10 Photochemistry By W. M. HORSPOOL Department of Chemistry The University Dundee This chapter covers a few photochemical topics selected from the many thousands reported which in the author's opinion are either of general interest to photo- chemists or else are of special interest in a specific area of the subject. Inevitably the report is subjective and tends to be biased towards the author's own interests. Interest in benzene photochemistry continues unabated. Several reports during the past year are worthy of selection. West et a[.' have reported the photoisomerization of 1,2,4,5-tetrakis(trimethylsilyl)benzene by irradiation in ether. The product mixture which differed somewhat in the distribution of pro-ducts from that obtained from alkyl benzene rearrangement did not contain 'Dewar' benzenes but gave 1,2,3,5-tetrakis(trimethylsilyl)benzene (12 %) the benzvalenes (la) (19 %) and (lb)(47%) the fulvene (2) (5 %) and another fulvene of uncertain structure.The first report of photoisomerization in the naphthalene series has been made' whereby 1,3,6,8-tetra-t-butyl naphthalene is converted (94%) into the 'Dewar' naphthalene (3). a; R' = SiMe, R2 = H (3) b; R' = H R2= SiMe (1) Ohashi3 has reported the enhanced ( x 10) formation of the 1,2-adduct between acrylonitrile and benzene when the irradiation is carried out in the presence of ' R. West M. Furue and V. N. Mallikarjuna Rao Tetrahedron Letters 1973 91 I. ' ' W. L. Mandella and R. W. Franck J. Amer. Chem. SOC.,1973,95971. M. Ohashi Tetrahedron Letters 1973 3395.310 Photochemistry 31 1 zinc chloride. Cornelisse and Srinivasan4" have studied the addition of cyclo- butene and cyclopentene to benzene and from quantitative measurements have come to the conclusion that the first excited singlet (IB2J of benzene is involved in the addition rather than a vibrationally excited ground state or excited states of valence isomers. This mode of addition has provided a route to an asterane (4) by the addition of cyclopentene to benzene yielding the adduct (5)(25% 0 = 0.011). This is transformed thermally or by action of acid into the asterane (4).4b A triplet biradical mechanism is involved in the addition of phenanthrene to dimethyl fumarate dimethyl maleate and maleic anh~dride.~ However the mechanism is complicated by the formation of only one product (with maleic anhydride) and the suggestion is that a triplet exciplex is formed which leads to the formation of only one of the two possible biradical inter- mediates.Complex formation (charge-transfer) is also important in the photo- chemical addition of tetracyanoquinodimethane to toluene.6 Irradiation into this band gives a single product (6) in high yield. This reaction is greatly ( x 9) enhanced in the presence of trifluoroacetic acid. Acid catalysis such as this has previously been noted in other benzene systems7 Bryce-Smith et aL8 have reported the formation of adducts (7) by meta-cyclization following the irradiation of phenylpropyl (or butyl) dimethylamine. -NMe H (7) n = 3 or 4 Interest in bichromophoric systems has continued during the past year.De Schryver and his co-workers' have examined the reactions of 1,l'-and 2,2'-linked dianthracenes. With the 1,l'-dianthracenes (8a and b) only one photo- product was obtained in each case but although head-to-head linking resulted no conclusion could be reached as to whether the products were the result of syn-or anti-addition. The 2,2'-linked dianthracenes (9a and b) give both head- to-tail and head-to-head dimers whereas the shorter chain-length linkage of (9c) gave exclusively the head-to-head dimer. The X-ray structure of the bis- thymine (10) has shown that the two thymine units lie over each other in the (a) J. Cornelisse and R. Srinivasan Chem. Phys. Letters 1973 20 278; (b) C.S. Angadiyvar J. Cornelisse V. Y. Merritt and R. Srinivasan Tetrahedron Letters 1973 4407. R. A. Caldwell J. Amer. Chem. SOC.,1973 95 1690. K. Yamasaki A. Yoshino T. Yonezawa and M. Ohashi J.C.S. Chem. Comm. 1973,9. D. Bryce-Smith M. T. Clarke A. Gilbert G. Klunklin and C. Manning Chem. Comm. 1971 916. D. Bryce-Smith A. Gilbert and G. Klunklin J.C.S. Chem. Comm. 1973 330. F. C. De Schryver M. De Brackeleire S. Toppet and M. Van Schoor Tetrahedron Letters 1973 1253. W.M. Horspool X X X= CO(CH,),OCIt0It 0 (8) a; n = 2 b;n=3 (9) a; n = 7 b;n= 9 c; n=5 0 correct environment to produce a trans-syn-dimer.' Solid-state irradiation of the bisthymine (10) does indeed lead to a material with trans-syn cyclobutane linkages but as the product was polymeric little could be done to fully character- ize the material.' ' Nevertheless the results from the photolysis and the structure determination clearly show that the thymine fragments are aligned in a stack within the crystal which permits interaction in a specific fashion.Irradiation of the same bisthymine (10) in acetone-water (1 :9) gave the cis-syn-dimer (1l).' Intramolecular addition has also been examined in the bismaleimides (12).12 The reaction arises from the triplet state and the intersystem crossing efficiency increased (to 0.15 from 0.025) with increasing chain length. The intramolecular cycloaddition of biscoumarins (1 3)' has also been studied. lo J. K. Frank and 1. C. Paul J. Amer. Chem. Soc. 1973,95,2324.N. J. Leonard R. S. McCreadie M. W. Logue and R. L. Cundall J. Amer. Chem. SOC. 1973 95 2320. l2 J. Pat and F. C. De Schryver J. Amer. Chem. Soc. 1973 95 137. l3 L. H. Leenders E. Schouteden and F. C. De Schryver J. Org. Chem. 1973 38 957. Photochemistry I/ (CH,) (12) n = 3,4 5,6 or 7 (11) Barltrop and his co-w~rkers'~.' have studied the photoreactions of pyrilium salts (14a and b) in aqueous solution. Both compounds afford ring-opened keto-aldehydes (15) as products. Pyrilium salt (14b) also gives the syn-and anti-isomers of the cyclopentenone (16).15The results are rationalized in terms of an oxoniabenzvalene intermediate (17). Other worked6 have examined the photochemistry of the pyrilium salts (14c and d) in sulphuric acid.Benzvalene R3 R' R2 R3 R4 R5 (15) (16) (14) a; Me b; Me c; Me d; H H H H Me Me Et HO Me H H H H Me Me Me HO fi e; Me g; Mef; Me Ph Me Ph HO HO HO Ph Me H Me Me Me 9 :fiR3+/OH R' R2 0 R3 (I7) (18) a; Me Ph Ph b; Me Ph H c; Me Me Me intermediates do not seem to fit for these reactions. However in a later report Pavlik and KwongI7 propose the rearrangement of the salts (14e f and g) into the isomeric species (18a-c) via an oxoniabenzvalene intermediate. l4 J. A. Barltrop K. Dawes A. C. Day and A. J. H. Summers J.C.S. Chem. Comm. 1972 1240. l5 J. A. Barltrop K. Dawes A. C. Day S. J. Nuttall and A. J. H. Summers J.C.S. Chem. Comm. 1973 410. l6 J. W. Pavlik and E. L. Clennan J. Amer. Chem. Soc. 1973,95 1697. " J. W. Pavlik and J. Kwong J.Amer. Chem. Soc. 1973 95 7914. W.M. Horspool Over recent years interest has been shown in the nature of the excited state involved in nucleophilic aromatic substitutions. Present work has established that the photosubstitution of m-nitroanisole by hydroxide ion can be sensitized by benzophenone or quenched by oxygen. These results suggest the inter- mediacy of a n+ n* triplet state." On the other hand photosubstitution in anthraquinones (19) arises from the singlet manif01d.'~ Wubbels et ~1.~'have examined the photoreduction of nitrobenzene in HC1-water-propan-24. Contrary to an earlier report there is no evidence for protonation of the excited state and the reduction is effected by electron transfer from the chloride ion. 0 a; R' = MeO R2 = H b; R' = H R2 = Me0 The question of non-vertical energy transfer has been re-examined.Originally the concept arose from the examination of the kinetics for the cis-trans-photo- sensitized isomerization of stilbene.2 Yamauchi and Azumi22 have questioned the original interpretation and suggested that although the non-vertical concept is valid the process can be accommodated within the normal spectroscopic framework. The authors22 deduce that the spectrum of cis-stilbene tails below 48.6 kcal mol-' (204 kJ mol-') whereas the spectrum of the trans-isomer cuts off sharply at this value. In view of this fact and also that the rate of energy transfer from sensitizer to stilbene is dependent on the overlap between the spectra of the stilbene and that of the sensitizer then the transfer rate to the trans-isomer will fall off more rapidly than that to the cis.A comparison of the irradiation of cycloalkenes in the presence of hydroxylic solvents by both direct and sensitized photolysis has shown that there are similarities of beha~iour.~~ Thus 1-methylcyclo-hex-,-hept-,and -oct-enes give a similar range of products by either mode of irradiation. However 1-methylcyclopentene forms ethereal products upon direct irradiation but not under sensitized irradiation. The dimerization of norbornene yielding (20) has been carried out in high yield (88 %) by irradiation in the presence of copper(1) trifluoromethanesulphonate as catalyst.24 This catalyst is superior to other Cu' halides normally used for this purpose.J. den Heijer T. Spee G. P. de Gunst and J. Cornelisse Tetrahedron Letters 1973 1261. l9 J. Griffiths and C. Hawkins J.C.S. Chem. Comm. 1973 11 1. 2o G. G. Wubbels J. W. Jordan and N. S. Mills J. Amer. Chem. SOC.,1973 95 1281. 21 G. S. Hammond and J. Saltiel J. Amer. Chem. SOC.,1963 85 2516. 22 S. Yamauchi and T. Azuni J. Amer. Chem. SOC.,1973,95 2709. " P. J. Kropp E. J. Reardon,jun. Z. L. F. Gaibel K. F. Williard and J. H. Hattaway J. Amer. Chem. Sac. 1973 95 7058. 24 R. G. Salomon and J. K. Kochi Tetrahedron Letters 1973 2529. 315 Photochemistry A re-in~estigation~~ of the photochemical reaction of /I-t-butylstyrenes has been made.26 Using the p-cyano-derivative for ease of analysis the photo- chemical conversion of either the cis- or the trans-olefin gives a trans-cyclo- propane (21) uia a singlet-state reaction.This clearly shows the reaction to be non-concerted. Further study showed that the substituent on the aryl group of the styrene can influence the ease with which the methyl group migrate^.^' Considerable effort has been expended upon the solving of the mechanistic and stereochemical problems posed by the remarkable di-n-methane rearrange- ment. The studies have revealed that in the conversion of (22) into (23),there is a preferential formation of the three-membered ring by disrotatory motion of the orbitals at C-1and C-3anti to the migrating olefinic group. This conversion will occur only if the stereochemistry of the molecule will permit. The recognition of a syn-disrotatory conversion has been achieved in the direct irradiation (the sensitized reaction afforded no products) of the diene (24) which yielded the cyclopropyl olefin (25).28 It is reasoned that in the absence of steric restraints the preference for a given path lies in the better orbital overlap from the back of C-3with C-1.This situation does not obtain in the present example and an alternative reaction mode is followed.Zimmerman and Pincock2’ have examined the photochemistry of 3,3-dimethyl-l,5-diphenylpenta-1,4-diyne, which does not follow a di-n-methane route to the products 1,l -dimethyl-3,4-diphenylcyclopenta-2,4-diene and the ene-yne (26). The products arise uia a biradical intermediate and hydrogen abstraction from the solvent. Another potential di-n-methane reaction was sought for in the irradiation of the allene (27).In this example [2 + 21 cycloaddition predominates giving (28) and the di-n-methane path is of 25 S. S. Hixson and T. P. Cutler J. Amer. Chem. SOC., 1973 95 3031. 26 H. Kristinsson and G. W. Griffin J. Amer. Chem. SOC.,1966 88 378. 27 S. S. Hixson and T. P. Cutler J. Amer. Chem. SOC., 1973,95 3032. 28 P. S. Mariano and R. B. Steitle J. Amer. Chem. SOC.,1973 95 61 14. 29 H. E. Zimmerman and J. A. Pincock J. Amer. Chem. SOC.,1973,95 3246. W.M. Horspool minor imp~rtance.~’ The discovery of a ‘walk rearrangement’ of a cyclopropane ring in a heterocyclic system (29) has been reported?1 This rearrangement affords (30) (which is also photoreactive) as the primary photochemical product from the singlet excited state of (29).Mazzocchi and Lust$’ have observed the photochemical conversion of optically active l-deuterio-2,2-dimethyl-l-phenylcyclopropane into 4-deuterio-2-methyl-4-phenylbut-l-ene with complete retention of optical activity. This is interpreted as arising by fission of the C-l- C-2 bond concomitant with rotation about the C-2-C-3 bond which brings the methyl groups on C-2 into close proximity with the developing free radical at C-1. The Norrish Type I1 reaction of ketones in their nn* excited states is probably the most studied photochemical reaction. Related to this reactivity is the report of the irradiation (253.7nm) of a methanol solution of l,l-dideuterio-2-phenyl-2-o-tolylethylene which gave product (31) where an exchange of a deuterium atom had taken place.33 The reaction is thus similar to a Norrish Type I1 hydrogen abstraction in carbonyl compounds and the authors33 suggest that there is a similarity between the reactivities of the nn* state of a carbonyl group and of the nn* state of the ethylene.CHD CHzD Another problem associated with the interpretation of results from Norrish Type I1 reactions of ketones has been uncovered following a re-in~estigation~~ of the photochemistry of 4-methyl- 1-phenylpentan-1 -one. Usually care is taken in such experiments to exclude oxygen to prevent quenching of the ketonic triplet ~tate.~’,~~ However Grotewold et have observed that oxygen does not 30 D. C. Lankin D. M. Chihal G. W.Griffin and N. S. Bhacca Tetrahedron Letters 1973 4009. 3’ H. E. Zimmerman and W. Eberbach J. Amer. Chem. Soc. 1973,95 3970. 32 P. H. Mazzocchi and R. S. Lustig J. Amer. Chem. Soc. 1973 95 7178. 33 F. Scully and H. Morrison J.C.S. Chem. Comm. 1973 529. 34 J. Grotewold C. M. Previtali D. Soria and J. C. Scaiano J.C.S. Chem. Comm. 1973 207. 35 J. A. Barltrop and J. D. Coyle J. Amer. Chem. Soc. 1968 90 6584. 36 F. D. Lewis and T. A. Hilliard J. Amer. Chem. Soc. 1972,94 3852. Photochemistry 317 quench the reaction of the ketone and in fact the yields of acetophenone cyclo- butanol (32) and 2-methylpropene are increased. The authors34 suggest that PhHb (32) (34) (33) a; R' = OCOCH,Ph R2 = H b; R' = H RZ = OCOCH,Ph c; R' = CH,COMe R2 = H the triplet ketone reacts with oxygen producing a biradical species which sub- sequently breaks down to products.A peroxide is formed in the reaction and this appears to support the mechanism. Lewis and his co-~orkers~~ have examined the differences in the photochemical reactivity of some arylalkyl ketones. Their results indicate that radiationless decay of the triplet states of the ketones will not compete with y-hydrogen abstraction in the Norrish Type I1 process since the triplet state is too short-lived. They reason37 that the differences in reactivity result from differences in entropy. The singlet-state-induced Norrish Type I1 elimination reactions of the esters (33a and b) give a low yield of the highly reactive adamantene (34).38The success of this route is to be contrasted with the failure of the triplet-state reaction of (33c) which yielded cycl~butanols.~~ Kanaoka et uI.~'.~' have reported the photochemical reactions of N-substituted phthalimides.Norrish Type I reactions of the cyclic ketones (35) have been used in the synthesis of some rna~rolides.~~ 0 0 ,Li3 4 (35) n = I. 2 3 or 4 MO treatment of the photochemistry of fl,y-unsaturated ketones has been reported.43 This demonstrates that in the singlet state (m*) of the model com- pound (36) there is weak bonding between C-2and C-5 but antibonding between C-2 and C-4 with considerable weakening of the C-2-C-3 bond. Thus in this 37 F. D. Lewis R. W. Johnson and D. R. Kory J. Amer. Chem. Soc. 1973 95 6470. " J.E. Gano and L. Eizenberg J. Amer. Chem. Soc. 1973 95 972. 39 R. B. Gagosian J. C. Dalton and N. J. Turro J. Amer. Chem. Soc. 1970 92 4752. 40 Y.Kanaoka Y.Migita K. Koyama Y.Sato H. Nakai and T. Mizoguchi Tetrahedron Letters 1973 1193. 41 Y. Kanaoka and Y. Migita Tetrahedron Letters 1973 51. 42 R. G. Carlson J. H.-A. Haber and D. E. Henton J.C.S. Chem. Comm. 1973 223. 4' K. N. Houk D. J. Worthington and R. E. Duke jun. J. Amer. Chem. Soc. 1972 94 6233. W. M. Horspool state a-fission or 1,3-aryl shifts are predicted. The triplet nn* state is similar. However the triplet nn* state has little weakening of the C-2-C-3 bond but the interaction between C-2 and C-4 is bonding. Thus from this state 1,2-aryl shifts are predicted. The explanation given here is different from that proposed by Schuster et Interest in the difference between singlet and triplet reactivity of enones has continued.Direct irradiation of 2-methyl-2-(cyclopent- 1-enyl) cyclopentanone has been shown to give the hydrazulene (37) by a 1,3-migration whereas acetone sensitization gives the tricyclic ketones (38) by a 1,2-migration (an oxa-di-n-methane rea~tion).~' The structural effects in the triplet state rearrangement of some /?,penones have been Wolff and Agosta4' have continued their examination of the intramolecular hydrogen abstracting capabilities of the triplet state ofa,/?-unsaturated ketones. The current report deals with the formation of the bicyclic ketone (39) from the irradiation of the enone (40). This ketone (39) arises by loss of methylene from the biradical intermediate (41) followed by cyclization.The effect of the electron- withdrawing groups on the photochemistry of cross-conjugated cyclohexa- dienones has been reported.48 0 I1 n 0 I (43) A 1,4-addition product (42) has been isolated from the argon-laser-induced photoaddition of p-benzoquinone (n +n* excitation) to cy~lo-octatetraene.~~ This structure (42) is a reassignment of an earlier report where 1,2-addition was proposed.'' An enedione (43) has been isolated from the mixture of products formed from the irradiation of 2,6-di-t-butyl benzoquinone.' ' The continuation of the elegant studies of Chapman and his co-workers on matrix isolation photochemistry of organic molecules merits special mention.44 D. I. Schuster G. R. Underwood and T. P. Knudsen J. Amer. Chem. SOC.,1971 93 4304. 45 R. G. Carlson R. L. Coffin W. W. Cox and R. S. Givens J.C.S. Chem. Comm. 1973 501. 46 H. Sato K. Nakanishi J. Hoyashi and Y. Nakadaira Tetrahedron 1973 29 275. " S. Wolff and W. C. Agosta J.C.S. Chem. Comm. 1973 502. 48 D. Caine P. F. Brake J. F. De Bardeleben and J. B. Dawson J. Org. Chem. 1973,38 967. 49 E. J. Gardner R. H. Squire R. C. Elder and R. M. Wilson J. Amer. Chem. Sor. 1973 95 1693. 50 D. Bryce-Smith A. Gilbert and M. G. Johnson J. Chem. Soc. (0,1967 383. 51 T. J. King A. R. Forrester M. M. Ogilvy and R. H. Thomson J.C.S. Chem. Comm. 1973 844. Photochemistry 319 In this work the study of a-pyrone photochemistry has shown that keten (44) formation is the primary photochemical path and that lactone (45) formation is only a minor path~ay.’~ However prolonged irradiation of the a-pyrone using Pyrex-filtered light at 8 K eventually gives a high yield of the lactone (45).Subsequent irradiation of the lactone with quartz-filtered light leads to CO extrusion and the formation of cyclobutadiene (i.r. absorptions at 1240 650 570 cm-1).53 The simplicity of this spectrum suggests that cyclobutadiene is square planar with D, symmetry. Other workers have arrived at the same con~lusion.’~ This symmetry is further suggested by the synthesis and i.r. investigation of deuteriocyclobutadienes. The experiments demonstrated that while irradiation of isomeric monodeuterio-a-pyrones (46) gave different /?-lactones the same monodeuteriocyclobutadiene was obtained.’ ’ (44) (46) a; R’= H R2 = D (47) b; R’ = D R2= H (46) Benzyne has been isolated in an argon matrix from the prolonged irradiation of phthaloyl peroxide or benzocyclobutenedione.56 Irradiation (at 8 K) of the peroxide for shorter times produces the elusive benzopropiolactone (47).’ The photochemistry of N-oxides continues to produce interesting results.Ullman and his co-w~rkers~~.~~ have studied the photoconversion of the radical (48)into two new radicals (49) and (50). The ring-contraction [to (49)] encountered in this sequence is all the more interesting as a result of the photorearrangement of the related N-oxide (51) into ketone (52).60 Other workers6’*62 have reported novel ring-contractions in the photochemistry of 4-substituted 1,2,3-benzotria- zine 3-N-oxides (53) and 4-methylcinnoline 2-N-oxide.Irradiation of 4-amino- naphthotriazinone in acetonitrile leads to the formation of the relatively stable azetinone (54).63 The mechanism of conversion of oxazoles into isoxazoles has been studied6* and shown to involve the formation of isocyanides. 52 0. L. Chapman C. L. McIntosh and J. Pacansky. J. Amer. Chem. SOC.,1973,95 244. 53 0. L. Chapman C. L. McIntosh and J. Pacansky J. Amer. Chem. SOC.,1973,95 614. 54 A. Krantz C. Y. Lin and M. D. Newton J. Amer. Chem. SOC.,1973 95 2744. 55 0.L. Chapman D. De La Cruz R. Roth and J. Pacansky J. Amer. Chem. SOC.,1973 95 1337. 56 0. L. Chapman K. Mattes C.L. McIntosh J. Pacansky G. V. Calder and G. Orr J. Amer. Chem. SOC.,1973 95 6134. 57 0.L. Chapman C. L. McIntosh J. Pacansky G. V. Calder and G.Orr J. Amer. Chem. SOC.,1973 95 4061. ’’ E. F. Ullman L. Call and S. S. Tseng J. Amer. Chem. SOC.,1973 95 1677. 59 L. Call and E. F. Ullman Tetrahedron Letters 1973 961. 6o R. Felden 0. Meth-Cohn and H. Suschitzky J.C.S. Perkin I 1973 702. 61 W. M. Horspool J. R. Kershaw A. W. Murray and G. M. Stevenson J. Amer. Chem. SOC.,1973 95 2390. 62 W. M. Horspool J. R. Kershaw and A. W. Murray J.C.S. Chem. Comm. 1973 345. ‘’ N. Bashir and T. L. Gilchrist J.C.S. Perkin I 1973 868. 64 J. P. Ferris F. R. Antonucci and R. W. Trimmer J. Amer. Chem. SOC.,1973,95 919. W.M. Horspool ?-0 0 (49) I 0' (48) (50) 0- 'Ph Ph (53) R' = Et or Me RZ = H R' = Ph R2= H or C1 (51) (52) Interest in the photochemistry of thiocarbonyl compounds has continued during the past year.Thiophosgene adds photochemically to 2,3-dimethylbut- 2-ene to afford the thietan (55) in 51 % yield.65 The dithiolethione (56) is also photochemically reactive and irradiation (A > 420nm) in benzene gave the product (57).66 This is formed by the intramolecular addition of the photo- chemically excited thiocarbonyl group to the benzene ring. The excited thio- carbonyl group can be trapped intermolecularly by irradiation in the presence of 1,l -diphenylethylene when (58) is formed. Study of adamantanethione photo- chemistry has led to the proposal that there is significant intersystem crossing (54) (55) 65 H.Gotthardt Tetrahedron Letters 1973 1221. 66 P. de Mayo and H. Y. Ng Tetrahedron Letters 1973 1561 Photochemistry 321 to the triplet manifold from the second excited m*singlet ~tate.~’ The first example of a stable a-dithione (59) has been prepared by the photo-decarbonyla- tion of the corresponding diary1 dithiovinylene carbonate.68 Ar S (59) Ar = p-dimethylaminophenyl SxAr Several unrelated topics make up a final paragraph. Zimmerman et have published an account of a method for the determination of ultra-fast (in the picosecond range) singlet reaction rates. A report by Williams et suggests the use of a photosensitizer dye (Eosin or Rose Bengal) complexed upon a basic anion exchange resin (or Methylene Blue upon an acidic exchange resin).This method gets around the separation problems in dye-sensitized singlet oxygen reactions. The thermal population of electronicalIy excited states has been further studied and a report on the production of benzene triplets from the thermolysis of ‘Dewar’ benzenes has been p~blished.~’ An account of ‘anti- Stokes’ sensitization of the triplet Norrish Type I1 elimination of valerophenone by energy transfer from the biacetyl-tetramethyl 1,2-dioxetan is also of interest.72 Acetone has been used for the facile epimerization of unactivated tertiary centres such as the conversion of cis-decalin into trans-decalin. 67 A. H. Lawrence and P. de Mayo J. Amer. Chem. SOC.,1973,95,4084. 68 W. Kusters and P. de Mayo J.Amer. Chem. SOC.,1973,95 2383. 69 H. E. Zimmerman D. P. Werthemann and K. S. Kamm J. Amer. Chem. SOC.,1973 95 5094. ’O J. R. Williams G. Orton and L. R. Unger Tetrahedron Letters 1973 4603. 71 P. Lechtken R. Breslow A. H. Schmidt and N. J. Turro J. Amer. Chem. SOC.,1973 95 3025. 72 N. J. Turro and P. Lechtken Tetrahedron Letters 1973 565. 73 R. G. Salomon and J. K. Kochi Tetrahedron Letters 1973 4387.