6 Photochemistry By J. D. COYLE Chemistry Department The Open University Milton Keynes MK7 6AA 1 General A collection has appeared’ of short ‘state-of-the-art’ reports covering a wide range of physical and organic photochemistry each written by an active leader in the field. There is a continuing strong interest in the study of photochemical reactions carried out in a heterogeneous medium and it is suggested2 that examination of radical processes on silica or other porous surfaces using cage and magnetic effects can be used not only to study radical reactions but also to obtain useful information about the surfaces. In a synthetic application of heterogeneous photochemistry irradiation of ethyl cinnamate or benzalacetone (4-phenylbut-3-en-2-one) in a stirred suspension of aluminosilicate or a solid acidic material such as Nafion leads to a much higher proportion of the cis isomer than can be achieved in homogeneous solution;3 the effect is attributed to the different relative degree of adsorption by the two isomers.Titanium dioxide is widely used in different contexts as a heterogeneous promoter of photochemical reactions and a careful study4 of amorphous and colloidal forms suggests that the properties are not adequately explained by a semi-conductor model but better by postulating the existence of Ti”’ species. 2 Alkenes Irradiation of peduoronorbornadiene gives the corresponding bicyclo-[3.2.0]heptadiene but it is now revealed’ that at low temperature the expected quadricyclane is formed (Scheme l) which undergoes a sequential thermal iso- merization to perfluorocycloheptatriene;this is converted photochemically into the bicyclo[3.2.0]heptadiene or on treatment with boron trifluoride it yields the hepta- fluorotropylium cation.The copper( 1)-promoted internal photocycloaddition of myrcene leads to a bicyclo[3.2.0]heptane (1) as a significant product;6 this compound is not formed on direct photolysis nor on triplet-sensitization demonstrating again the usefulness of copper(1) in enhancing this mode of cycloaddition. A different use of a copper ‘Photochemistry Past Present and Future,’ ed. R. P. Wayne and J. D. Coyle J. Photochem. 1984 25 issue I. ’ N. J. Turro and Chen-Chih Cheng J. Am. Chem. Soc. 1984 106 5022. R. F. Childs B. Duffey and A.Mika-Gibala J. Org. Chem. 1984 49 4352. R. S. Davidson C. L. Morrison and J. Abraham J. Photochem. 1984 24 27. ’ W. P. Dailey and D. M. Lemal J. Am. Gem. Soc. 1984 106 1169. K. Avasthi S. R. Raychaudhuri and R. G. Salomon J. Org. Chem. 1984,49,4322. 97 J. D. Coyle hv I25°C 25 'C/ Scheme 1 reagent this time copper( 11) in conjunction with an electron-transfer photosensitizer is seen in the oxidative addition of methanol to certain cy~lopropylstyrenes,~ which gives ketones (2) derived by a mechanism that includes a 1,2-shift of the cyclopropyl group. p h ! hv,MeOH CU~',C,,H&CN) qB R (R= Me c-C,H,) O -55% A study of the photo-oxidation of 2,5-dimethylhexa-2,4-dieneby singlet oxygen highlights the considerable effect of solvent on the products observed.8 In methanol the major product (53%) is a 1,2-dioxetane; in acetonitrile the predominant pathway leads to the 'normal' allylic hydroperoxide (89O/0 ); however in benzene the major product (45%) is a different hydroperoxide (3) formed in a previously unreported vinylogous ene-reaction possibly by way of a non-polar singlet biradical.The use of electron-transfer photosensitizers for the ring-opening oxidation of cyclobutanes was described recently; now the same 1,2-dioxan products (4) are isolated in very high yields starting from the aryl-alkene rather than the cyclobutane.' ' K. Mizuno K. Yoshioka and Y. Otsuji J. Chem. SOC.,Chem. Commun. 1984 1665. K. Gollnick and A. Griesbeck Tetrahedron 1984,40 3235. K. Gollnick and A.Schnatterer Tetrahedron Lett. 1984 25 185 and 2735. Photochemistry 3 Aromatics The valence isomerizations of benzene continue to interest theoretical chemists and a MIND0/3 and ab initio study of the triplet benzene + benzvalene surface suggests that the prefulvene biradical may be a relatively stable species.” The aromatic ring photoisomerizations that find synthetic applications are more usually of systems such as 2-pyridones and such a photoreaction of a 4-acetoxypyridone (Scheme 2) gives a product that after catalytic hydrogenation and alkaline reductive hydrolysis provides in good yield an intermediate for carbapenam synthesis.” The photoreduc- tion of benz[ alanthracene in the presence of N N-dimethylaniline and sodium borohydride yields 6 1‘/o of the 7,12-dihydro derivative.I2 This method for reducing polycyclic aromatic hydrocarbons overcomes the problem of low solubility in ammonia that hinders the use of Birch reduction; the reducing agent is the tertiary amine and the sodium borohydride is said to serve as a scavenger to reduce the formation of by-products.Scheme 2 Little work has been reported with regard to photochemical electrophilic substitu- tion of aromatic compounds but an important study13 shows that tryptophan irradiated in D20 exchanges deuterium for hydrogen at qng carbon number 4 in high yield (96% 4 = 0.14). Attack occurs from the -NH3 substituent on the side-chain and the result is significant in understanding the mechanisms for non- radiative decay of tryptophan one of the amino-acids most commonly implicated in photochemical damage to proteins.An interesting rep~rt,’~ that deals with the much more widely investigated nucleophilic photosubstitution reactions of aromatic compounds shows that methylamine replaces the methoxy group meta to nitro in 1,2-dimethoxy-4-nitrobenzene as expected but dimethylamine replaces the methoxy group para to nitro (Scheme 3). The authors suggest that charge-density calculations provide a satisfactory explanation of reactivity for small nucleophiles such as ammonia methylamine or hydroxide ion but that with larger (‘softer’) nucleophiles frontier orbital considerations become important. lo S. Oikawa M. Tsuda Y. Okamura and T. Urabe J. Am. Chem Soc. 1984 106 6751.I’ C. Kaneko T. Naito and A. Saito Tetrahedron Left. 1984 25 1591. I’ N. C. Yang W.-L. Chiang and J. R. Langan Tetrahedron Lett. 1984 25 2855. l3 I. Saito H. Sugiyama A. Yamamoto S. Muramatsu and T. Matsuura J. Am. Chem. Soc. 1984,106,4286. J. Cervell6 M. Figueredo J. Marquet M. Moreno-Mahas J. Bertran and J. M. Lluch Tetrahedron Lett. 1984,25 4 147. 100 J. D. Coyle OMe ON""' No2 81% NO 68% Scheme 3 Chlorobenzene is hydrolysed to phenol (100%,$I = 0.1) when irradiated in dilute aqueous solution.' The reductive dehalogenation of halogenoaromatics (as well as vinyl or cyclopropyl halides) can be carried out with improved yield and efficiency when lithium aluminium hydride is present. l6 For example p-bromochlorobenzene gives chlorobenzene in 77% yield or more usefully 7,7-dichloronorcarane gives the 7-chloro compound in a similar yield.The related reductive decyanation of cyanoaromatics occurs by way of the aromatic radical-cation and this process is enhanced by the additon of a dialkyl ~u1phide.l~ One cyano group in 1,2- or 1,4-(but not 1,3-) dicyanobenzene can be replaced by a 2-methoxyalkyl group (Scheme 4) by irradiating with an alkene and methanol in acetonitrile so1ution.l8 The presence of phenanthrene as sensitizer improves the yield and efficiency which is consistent with a mechanism involving radical ions. Electron transfer is also invoked to account for the side-chain substitution in which toluenes are converted into benzyl nitrates (96% for the p-methyl system) on irradiation with cerium( IV) ammonium nitrate." A very extensive compilation of the photocyclization reactions of stilbenes and related compounds has been published.20 An unusual photochemical cyclization is I CN CN 70% Scheme 4 l5 A.Tissot P. Bode and J. Lemaire Chemosphere 1984 13 381. 16 N. Shimizu K. Watanabe and Y. Tsuno Bull. Chem. SOC.Jpn. 1984 57 885. l7 R. A. Beecroft R. S. Davidson D. Goodwin and J. E. Pratt Tetrahedron 1984 40,4487. I* R. M. Borg D. R. Arnold and T. S. Cameron Can J. Chem 1984 62 1785. 19 E. Baciocchi C. Rol G. V. Sebastiani and B. Serena Tetrahedron Lett. 1984 25 1945. 20 F. B. Mallory and C. W. Mallory Org React. 1984 30 I. Photochemistry 0 0 72‘10 Scheme 5 involved in the reaction (Scheme 5) that leads from phenacyl chloride to a 1-tetralone in the presence of an alkene and silver triflate.2’ 4 Carbonyl Compounds There have been several reports of interesting reactions that involve photochemical cleavage in a carbonyl compound.A mechanistic study22 of the replacement of aromatic aldehyde C-H by C-D on irradiation in D20 shows that the process is efficient ( = 0.95 and benzoin derivatives are formed in only small amounts) and occurs through the (n,T*)triplet state. The photolysis of glycylglycine or alanylgly- cine causes loss of both carbon dioxide and ammonia and the major organic product after thermal treatment is acetamide or propionamide (-50% isolated yield).23 These results together with the structures of minor products point to the involvement of intermediates obtained by electron transfer and not a-carbon radicals such as those detected in e.s.r.studies of irradiated peptides. Photochemical a-cleavage in cyclic ketones generally occurs on the more heavily substituted side of the carbonyl group but for a pair of epimeric tetracyclic cyclopentanones (Scheme 6) there is very strong control by the stereochemical c~nfiguration,~~ leading in one case to almost complete cleavage on the less substituted side. a-Cleavage is also involved in the reactions that lead from 6-(w-hydroxyalkyl)cyclohexa-2,4-dienonesto macrocyclic lactones (Scheme 7).25In some systems (e.g. R = H n = 4)dilactones rCOOMc Scheme 6 2’ T. Sato and K. Tamura Tetrahedron Lett.. 1984 25 1821. 22 A.Defoin R. Defoin-Straatmann and H. J. Kuhn Tetrahedron 1985 40,2651. 23 D. Birch J. D. Coyle R. R. Hill G. E. Jeffs and D. Randall 1.Chem. Soc,Chem. Commun.,1984,796. 24 J. M.Trendel and P.Albrecht Tetrahedron Lett. 1984 25 1175. 25 G. Quinkert G. Fischer U.-M. Billhardt J. Glenneberg U. Hertz G. Diirner E. F. Paulus and J. W. Bats Angew. Chem. Int. Ed. Engl. 1984 23,440. 102 J. D. CoyZe 0 hv(R = Me n = 9) DABCO Go 73 % / Scheme 7 (22-membered 30%) and trilactones (33-membered 6%) are formed by inter- molecular reaction of the intermediate ketene. Photochemical &cleavage can occur in carbonyl compounds where the a-/3 bond is relatively weak as with N-halogenoimides to give halogen atoms and imido radicals.N-Bromo- and N-chloro-succinimides give reasonable yields of photo- adducts such as (5) with cyclohexene by such a route.26 Extensive use has been made of the photochemical cleavage of N-acyloxypyridine-2-thiones for the decar- boxylation of N-protected amino-acids (Scheme 8);27 a modification of the pro- cedure allows halogen derivatives RBr to be formed. A 0 NHBoc e.g. R = CH / 78% 'CH2CH2SMe Scheme 8 A re-investigation is reported28 of the photoreduction of ketones by tributylstan- nane; the major products are alkoxytributylstannanes (Bu3SnOCHR2 from O=CR2) the process involves a chain mechanism and some ring reduction occurs for aromatic ketones. The Norrish type 2 biradicals from aromatic ketones PhCO(CH,),Me can be oxidized by chromium(v1) or maganese(vI1) species to give good yields (around 26 J.Lessard Y. Couture M. Mondon and D. Touchard Can. J. Chem 1984 62 105. 27 D. H. R.Barton Y. HervC P. Potier and J. Thierry 1. Chem. Soc. Chem. Commun. 1984 1298. 28 M. H. Fisch J. J. Dannenberg M. Pereyre W. G.Anderson J. Rens,and W. E. L. Grossman Tetrahedron 1984,40 293. Photochemistry 70% ) of 1,4-diketones PhCO( CH2)2CO(CH2) -3 Me.29 Related biradicals from o-alkylbenzyl phenyl ketones ArCH2COPh can be excited photochemically in solution if a laser source is used for their generation;30 the process leads to biradical cleavage products. Such photochemistry of a transient intermediate in fluid solution is uncommon. The Norrish type 2 elimination process continues to find applications in generating multiply bonded species and a patented example involves the oxidation of a 4-mercapto p-lactam to the corresponding thione by way of the S-phenacyl derivative (Scheme 9).31 Scheme 9 Intramolecular hydrogen abstraction by the oxygen of a 2-pyridone is responsible (Scheme 10) for converting rhombifoline a common lupin alkaloid to tsukush- inamines A and B which co-exist in small quantities in the plant.32 In many a,p-enones however the abstracted hydrogen is taken by the p-carbon atom; this is responsible for the production of tricyclic ketones from certain unsaturated bicyclic ketones (Scheme 11),33 and for the formation of spiro-P-lactams in reasonable yield (28-65% ) from 2-( acylamino)cyclohex-2-enones.34 0 Scheme 10 Scheme 11 29 M.Mitani M. Tamada S. Uehara and K. Koyama Tetrahedron Lett. 1984 25 2805. 30 J. C. Sciano and P.J. Wagner J. Am. Chem. SOC.,1984 106 4626. 31 L. Re A. Brandt and L. Bassigani Pat. Spec$ (Aust.) AU 532 930 (Chem. Abstr. 1984 101 23212). 32 S. Ohmiya H.Otomasu and I. Murakoshi Chem. Pharm Bull. 1984 32 815. 33 Y. Tobe T. Iseki K. Kakiuchi and Y. Odaira Tetrahedron Lett. 1984 25 3895. 34 M. Ikeda T. Uchino H.Ishibashi Y.Tamura and M.Kido J. Chem. SOC.,Chem. Commun. 1984,758. 104 J. D. Coyfe Intramolecular photocycloadditions of carbonyl compounds like their more widely studied intermolecular counterparts have been employed in synthetic approaches to natural products. A y-vinyloxy ketone gives as minor product a homo-analogue to thromboxane A2 (Scheme 12) accompanying the major dioxabicyclo[4.2.0] prod~ct.~’ In an attempt to make perhydrohistrionicotoxin a route has been developed that involves as a key stage the internal photocycloaddition of an alkyne to an enone in the cyclohexenone (6).36 Scheme 12 0 5 Sulphur Nitrogen and Other Compounds The irradiation of N-vinylthiobenzamides gives 2-arylthiazolidines in good yield (Scheme 13) possibly by way of the thiol ta~tomer.~’ N-Alkylthioimides (which are red) are converted photochemically into isomeric a-amidothioacetophenones (which are purple); if the reaction is carried out at low temperature the reaction mixtures are colourless reflecting the existence of aziridinethiols as intermediates (Scheme I 4).38Thioimides derived from methacrylic acid have been used in another Scheme 13 Scheme 14 35 H.A. J. Carless and G. K. Fekarurhobo J. Chem. SOC.,Chem. Commun. 1984. 667. 36 E. R.Koft and A. B. Smith J. Org. Chem. 1984,49 832. 37 A. Couture R. Dubiez and A. Lablache-Combier J. Org. Chem. 1984. 49 714. 38 M. Sakamoto H.Aoyama and Y.Omote J. Org. Chem. 1984.49 1837. Photochemistry 0 5 5-95 % Scheme 15 photochemical approach to p-lactams (Scheme 15),39 but one that employs the photocycloaddition reaction with alkenes rather than a hydrogen abstraction process. The photochemistry of C=N compounds has developed into an area of consider- able interest. Hydrogen transfer reactions involving this group have occasionally been reported and an intramolecular example is seen in the conversion of 1-(0-toluoyl)-3,4-dihydroquinolinesinto spiro-indanones (Scheme 1 6).40 The electron- transfer photoaddition of allylsilanes or benzylsilanes to iminium salts has proved to be especially useful.In the basic intermolecular reaction with allylsilanes 2-allylpyrrolidines are formed from pyrrolinium perchlorates (Scheme 17).41Several intramolecular examples are reported including the use of o-substituted N-benzyl-pyrrolinium salts (Scheme 18) where the absence or presence of a trimethylsilyl group controls the direction of reaction and hence the size of ring formed.42 Me0 Me0 / hv -Meo?ilH R Scheme 16 Scheme 17 As part of a strategy to prepare larger quantities of 20-methylisobacteriochlorins a mild synthetic route has been developed43 that involves in the final stage a photochemical ring-closure between two C=N functions one of which carries a methylthio group (Scheme 19).The photochemistry of N-oxides has been reviewed,44 39 M.Sakamoto Y. Omote and H. Aoyama J. Org. Chem. 1984 49 396. 40 Y. Hirai H. Egawa and T. Yamazaki Heterocycles 1984 22 1359. 41 K. Ohga U. C. Yoon and P. S. Mariano J. Org. Chem. 1984,49 213. 42 A. J. Y. Lucan S. L. Quillen R.0.Heuckeroth and P. S. Mariano J. Am. Chem. Soc. 1984,106 6439. 43 D. M. Amott A. R. Battersby P. J. Harrison G. B. Henderson and Zhi-Chu Sheng J. Chem. Soc. Chem. Commun. 1984 525. 44 A. Albini and M.Alpegiani Chem. Rev. 1984 84 43. 106 J.D. Coyle 90% b R (R = CH,SiMe,) F Scheme 18 Scheme 19 and an interesting example is reported4’ of an azoxy compound that undergoes intramolecular metathesis with a neighbouring alkene [-N=N(O)-+ C=C +-N=C-+ -C=N(O)-1 whereas the corresponding azo-compound gives a 1,2-diazetidine in quantitative yield. Alkyl halides in solution undergo reactions that involve radicals carbocations or carbenes as intermediates and these processes have been reviewed.46 The use of deuterium-labelled 1-iodo-octane helps to elucidate the various pathways for this primary halide.47 Nucleophiles such as alcohols or even acetonitrile or benzene can replace the phosphate group in benzyl diethyl phosphates irradiated in and these reactions most probably occur through a benzyl cation intermediate.45 G. Fischer D. Hunkler and H. Prinzbach Tetrahedron Lett 1984 25 2459. 46 P.J. Kropp Acc. Chem. Res. 1984 17 131. 47 P. J. Kropp J. A. Sawyer and J. J. Snyder J. Org. Chem. 1984 49 1583. 48 R. S. Givens and B. Matuszewski J. Am. Chem. Soc. 1984 106 6860.