7 Photochemistry By A. COX Department of Chemistry and Molecular Sciences University of Warwick Coventry CV4 7AL 1 Introduction Reviews have appeared of a number of important fields including the excited-state chemistry of cyclopropene derivatives,' photochemical rearrangements of benzene derivatives and annelated arenes,2 the photochemistry of protonated unsaturated carbonyl c~mpounds,~ the photo- the photochemical decomposition of a~oalkanes,~ chemistry of organic bichromophoric molecule^,^ and the control of unimolecular photorearrangements that is exerted by the crystal lattice.6 2 Alkenes A hydrocarbon analogue of the Type I1 photoeliminations of ketones has been reported.' Direct irradiation of 1,4-diphenylpent-4-en-1-01leads to 2-methyl-2,S- diphenyltetrahydrofuran probably via the radical anion of the alkene.However on sensitization using benzophenone the products are a-methylstyrene aceto-phenone and 1,4-diphenylpentan-1 -one. Following excitation these arise by abstraction of hydrogen by the methylene carbon of the excited alkene which then collapses to products. Similar results have been obtained for a number of other substituted phenylpentenes. Photoaddition of methanol to 1-phenylcyclohexene to give a Markownikov-type ether has been shown' to involve an intermediate that is common to both the direct and the sensitized pathways. Low-temperature studies have enabled this intermediate to be identified as trans-1 -phenylcyclohexene. Exciplex isomerizations of dewarbenzene have been discussed,' together with the features which control exciplex formation and the efficiency of diabatic and adiabatic isomerizations.Evidence is cited which suggests that isomerization of the exciplex of hexamethyldewarbenzene occurs adiabatically and this seems to be favoured when a reaction leading to an excited-state product is least endothermic. Irradiation of certain quadricyclenes in the presence of electron acceptors such as 9,lO-dicyanoanthracene brings about valence isomerization to the corresponding ' A. Padwa Acc. Chem. Res. 1979 12 310. G. Kaupp Angew. Chem. Int. Ed. Engl. 1980,19,243. R. F.Childs Rev. Chem. Zntermed. 1980 3,285. P. S.Engel Chem. Rev. 1980,80,99. H. Morrison Acc. Chem. Res. 1979 12 383. J. R. Scheffer Acc. Chem. Res.1980,13,283. J. M.Hornback and G. S. Proehl J. Am. Chem. SOC. 1979,101,7367. W.G. Dauben H. C. H. A. van Riel J. D. Robbins and G. J. Wagner J. Am. Chem. Soc. 1979,101 6383. G.Jones and S.-H. Chiang J. Am. Chem. SOC.,1979,101,7421. 96 A. Cox norbornadiene." Various other strained hydrocarbons are however found to be rather less effective. Sensitized valence isomerization also occurs in the presence of fumaronitriles but geometrical isomerization is not apparent. The photochemistry of bicyclo[4.l.O]hept-3-ene has been investigated," using light of wavelength 185 nm (6.7 eV photons) and it was found that five isomeric compounds account for over 92% of the product. Two important conclusions have been drawn from the study. It has been shown that although the photon is absorbed by the .n-bond reaction largely occurs at the cyclopropyl group.Secondly there are two examples of secondary products being formed by carry-over of excitation energy to the initial product. One is hepta-1,3,6-triene whose precursor is the cis-isomer and the other is bicyclo[4.1 .O]hept-2-ene which comes from an electronically 'hot' cyclohepta-1,4-diene. A study has been madeI2 of cis-fused bicyclo[4.n.0]-2,4- dienes (n =3 or 4) and bicyclo[5.4.0]undeca-8,lO-diene.Depending on wavelength and temperature bicyclo[4.3.0]nona-2,4-dienegives rise either to cyclic trienes or to cyclobutenes. The initial products of photolysis of bicyclo[4.4.0]deca-2,4-diene could not be detected but bicyclo[5.4.0]undeca-8,lO-diene also gives a triene and a cyclobutene as primary photoproducts.These results suggest that the conformation of the ground state exercises a degree of control over the outcome of these photoreactions. A new practical synthesis of semibullvalene has been rep~rted.'~ Irradiation of cyclo-octatetraene in the vapour phase leads to the desired product in essentially quantitative yield. For its success however the reaction requires a light source that has the majority of its emission at 300 nm as well as an experimental arrangement that is designed such that liquid cyclo-octatetraene is not irradiated. The dynamics of cis-trans photoisomerization of alkenes are usually explained in terms of a twisted 'common intermediate' whose vibrational levels are in thermal equilibrium.This description fails however if the lifetime of the excited state is too short for vibrational equilibrium to be attained. The semi-classical trajectory approach has now been found to be more satisfactory for these cases and it has been appliedI4 to the study of the cis-trans isomerization of alkenes and polyenes. The mechanism of the redox-photosensitized cycloreversion of the trans,syn-dimer of indene (1)has been the subject of a kinetic study.15 The results show that following photochemical transfer of an electron to dicyanobenzene or to the radical cation of phenanthrene or another selected aromatic hydrocarbon a ?r-complex is formed with (l),and this complex plays a key role in the cycloreversion (see Scheme I). This sequence may provide a model for enzymatic photo-re-activation of damaged DNA.3 Aromatic Hydrocarbons A report has appearedI6 in which it is suggested that in the photocyclization of cata-condensed polynuclear aromatic hydrocarbons with cyclohexa-193-diene the lo G. Jones S.-H. Chiang W. G. Becker and D. P.Greenberg J. Chem. SOC.,Chem. Commun. 1980,681. R. Srinivasan and J. A. Ors J. Am. Chem. SOC.,1979,101 3411. '* W. G. Dauben and M. S. Kellogg J. Am. Chem. SOC.,1980,102.4456. l3 N.J. Turro J.-M. Liu H. E. Zimmerman and R. E. Factor J. Org. Chem. 1980 45,3511. l4 R.M. Weiss and A. Warshel J. Am. Chem. SOC.,1979,101,6131. '' T. Majima C. Pac and H. Sakurai J. Am. Chem. SOC.,1980,102 5265. l6 N.C. Yang J. Masnovi and W. Chiang J. Am. Chem. SOC.,1979,101,6465.Photochemistry 'S* (1) DCNB =dicyanobenzene Scheme 1 favourable pathway is determined by the local symmetry of the frontier orbitals about the reactive positions. Thus in the presence of cyclohexadiene irradiation of dibenz[a,c]anthracene which is a compound that is known to react readily with maleic anhydride gives the [T +T:] adduct (2) as the major product. A new photochemical method for preparation of a [3.l]metacyclophane and of a [3.2]metaparacyclophane has been described." Irradiation of the benzyl alcohol (3 ; n =3) gives (4) together with other products and irradiation of (3; n =2) gives N-methyl-l-aza[3.2]metaparacyclophane(5). The reaction probably occurs by an internal electron-transfer mechanism to give an ion pair which can collapse by two different routes leading to (4) and (5).hv @-\\ \ CH20H Q (3; n=3) (CH2)" (3) n =2 or 3 (4) " C.-I. Lin. P. Singh M. Maddox and E. F. Ullman. J. Am. Chern. Soc. 1980 102 3261. 98 A. Cox In view of the known intermolecular photosubstitution reactions of amines on the 3-and 4-nitroanisoles a study has been made1* of the photoreactions of the ortho- meta- and para-isomers of P-(nitrophen0xy)ethylamine.The results show that although the ortho-and para-isomers are photosensitive no product appears to arise as a result of a Smiles rearrangement. However it is observed that the meta-isomer photorearranges to P-(3-nitroanilino)ethanol with a quantum yield of 0.23 at less than 10% conversion. Compounds that are known as [1]-and [2]-rods consisting of bicyclo[2.2.2]octane moieties and having suitable donor and acceptor groups at terminal bridgehead sites have been prepared." As these molecules have little or no molecular flexibility they have been used to provide a test of mechanisms of energy transfer.It is shown that energy transfer over short distances occurs partly by dipole-dipole coupling between the chromophoric groups and also by trans- mission of excitation energy through the molecule. The first piece of direct evidence2' is now available for the existence of a biradical intermediate in the di-wmethane rearrangement of the bicyclo[3.2.2]nonanaph- thalene (6) using low-temperature spectroscopy. Irradiation of (6) in an MTHF matrix at 77 K with light of wavelength 313 and >340 nm leads to a species that has an e.s.r.spectrum consistent with its being the triplet state of (6). A second species also identified by e.s.r. spectroscopy is probably the extensively delocalized biradical (7). D ? A report has appeared2* on the ability of cadmium sulphide and titanium dioxide to function as photocatalysts for the oxygenation of aromatic olefins. Typically a suspension of the semiconductor powder is irradiated in an olefin solution and under an atmosphere of oxygen. In the particular case of 1,l-diphenylethylene the prod- ucts are benzophenone 2,2-diphenyloxiran and 2-methoxy-2,2-diphenylethanol this last product being derived by methanolysis of the oxiran. The oxygenation appears to involve free-radical species rather than singlet oxygen and the reaction pathway shown in Scheme 2 has been suggested.CdS+hv + CdS" CdS*+02 + CdS++02' CdS++ PhZC=CHz -* CdS + [Ph2C=CH$ [Ph2C=CH2]++ 0z7 + PhZC-CH202. Scheme 2 l8 G. G. Wubbels A. M. Halverson and J. D. Oxman J. Am. Chem. SOC.,1980,102,4848. l9 H. E. Zimmerman T. D. Goldman T. K. Hirzel and S. P. Schmidt,J. Org. Chem. 1980,45 3933. 2o M. Demuth D. Lemmer and K. Schaffner J. Am. Chem. SOC.,1980,102,5407. 21 T. Kanno T. Oguchi H. Sakuragi and K. Tokumaru TetrahedronLett. 1980,21,467. Photochemistry 4 Carbonyl Compounds In an attempt to characterize some of the species involved in the photo-enolization of ortho-alkyl-substituted carbonyl compounds electron-transfer properties of the intermediate biradicals have been examined.22 Paraquat (Pa) has been used as the electron trap and the formation of PQ? has been monitored in time-resolved experiments.In this way o-methylbenzaldehyde has been shown to have two triplet states that are responsible for biradical production and these have lifetimes of 1.1 and 9.5 ns accounting for 58% and 42% of the reaction respectively. The biradical itself has a lifetime of 1500ns and undergoes electron transfer to paraquat dications with a rate constant of 6.2 x lo91mol-' s-l. The photochemistry of 1,5-diaryl-1,5-diketones of the form (8)-(10) has been in~estigated,~~ stimulated by an interest in energy-migration processes. In (8) and (9) the triplet energy is found to migrate between the two chromophores with a frequency that is in excess of lo9s-' so that there is complete excitation equilibra- tion.However equilibrium is not reached between the two triplet states of (lo) (8) R1=R2=H (9) R' = H R2 = Me (10) R' =Me R2 = H which in this case correspond to its syn and anti conformers. Energy migration of the type that has been described causes a decrease in yield of both modes of Type I1 photocleavage. The primary intermediates generated in the photoreduction of benzophenone by EtOH have been to be EtO' and the p-benzosemiquinone anion by using phenyl N-(t-buty1)nitrone as a spin trap. This observation provides support for the 'anionic' mechanism for the conversion of quinone into hydroquin- one that is shown in Scheme 3.3BQ* + EtOH + BQ-+ Et6H 2BQ-B BQ+BQ2-BQ2-+2H+ + BQH2 Scheme 3 Flash photolysis studies have revealed25 that the primary reaction of the benzo- phenone triplet with a variety of aliphatic amines in benzene is conversion into the benzophenone ketyl radical (a= 0.9-1.0). The low overall yields for reduction of ketones may not be assigned to partial quenching within the primary excited reaction complex and they must result from disproportionations which regenerate the starting materials. 22 P. K. Das M. V. Encinas R. D. Small and J. C. Scaiano J. Am. Chem. SOC.,1979,101,6965. 23 J. P. Bays M. V. Encinas R. D. Small and J. C. Scaiano J. Am. Chem. SOC.,1980,102,727. 24 S.Noda T. Doba T. Mizuta M. Miura and H. Yoshida J. Chem. SOC.,Perkin Trans.2,1980,61. *' S.Inbar H. Linschitz and S. G. Cohen J. Am. Chem. Soc. 1980,102 1419. 100 A. Cox Benzocyclobutanols have been preparedz6 by irradiation of benzocycloalkenones in t-butyl alcohol (Scheme 4). If R = MezCH a second tricyclic benzocyclobutenol is formed and the results suggest that this is a good method of synthesizing both of these structural types. A biradical intermediate seems to be involved which can collapse along the two competing pathways of cyclization to benzocyclobutenol and formation of a dienol. hv -& H n=5-7 Scheme 4 Although the photochemistry of ketones has been extensively investigated the same is not true of esters and this has stimulated a study of the photochemical behaviour of (R,S:S,R)-and (R,R:S,S)-1,2-dimethylbutyl trifluoroa~etate.~~ Broadly the photochemistry of alkyl trifluoroacetates parallels that of similar ketones.Differences are however found to exist. Ester singlets are much more reactive than ketone singlets and this leads to a greater proportion of reactions through S1 than through TI states. The ester biradical is probably longer-lived than T*). the ketone counterpart and the ester triplet of lowest energy is the '(7 Moreover simple esters do not undergo a Type I1 reaction exclusively through the triplet state. The photochemical kinetics of salicylideneaniline have been examined,z8 in both protic and aprotic solvents in order to determine the time-scale for transfer of a proton within the excited state and to identify the intermediate species before the photochromic species.At room temperature the quinoid fluorescence produced by excitation of the enol is found to have a rise time of 4ps and as such is in close agreement with the rate of tautomeric proton transfer. The fluorescence at low temperature shows a short-lived component (which is probably a vibrationally excited fluorescence) and a component of much longer lifetime which has the short-lived component as precursor. A new procedure has been announcedz9 for the one-step synthesis of y-keto- carboxylic acids and esters by irradiation of an aldehyde and an ap-unsaturated ester in the presence of benzophenone as sensitizer. The transformation probably occurs by abstraction of the aldehydic hydrogen by the triplet state of benzophenone to give an acyl radical which then adds to the carbon-carbon double-bond of the enone.Yields are good for those substrates having a P-substituent that is not in conjugation with the enone moiety and the reaction should find application in the synthesis of butenolides. It has been established,'' by studies of lifetimes and of quenching using laser flash photolysis that transient species having lifetimes of the order of tens of nanoseconds 26 M.-C. CarrC M.-L. Viriot-Villaume and P. Caubkre J. Chem. SOC.,Perkin Trans. 1,1979,2542. '' J. E.Gano and D. H.-T. Chien J. Am. Chem. SOC.,1980,102,3182. 28 P.F.Barbara P. M. Rentzepis and L. E. Brus J. Am. Chem. Soc. 1980,102,2786. 29 H. Cerfontain and P. C. M. van Noort Synthesis 1980,490.'O R.Bonneau J. Am. Chem. Soc. 1980,102,3816. Photochemistry 101 and produced on excitation of enones such as methyl vinyl ketone acetyl- cyclohexene cycloheptenone cyclohexenone cyclopentenone and testosterone are orthogonal triplet states. The angle of twist is found to vary with the rigidity of the molecule and the results indicate that the relaxed triplet is not the reactive intermediate that leads to the cyclodimerization of cyclohexenone or cyclopen- tenone. Substituent effects on the photochemistry of some hexa-l,5-dien-3-ones have been examined31 and have been shown to parallel the substituent effects observed in the cyclization of hex-5-enyl radicals. A model is used which treats the &carbon of the enone system as a radical centre and this has enabled variations in the regiospecificity of [2 +21 cycloadditions of those dienones to be rationalized.The key step in a new method3* providing access to a variety of substituted cyclohexenones is a four-carbon annelation sequence in which alkenes are photo- cyclo-added to 2,2,6-trimethyl- 1,3-dioxolenone. Following mild reduction (using di-isobutylaluminium hydride) and aldol cyclization cyclohexenones are obtained in high yield. A broad spectrum of regioselectivities is obtained from unsymmetrical alkenes and although a rationalization for this behaviour is not clear at the present time the outcome does seem to be especially sensitive to the nature of the substituent at the &position. Irradiation of (11)is to bring about cycloaddition to form the cage structure (12) which is readily converted into the acetoxy-cyclobutane (13).In essence this transformation is the [2 +21 cycloaddition of the enol acetate of formyl acetic ester to the double-bond of 4-hydroxycyclohexene. Furthermore since (13) will undergo retro-aldolization this sequence constitutes a method of placement of vicinal carboxaldehyde and acetic ester groupings on to a double-bond and may have implications for the synthesis of prostaglandins. An example has been of crystal-lattice restraints controlling the outcome of unimolecular photorearrangements. Thus direct and benzophenone- sensitized irradiation (A >330 nm) of the hydroxycyclohexenone (14) gives high yields of the [2 +21 intramolecular cycloaddition product (15).In the solid state however the outcome is the tricyclic ketone (16). It has been established that solid-phase reactions are controlled by the crystal lattice and are least-motion 31 W. C. Agosta and S. Wolff J. Org. Chem. 1980,45,3139. 32 S. W. Baldwin and J. M. Wilkinson J. Am. Chem. SOC.,1980 102. 3634. 33 B. A. Pearlman J. Am. Chem. SOC.,1979,101,6398. 34 W. K.Appel T. J. Greenhough J. R. Scheffer J. Trotter and L. Walsh J. Am. Chem. SOC.,1980,102 1158. 35 W. K. Appel T. J. Greenhough J. R. Scheffer J. Trotter and L. Walsh J. Am. Chem. SOC.,1980,102. 1160. 102 A. Cox Me processes. Reaction is thus limited to one stable conformational isomer of a given substrate in contrast to the reaction in the liquid phase where a minor high-energy conformation is reacting.The behaviour of duroquinone triplets has been to vary widely in the range pH 12 to Ho= -2. This is reflected in their reactivity towards inorganic ions such as halide and hydroxide and suggests the formation of a charge-transfer complex followed predominantly by a back-reaction rather than complete transfer of an electron. The reactivity towards some amines was also examined. Evidence has been made available37 to suggest that the intermediate in the photocycloaddition of 3(n T*)p-benzoquinone to olefins to give oxetans is not always the pre-oxetan 1,4-biradical as previously thought (see Scheme 5). The experimental results are best interpreted in terms of a quinone-olefin charge- transfer complex and suggest the possibility of new ionic photochemical reactions.Insupport of this irradiation of p-benzoquinone in the presence of pent-4-en01 leads to the cyclic ether (17). In addition to the well-known [2+6] and [2+4] carbocyclic adducts that are formed on irradiation of p-benzoquinone with either cycloheptatriene or cyclo- octatetraene (COT) duroquinone has now been reported3* to yield new cage oxetans 0 1,4-biradicals CT complexes peroxides sulphones acetates HO (17) Scheme 5 36 J. C. Scaiano and P. Neta J. Am. Chem. SOC.,1980,102 1608. 37 R.M. Wilson and A. K. Musser J. Am. Chem. SOC.,1980,102,1720. 38 K.Ogino T. Minami and S. Kozuka J. Chem. SOC.,Chem. Commun. 1980,480. Photochemistry 103 compounds on photoreaction with cyclic polyenes.Good yields of [2 +21 photo- adducts are obtained from cycloheptatriene but conversion is low with COT. Investigations indicate that in the case of COT the reaction that occurs is with the thermal dimer of the cycloalkene. It has been argued that planar enone triplets relax by twisting around the C=C bond following which rapid crossing to the potential surface of the ground state is possible. From this it follows that enones which by reasons of structural rigidity are constrained from twisting around the C=C bond should be inhibited from displaying typical enone molecular rearrangements. Thus irradiation of (1 8) in degassed Bu'OH with light of wavelength 300nm does not lead to products of a lumiketone rearrangement but rather gives3' a mixture of (19) and (20).0 hu + BU'OH or Pr'OH Earlier work has shown that an enhanced cage effect of radical pairs in a micelle and a magnetic isotope effect ("C) are responsible for the 13C enrichment that is discovered in recovered dibenzyl ketone after partial photolysis in micellar solution. A number of striking predictions have been made about the quantum yields €or the cage reaction and for product formation for the photolysis of dibenzyl ketone in HDTCl micellar solutions and a correlation has been established4' between 13C- enrichment parameters and measurements of quantum yields. Studies of structural and viscosity effects on 13C isotope enrichment have also been made.4' An interest- ing paper has appea~ed,~' describing an investigation of the effect of microemulsions on aspects of photochemical reactions and comparing the effects with those apparent in micelles.The effect of microemulsions on the photophysical properties of probe molecules was also examined. Nucleophilic substitution reactions between 2-methoxy- 1,4-naphthoquinone and methylamine have been induced thermally and photochemically and have been found to follow different pathways with high regio~pecificity.~~ In the thermal process it is the 2-methoxy-group which is replaced by the amine whereas the photoreaction leads to replacement of the 3-hydrogen atom. The transformation probably occurs as shown in Scheme 6. As 1,4-naphthoquinone shows a well- resolved n + v* band and is unreiictive towards methylamine and as the reaction is inhibited by benzophenone it has been concluded that the reaction occurs from the first (v,T*)singlet state of (21).The photochemical nucleophilic substitution reactions of 2,6-dimethoxy-l,4-benzoquinone have also been examined. The photochemistry of a number of selenoketones is reported44 to be similar to that of thioketones in that they abstract hydrogen from an upper excited state. The lifetime of the abstracting state of di-t-butyl selenoketone is lo-" s and irradiation into the Sz band gives a diselenide. 39 D. I. Schuster and S. Hussain J. Am. Chem. SOC.,1980,102,409. 40 N. J. Turro B. Kraeutler and D. R. Anderson J. Am. Chem. SOC., 1979,101,7435. 41 N.J. Turro D. R. Anderson and B. Kraeutler Tetrahedron Lett.1980 21 3. 42 M.Almgren F. Grieser and J. K. Thomas J. Am. Chem. SOC.,1980,102 3188. 43 S.M.Drew J. Griffiths and A. J. King J. Chem. SOC.,Chem. Commun. 1979 1037. 44 N. Y. M. Fung P. de Mayo B. Ruge A. C. Weedon and S. K. Wong Can. J. Chem. 1980,58,6. 104 A. Cox @Me +{+;Me -{$OMe .$'". KNMe NHMe NHMe NHMe 0 0 OH 0 ?7T 7T*> (21) Scheme 6 5 Singlet Oxygen The results of ab initio calculations on the mechanism of the ene reaction of singlet oxygen with olefins have been rep~rted.~' These suggest that the reaction involves a biradical intermediate of the peroxy type whose zwitterionic character can be greatly enhanced by both solvent and substituent. Solvent variation has a predictable effect on the outcome of the reaction as shown in Scheme 7 and it is also claimed that the known directing effect of Me0 substituents should also be exhibited by F or C1 substitution.A new view of the mechanism of the ene reaction of singlet oxygen has been and it incorporates the existence of a complex whose structure is dominated by a HOMO-olefin/LUMO-oxygen interaction. Such interactions might be especially important in a system such as but-2-ene in which there are contributions from the olefin .rr-orbitals and the CH pseudo-+orbitals to give an orbital that is similar to 9b3 in butadiene. This proposal satisfactorily explains a number of recent experimental observations. lo2 Non-polar Polar -00 M~OR~ ~ ~ M~OHM~ solvent solvent M~O&M~ Me H Me H Me H Scheme 7 Results have been to show that the singlet oxygen-ene reaction is a highly stereospecific suprafacial process.Oxidation of (22) followed by its reduc- tion may if the process is concerted give two products namely (23)-(R H) and (23)-(S D). These arise from approach of the oxygen from above to give (following removal of D) (23)-(R H) and following removal of H the (S)akohol'is obtained. Crossover products (R D) and (S H) are possible only to the extent that the other H (22) bottom attack Me* %..Ph HO D (23)-(S,D) " L. B. Harding and W. A. Goddard J. Am. Chem. SOC.,1980 102,439. 46 L. M. Stephenson Tetrahedron Lett. 1980,21 1005. '' M. Orfanopoulos and L. M. Stephenson J. Am. Chem. SOC.,1980,102,1417 Photochemistry 105 isomer of the olefin or the opposite enantiomer is present in the starting material.Precisely the expected quantity of crossover products is obtained and no isotope discrimination is apparent. The dye-sensitized photo-oxygenation of imidazole has attracted much interest stimulated by the knowledge that photo-oxidative damage to histidine residues causes loss of activity in many enzymes. 2,5 -Endoperoxides have now been reported48 as the initial oxidation products of the dye-photosensit- ized oxygenation of imidazoles and of histidines themselves. These intermediates may suffer a number of different fates depending upon the precise nature of the substituents. Loss of oxygen from the initial adduct and regeneration of the starting material appear to feature importantly.1,2-Benzodithiole-3-thione has been used to study4’ the singlet oxygenation of thiones. Irradiation of a solution of the substrate containing 1YO of cross-linked polystyrene-anchored Rose Bengal leads to 4.5% of a sulphine together with about 3% of 1,2-benzodithio1-3-one. Although this mechanism may not be general it is the first evidence supporting a sulphine mechanism for the singlet oxygenation of unhindered thiones. 6 Heterocycles Interest is still clearly evident in permutation-pattern analysis. A study of the phototranspositions of the first excited singlet states of cyano-thiophens suggestsso that 2,5-bonding occurs followed by a ‘walk’ of a sulphur atom; the rate of the walk is similar to that of re-aromatization. These conclusions are supported by the isolation of furan-thiabicyclopentene adducts and suggest that there are similarities to the photochemical behaviour of cyano-pyrroles.Evidence has been presented51 for the intermediacy of oxa-azabicyclo[2.2.0]hexenones in the photoisomerization of a six-membered heterocycle. Thus irradiation of oxazinone (24) establishes an equilibrium with the isomeric oxazinone (25). It is suggested that (26; R1 =Me R2=Ph) is thermodynamically more stable than (26; R’=Ph R2 =Me) and that their mutual interconversion takes place through the zwitterion (27). n2 3 Yk R1 Me ~ (24) (25) (26) (27) The photolysis and thermolysis of the tricyclic azoalkane (28) has been studied5* with a view to generating (29) and hence providing a direct entry into the biradical manifold of the di-vmethane rearrangement of benzonorbornadiene.Direct photo- lysis gives the biradical(29) which collapses to 2,3-benzotricyclo[2.2. 1.05*’]hept-2- ene rather than rearranging to (31).This suggests that (29) probably lies at an energy minimum on the energy surface of the singlet excited state of the alkane. Sensitized photolysis leads to (30) which is a transformation analogous to the photochemical conversion of norbornene into norcar-2-ene. 48 Ryang and C. S. Foote J. Am. Chem. SOC.,1979,101,6683. H.-S. 49 S. Tamagaki and K. Hotta J. Chem. SOC.,Chem. Commun. 1980,598. 50 J. A. Barltrop A. C. Day and E. Irving J. Chem. SOC.,Chem. Commun. 1979 881. ” P. de Mayo A. C. Weedon and R. W. Zabel J. Chem. SOC.,Chem.Commun. 1980,881. 52 W. Adam and 0.De Lucchi J. Am. Chem. SOC.,1980,102.2109. 106 A. Cox F A general oxindole synthesis has been de~cribed.’~ Treatmentof N-alkyl-N-acyl-o-chloroanilines or N-acyl-o-chloroanilines with an excess of lithium di-isopropyl- amide in THF-hexane gives the corresponding anion which on irradiation cyclizes to the oxindole. Preliminary investigations have shown that the reaction may proceed by ‘an intramolecular SRNlmechanism. ’’ J. F. Wolfe M. C. Sleevi and R. R. Goehring J. Am. Chem. Soc. 1980,102,3646.