiaromatic lH-azirines. An interesting rearrangement of a (trappable) aminonitrene (74) to give a pyridazine has been described and a mechanism suggested.' 32 Ph phx?ph Ph Nitrenium ions are analogues of carbenes and nitrenes. The amine (75) is probably formed from the triplet state of the nitrenium ion (76) by hydrogen abstraction.In agreement with this theory the proportion of (79 relative to singlet derived products is increased in the presence of solvents containing heavy 12' W. D. Crow and C. Wentrup Chem. Comm. 1969 1387. lz8 C. D. Campbell and C. W. Rees Chem. Comm. 1969 537. lZ9 J. 1. G. Cadogan and J. B. Thomson Chem. Comm. 1969 770; C. J. Overberger M. Valentine and J.-P. Anselme J. Amer. Chem. SOC.,1969 91 687. D. J. Anderson T. L. Gilchrist D. C. Horwell and C. W. Rees Chem. Comm. 1969 146; L. Hoesch and A. S. Drieding Chimia (Switz.) 1969 23,405; see R. S. Atkinson and C. W. Rees J. Chem. SOC.(0,1969 772 778 for further references. 13' D. J. Anderson T. L. Gilchrist and C. W. Rees Chem. Comm. 1969 147. 132 C. W.Rees and M. Yelland Chem. Comm. 1969 377. 162 B. G. Ode11 atoms which are known to facilitate intersystem cr0s~ing.I~~ Other examples of carbene analogues are oxygen and sulphur atoms. A theoretical study of their reactions with olefins has been reported and compared to experimental work.' 34 133 P. G. Gassman and R. L. Cryberg J. Amer. Chem. SOC. 1969 91 5176. L34 E. Leppin and K. Gollnik Tetrahedron Letters 1969 3819.

ISSN:0069-3030    

DOI:10.1039/OC9696600143

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

4 Free-radical Reactions‘ By M. J. PERKINS Department of Chemistry King‘s College Strand London WC2R 2LS THEfundamental question of the geometry of alkyl radicals continues to attract considerable attention. Symons has considered the available e.s.r. evidence and concludes in favour of a preferred planar geometry for alkyl radicals.2 Furthermore a new non-spectroscopic result points to appreciable destabilisation in bridgehead adamantyl radicals ;in a study of the relative ease of fragmentation of radicals (1) to alkyl radicals and benzaldehyde it was found that the ease of formation of 1-adamantyl radicals was considerably less than that of formation of t-butyl radicals and indeed was only a little greater than the ease of formation of methyl radical^.^ Considerable angle strain has also been identified in bridge- head tripticyl radical^.^ ROCHPh + R.+ PhCHO (1) * Recent reviews have dealt with Free Radical Thermochemistry D.M. Golden and S. W. Benson Chem. Rev. 1969 69 125; Radical Reactivity Y. L. Spirin Uspekhi Khim. 1969,38 1201 ;Organic Electron Transfer Reactions K. A. Bilevich and 0.Y. Okhlobystin ibid. 1968 37 2162; Peroxy Radical Chemistry K. U. Ingold Accounts Chem. Research 1969,2 1 ;Autoxidation I. Seree de Roch Ind. chim. belge 1968,33 994; N. M. Emanuel Z. K. Maizus and I. P. Skibida Angew. Chem. Internat. Edn. 1969,8,97; Antioxidants L. R. Mahoney ibid. 1969,8,547; Intramolecular Hydrogen Transfer and Related Processes P. A. Verlrugge Chem. Tech. (Amsterdam) 1968 23 286; Fragmentation and Isomerisation of Radicals J.A. Kew and A. C. Lloyd Quart. Rev. 1968,22,549; Aspects of Radical Additions N. Isenberg and M. Grdinic J. Chem. Educ. 1969,46,601; W. G. Lloyd ibid. 1969,46,299; I. B. Afanasev and G. I. Samokh-valov Uspekhi Khim. 1969,38,687; Cation Radicals in Polymer Synthesis A. Ledwith Ann. New York Acad. Sci. 1969,155 385; Chemiluminescence of Radical Ions E. A. Chandross Trans. New York Acad. Sci. 1969 31 571 ;Chemiluminescence in Kinetic Studies L. Matisova Chem. Listy. 1968 62 1417; High Temperature Pyrolysis of Nitro-Compounds E. K. Fields and S. Meyerson Accounts Chem. Res. 1969 2 273; Methods for Production and Study of Short-Lived Radicals S. Kirkiachorian Ann. Chim. (France) 1968 3 403; Use of E.S.R. in Precision Gas Kinetics Studies; A.A. Westenberge Science 1969 164 381 ; Spin-Labelling in Studies of Bipolymers 0. H. Grifith and A. S. Waggoner Accounts Chem. Res. 1969 2 17; Substituent Effects in E.S.R. Spectra and Radical Stability E. G. Janzen ibid. 1969,2,279; Uses of N.M.R. to Study Radicals and Radical Ions A. L. Buchachenko and N. A. Sysoeva Uspekhi Khim. 1968 37 1852. See also ‘Oxidation of Organic Compounds-2 and 3’ ed. R. F. Gould Advances in Chemistry Series 76 and 77 Amer. Chem. SOC. Washington 1968. ’M. C. R. Symons Nature 1969 222 1 123. W. H. Chick and S. H. Ong Chem. Comm. 1969 216; R. L. Huang Tong-Wai Lee and S. H. Ong. J. Chem. SOC.(0,1969,40. S. F. Nelsen and E. F. Travacedo J. Org. Chem. 1969,34 3651. 164 M. J. Perkins One argument in favour of planar geometry for alkyl radicals derives from the observation5 that the ease of formation of cycloalkyl radicals as a function of ring-size varies in a manner reminiscent of the ease of formation of cycloalkyl cations as a function of ring-size.Ruchardt and his colleagues have presented new results on the formation of cycloalkyl radicals (R".) employing t-butyl cycloalkanepercarboxylates(R"C0,OBu') as the radical precursors.6 The new data support the older results but from the relatively small rate dependence found in the perester decompositions it was clear that this type of reaction is much less sensitive to the stability of the incipient radical than is symmetrical azo-compound decomposition. This is consistent with results noted last year on the ease of formation of 1-adamantyl radical^.^ However related work also discussed last year,' in which the observation that 1-adamantyl radicals are relatively indis- criminate in halogen abstraction reactions had led to the conclusion that they were unusually reactive has now been questioned with the discovery that selectivity in this type of reaction does not appear to correlate with radical stability.8 Some evidence that cyclopropyl radicals may not be planar has been found in the discovery of partial retention of configuration in reactions involving such ~pecies.~ A detailed survey of examples of cyclopropylcarbinyl radicals has been reported from which a general preference for the bisected conformation (2) has emerged.' This conformational preference is enhanced by electron-withdrawing subs ti tuen ts.(2) (3) R = Ph (4) R = Me The geometry of vinyl radicals has also received further attention this year. Last year's report mentioned work in which it appeared to have been established that 1,2-diphenylvinyl radicals exist as a rapidly equilibrating mixture of geo- metric isomers. Efficient radical scavenging gave stilbenes with predominant retention of the geometry of the peroxidic precursor. An alternative linear formulation (3) therefore appeared to be excluded.' ' However new results on 2-methyl-1-phenylvinyl radicals appear to be interpretable only in terms of the linear (sp-hybridised) structure (4).'* Clearly there is scope here for further C. G. Overberger H.Biletch A. B. Finestone J. Lilker and J. Herbert J. Amer. Chem. SOC.,1953,75 2078. P. Lorenz C. Ruchardt and E. Schacht Tetrahedron Letters 1969 2787. ' See Ann. Reports (B) 1968 p. 172. C. Ruchardt K. Herwig and S. Eichler Tetrahedron Letters 1969 421. J. Jacobus and D. Pensak Chem. Comm. 1969,400; M. J. S. Dewar and J. M. Harris J. Amer. Chem. SOC.,1969,91 3652; but see also L. J. Altman and B. W. Nelson ibid. 1969,91,5163. lo N. L. Bauld J. D. McDermed C. E. Hudson Y. S. Rim J. Zoeller R. D. Gordon and J. S. Hyde J. Amer. Chem. SOC.,1969,91,6666. I1 See Ann. Reports (B) 1968 p. 173. L. A. Singer and J. Chen Tetrahedron Letters 1969 4849. Free-radical Reactions research. In an independent study of the reactions of 1,2-dialkylvinyl radicals produced by photolysis of the corresponding vinyl iodides in hydrogen-donor solvents it was noted that there was a slight tendency towards inversion of configuration at C-1 in the olefinic product^.'^ This was tentatively ascribed to the initial formation of vibrationally-excited vinyl radicals.It has now been found that photolysis of acetylenic iodides constitutes a means of generating acetylenic radicals; this is exemplified by the photolysis of the acetylene (5) in benzene which produces tolan by aromatic sub~titution.'~ The unpaired electron in the acetylenic radical is presumably in an sp hybrid orbital and in agreement with this the relative reactivities of a series of substituted benzenes revealed appreciable electrophilic character in the new radical.PhH ICrCPh !% I. + CrCPh -D PhCGCPh (5) Last year's report that 7-norbohenyl radicals are bridged ('non-classical') (6) has been refuted. The claim that these radicals react with a trialkyltin deuteride to give exclusively anti-7-deuterionorbornene has been shown to be in error,I5 though the proportion of syn-product is only about 25 %. The stereo-selectivity may be a consequence of non-planar geometry in this strained radical. In contrast with the new experimental data molecular orbital calculations have been des- cribed which suggest that structure (6) might be more stable than a classical non-bridged structure despite the location of the unpaired electron in (6)in an antibonding orbital.16 It would be interesting to observe this radical directly by em.in an attempt to obtain direct evidence on the possibility of bridging. Obtaining a solution e.s.r. spectrum of 7-norbornenyl should now be a fairly straightforward matter with the rapid strides taken by Kochi and Krusic and others in the development of photochemical procedures for the direct observation of reactive radicals in solution by e.s.r. Among the achievements of this work have been direct observation of cyclopropylcarbinyl radicals (formed by hydrogen abstraction from methylcyclopropane when di-t-butyl peroxide is photolysed in its presence). These show no evidence of non-classical behaviour though at temperatures above -100 "Cisomerisation to allylcarbinyl is very rapid.' Direct observations of cis-methallyl radicals showed them to remain isomerically pure l3 R.C. Neuman and G. D. Holmes J. Org. Chem. 1968,33,4317. l4 G. Martelli P. Spagnolo and M. Tiecco Chem. Comm. 1969 282. Is G. A. Russell and G. W. Holland J. Amer. Chem. SOC.,1969 91 3968; S. J. Cristol and A. L. Noreen ibid. 3969. l6 H. 0. Ohorodnyk and D. P. Santry Chem. Comm. 1969 510; J. Amer. Chem. SOC. 1969,91,4711. "J. K. Kochi P. J. Krusic and D. R. Eaton J. Amer. Chem. SOC.,1969,91 1877 1879; See Ann Reports (B) 1968 p. 183. 166 M. J. Perkins on the time scale of their survival in the experiment ; cis to trans isomerisation in these radicals was estimated to have a rate constant of less than lo2sec-' at Oo.I8 Alkyl radicals were also observed when t-butyl peroxide was photolysed in the presence of other hydrocarbon^,'^ or in the presence of alkylboranes2' or other metal alkyls.2' The reactions with the organometallic compounds involve displacement of an alkyl radical from the metal by a butoxy radical (Equation l) Bu'O.+ MR + Bu'OM + R. (1) in direct analogy with the displacement by peroxy radicals (Equation 2) known ROO-+ MR -+ ROOM + R-(2) to be involved in the autoxidation of many organometallic compounds.22 This procedure constitutes a means of generating specific alkyl radicals for e.s.r. examination provided that the appropriate metal alkyl is accessible. (Reaction of butoxy radicals with hydrocarbons may give mixtures of several radicals if several non-equivalent hydrogen atoms can be abstracted). An alternative means of generating specific alkyl radicals for this type of study involves photolysis of the appropriate diacyl peroxide [(RC02)2].23 In the course of this work P-phenylethyl radicals were studied but no evidence was obtained for the bridged structure (7) proposed24 to account for anomalous behaviour of this radical in studies of aromatic alkylation.In view of the ready accessibility of the necessary materials the most convenient method for generating 8 specific alkyl radicals appears to be that reported by Hudson and which depends on halogen abstraction from alkyl halides by silyl radicals; the silyl radicals- are formed from a trialkylsilane by photochemically-produced butoxy radicals Bu'O. + Et,SiH -P Bu'OH + Et,Si. Et,Si. + RBr -+ Et,SiBr + Re J.K. Kochi and P. J. Krusic J. Amer. Chem. SOC.,1968 90,7157. P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC.,1968 90,71 55 ; P. J. Krusic J. P. Jesson and J. K. Kochi ibid. 1969 91 4566; see also A. Hudson and H. A. Hussain J. Chem. SOC.(B) 1969 793; Mol. Phys. 1969 16 199; A. Hudson and K. D. J. Root Tetrahedron 1969 25 531 1. 2o A. G. Davies and B. P. Roberts Chem. Comm. 1969 699. 21 A. G. Davies and B. P. Roberts J. Organometaffic Chem. 1969,19 P17; P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC.,1969 91 3942. 22 For recent work see P. G. Allies and P. B. Brindley J. Chem. SOC.(B) 1969 1126; J. Grotewold E. A. Lissi and J. C. Scaiano ibid. 1969 475; A. G. Davies and B. P. Roberts ibid. 1969 311 317; K. U. Ingold Chem. Comm. 1969 911. 23 J.K. Kochi and P. J. Krusic J. Amer. Chem. SOC.,1969,91 3940. 24 D. I. Davies J. N. Done and D. H. Hey J. Chem. SOC.(C),1969,2021. 2s A. Hudson and R. A. Jackson Chem. Comm. 1969 1323. Free-radical Reactions 167 In other work silyl radicals themselves were observed,26 and in a study of the reaction of t-butoxy radicals with trivalent phosphorus compounds direct observation of phosphoranyl radicals was possible in some instances (e.g. Me3POB~‘).27 Phosphoranyl radicals were assumed to be intermediates in all the reactions with trivalent phosphorus compounds whether they could be detected spectroscopically or not. However displacement of alkyl radicals from boron or other organometallic compounds was considered to be a concerted process not involving a tetraco-ordinate intermediate.Several other recent reports on homolytic reactions of trivalent phosphorus assume that phosphoranyl radicals participate in the reactions.28 Displacement of alkyl radicals from sulphoxides has also been studied by e.s.r. both directly using hydroxyl radicals generated in a flow system,29 and indirectly using the nitroso-compound (8)as a ‘spin-trap’ and observing the spectrum of H202 2HO. HO. + Me2S0 -+ Me. + MeS02H Me. + Me2C-N:O -P Me2C-N-0. I II MeCO MeCO Me (8) (9) the derived nitroxide (e.g.9).30 This ‘spin-trapping’ technique in which radical intermediates are scavenged by C-nitroso-compounds or by nitrones to give relatively stable nitroxides which are easily detected by e.s.r. has been further exploited to obtain evidence for the formation of succinimidyl radicals from N-bromos~ccinimide,~ and to study the radicals formed by photo-induced oxidation of alcohols and carboxylic acids by lead tetra-a~etate,~~ as well as to examine a variety of other reactions.33 However a note of caution in the inter- pretation of spin-trapping results is introduced by the apparent ease with which the extremely short-lived acetoxy radicals are ~cavenged,~~‘ as well as by the direct formation of nitroxides when certain aromatic nitroamines are mixed with solutions of the benzylidene nitrone PhCH:N(+ O)Bu‘ in the dark.34 26 P.J. Krusic and J. K. Kochi J. Amer. Chem. SOC. 1969 91 3938; S. W. Bennett C. Eaborn A. Hudson A. Hussain and R. A. Jackson J. Organometallic Chem.1969 16 P36. 2’ J. K. Kochi and P. J. Krusic J. Amer. Chem. SOC. 1969 91 3944. ’* R. E. Atkinson J. I. G. Cadogan and J. T. Sharp J. Chem. SOC. (B) 1969 138; W. G. Bentrude J. H. Hargis and P. E. Rusek Chem. Comm. 1969 296; W. G. Bentrude and R. A. Wielesek J. Amer. Chem. SOC.,1969,91,2406; R. S. Davidson Tetrahedron 1969 25 3383; W. G. Bentrude and J. J. L. Fu Tetrahedron Letters 1968 6033; K. Terauchi and H. Sakurai Kogyo Kagaku Zasshi 1969,72 215. ” W. Damerau G. Lassmann and K. Lohs 2. Chem. 1969,9 343. 30 C. Lagercrantz and S. Forshult Acta. Chem. Scand. 1969 23 81 1. 31 C. Lagercrantz and S. Forshult Acta. Chem. Scand. 1969 23 708. 32 S. Forshult C. Lagercrantz and K. Torssell Acta. Chem. Scand. 1969 23 522. 33 (a) I. H.Leaver G. C. Ramsay and E. Suzuki Austral. J. Chem. 1969 22 1891 ; I. H. Leaver and G. C. Ramsay ibid. 1969 22 1899; (6) S. Terabe and R. Konaka J. Amer. Chem. SOC.,1969 91 5655; (c) E. G. Janzen and B. J. Blackburn ibid. 1969 91 4481. 34 E. G.Janzen and J. L. Gerlock J. Amer. Chem. SOC. 1969,91 3108. 168 M. J. Perkins Nitroso-compounds and nitrones are susceptible to nucleophilic attack ;this type of reaction coupled with electron transfer is one of several alternatives to radical scavenging which could conceivably yield nitroxides in some spin-trapping experiments. On the other hand nucleophilic attack on for example a nitroso- compound may itself involve initial one-electron transfer from the nucleophile to the nitroso-compound. Certainly an increasing number of reactions which have conveniently been classified as nucleophilic displacements are being identified as involving a one-electron transfer step.Among these are the reactions of alkali metal alkyls R'M with alkyl halides R2X in which the probable precursor of alkyl coupling product is the caged complex [pel M X R.2].35y36 The study of these reactions and related halogen-metal exchange processes led to the discovery of the phenomenon of chemically induced dynamic nuclear polarisation (CIDNP),37 in which nuclear spin polarisation is observed in the products of reactions which involve reactive free radical intermediates. Manifest in anomalous n.m.r. spectra of the reaction products provided that these are recorded before spin relaxation is complete (enhanced absorption or emission spectra are observed) this phenomenon was discovered independently in observations on peroxide decomp~sition.~ Recently the phenomenon has been increasingly utilised to identify homolytic pathways for organic reaction^,^^.^^ particularly for certain reactions which whilst superficially sigmatropic rearrangements are in fact symmetry f~rbidden.~' Several new fundamental observations regarding the CIDNP phenomenon have also been made,41,42 including the discovery that the polarisation effects in radical coupling products depend on spin multiplicity in the precursors of combining radical pairs.42 This has been rationalised in a new theoretical description of CIDNP.43 One-electron transfer has now been proposed as a key step in many induced decomposition reactions of peroxides44 and diazonium but a distinction between this type of mechanism and one-step homolytic processes 35 H.R. Ward R. G. Lawler and R. A. Cooper J. Amer. Chem. SOC.,1969 91 746; see also Reference 1 (Bilevich and Okhlobystin). 36 J. F. Garst and J. T. Barbas Tetrahedron Letters 1969 3125. 37 H. R. Ward and R. G. Lawler J. Amer. Chem. SOC.,1967,89 5517 5518 5519. 38 J. Bargon H. Fischer and U. Johnsen Z. Naturforsch. 1967 22a 1551; J. Bargon and H. Fischer ibid. 1967 22a 1556; Accounts Chem. Res. 1969 2 110. 39 A. R. Lepley J. Amer. Chem. SOC.,1969,91. 1237; J. E. Baldwin and J. E. Brown ibid. 1969,91 3647; U. Scholkopf V. Ludwig G. Ostermann and M. Patsch Tetrahedron Letters 1969 3415; U.Scholkopf G. hermann and J. Schossig ibid. 1969 2619; R. W. Jemison and D. G. Morris Chem. Comm. 1969 1226; D. G. Morris ibid. 1969 1345. 40 A. G. Lane C. Ruchardt and R. Werner Tetrahedron Letters 1969 3213; A. Rieker P. Niedener and D. Leibfritz ibid. 1969 4287. 41 H. R. Ward R. G. Lawler H. Y.Loken and R. A. Cooper J. Amer. Chem. SOC.,1969 91 4928. 42 G. L. Closs and L. E. Closs J. Amer. Chem. SOC.,1969,91,4549,4550;G. L. Closs and A. D. Trifunac ibid. 1969 91 4554. 43 G. L. Closs J. Amer. Chem. SOC.,1969 91 4552. O4 K. Tokumaru and 0.Simamura Bull. Chem. SOC.Japan 1963,36 333. 45 J. F. Bunnett and H. Takayama J. Org. Chem. 1968 33 1924; J. Amer. Chon. SOC. 1968,90 5173. Free-radical Reactions such as those discussed recently for Reaction (3),46 and for the formal hydrogen- transfer process (4),47may be very difficult to establish.In the latter case for example the failure of Ph,cOMe radicals to induce a similar decomposition may be a consequence of reversal of an initial electron transfer (Reaction 5) which competes more effectively with the onward reaction when R = Me than it does when R = H. R3Sn. + (RC02)2 -+ R3SnOCOR + RCOz-PhZCOH + (PhC02)2 -+ Ph2CO + PhC02H + PhCOz. (3) (4) Ph2cOR + (PhC02)2 [PhzdOR (PhCOZ),-] -+ Products (5) The molecule-induced homolysis of peroxides by tertiary amines has often been cited as an electron transfer reaction.48 Interesting new results have been presented on the ease of oxidation of triethylamine by a wide range of one- electron oxidants (excluding organic peroxides).The rate of oxidation correlates well with the oxidation potential of the oxidant consistent with slow one- electron transfer as the rate-determining step. In the case of oxidation by ferri- cyanide the electron transfer was shown to be reversible by demonstrating an inhibitory effect of added ferr~cyanide.~' Further support for the feasibility of electron transfer to peroxides was obtained recently with the direct observation at low temperature of the radical anion of di-t-butyl peroxide. 50 Reversible one-electron reduction of diazonium ions has now been established p~larographically,~' and electron transfer was postulated to be the first step in the molecule-induced homolysis (6). PhNZ+BFa-+ PhZNOH + Ph.+ PhZNO-+ N + HBF (6) This reaction provides an analogy for a key step in the decomposition of nitroso- acetanilide in benzene for which it has been suggested that decomposition of the rearranged diazoacetate (10) may be induced by a substituted hydroxylamine. The mechanistic details of reactions of nitrosoacetanilide are very complex and new e.s.r. data appear to have established the involvement of the diazotate radical PhN :NO.,53in addition to the previously identified nitroxide N:O I PhN-COMe -+ PhN = NOCOMe ++ [PhNz+ OCOMe] 46 K. Rubsamen W. P. Neumann R. Sommer and U. Frommer Chem. Ber. 1969,102 1290 and earlier papers by these authors. 47 W. F. Smith and B. W. Rossiter Tetrahedron 1969 25 2059. 48 For a recent example see D. G. Pobedimskii A.L. Buchachenko and A. L. Neuman Zhur. fiz. Khim. 1968,42 1436. 49 L. A. Hall G. T. Davis and D. H. Rosenblatt J. Amer. Chem. SOC.,1969 91 6247. 50 T. Shida J. Phys. Chem. 1968 72 723. R. M. Elafson and F. F. Gadallah J. Org. Chem. 1969,34,855. 52 R. M. Cooper and M. J. Perkins Tetrahedron Letters 1969 2477. 53 J. I. G. Cadogan R. M. Paton and C. Thomson Chem. Comm. 1969,614. 170 M. J.Perkins PhN(O.)NPhCOMe as an important paramagnetic intermediate. The radical detected during homogeneous diazotization of aniline by amyl nitrites4 is almost certainly a nitroxide and not the diazotate suggested. There have been several recent reports of the study of cage effects in the decomposition of peroxides and azo-compounds,ss and Koenig has presented a new theoretical description of the diffusive separation of radical pairs the rate of which is found to vary inversely with the square root of the viscosity of the medium.56 This is in agreement with experimental data.Cage recombination of benzoyloxy radicals has been detected during the decomposition of dibenzoyl peroxide ;" surprisingly this occurs to a lesser extent than does cage recombina- tion of acetoxy radicals from acetyl peroxide. Other studies of acyl peroxide decomposition have concentrated on the nature and magnitude of secondary deuterium isotope effect^.^^,^^ There is an isotope effect (kH/kD) of ca. 1.06 per deuterium atom on the rate of decomposition of PhCD2C020Bu';58 (this is significantly less than the figure quoted in a preliminary report).In the course of attempts to make direct measurement of the rates of dimerisa- tion of tertiary alkyl radicals by an e.s.r. technique,60 no evidence could be found for a correlation between dimerisation rate and radical stability. A gas-phase technique for direct measurement of radical-molecule reaction rates has also been described.61 Experiments on the oxidation of tetrahydrofuran with iodine yielded a bond dissociation energy for the aC-H bond of 91-5kcal mol- only slightly less than that for a C-H bond in cyclopentane.62 The failure of a-oxygen substitution to impart significant stabilisation to an alkyl radical has also emerged from a study of the rates of decomposition of azo-compounds (11;X = CH2,0 or S).63There is little difference between the rates of decomposition of the two compounds with X = CH2 and X = 0.However when X = S the decomposition is considerably accelerated consistent with substantial stabilisation by a-sulphur substitution in the incipient radicals.64 Me Me I I PhX-C-N = N-C-XPh I I Me Me 54 A. F. Levit and I. P. Gragerov Zhur. org. Khim. 1969 5 310. 55 R. C. Neuman and J. V. Behar J. Amer. Chem. SOC.,1969 91 6024; T. Koenig and M. Deinzer ibid. 1968,90,7014; R. C. Neuman and R. J. Bussey Tetrahedron Letters 1968 5859; K. R. Kopecky and T. Gillan Canad. J. Chem. 1969,47,2371. 56 T. Koenig J. Amer. Chem. SOC.,1969,91 2558. 57 J. C. Martin and J. H. Hargis J. Amer. Chem. SOC., 1969 91 5399. 58 T. Koenig and R. Wolf J. Amer. Chem. SOC.,1969,91 2568 2574.s9 T. Koenig and R. Cruthoff J. Amer. Chem. SOC.,1969,91 2562. 6o S. A. Weiner and G. S. Hammond J. Amer. Chem. SOC.,1969 91 986. 61 N. A. Grac D. M. Golden and S. W. Benson J. Amer. Chem. SOC.,1969,91 3091. 62 F. R. Cruickshank and S. W. Benson J. Amer. Chem. SOC.,1969,91 1289. 63 A. Ohno and Y.Ohnishi Tetrahedron Letters 1969 4405. 64 See also W. Tagaki T. Tada R. Nomura and S. Oae Bull. Chem. SOC.Japan 1968 41. 1696. Free-radical Reactions 171 Exceptional selectivity has previously been reported for chlorination reactions employing metal salt-catalysed decomposition of chloramines in strong acid,65 and this was attributed to hydrogen abstraction by aminium cation radicals. This mechanis? has now been discredited by the discovery that some of the iso- meric chlorination products are unstable under the conditions of the reaction and are selectively destroyed.66 The dichlorobutanes are stable under these reaction conditions and it has been found that chlorination of 1-chlorobutane gives identical mixtures of products whether the reaction is effected by a chloramine and metal salt or photolytically with chlorine itself.This suggests that hydrogen abstraction must be by chlorine atoms.66 Chlorine atom chains are also involved in many chlorination reactions employing t-butyl hypochlorite a reaction long believed to involve hydrogen abstraction by butoxy radicals.67 Much of the quantitative data on butoxy radical reactivity which has been obtained with this reagent therefore requires reappraisal.Bromination with N-bromosuccinimide is now generally believed to involve abstraction by bromine atoms but a clear case of a chain reaction involving hydrogen abstraction by succinimidyl radicals has been found in the oxidation of a-phenyl-ethanol to acetophenone by N-iodosuccinimide.68 It is interesting in this context that quite long chains are observed in the intramolecular halo- genation reactions of N-haloamides for which hydrogen abstraction must be by amido radicals. It has now been demonstrated that in these reactions the intramolecular hydrogen transfer occurs exclusively to the nitrogen of the amido radical and not to oxygen followed by tautomeri~ation.~~ There have been a number of new studies of linear free energy relationships in radical reactions both for hydrogen abstraction from benzylic compounds7o and also for compounds in which the substituent effect is transmitted across a-b~nds.~' There has also been an interesting report of a pronounced solvent effect on the selectivity of chlorination of branched aliphatic esters.72 The effect of added aromatic solvents in these reactions is markedly dependent on the polar nature of any substituent present in the aromatic compound.The reactions of trialkyltin hydrides with alkyl halides is probably now the best understood two-step radical chain reaction as a result of a detailed kinetic study by Carlsson and Ing01d.~~ In the course of this study rate constants for 65 See Ann. Reports (B) 1968 p. 178. 66 D. D. Tanner and M.W. Mosher Canad. J. Chem. 1969,47 715. 67 C. Walling and J. A. McGuinness J. Amer. Chem. SOC. 1969 91 2053. 6a T. R. Beebe and F. M. Howard J. Amer. Chem. SOC. 1969,91 3379. 69 Y. L. Chow and T. C. Joseph Chem. Comm. 1969,490. 'O K. H. Lee Tetrahedron 1969,25,4357,4363;E. P. Chang R. L. Huang and K. H. Lee J. Chem. SOC.(B) 1969,878; S. E. Friedrich E. C. Friedrich L. J. Andrews and R. M. Keefer J. Org. Chem. 1969 34 900; W. A. Pryor V. Tonalatto D. J. Fuller and S. Jumonville ibid. 1969,34,2018 ;J. D. Unruh and G. J. Gleicher J. Amer. Chem. SOC. 1969,91,6211. 71 G. J. Gleicher J. L. Jackson P. H. Owens and J. D. Unruh Tetrahedron Letters 1969 833; I. Tabushi T. Okada Y. Aoyama and R. Oda ibid. 1969 4069; L. Harvey G. J. Gleicher and W. D. Totherow Tetrahedron 1969 25 5019; J.P. Soumillion and A. Bruylants Bull. Soc. chim. belges 1969 78 169. l2 J. P. Soumillion and A. Bruylants Bull. Soc. chim. belges 1969 78 425 435. 73 D. J. Carlsson and K. U. Ingold J. Amer. Chem. SOC. 1968,90 7047. 172 M. J. Perkins fast unimolecular processes which compete with chain propagation were estimated. For example at 25 "C the rate constant for phenyl migration in 2,2,2-triphenyl- ethyl radicals is ca. 5 x lo7sec-' and that for the cyclisation of hex-5-enyl radicals (12) to cyclopentylmethyl radicals is ca lo5sec-'. An independent estimate has confirmed the latter value.74 Other examples of the cyclisation of RC=C(CHJ,S. _+ (13A) R = Ph (13B) R = H radicals related to (12) have been discussed in which kinetic control invariably leads to a 5-membered rather than a 6-membered ring.75 The geometry of intra- molecular addition to acetylenes is different and whilst 5-membered ring forma- tion is the exclusive reaction path for cyclisation of the thiyl radical (13A) the terminal acetylene (13B) gives almost entirely dihydrothiapyran.76 A kinetic preference for axial addition of radicals to cyclic olefins has been empha~ised.~~ New examples of radical cyclisation on to an aromatic ring are represented by the lead tetra-acetate oxidation of (14) and related and the intra- molecular arylation effected by photolysis in benzene of the aryl halides (15).79 This last reaction gives fluoranthene and 2-(a-naphthyl)biphenyl as major products but the relative yields of these compounds depends on the nature of X (Cl Br or I).A radical arylation mechanism was considered to operate with all three halides but the cyclohexadienyl radicals (16) and (17) were supposed to be formed reversibly with a kinetic preference for (16) and thermodynamic preference for (17). The more reactive counter radicals (C1-) then give predominantly fluoranthene whilst in the case of X = I the major product is the naphthyl- biphenyl. Reversibility in the addition of aryl radicals to aromatic substrates 74 J. F. Garst and F. E. Barton Tetrahedron Letters 1969 587. 75 M. Julia and M. Maumy Bull. SOC. chim. France 1969 2415 2427; N. 0. Brace J. Org. Chem. 1969 34 2441 ; R. D. Rieke and N. A. Moore Tetrahedron Letters 1969,2035; J.M. Surzur M. P. Bertrand and R. Nougonier ibid. 1969,4197; A. L. J. Beckwith and W. B. Gara J. Amer. Chem. SOC. 1969,91 5689,5691 ; B. A. Trofimov A. S. Atavin G. M. Gauvilova and G. A. Kalabin Zhur. obschei Khim. 1968,38,2344. ''J. M. Surzur C. Dupuy M.-P. Crozet and N. Amor Compt. rend. 1969 269 C 849. "J. G. Traynham A. G. Lane and N. S. Bhacca J. Org. Chem. 1969,34 1302; N. A. LeBel R. F. Czaja and A. DeBoer ibid. 1969 34 31 12. 78 J. C. Chottard M. Julia and J. M. Salard Tetrahedron 1969 25,4967. 79 W. A. Henderson R. Lopresti and A. Zweig J. Amer. Chem. SOC.,1969 91 6049. Free-radical Reactions * Q 02 C02H CO2Me Me C02Me (14) at or near ambient temperatures has usually been discounted but another example has been claimed recently following detection of isotope effects in the intramolecular arylation of deuteriated aromatics in dimethylsulphoxide.80 The homolytic hydroxylation of aromatic compounds by Fenton's reagent shows an interesting pH-dependence.*l For example in the reaction with toluene the yield of bibenzyl increases with acidity and this has been attributed to acid- catalysed diversion of hydroxycyclohexadienyl radicals to benzyl radicals as indicated in equation (7).Several new examples of radical rearrangements have been disclosed in- cluding instances of 1,Zshifts of aryl groups,82 and of the now familiar cyclo- 8o M. Kobayashi H. Minato and N. Kobori Bull. Chem. SOC.Japan 1969,42 2738. C. R. E. Jefcoate J. R. L. Smith and R. 0.C. Norman J. Chem.SOC.(B) 1969 1013. R.L.Dannley and G. C. Farrant J. Org. Chem. 1969,34,2432;M.Abramovici and H. Pines ibid. 1969 34 266. 174 M. J. Perkins H H OH g- H+ propyl allylcarbinyl rearrangement,83,84 though a cyclopropylcarbinyl unit in a biradical survived intact into the prod~cts.~' There appears to be substantial stabilisation of a radical by an adjacent cyclopropyl substituent not only in the case of carbon radicals,84 but also with nitrogen86 and oxygen radical^.^' Thus homolytic oxidation of cyclopropanols has been found to occur at the hydroxy- group ;this is accompanied by ring-opening e.g. (18) -+( 19). Me OCOMe oh OCOMe I Me&CH,. -Me&dH -Me -CH Me (20) Me Me 1 Me3COCOMe Me2CHCH20COMe S.J. Cristol and R. W. Gleason J. Org. Chem. 1969 34 1763; A. J. Davidson and A. T. Bottini ibid. 1969,34 3642; G. A. Gray and W. R. Jackson J. Amer. Chem. SOC. 1969 91 6205; C. Cueille R. Fraisse-Jullien and A. Hunziker Tetrahedron Letters 1969 749. 84 E. C. Friedrich J. Org. Chem. 1969 34 528 1851. 85 T. Sanjiki M. Ohta and H. Kato Chem. Comm. 1969,638; but see J. J. McCullough and P. W. W. Rasmussen ibid. 1969 387. 86 E. E. J. Dekker J. B. F. N. Engberto and T. J. DeBoer Tetrahedron Letters 1969 265 1. '' D. H. Gibson and C. H. DePuy Tetrahedron Letters 1969 2203. Free-radical Reactions ‘H The first example of a homolytic migration of an acetoxy-group has been disclosed [Equation The bridged structure (20) is a likely intermediate but is probably not the immediate precursor of the products.Electrocyclic rearrangements of carbocyclic radical anions (21),8g (22)” and (23)” have been described and whilst the stereochemistry of the first two is unknown that of (23) is necessarily disrotatory and is apparently symmetry forbidden. A related disrotatory opening is required of the radical (24) in the minor reaction path which leads to na~hthalene,~~ but in this case orbital symmetry is conserved. A novel rearrangement mechanism appears to be involved in the decomposition of cyclic peroxides (25).92 The evidence points to initial formation of a biradical which then suffers decarboxylation in concert with migration of an alkyl group (phenyl migration is a minor pathway). 9I -+ -.c<.o (Ph)R.::I\ (R)Ph “0 C02 + PhCOCH,R + C02 + RCOCHZPh The rearrangements sometimes encountered during Clemmensen reduction of up-unsaturated ketones have been suggested to involve intermediate formation of cyclopropanol by cyclisation of an intermediate diradical (e.g.26).This is now supported by the isolation of (27) when ketones (28) and (29) are reduced in the presence of acetic anh~dride.’~ Cyclopropane formation was also found in the lithium/’ammonia reduction of (30).94 D. D. Tanner and F. C. P. Law J. Amer. Chem. SOC.,1969,91 7535. 89 N. L. Bauld and F. Farr J. Amer. Chem. SOC.,1969,91 2788. 90 N. L. Bauld and G. R. Stevenson J. Amer. Chem. SOC.,1969 91 3675. 91 L. L. Miller and L. J. Jacoby J. Amer. Chem. SOC.,1969,91 1130. 92 W. Adam and Y.M. Cheng J. Amer. Chem. SOC.,1969 91 2109; W. Adam Y.M. Cheng C. Wilkerson and W. A. Zaidi ibid. 1969 91 21 11. 93 I. Elphimoff-Felkin and P. Sarda Tetrahedron Letters 1969 3045. 94 W. Reusch and D. P. Priddy J. Amer. Chem. SOC.,1969 91 3677; T. J. Curphey C. W. Amelotti T. P. Layloff R. L. McCartney and J. H. Williams ibid. 1969 91 28 17. 176 M. J. Perkins Retention of configuration at silicon has been reported in reactions of optically active silyl radicals,95 but an example of a phosphinium radical cation (iso- electronic with silyl radicals) has been reported to be configurationally unstable ;96 the lifetime of the phosphorus species was however almost certainly several orders of magnitude greater than that of the silyl radicals.New stable radicals include several biradi~als,~’ a a ferrocenyl nitr~xide,~~ substituted diarylamino radical,99 and the first example of a stable tetrazolinyl radical. With an annual reference list for free-radical chemistry running into four figures space does not permit more than a very selective coverage in this section and I apologise for what some readers may regard as a highly personal choice of material. O 95 H. Sakurai M. Murakami and M. Kumada J. Amer. Chem. SOC., 1969 91 5191. 96 R. L. Powell and C. D. Hall J. Amer. Chem. SOC.,1969,91 5403. 97 E. F. Ullman and D. G. B. Boocock Chem. Comm. 1969 1161 ; A. Calder A. R. Forrester P. G. James and G. R.Luckhurst J. Amer. Chem. SOC.,1969 91 3724; see also P. W. Kopf and R. W. Kreilick ibid.1969,91 6569 and K. Leibler K. Okon G. Anaraska and E. Bamburski Roczniki Chem. 1969 43 585. 98 A. R. Forrester S. P. Hepburn R.S. Dunlop and H. H. Mills Chem. Comm. 1969,698. 99 L. Dulog and G. Baum Chem. Ber. 1969,102 1626. loo F. A. Neugebauer Angew. Chem. Internat. Edn. 1969,8,520; seealso F. A. Neugebauer Chem. Ber. 1969 102 1339. A more extensive coverage of current free radical chemistry will be found in ‘Organic Reaction Mechanisms 1969’ B. Capon and C. W. Rees Eds. Wiley London 1970.

ISSN:0069-3030    

DOI:10.1039/OC9696600163

出版商:RSC    

年代:1969

数据来源: RSC