Abstract. The photochemistry of the series of a-phenyl P,y-enones 6-10 has been studied under conditions of both direct (A 300 nm) and triplet-sensitized irradiation with the aim of determining the reactivity patterns of these "mu1ti"-chromophoric systems. Upon direct irradiation, the reactants exhibit the typical photoreactions of P,y-enones, viz. the 1,3-acyl shift, affording the corresponding (E)-and (Z)-5-phenyl-4-hexen-3-ones, decarbonylation of the radicals formed by a-cleavage and recombination of the resulting alkyl radicals and, in addition, a new type of reaction from the triplet-excited state yielding small amounts of the corresponding acetophenones. The acetophenones are thought to be formed by initial P-bridging between the carbonyl and the phenyl group, followed by extrusion of the C,H, fragment from the 1,4-or 1,3-0xa-diradical. Upon sensitized irradiation, the o-methoxy-and p-cyano-substituted reactants 9 and 10 exhibit the di-n-methane rearrangement, leading to mixtures of the corresponding cis-and trans-1 -acetyl--1 -methyl-2-phenylcyclopropanes, with quantum yields of 0.03 and 0.10, respectively. The formation of the 1,3-AS and decarbonylation products illustrates the occurrence of a-cleavage, whereas the acetophenones and di-n-methane products are formed via initial aryl-carbonyl and aryl-vinyl bridging, respectively. The inability of the other a-phenyl P,y-enones to undergo photocyclopropanation is discussed in terms of excitation-energy partition. In the o-methoxy-and p-cyano-substituted reactants, the excitation energy may be predominantly concentrated in the aryl moiety in contrast to the other two systems in which the excitation energy may be mainly localized on the P,y-enone moiety. Subsequent triplet-energy dissipation by a free-rotor mechanism would then account for the stability of these systems. This was observed for (E)-7 which, upon triplet sensitization, affords only the (Z) isomer, whereas this process is degenerate for the other substrates studied.
The photoisomerization of a series of (highly) crowded α‐oxo oxime ethers (3–8) has been studied under conditions of direct irradiation (λ 300 and 350 nm) and of triplet photosensitization. The decay ratios k5/k6 for the two conditions are the same for a given compound, illustrating that the isomerization of the α‐oxo oxime ethers, upon direct irradiation, also proceeds from the (lowest) excited triplet state. The excited singlet and triplet state energies are reported, as well as ΔG0(E)‐ΔG0(Z) for 4 (‐ 8.2 ± 0.5) and 6 (7.7 ± 0.5 kJ·mol−1), determined by thermal equilibration with I2 at 353 K. The decay ratio k5/k6 is significantly smaller for 4 and 5 than for 3 and 6‐8 due to steric hindrance upon relaxation of the perpendicular ground state to the (E)‐isomer of 4 and 5.
The photochemistry of the diphenyl P,y-enones 2 and 3 and the methyl-and phenyl-substituted a-phenyl ketones 4-9 has been studied, using mainly benzene as solvent. Irradiation of 2 with h 300 nm leads to the 1,3-acyl shift (1,3-AS) product 17, relatively small amounts of the decarbonylation products 15 and 16, and the photo-oxidation products benzophenone (18) and the a$unsaturated aldehyde 19. Direct irradiation of the 1,3-AS product 17 yields 15, 16, 18 and 19, but no 2. Direct irradiation of 3 with h 350 nm leads mainly to the formation of 1,3-AS products (E)and (2)-22, the decarbonylation products (E)and (2)-20 and 21 and, in addition, a small amount of the cis-di-n-methane isomer 23. Direct irradiation of the substituted a-phenyl ketones 4, 5, 8 and 9 with h 300nm leads predominantly to the formation of the radical coupling products 2,3-butanedione and 24-27, respectively, whereas 6 and 7 yield 1,2-diphenyl-1,2-ethanedione and the respective 24 and 25. The acetonesensitized irradiation of 4 leads to 4-phenyl-2-butanone (33) in addition to 2,3-butanedione and 1,2-diphenylethane (24).The syntheses of 2 and 3 are outlined in Scheme 1. Since phenyl vinyl sulfoxide is a strong and aselective reagent2', Scheme 2 . Photoproducts resulting from 2 upon direct irradiation 1 300 nm) in 2-propanol.
The photochemical rearrangement of the two 2-(2-alkenyl)-2-( 1-cyc1opentenyl)cyclopentanones @)-la and (E)-2a have been studied under conditions of both direct ( h 300 nm) and tripletsensitized irradiation in order to determine the applicability of 2-alkenyl moieties as intramolecular quenchers of the lowest excited triplet states of the P,y-enone chromophore. The P,y. yr,6'-dienone @)-la, upon direct irradiation, exhibits a reversible 1,3-acyl shift (1,3-AS) leading to an eventual photostationary state { [(E)-la]/[(E)-lb]},,, = 4.0 f 0.8, whereas triplet photosensitization of (E)-la gives the ODPM isomer (E)-lc. Both processes occur without ( E ) -( Z ) isomerization of the 2-butenyl side-chain. Direct irradiation of (E)-2a yields, albeit very inefficiently, the 1,3-AS compound (E)-2b (@ < 0.001) as the primary product which, upon electronic excitation, forms, in competing reactions, (E)-2a (@ = 0.18) and (Z)-2b (@ = 0.035). Upon triplet photosensitization of both (E)-2a and (E)-2b, only ( E ) -( Z ) isomerization of the styryl side-chain is observed. The eventual photostationary state in the direct irradiation equals {[(E)-2a]/[(Z)-2a]/[(E)-2b]/[(Z)-2b]},,, =(33 f 1)/(33 f 1)/(17 f ])/(I7 f 1).The differences in photochemical behaviour of the P,y. y',6'-dienone substrates and their 1,3-AS products are discussed with special emphasis being placed upon the triplet energies of the 2-alkenyl chromophores relative to those of the P,y-enone chromophores and the occurrence of CT interaction which enhances the internal conversion of the S'(nrr*) states of the P,y-enone chromophores. , J. Am. Chem. SOC. 104,6397 (1982). * 3a does not show phosphorescence, rendering a direct estimation 24aN. J . Turro, "Modern Molecular Photochemistry", the Benjamin of its triplet energy impossible.
Sowohl bei direkter als auch bei sensibilisierter Anregung ergeben sich für einen Oximether die gleichen Verhältnisse der Zerfallsgeschwindigkeitskonstanten, was auf die Bildung der Isomere über den gleichen (π,π*)‐Anregungszustand schließen läßt (E/Z‐Isomerenverhältnisse für (Ia), (Id) und (III): 1.5, 0.5 bzw. 8.6).
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