Photochemical reactivity of α-phenyl β,γ-enones. Singlet 1,3-acyl shift, triplet aromatic di-?-methane (DPM) rearrangement and triplet aryl-carbonyl bridging

Article abstract of DOI:10.1002/recl.19881070404

The photochemistry of the series of α-phenyl β,γ-enones 6-10 has been studied under conditions of both direct (λ 300 nm) and triplet-sensitized irradiation with the aim of determining the reactivity patterns of these "multi"-chromophoric systems.Upon direct irradiation, the reactants exhibit the typical photoreactions of β,γ-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 α-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 β-bridging between the carbonyl and the phenyl group, followed by extrusion of the C4H6 fragment from the 1,4- or 1,3-oxa-diradical.Upon sensitized irradiation, the o-methoxy- and p-cyano-substituted reactants 9 and 10 exhibit the di-?-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 α-cleavage, whereas the acetophenones and di-?-methane products are formed via initial aryl-carbonyl and aryl-vinyl bridging, respectively.The inability of the other α-phenyl β,γ-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 β,γ-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.