7 Photochemistry By A. COX Department of Chemistry and Molecular Sciences University of Warwick Coventry CV4 7AL A review has appeared' surveying some of the more complex adiabatic photo- reactions of organic molecules such as fragmentations electrocyclic rearrangements and geometrical isomerizations. The effect of pulsed infrared radiation on cis-3,4-dichlorocyclobutene has been examined.* Despite the fact that the experimental conditions used were probably sufficient to raise the substrate to vibrational levels considerably in excess of the normal lowest reaction channel cis,trans-1,4-dichlorobuta-1,3-dienewas the only product detected clearly showing an adherence to the Woodward-Hoff mann Rules for this thermally-allowed transformation. Further investigations into the field of cyclopropene photochemistry have been disc~ssed,~ with particular reference to 1-methyl-2,3 -diphenyl-3 -isobutenylcyclopropene and 1,2-diphenyl- 3 -methyl-3 -isobutenylcyclopropene.The results are interpreted in terms of the participation of a biradical rather than a carbene pathway in both the direct and triplet reactions. Interconversions of different vinylcyclopropenes are seen as an incipient di-r- methane rearrangement involving a biradical mechanism. It has been shown4 that in contrast to its thermal processes the photoreactions of bicyclo[4.2.0]octa-2,4,7-trienes are substituent-dependent. Thus irradiation of the 2,5-dimethyl-3,4- diphenyl-substituted compound leads to a mixture of aromatic and acetylenic products whose formation can be rationalized in terms of thermal equilibria and further photoprocesses.The corresponding tetraphenyl-substituted triene however yields only the 1,2,3,4-tetraphenylcyclo-octatetraene. A good deal of interest is still being shown in the di-r-methane rearrangement. An investigation of some 5-methylenenorborn-2-eneshas revealed' that in com- mon with many other rigid 1,4-diene systems di-r- methane rearrangements can occur from both singlet and triplet excited states. The efficiency of the triplet reaction is highly substituent-dependent and it is demonstrated that the structural constraints of the product control the regiospecificity. Direct irradiation of (-)-3-(p-methoxyphenyl)-3-methyl-l,l-diphenylpent-l-ene has been found6 to lead to a mixture of ( -)-cis-and ( +)-trans-2-ethyl-2-methyl-3,3-diphenyl-l-(p-methoxy-phenyl)cyclopropane in which the methane carbon has undergone an inversion ' N.J.Turro J. McVey V. Ramamurthy and P. Lechtken Angew. Chem. Internat. Edn. 1979,18 572. W. C. Danen D. F. Koster and R. N. Zitter J. Amer. Chem. Soc. 1979,101,4281. H.E.Zimmerman and M. C. Hovey. J. Org. Chem. 1979,44,2331. R. N.Warrener I. W. McCay R. Y. S. Tan and R. A. Russell Tetrahedron Letters 1979,3183. Z. Goldschmidt and M. Shefi J. Org. Chem. 1979,44 1604. H. E.Zimmerman T. P. Gannett and G. E. Keck J. Org. Chem. 1979,44,1982. 114 A. Cox h Me0 + Me0 t I J I pentadienyl;orbital f Ph Ph." Scheme 1 (Scheme 1). This stereochemistry is in accordance with the predicted allowed stereochemistry arising from a Mobius transition state.The photoreactions of a series of 5,6-benzo-2-azabicyclo[2.2.2]octa-7,8-dien-3-ones have been used' as a probe to study structural and electronic substituent effects on the di-rr-methane rearrangement. A phenyl substituent is found to reverse the previously observed regioselectivity and a solvent effect is also apparent. The synthesis of a cyclopropane sesquiterpenoid taylorione has been reported,8 using the di-n-methane rearrangement. This represents only the second example of the use of this reaction in natural product synthesis. An investigation of the photochemistry of cross-conjugated trienes has shown' that whereas irradiation of the dicyano-compound (1) leads to a 'toluene' type of product the corresponding tetraphenyl compound (2) reacts by a vinyl-vinyl bridg- ing process (Scheme 2).Using SCF-CI calculations along various reaction co- ordinates a theoretical analysis of the system has been made. This suggests that in the excited state phenyl-substitution on the em-methylene group alters vinyl-vinyl bridging from a forbidden to an allowed process. Ph Ph Ph Ph-'Ph The photochemistry of 2-halogeno-1,l -diphenylethylenes has been examined" and the product distribution found to depend on both the halogen and the solvent. Cleavage of the C-X bond may occur to give a radical pair and may be followed by eIectron transfer to give an ion pair. This leads to competing ionic and radical behaviour. Products derived from a stabilized triplet state are also possible and the proportions of the three pathways are dependent on the halogen.A report has appeared" of the first example of an olefin analogue of the Type I1photoelimination 'M. Kuzuya M. Ishikawa T. Okuda and H. Hart Tetrahedron Letters 1979 523. * G. Pattenden and D. Whybrow Tetrahedron Letters 1979 1885. H. E. Zimmerman and D. R. Diehl J. Amer. Chem. Soc. 1979,101,1841. lo B. Sket and M. Zupan J.C.S. Perkin Z 1979,752. H. Aoyama T. Hasegawa M. Okazaki and Y. Omote. J.C.S. Perkin Z 1979 263. Photochemistry 115 of ketones. Using a low-pressure mercury arc photocyclization of NN-dibenzyl- methacrylamide (3a) to the p-lactam (4) and photodealkylation of NN-di-iso-propylmethacrylamide (3b) to N-isopropylisobutyramide both proceed via dimethylketen and the corresponding amine.The first step in both of these trans- formations is intramolecular hydrogen abstraction by the p-carbon (Scheme 3). 0 0 ,CHR1R2 (3) (a) R'=H R2=Ph (b) R'=Ri=Me J Me \ c=c=o / Me (R1=H,R2/ R> C=NCHR~R~ + \ RZ 0 Me,EAN,CHR'R2 II Ph CH,Ph Me H (4) Scheme 3 Chemical evidence has now been presented12 which adds further weight to an earlier rep~rt'~ that direct irradiation of cis-1-phenylcyclohexene affords the corresponding trans-isomer. At -75 "C and in methanol irradiation of cis-1-phenylcyclohexene gives rise to a dimer (5) thought to arise by a Woodward- Hoffmann-allowed concerted [7r4,+ 7r2,] addition of cis-1-phenylcyclohexene to the endocyclic trans double-bond of trans- 1-phenylcyclohexene.(5) 12 W. G. Dauben H. C. H. A. van Riel C. Hauw F. Leroy J. Joussot-Dubien and R. Bonneau J. Amer. Chem. SGC., 1979 101 1901. 13 R. Bonneau J. Joussot-Dubien L. Salem and A. J. Yarwood J. Amer. Chem. SOC.,1976 98,4329. 116 A. Cox It has been ob~erved'~ that the quantum yields of the direct trans-cis photo- isomerization of some p- rn- and m,m'-bromine-substituted stilbenes show larger quenching effects by azulene than do the trunslcis-photostationarycompositions. Together with measurements of the temperature-dependence of the quantum yield of fluorescence of each trans-isomer it is concluded that the positional dependence previously inferred for the heavy-atom effect is incorrect.It also emerges that 'p* cannot be the exclusive intermediate in the mechanism of photoisomerization of the cis-bromostilbenes. In some related work evidence has been presented" support- ing the view that between -155 and -100 "C the trans-cis photoisomerization of 4-bromostilbene occurs mainly via an upper excited triplet state. At temperatures above -100 "C it is suggested that the upper excited triplet mechanism competes with twisting in the lowest excited singlet state and even at room temperature this may still be the case. The photocyclization of stilbenes has been used16 in the synthesis of juncusol an anti-cancer agent but in the key step (6)+(7) the unwanted isomer (8) is found to predominate (Scheme 4).This result is consistent hv ,Me09 + MeOq / NC\ OMe NC\ OMe MeO\ CN Me Me Me (6) (6:1,E:Z) (7) Scheme 4 with both the Giisten-Klasinc ground-state model and with simple frontier MO calculations. The trans *cis photointerconversion of 1,2-di-(2'-naphthyl)ethyl-enes and the photocyclization of the cis-isomer to 4a,4b-dihydrophenanthrene derivatives has been investigated" over the temperature range +25 to -180 "C. For the cis-isomers the presence of conformer equilibria has been established spec- troscopically and the results also indicate the presence of potential barriers on the pathway between the excited-state starting materials and the products. CIDNP effects have been used" to examine the photosensitized photoisomerization of electron-donor and electron-acceptor styrenes and two different mechanisms of isomerization seem to operate.For the electron-transfer-induced isomerization of donor olefins the effects are compatible with the interconversion of two different radical ions of different energies. On the other hand the effects for acceptor styrenes with photoexcited electron donors suggest a single intermediate in the isomerization. Irradiation of 1,2,4,5-tetracyanobenzenein either diethyl ether tetrahydrofuran or tetrahydropyran leads to replacement of one cyano-group by an ether residue." l4 J. Saltiel A. Marinari D. W.-L. Chang J. C. Mitchener and E. D. Megarity J. Amer. Chem. SOC.,1979 101,2982. H. Gorner and D. Schulte-Frohlinde J. Amer. Chem. SOC.,1979 101,4388. l6 A.S. Kende and D. P. Curran J. Amer. Chem. SOC.,1979 101 1857. " T. Wismonski-Knittel and E. Fischer J.C.S. Perkin II 1979,449. H. D. Roth and M. L. M. Schilling J. Amer. Chem. SOC.,1979,101 1898. l9 M. Ohashi K. Tsujimoto and Y. Furukawa J.C.S. Perkin I 1979 1147. Photochemistry 117 Three possible mechanisms are considered One of these in which a tetrahydrofuran radical cation reacts with a neighbouring tetrahydrofuran molecule was definitely eliminated on kinetic grounds. However no decision was possible between a path involving charge-transfer exciplexes and a zwitterionic mechanism. A novel photo- chemical aromatic amination has been reported2' by irradiation of a solution of 9,lO-dicyanoanthracene and an amine in acetonitrile. The initial step appears to be transfer of an electron from amine to anthracene via an encounter complex or exciplex.Water is also apparently necessary to hydrolyse the intermediate imine. Evidence has been presentedz1 on the mechanism of the diene-induced photodech- lorination of chloro-aromatics. The reaction involves both exciplex and triplex formation and also a protonation step and is shown to proceed via a long-lived ion-radical intermediate. A study has been made22 of the orbital topology in a photochemical carbanionic ring-opening using the anion generated from cis,fruns-2,3-diphenylcyclopropanecarbonitrileby treatment with lithium di-isopropylamide. The results show that the process is stereochemically non-random but fail to distinguish between disrotatory photochemical opening of an anion and thermal opening of a radical.Irradiation of naphthalene in trifluoroacetic acid has been to lead to bimolecular hydrogen exchange via a singlet-state process. Consistent with the observed small kinetic isotope effect the incoming hydrogen is found to be lost about one hundred times more readily than one originally present and it is concluded that a wcomplex is formed. The three-membered ether chain CH2-0-CH2 has been to hold two anthracene chromophores in close proximity while the photobehaviour of the entire system is studied at low tempera- ture. The spectroscopic properties of a series of derivatives of bis-9-anthrylmethyl ethers and their photocyclodimerization reactions have been studied in this way. A study has been madez5 of the photochemistry of a series of bicyclic Py-cyclopropyl ketones.The behaviour of these ketones suggests a stepwise sequence of events involving opening of the three-membered ring by a-cleavage followed by rearrangement of the initial cyclopropylcarbinyl radical under stereoelectronic control to a homoallylic radical. The photochemistry of Py -cyclopropyl ketones derived from bicyclo[4.2.l]nona-2,4,7-trien-9-oneand possessing structural fea- tures which under irradiation tend to promote decarbonylation has been examined.26 Relative rates of decarbonylation 1,3-acyl shifts and a-cleavage are established; for the compounds chosen this sequence is found to be in agreement with qualitative information in other systems. Photoisomerization about a carbon-xygen partial double bond has been obser- ~ed~~ (Scheme 5).Irradiation of (9) in FS03H at -7O"C promotes partial iso- merization to give a photostationary state comprising (10) (73%) and (11)(27%). It has been suggested that this same photoinduced stereomutation might occur with protonated carbonyl compounds and that this could be a way for protonated 2o M. Ohashi H. Kudo and S. Yamada J. Amer. Chem. SOC.,1979,101,2201. 21 W. K. Smothers K. S. Schanze and J. Saltiel J. Amer. Chem. SOC.,1979 101 1895. 22 M. A. Fox J. Amer. Chem. SOC.,1979,101,4008. 23 N. J. Bunce Y. Kumer and L. Ravanal J. Org. Chem. 1979,44 2612. 24 A. Castellan J.-M. Lacoste and H. Bouas-Laurent J.C.S. Perkin 11,1979,411. 25 I. M. Takakis and W. C. Agosta J.Amer. Chem. SOC.,1979,101 2383. 26 I. M. Takakis and W. C. Agosta J. Org. Chem. 1979 44 1294. " R. F. Childs and M. E. Hagar J. Amer. Chem. SOC.,1979,101 1052. 118 A. Cox (9) R=HorMe Scheme 5 carbonyls to relax to their ground state. In a related paper2* the photoisomerization of protonated 4,4-dimethylcyclohex-2-enonein FS03H at -85 "C is reported. Incorporation of the chromophore into a ring system renders isomerization about the double bond more difficult and protonated 6,6-dimethylbicyclo[3.1 .O] hexan-2-one is produced. The effect of methyl substituents on the stereochemistry of the photoinduced addition of methanol to seven- and eight-membered cycloalk-2- enones has been st~died.~' It is concluded that the photoinduced addition of the nucleophile to cis-cyclohex-2-enones involves photoisomerization to the trans- isomer followed by syn-addition of the nucleophile.Proton transfer is probably the rate-determining step. trans-Fused [2s +2a] addition products are formed3' in the photocycloaddition of 17P-hydroxyandrosta-4,6-dien-3-one acetate to the elec- tron-deficient olefin methyl acetate. This breakdown in stereospecificity of cyclo- addition confirms a theoretical prediction31 based on a calculation of the potential energy surface for 3(7r,7r*) cycloadditions which indicates that if the enone and olefin are a good donor-acceptor pair then non-Woodward-Hoff mann addition should occur. The regioselectivity of enone photodimerization appears to be profoundly influenced by irradiating in potassium dodecanoate micelle~.~* Thus 3-alkyl- pentenones are observed to give ratios of head-to-head to head-to-tail dimers of up to 98 :2.The efflciency of the dimerization is also greatly increased. Photolysis of 2-arenesulphonyloxy-cyclohex-2-~nones leads33 to 2-hydroxy-3- phenylcyclohex-2-enones in about 40% yield. The reaction which probably pro- ceeds uia a triplet state is not of the photo-Fries type but may involve S-0 cleavage as the primary photochemical step However a mechanism involving cyclization to a biradical is also a possibility. A new method for the preparation of bicyclo- [3.2.0]heptan-2-ones has appeared34 and is especially interesting as on thermal fragmentation these compounds lead cleanly to cyclopentenones (Scheme 6).The Scheme 6 R. F. Childs K. E. Hine andF. A. Hung Canad. J. Chem. 1979,57 1442. 29 H. Hart B. Chen and M. Jeffares J. Org. Chem. 1979,44,2722. 30 G. R. Lenz and L. Swenton J.C.S. Chem. Comm. 1979,444. 31 S. Shaik and N. D. Epiotis J. Amer. Chem. SOC.,1978,100 18. 32 K.-H. Lee and P. de Mayo J.C.S. Chem. Comm. 1979,493. 33 A. Feigenbaum J.-P. Pete and D. Scholler Tetrahedron Letters 1979 537. 34 R. G. Salomon D. J. Coughlin and E. M. Easler J. Amer. Chem. SOC.,1979 101 3961. Photochemistry 119 key reaction is the photobicyclization of hepta- 1,6-dien-3-0ls using copper(1) trifluoromethanesulphonate as the catalyst. In the absence of copper(I) this reaction is suppressed. Hitherto it has generally been accepted that whereas electroisomeric linearly conjugated cyclohexa-2,4-dienones of predominantly n,v* character afford iso- meric dienylketens those of predominantly v,v* character lead to isomeric bicy- cl0[3.1 .O]hex-3-en-2-ones.However a recent paper3' now suggests that bicyclic ketones may also be the products of n,v* states and a study of a family of methylated o-quinol acetates indicates that the reaction channel depends on such factors as reaction r,iedium and substitution pattern. A paper describing the photoinduced cycloaidition of sterically hindered p-quinones to diphenylketen has appeared.36 This reaction the first example of its type involving diphenylketen affords a mixture of a spiro-oxetanone and a cyclobutanone. Both products are considered to arise from the 3(7r,7r*) state of the quinone and the predominance of the oxetanone product is attributed to the degree of dipole-dipole interaction between the excited state of the quinone carbonyl and the ground state of the diphenylketen.In a related paper,37 the photoreaction of p-benzoquinone with unsymmetrical keten dimers is discussed. A temperature-dependent photo-CIDNP study has revealed3' that on irradiation certain by- unsaturated ketones yield radical pairs (Scheme 7) from either the S1 or T2state depending upon the reaction temperature. Before collapsing to products one reaction channel that is open to the radicals is a 1,3-acyl shift. * Y eOMe 9I I X=Y=Me 1 MeCHO X = Me,Y= H X=Y=H 0 Scheme 7 As part of a series of studies in the field of py-unsaturated ketones the stereo- chemistry of the oxa-di-v- methane rearrangement of 2-substituted-2-(cyclopent-1-eny1)cyclopentanones has been in~estigated.~~ The main conclusions drawn are that the triplet-sensitized oxa-di-v- methane rearrangement involves [1,2]-supraf acial carbonyl migration and that this proceeds with inversion of configuration at the 35 G.Quinkert F. Cech E. Kleiner and D. Rehm Angew. Chem. Internat. Edn. 1979 18 557. 36 K. Ogino T. Matsumoto and S. Kozuka J.C.S. Chem. Comm. 1979 643. 37 K. Ogino T. Matsumoto T. Kawai and S. Kozuka J.C.S. Chem. Comm. 1979 644. 38 A. Henne N. P. Y.Siew and K. Schaffner J. Amer. Chem. SOC., 1979,101,3671. 39 R. L. Coffin W. W. cox R. G. Carlson and R. S. Givens J. Amer. Chem. SOC.,1979 101 3261.120 A. Cox methane carbon. [1,3]-Rearrangement of the singlet state seems to occur suprafacially to give a substituted bicyclo[5.3.0]dec-l-en-6-one. Flash photolysis of various methylated indan-2-ones indicates4' the presence of a transient identified as an o-xylylene and which in the case of 1,1,3,3-tetra-methylindan-2-one has been shown to be 7,7,8,8-tetramethyl-o-xylylene.Thermal decay of these transient non-planar xylylenes may proceed via the hitherto unknown antarafacial [1,5]-shift to give styrenes. An examination of the photochemistry of the 2-spirocyclopropylindan-1-one (12) in a rigid matrix at 77K affords4' (E)-2- ethylideneindan-1-one formed exclusively from the 3(n,7r*)state in a monopho- tonic process; no reaction however occurs in fluid solution.It is suggested that since the rate of ring-closure in fluid solution is rapid under these conditions the biradical simply returns to the starting material (Scheme 8). O* 0 hv and iMf ring fission T[~,~IH shift 6-ringclosllre \ \ Scheme 8 It has been reported4* that irradiation of certain methoxy-acetophenones together with cyanide ion in acetonitrile can lead to the formation of methoxy-benzonitriles in high yield. The reaction is inhibited by water and although no mechanism has yet been suggested the transformation is known not to involve a ground-state complex between nucleophile and aromatic substrate. The triplet state of P-(dimethyl- amino)propiophenone decays in aqueous solution by a charge-transfer interaction to give intramolecular electron transfer and results in a biradical zwitterionic product which has been trapped43 by the l,l'-dimethyl-4,4'-bipyridyliumdication acting as an electron acceptor.The lifetime of this triplet is dependent on the pH of the medium and at values below the pK of the dication the triplet may decay before acid-base equilibrium has been established. CIDNP studies of the photoreduction of 4-substituted acetophenones by NN-dimethylanilines suggest the presence of two pathways for formation of radical i0n-pai1-s.~~ Triplet ion-pairs are formed by excitation of the ketone followed by electron abstraction from the ground-state amine and electron-transfer from the singlet excited amine to the ground-state ketone gives a singlet ion-pair.The influence of mercaptans on the photoreduction of various aromatic ketones by alcohols has been st~died.~' Retardation of the 40 K. K. De Fonseka J. J. McCullough and A. J. Yarwood J. Amer. Chem. SOC.,1979,101,3277. " R. 0.Loutfy and P. Yates J. Amer. Chem. SOC.,1979 101,4694. 42 A. L. Colb J. Amer. Chem. SOC.,1979,101,3416. 43 M. V. Encinas and J. C. Scaiano J. Amer. Chem. SOC.,1979 101,2146. B. M. P. Hendriks R. I. Walter and H. Fischer J. Amer. Chem. SOC.,1979,101,2378. 45 S. G. Cohen A. W. Rose P. G. Stone and A. Ehret J. Amer. Chem. SOC.,1979 101 1827. Photochemistry 121 process occurring between triplet benzophenone and propan-2-01 is largely due to hydrogen-transfer reactions from mercaptan to the 2-hydroxy-2-propyl radical and from the benzophenone ketyl radical to the thiyl radical.For 2-mercaptomesitylene the decrease in rate using deuteriated propan-2-01 is greater than in the protio system. A study of the photochemistry of a-benzoyl-w-azido-ketones of the type PhCO(CH2).N3 has revealed46 the existence of a competition between the carbonyl and azide (nitrene) photoprocesses namely y-hydrogen abstraction to give Type I1 products and energy transfer from the triplet ketone to the azide to form nitrene products. The positions of the two chromophores on the molecular skeleton have been varied systematically so that the rate constants for interaction between the two groups could be compared with their distances apart. The rate variations for n = 3 to 5 have been interpreted in terms of the strain present in medium-sized rings.A hydrophilic polymer-immobilized photosensitizer that is useful for photo- oxygenations in aqueous solution has been described.47 It is prepared by copoly- merization of chloromethylstyrene and the monomethacrylate ester of ethylene glycol as cross-linking agent. This is then heated with Rose Bengal in dry NN-dimethylformamide giving the hydrophilic sensitizer @-Rose Bengal. Singlet oxygenation of cyclo-octa-1,3,5-triene leads48 to (2 +4) adducts (Scheme 9). This represents a convenient route to novel [4.2.2]- and [2.2.2]-type endoperoxides which themselves have synthetic potential. There is considerable current interest in the quenching and reaction of singlet molecular oxygen with a-tocopherol as this may be the basis of its biological function as an anti-oxidant.It is now that at low temperatures the primary product of photo-oxygenation is the relatively stable hydroperoxy-dienone (13). Although simple acetylenes do not react with singlet oxygen that has been generated by using Methylene Blue or Rose Bengal in methanol photosensitized oxygenation of aromatic acetylenes has been shown” to yield benzils (Scheme 10). An electron-transfer mechanism similar to that suggested for the photosensitized oxygenation of alkenes has been advanced and the reactivity (13) 46 P. J. Wagner and B. J. Scheve J. Amer. Chem. SOC.,1979,101,378. 47 A. P.Schaap A. L. Thayer K. A. Zaklika and P. C. Valenti J Amer. Chem. SOC.,1979,101,4016. 48 W. Adam and I.Erden Tetrahedron Letters 1979,2781. 49 R.L.Cough B. G. Yee and C. S. Foote J. Amer. Chem. SOC.,1979,101,683. N. Berenjian P. de Mayo F. H. Phoenix and A. C. Weedon Tetrahedron Letters 1979,4179. 122 A. Cox sens sens* RCfCR_ [sens'] C[RC=CR'] RC0,H Scheme 10 of the acetylenes is in accordance with the predictions of the Weller equation. Results have now been published5' which cast doubt on earlier suggestions concerning the dye-sensitized photo-oxygenation of a-0x0-carboxylic acids in which decarboxyl- ation occurs. Measurements of the rates at which a-0x0-acids react with singlet oxygen in competition with 1,3-diphenylisobenzofuranindicate that reaction with the triplet dye is more efficient than with singlet oxygen. Consequently singlet oxygen appears not to play a significant role.The Rose-Bengal- or Methylene-Blue- sensitized photo-oxidation of 2-(2-quinolyl)indane-1,3-dionein solution has been showns2 to lead to a mixture of products including phthalic acid quinoline-2- carbaldehyde and quinoline-2-carboxylic acid (Scheme 11).Of the two forms of the fly)JpJ \ I \ 0 .. . . . ..... . H 0-H (14) 110 0 Scheme 11 quinophthalone derivative which exist in equilibrium in solution (14) is the more susceptible to photo-oxidation. On oxidation an unstable peroxide is produced by an ene reaction but a perepoxide-type intermediate is also a possibility. Further photolysis or possibly thermolysis leads to a breakdown to products. Photo-oxygenations of different types of sulphides have been carried out using both dye and dicyanoanthracene (DCA) as sensitizers and it appearss3 that although those reactions involving DCA do not proceed by singlet oxygen both involve the same intermediate.A superoxide may participate. Photosensitized oxygenations of various diazo-compounds in the presence of oxygen-atom acceptors have showns4 51 R. S. Davidson D. Goodwin and G. Smith J.C.S. Chem. Comm. 1979,463. '* N. Kuramoto and T. Kitao J.C.S. Chem. Comm. 1979,379. 53 W. Ando T.Nagashima K. Saito and S. Kohmoto J.C.S. Chem. Comm. 1979,154. 54 W. Ando. H. Miyazaki and S. Kohmoto Tetrahedron Letters 1979 1317. Photochemistry 123 that the products depend on the electrophilicity of the oxenoid intermediate. Thus the yields of diphenyl sulphoxide formed in the photosensitized oxygenation of benzoylphenyldiazomethane are very different from those obtained from diphenyl- diazomethane and diazofluorenone.A simple device for moving the position of a keto-group with concomitant introduction of a double bond has been ann~unced.’~ Thus dye-sensitized oxygenation of vinylsilanes leads to a hydroperoxide which can be transformed into an allylic alcohol (Scheme 12). This transformation depends on OOH SiMe 2 SiMe --* Reagents i ‘02; ii NaBH,; iii Bu”,N’F- dry MeCN Scheme 12 the fact that cup-epoxysilanes ring-open with a regioselectivity opposite to that followed by epoxides lacking carbon-metal bonds. A new reaction has been reported56 in which carbonyl oxides from diazo-compounds oxidize silyl ketones to esters.Thus irradiation of a solution of diphenyldiazomethane phenyltrimethylsilyl ketone and meso-tetraphenylporphin under oxygen leads to a high yield of tri-methylsilyl benzoate. A plausible mechanism is shown in Scheme 13. Ph2CN2 SiMe3 + I + -+ Me3SiCOR + Ph,C=O-O-C-O-+ Ph2C=O-O-C-OSiMe3 I I R 1 Ph2C0 + Me3SiOCOR Scheme 13 Irradiation of 4-cyanopyridine in 2,3-dimethylbut-:!-ene a mixture of 4- (1’,1’,2’-trimethylprop-2‘-enyl)pyridine and 4-(2’,3’-dimethylbut-2’-enyl)pyridine; 2-cyanoquinoline undergoes an analogous reaction. It is suggested that the addition proceeds by electron transfer from the r-system of the olefin to the r-system of the aromatic base. However since no products of photocyclization are apparent monophotonic hydrogen abstraction from the olefin by the n,r* state of the base followed by cross-dimerization of the radicals is also a possibility.Formation of 1H-1,3-benzodiazepines in the photolysis of isoquinoline N-imides has been anno~nced,~~ and probably occurs via ring-expansion of an aziridine intermediate (Scheme 14). Although this process is known for aromatic amine N-oxides it is the first example reported involving aromatic amine N-imides. A new high-yield synthesis of quinolines has been p~blished.~’ This transformation involves an ” W. E. Fristad T. R. Bailey L. A. Paquette R. Gleiter and M. C. Bohm J. Amer. Chem. SOC.,1979,101 4420. ’‘ A. Sekiguchi Y.Kabe and W. Ando J.C.S. Chem. Comm. 1979,233. 57 T. Caronna S. Morrocchi P.Traldi B. M. Vittimberga J.C.S. Chem. Comm. 1979,64. ’’ T. Tsuchiya M. Enkaku J. Kurita and H. Sawanishi,J.C.S. Chem. Comm. 1979 534. 59 P. de Mayo L. K. Sydnes and G. Wenska J.C.S. Chem. Comm. 1979,499. 124 A. Cox .\ C0,Et I 1 T Qy -Q N\ IR C0,Et C0,Et R = Meor COzEt Scheme 14 intramolecular cycloaddition of substituted thioanilides and is thought to proceed via a ‘(T,w*) state. High concentrations of HC104 or &SO4 increase the fluorescence of a variety of quinolinium ions in the Hammett acidity region but without any shift in the fluorescence or absorption wavelength.60 These observations have been inter- preted in terms of the formation of an adduct on substitution of the solvation sphere around the cation by entering C104-or HS04-ions.Quenching of the excited state by water molecules is consequently reduced leading to fluorescence enhancement but no fluorescence shift. The photocyclization of 1,3-diphenyl-5-(2’-halogeno-pheny1)pyrazoles has been shown61 to proceed with high quantum yield through the So”state and to involve homolysis of a carbon-halogen bond affording (15) as an intermediate. Replacement of the halogen by groups having a higher bond strength leads to a change in mechanism. This could be an electrocyclic process proceeding via (16) and if so would be consistent with the observed polarity dependence. The first report of a photoinduced [4 + 21-cycloreversion of a hexasubstituted 2,3-diazabicyclo[3.1 .O]hex-2-ene has appeared6* in which large groups are present at C-4 and C-6.Interest in this process stems from the fact that the [~f +v:] has hitherto been seen only for carbon systems. The photochemical rearrangement of oxaziridines to amides has been investigated theoretically using an ab initio 6o S. C. Chao J. Tretzel and F. W. Schneider J. Amer. Chem. SOC.,1979,101 134. 61 J. Grimshaw and A. P. de Silva J.C.S. Chem. Comm. 1979 193. 62 G. Ege. K. Gilbert and B. Hahn Tetrahedron Letters 1979 1571. Photochemistry 125 method.63 The lowest energy transition results from an n.,. +cr& excitation promoting rupture of the N-0 bond. Migration of hydrogen occurs in a separate step and probably does so on the ground-state energy surface. Irradiation of 6-methylflavone in aqueous methanol containing sodium sulphite two dimers.The mechanism of this process is unclear but it is suggested that following excitation of the flavone an electron is transferred from SO:-to give a radical anion; after abstraction of a proton from the solvent this dimerizes. The photoaddition reactions of the electron-deficient alkene thiochromone 1,l-dioxide with benzene and some simple derivatives have been inve~tigated~~ and an attempt has been made to establish a link between the ionization potential of the aromatic and the course of the phototransformation. It has been shown that the presence of electron-donating groups suppresses the formation of 2 1 adducts the degree of charge transfer involved possibly promoting dissociation to the ground state.If the ionization potential of the aromatic lies closer to that of thiochromone 1,l-dioxide the extent of formation of 2 :1adduct correlates well with the difference between the ionization potentials of the arene and the dioxide. Irradiation of 2-methylthianaphthalene 1,l-dioxide affords66 anti-head-to-head (HH) and anti-head-to-tail (HT) dimers. It has been suggested that the (HH) dimer results from a monomeric excited triplet and that an excimer is the precursor of the (HT) dimer. An external heavy-atom effect was detected and this has been interpreted in terms of a spin-orbital perturbation on the intersystem crossing in the 1,4-biradical. 63 E. Oliveros M. Riviere J. P. Malrieu and C. Teichteil J. Amer. Chern. SOC., 1979,101 318. I. Yokoe M. Taguchi Y.Shirataki and M. Komatsu J.C.S. Chem. Comm. 1979,333. 65 I. W. J. Still and T. S. Leong Tetrahedron Letters 1979 1097. 66 M. J. Hopkinson W. W. Schloman B. F. Plummer E. Wenkert and M. Raju J. Amer. Chem. SOC., 1979 101,2157.