7 Photochemistry ByA. COX Department of Chemistry and Molecular Sciences University of Wawick Coventry CV4 7AL 1 Introduction Reviews have appeared on a number of topics including photosensitization in organic synthesis,’ phototransformations of heterohexa-l,3,5-trienes,*and of mag- netic field and magnetic isotope effects in organic photochemical reaction^.^ A three-volume work on rearrangements in ground and excited states has also been p~blished.~ 2 Alkenes A study has been reported’ of the photobehaviour of some 3-(o-alkylphenyl)- substituted cyclopropenes that contain a benzylic hydrogen in the y-position of the side chain. The results of this investigation which is aimed at providing more information about the reactivity of excited olefins towards hydrogen abstraction show that triplet states of tetra-substituted cyclopropenes possessing y-hydrogens readily undergo intramolecular hydrogen transfer.For 1,2-diphenyl-substituted cyclopropenes the primary isotope effect is significantly higher than previously observed for H-transfer to an excited state. Irradiation of alkyl-substituted hexa- 1,5 -dienes induces6 allylcyclopropane formation and rearrangement to an isomeric hexa-1,5-diene. These products arise via competing [1,2]-and [1,3]-sigmatropic allyl shifts. Other products which are characteristic of the photochemical behaviour of isolated double bonds are also formed. It is suggested that on direct irradiation the sigmatropic allyl migrations occur from an excited state which arises when the diene has a conformation in which an orbital interaction exists between the two double bonds and the central C-3 -C-4 bond.Some substituted cycloheptatrienes are known to undergo regioselective [1,7]-sigmatropic shifts as well as electrocycliz- ations from singlet states and a model has been suggested’ that accounts for the reactivity and selectivity trends as well as for some wavelength-dependent photo- reactions. Regioselectivities are explained as a function of substituents with electron ’ A. Albini Synthesis 1981 249. M. V. George A. Mitra and K. B. Sukumaran Angew. Chem. Int. Ed. Engl. 1980,19,973. N. J. Turro and B. Kraeutler Acc. Chem. Res. 1980 13 369. Organic Chemistry Vol. 42-3 ‘Rearrangements in Ground and Excited States’ ed. P. de Mayo Academic Press New York 1980.’ A. Padwa C. S. Chou R. J. Rosenthal and B. Rubin J. Am. Chem. Soc. 1981,103 3057. T. D. R. Manning and P. J. Kropp J. Am. Chem. Soc. 1981,103,889. ’T. Tezuka 0.Kikuchi K. N. Houk M. N. Paddon-Row C. M. Santiago N. G. Rondan J. C. Williams and R. W. Gandour J. Am. Chem. SOC.,1981,103 1367. 133 134 A. Cox donors favouring electrocyclizations and electron acceptors promoting sigmatropic rearrangements. The model involves an excited singlet state in which there is 90" rotation about one terminal double bond accompanied by 'sudden polarization' to form a zwitterionic species. Using an ene reaction an intermediate has been trapped' in the photosensitized electron-transfer dimerization of 1,l-diphenylethylene and in the cross-cyclo- addition of 1,1-diphenylethylene with methylpropene (Scheme 1).The products of Ph H R sens.Ph H R H (a) R = Ph Scheme 1 the ene reaction are not readily available by alternative routes and consequently the transformation may be useful in synthesis. Photochemical reduction of carbon- carbon and carbon-nitrogen double bonds has been achieved using benzene- selen01.~ For example irradiation of @-aryl-cup-unsaturated carbonyl compounds and of some imino-derivatives of benzaldehyde in the presence of benzeneselenol leads to the formation of the corresponding saturated compounds. These reactions seem to occur either by an SH2displacement of the benzylic radicals from the selenium by PhSe or by direct photolysis of the benzyl-Se bond.3 Aromatics Photoionization techniques have been used to produce a number of novel species trapped in an argon matrix including cycloheptatriene benzyl bromide and toluene cations. The behaviour of the cycloheptatriene cation under photolysis has been examined" and excitation at 470 nm was found to be most effective in promoting rearrangement to the toluene cation. Photolysis of the benzyl cation brings about rearrangement to the tropylium cation and photochemical interconversion of these two species has also been demonstrated." An investigation of the photoreduction of chlorobenzene and of chloroanisoles in methanol under conditions in which photosubstitution also occurs has shown'* that the solvent behaves both as a hydrogen donor and as a nucleophile.The key step is thought to be formation of D. R. Arnold R. M. Borg and A. Albini J. Chem. SOC., Chem. Commun. 1981 138. M. J. Perkins B. V. Smith and E. S. Turner J. Chem. SOC.,Chem. Commun. 1981,977. lo L. Andrews and B. W. Keelan J. Am. Chem. SOC., 1980,102,5732. l1 L. Andrews and B. W. Keelan J. Am. Chem. SOC.,1981,103 99. l2 J. Ph. Soumillion and B. De Wolf J. Chem. SOC., Chem. Commun. 1981,436. Photochemistry 'ArCl* homolytic rupture 1 lNU 1 ArCl Substitution Reduction product product via H-abstraction Scheme 2 a pair of radical ions via a triplet excimer of which the radical cation leads to the substitution product (Scheme 2). A number of other interesting photosubstitutions have been reported.An examin-ation of the photocyanation of anisole in the presence and absence of electron acceptors such as terephthalonitrile has shownI3 that polyethylene glycol (PEG) can supplant a crown ether in the photoinduced replacement of the methoxy-group using KCN in CH2C12. The success of the reaction is ascribed to the ability of PEG to complex with K' as do crown ethers with the consequent activation of the CN- ion in an aprotic solvent. Addition of the electron acceptor improves both the yield of photocyanation products and also the specificity of the substitution and these observations are rationalized in terms of charge-transfer complex formation between substrate and added electron acceptor. Enolate anions derived from simple ketones and esters have been rep~rted'~ to undergo an efficient photo-S,,l reaction.However 2-lithio-1,3-dithianes give low yields as do dialkyl-substituted ketone and ester enolates. This is because of hydrogen atom transfer from a carbon situated adjacent to the enolate anion to the transient phenyl radical. Intramolecular reaction of enolate anions with aryl halides can also be very efficient and if @-hydrogen transfer is blocked the cyclization can proceed with yields of 70-90% even for eight-membered rings. A report has appeared" of the photo-Birch reduction of some arenes with NaBH in the presence of rn-or p-dicyanobenzene. Thus irradiation of phenanthrene and of anthracene leads to the corresponding 9,1O-dihydroarenes and naphthalene and some substituted naphthalenes are exclusively reduced at C-1 and C-4.The mechan- ism involves electron transfer from the excited singlet-state of the arenes to dicyanobenzene followed by nucleophilic attack of borohydride on the arene radical cation. The reaction may be of wide applicability to the reduction of electron donating arenes. l3 N. Suzuki K. Shimazu T. Ito and Y. Izawa J. Chem. SOC.,Chem. Commun. 1980 1253. l4 M.F.Semmelhack and T. Bangar J. Am. Chem. SOC.,1980,102,7765. '' M.Yasuda C. Pac and H. Sakurai J. Org. Chem. 1981,46,788. 136 A. Cox The addition of nonsymmetrical tertiary amines to photoexcited trans-stilbene in its singlet state has been discussed.'6 It is shown that addition of highly branched amines is selective for formation of the least substituted a-amino-radical and that less highly branched amines are relatively non-selective in this process.The product selectivity is determined by the orientation during the deprotonation of the aminium radical by the stilbene radical anion and the oxidation selectivity of amines is a consequence of a stereoelectronic effect which depends on branching in at least one alkyl group. The bis(crown ether) (1)has been synthesized" and on irradiation undergoes trans-cis isomerization as expected; reversion to the original geometry then occurs thermally. A stable 1 1 sandwich-type complex is formed between cis-(1) and large alkali-metal cations and this enables ion extraction and ion transport through a liquid membrane to be controlled by light. (1) The photochemistry of the tetraphenylcyclopentadienylanion and of the fluorenyl anion has been examined.18 Although there is no reaction in homogeneous aprotic media if the photolysis is carried out at the surface of a potentiostated (0.0 eV us.Ag) n-Ti02 electrode both alkylation and dimerization occur. These new routes for carbon-carbon bond formation may be of value in synthesis and a possible mechanism for the photoinduced oxidative dimerization is shown in Scheme 3. The photoconversion of aryl vinyl ethers to dihydrofurans has been shownIg to occur via the triplet states of educts and zwitterionic ground-state intermediates followed by mono- or bimolecular 1,4-hydrogen shifts. This photocyclization is Photoanodic reaction Dark cathodic reaction Scheme 3 l6 F.D. Lewis T.-Ing. Ho and J. T. Simpson J. Org. Chem. 1981 46 1077. *' S.Shinkai T. Nakaji T. Ogawa K. Shigematsu and 0.Manabe J. Am. Chem. Soc. 1981 103 111. M. A. Foxe and R. C. Owen J. Am. Chem. Soc. 1980,102,6559. l9 T.Wolff J. Org. Chem. 1981,46,978. Photochemistry similar to that of aryl vinyl sulphides and of aromatic enamines and favourable conditions are suggested for the reaction on the basis of the results obtained. A new pathway to multibridged cyclophanes has been published” which avoids the laborious conventional routes and which depends upon an efficient stereochemically controlled photoreaction. Irradiation of (E,E,E)-1,3,5-tristyrylbenzene leads via a loose singlet excimer to a product (2; R = Ph) having c3h symmetry.Because of the involvement of this singlet excimer precursor it is believed that it is this product which is formed rather than the alternative structure having Dfhsymmetry and arising from threefold stereospecific head-to-tail cycloaddition. (2) 4 Carbonyl Compounds Picosecond absorption experiments have been carried out on the photoreduction of benzophenone by triethylamine.” These have enabled direct observation to be made of the charge-transfer complex (3) tl12 10 f 5 ps A,, 610 and of the intermediate amine radical (4) tII2 15 f 5 ps A,, 545 to which it decays. The intrinsic rate of electron transfer kIR,and the rate of proton transfer kH,have 0 \ *-\ +./ A(Ti)+ /CH-N( -% A + /CH-N \ (3) 0 \ OH \ A(&) ../ I\+ ../ + CH-N k-~ / \ /\ (4) Scheme 4 also been measured and the quantum yield for net hydrogen-transfer is shown to be one.In some related workz2 the quenching of excited benzophenone has been studied by a range of aliphatic amines. The results suggest that quenching normally occurs by H abstraction from N and/or a-Cwith radical formation. However this 20 J. Juriew T. Skorochodowa J. Merkuschew W. Winter and H. Meier Angew. Chem. Int. Ed. Engl. 1981,20,269. 21 C. G. Shaefer and K. S. Peters J. Am. Chem. SOC.,1980 102 7566. 22 S. Inbar H. Linschitz and S. G. Cohen I.Am. Chem. SOC., 1981,103,1048. 138 A. Cox is not true for quenching with DABCO in the case of which a triplet amine charge-transfer complex or radical-ion pair is formed. Photoexcited acetophenone (AH,) and photoexcited trifluoroacetophenone (AF,) are both known to attack p-cymene to yield a mixture of primary and tertiary radicals and it has now been reportedz3 that the primary/tertiary ratio varies as a function of ring substitution.Electron-withdrawing substituents increase this ratio and electron-donating substituents decrease it the magnitude of the effect being greater in AF than in AH,. These effects can be related qualitatively to the degree of positive charge on the p-cymene in the exciplex formed between the p-cymene as donor and the triplet ketone and this provides a unique probe for determining the extent of charge transfer in exciplexes generally. An examination of the photoreduction of acetophenone by 1-phenylethanol has revealedz4 that half of the ketone triplets are quenched by an OH bond.This finding has led to a reinvestigation of an early reportz5 that ketones photosensitize the oxidative cleavage of pinacols. Mechanistically the transformation probably involves hydrogen abstraction by the ketone triplet possibly via an exciplex to give an alkoxy-radical which undergoes p-scission very rapidly. This observation leads to the conclusion that in-cage disproportionation of the a-hydroxy-alkoxy radical pair is highly efficient. From a study of the reaction between xanthone triplets and propan-2-01 as a function of solvent composition,z6 the degree of solvent-hydrogen bonding is found to dominate the photochemistry. Clear evidence of this is provided by the values of the bimolecular rate constant for reaction of the triplet state which in propan-2- Ol-cCl4 and neat propan-2-01 are 1.1 x lo8 M-' s-' and 2.2 x lo5 M-' s-' respec-tively.This implies an inversion of the ,(n T*) and '(T T*)states and although the effect is well known its size is without precedent. The triplet lifetimes of hindered 4'-substituted-2,4,6-tri-isopropylbenzophenones have been to be influenced by substituents in a manner which is precisely opposite to that expected from the rate of intermolecular hydrogen abstraction reactions of unhindered benzophenones. A Hammett plot of log T~/T,against u+ gave a reasonably good correlation with p = -0.35 yet for unhindered benzo- phenones a similar plot gave p = +0.6,clearly suggesting a strong effect of the bulky isopropyl group at the ortho-position.This may be ascribable to the effect of hindered rotation about the C-C single bond between the carbonyl group and the tri-isopropylphenyl group on the lifetime of the triplet excited-state. Irradiation of both a-allylbutyrophenone and y-cyclopropylbutyrophenone gives 1,4-biradicals that undergo typical radical rearrangements in competition with conventional Norrish Type I1 reactions. An analysis of product distributions has shown28 that rearrangements of these biradicals occur with rates characteristic of analogous monoradicals. 23 P. J. Wagner and A. E. Puchalski J. Am. Chem. SOC., 1980,102,6177. 24 P. J. Wagner and A. E. Puchalski J. Am. Chem. SOC., 1980,102 7138. *' A. Schonberg and A. Mustafa J.Chem. SOC.,1944,67. 26 J. C. Scaiano J. Am. Chem. SOC., 1980,102,7747. 27 Y. Ito Y. Umehara Y. Yamada and T. Matsuura J. Chem. SOC.,Chem. Commun. 1980,1160. 28 P. J. Wagner K.-C. Liu and Y. Noguchi J. Am. Chem. SOC.,1981,103 3837. Photochemistry 139 The CIDNP technique has been usedz9 to study the reaction between excited benzophenone and phenols and the results show that the triplet state of the ketone abstracts H' from the phenol to give PhO'. However in [*H6]benzene the reactants form a complex leading to a predominantly singlet-state reaction. The same method has also been used to study the 1,4-biradical generated in the Norrish Type I1 reactions of valerophen~ne.~' Two intersystem-crossing mechanisms operate for this species one of which involves hyperfine coupling of the odd electron with the protons and a second which is product-selective and involves spin-orbit coupling.A new method of photoreducing ketones and aldehydes in yields varying between 70 and 80% and using hydrogen selenide in THF has been rep~rted.~' At the wavelengths used H,Se in THF has no absorption band and in the presence of triplet quenchers the photoreduction proceeds much more slowly than in their absence. These observations suggest the involvement of the triplet state of the carbonyl compound. Irradiation of biacetyl in the presence of alkenes such as indene 2,3-dimethylbut-2-ene furan and 1,2-dimethoxyethene promotes photo- addition via biacetyl triplets as well as the Paterno-Buchi rea~tion.~' In the case of biacetyl and 1,2-dimethoxyethene the photoaddition is non-stereospecific and isomerization of the starting alkene is observed to accompany oxetan formation.Biacetyl-alkene exciplexes are the primary photochemical intermediates. Hydrogen abstraction also features importantly in a newly developed33 synthesis of (&)-oestrone which has as its key step the photoinduced cyclization (5) -B (7). Excita-tion of (5) promotes photoenolization to the kinetically unstable o-quinodimethane (6),which undergoes intramolecular [4 + 21-cycloaddition to (7). This can be readily converted into (&)-oestrone (Scheme 5). 0 Scheme 5 29 M. L. M. Schilling J. Am. Chem. SOC.,1981,103 3077. 30 R. Kaptein F. J. J. de Kanter and G. H. Rist J. Chem. SOC.,Chem.Commun. 1981,499. 31 N. Kambe K. Kondo S. Murai and N. Sonoda Angew. Chem. Int. Ed. Engl. 1980,19,1008. 32 G. Jbnes M. Santhanam and S.-H. Chiang J. Am. Chem. SOC.,1980 102,6088. 33 G. Quinkert W.-D. Weber U. Schwartz and G. Durner Angew. Chem. In?. Ed. Engl. 1980,19,1027. 140 A. Cox In ether solution photochemical conversion of the spirodiketone 2,3-benzo- spiro[4.5]deca-2,6-diene-1,8-dione (8) into 3,4-dihydro-9-hydroxy-2( 1H)-anthracenone (10) has been reported34 to proceed uia 3,4-dihydro-2,9( lH 10H)- anthracenedione (9).This is the first example of the isolation of the ketone tautomer of a naphthol. Conversion of (9)into (10) is a thermal process and the photoinduced rearrangement of (8) into (9) may be an example of the oxa-di-7r-methane rearrangement of py-enones.hohu & -b II \ \ \/ (8) (9) H (10) Two interesting papers have appeared on the mechanistic photochemistry of acylsilanes. In propan-2-01 as solvent irradiation of acetyltrimethylsilane (11) at 366 nm leads3’ to the acetal(l2) as sole photoproduct in 80-90% yield. Excitation of the acylsilane to its triplet state induces a rapid migration of silicon from carbon to oxygen to generate a nucleophilic siloxycarbene (13) which is capable of 0 OCHMez II 1 Me -C -SiMe3 Me -CH -OSiMe3 Me-C-OSiMe3 (11) (12) (13) intermolecular reaction with a variety of reagents. In this particular case reaction with the alcohol solvent gives (12). It has also been that in some instances at least photoreaction of the acylsilane (11)with electron deficient olefins such as @)-dimethyl butendioate results from direct reaction of the ester with S1 and TI states of the acylsilane to give a substituted cyclopropane.An ‘umpolung’ of the excited state has been revealed3’ in the photolysis of sterically hindered dialkylketenes. Thus irradiation of (14) gives the typical C-H insertion product of a carbene (15). However on irradiation of (14) in methanol at -60 “C,an equimolar mixture of (15) and (16) is produced (Scheme 6). These (16) Scheme 6 34 M. Kimura and S. Morosawa J. Am.Chem. SOC.,1981,103,2433. ” R. A. Bourque P. D. Davis and J. C. Dalton J. Am. Chem. SOC., 1981,103 698. 36 J. C. Dalton and R. A. Bourque J. Am. Chem. SOC., 1981,103,699. 37 W. Kirmse and Walter Spaleck Angew.Chem. Int. Ed. Engl. 1981 20 776. Photochemistry 141 observations constitute experimental verification of the quantum mechanical sug- gestion that the carbonyl carbon of the ketene carries a positive charge in the ground state and a partial negative charge in the excited state. Aromatic ketones have been used to photosensitize the dissociation of di-t-butyl per~xide.~' No evidence has been found for the intermediacy of an exciplex and energy transfer seems to populate a repulsive state of the peroxide. It is suggested that if the sensitizer does not meet the energy requirements at the equilibrium 0-0 bond distance then energy transfer occurs by vertical excitation at non- equilibrium distances. Photoirradiation has been reported3' to lead to an increase Me I o=c 4-0-0 N=N trans-( 17) cis-( 17) 'shallow cavity' Scheme 7 'deep cavity' in the rate of hydrolysis of P-nitrophenyl acetate catalysed by azobenzene-capped P-cyclodextrin (17).The maximum value of the rate enhancement klight/kdark is 5.5 and the observed catalysis has its origin in a photoinduced cis-trans isomeriz- ation (Scheme 7). Two opposing considerations seem to be relevant. In trans-(17) the substrate is better placed for achieving the maximum rate-constant but in cis-(17) the deeper cavity results in enhanced binding. It appears that the second consideration is the more important. An interesting use of an intramolecular photochemical cycloaddition is in the synthesis of derivatives of tricyclo[4.2.0.0'*4]octane ([4.4.4]fenestrane) a member of the class of structures known as broken-windo-w (Scheme 8).5 Singlet Oxygen The mechanism of the ene reaction between singlet oxygen and olefins has been re~iewed.~' The behaviour of singlet oxygen towards trans-cyclo-octene has been reported42 to be very different from that towards the corresponding cis-isomer. Some of the 38 J. C. Scaiano and G. G. Wubbels J. Am. Chem. SOC.,1981,103,640. 39 A.Ueno K. Takahashi and T. Osa J. Chem. SOC.,Chem. Commun. 1981,94. 40 S. Wolff and W. C. Agosta J. Chem. SOC., Chem. Commun. 1981 118. 41 L. M.Stephenson M. J. Grdina and M. Orfanopoulos Acc. Chem. Res. 1980 13,419. ** Y.Inoue and N. J. Turro Tetrahedron Lett. 1980 21,4327. 142 A.Cox iSeveral steps Scheme 8 trans-alkene reacts stereospecifically in a [2 + 21 cycloaddition to form significant amounts of an unstable trans-fused bicyclic dioxetan. Differing yields of allylic hydroperoxides are obtained from the two isomers and this may be a consequence of oxidation pathways open to the trans-isomer which do not involve singlet oxygen. Dye-sensitized photo-oxygenation of (2,Z)-octadeca-9,12-dienoic 'acid methyl ester (methyl linoleate) gives rise43 to the monhydroperoxides (18) and (19) which are formed by a stereoselective alkenylperoxy radical cyclization. The cyclization of &-unsaturated lipid hydroperoxides to cis-1,2-dioxolanes could be general and such a transformation may be involved in prostaglandin biosynthesis. 0-0 0-0 R U P R R W R OOH I OOH (18) R = (CH2)7C02Me R' = (CH2)3Me R = (CH2)4Me R' = (CH2)&02Me The oxazole-triamide rearrangement has application in a general lactone synthesis for ring sizes from five-membered to macrocycles (Scheme 9).This transformation employs 2-methyl-4,5-diphenyloxazoleas a protected carboxy- function from which the carboxylate is generated under the mild conditions of photo-oxidation. The utility of this sequence of reactions is demonstrated by synthesis of recifeiolide. A re-investigation of the Methylene Blue-sensitized photo-oxygenation of trans- stilbene in MeCN has suggested4' that contrary to earlier reports,"6 the major pathway does not involve singlet oxygen but rather that the reaction proceeds by transfer of an electron from stilbene to the dye in its singlet excited-state.Other reactions sensitized by Methylene Blue e.g. the photo-oxidation of 2-methoxynor- bornene in MeOH could also occur by this mechanism and if so this is especially significant because the results from these experiments have been used as support for a zwitterionic peroxide intermediate in singlet oxygen chemistry. 43 E. D. Mihelich J. Am. Chem. SOC.,1980,102,7141. 44 H. H. Wasserman R. J. Gamble and M. J. Pulwer Tetrahedron Lett. 1981 1737. 45 L.E.Manring J. Eriksen and C. S. Foote J. Am. Chem. Suc. 1980 102,4275. 46 G.Rio and J. Bertholet Bull. SOC.Chim. Fr. 1969 3609. Photochemistry 143 Ph t 13-tridecanolide Scheme 9 6 Heterocycles 1-Azatriptycene is a di-?r-methane system carrying a heteroatom at the methane position and has been investigated photochemically under a variety of condition^.^' In addition to indenoacridine a product already known48 to occur in this process a number of new products have been found and these implicate a singlet nitrene- intermediate which arises from the excited singlet-state of the starting material.Carbenes however do not appear to be involved. The high bridging regioselectivity evident in this transformation may result from the C-N bond being shorter than the bridgehead-to-benzene C-C bond present in many triptycenes. Alternatively the determining consideration could be the greater electronegativity of nitrogen as compared with carbon which may stabilise the aziridine-2,3-dicarbinyldiradical with respect to the cyclopropylcarbinyl radical.(20) (21) (22) (23) Certain azoalkanes for example (20)-(23) all of which contain six-membered rings and which resist denitrogenation (& < 0.05) at 350 nm have been reported49 to undergo denitrogenation to the corresponding hydrocarbon at 185 nm probably as a result of n + T* and/or ?r -* T*excitation. Similar investigations involving the bichromophoric substrate 3,3,5,5-tetramethylpyrazolin-4-one have also been carried out and these suggest that denitrogenation of reluctant azoalkanes at 185 nm may be a general phen~menon.’~ The first report of the photochemistry of a l,l-diazene a group isoelectronic with the carbonyl function has appeared.’l Thus n -* T* excitation of N-(2,2,5,5-47 T.Sugawara and H. Iwamura J. Am. Chem. SOC.,1980,102,7134. 48 G.Wittig and G. Steinhoff Annalen 1964,676 21. 49 W. Adam and F. Mazenod J. Am. Chem. SOC.,1980,102,7131. ’’ W. Adam A. Fuss F. P. Mazenod and H. Quast J. Am. Chem. SOC.,1981,103,998. 51 P.G.Schultz and P. B. Dervan J. Am. Chem. Soc. 1981,103 1563. 144 A. Cox L Scheme 10 tetramethylpyrrolidiny1)nitrene in CFC13 at -78 "C leads to the fragmentations shown (Scheme 10). Both the S1and TI states are product precursors and in addition the S1 state may also be deactivated by fluorescence. Photochemical cyclization of an azimine to an aziridine has been achieveds2 and is the first reported example of a triaziridine. Thus irradiation of the acylazimines (24) obtained by addition of ethoxycarbonyl-nitrene to an excess of (E)-or (2)-azoisopropane gave l-ethoxycarbonyl-trans-2,3-di-isopropyltriaziridine(25).At room temperature this reverts thermally to (24a) and (24b) with a half life of 3.5 days (Scheme 11). v I I 0 EtO,C-N+ N\\N -\)_N."? v A f-o .A (24) a; 22 b; 2E Scheme 11 Evidence has been presented5 to suggest that in propan-2-01 5-bromouracil is reduced to uracil by a radical pathway in the singlet manifold and by an ion-radical pathway in the triplet manifold. The transformation probably occurs by an electron transfer from the solvent to the triplet state of the bromouracil somewhat analogously to the photo-oxidation of alcohols with one-electron photo-oxidizing agents.54 A study has been made of the effects of CN and CF substituents on pyrazole photo~hemistry.~~ Cyano-substituted pyrazoles undergo phototransposition by two 52 C.Leuenberger L. Hoesch and A. S. Dreiding J. Chem. SOC.,Chem. Commun. 1980 1197. '' B.J. Swanson J. C. Kutzer and T. H. Koch J. Am. Chem. SOC.,1981,103,1274. 54 A.Ledwith P. J. Russell and L. H. Sutcliffe Proc. R.SOC.London Ser. A 1973,332,151. 55 J. A. Barltrop A. C. Day A. G. Mack A. Shahrisa and S. Wakamatsu J. Chem. SOC.,Chem. Commun. 1981.604. Photochemistry 145 concurrent pathways namely 1,5-interchange and 2,3-interchange but by contrast phototransposition of 1,5-dimethyl-3-trifluoromethylpyrazoleoccurs only by the former pathway. Irradiation of a number of 2-(4-pyridylvinyl)-4H-chromen-4-ones in oxygenated benzene induce^'^ photocyclization to 12H-[ l]benzopyrano[2,3- h Jisoquinolin-12-one in 70% yield.This transformation which is also successful for a number of related substrates provides a synthetically convenient route to benzopyranoisoquinolines and benzopyranoquinolines. s6 I. Yokoe K. Higuchi Y. Shirataki and M. Komatsu J. Chem. SOC.,Chem. Commun. 1981,442.