Photochemical reactivities of cyclic α-phenyl-β,γ-enones. Singlet 1,3-acyl shift, decarboxylation and unquenchable oxa-di-?-methane reactions upon direct irradiation

Article abstract of DOI:10.1002/recl.19881070906

The photochemistry of the α-phenyl-β,γ-enones 3-6 has been studied under conditions of direct (λ 300 nm) and triplet-sensitized irradiation.Upon direct irradiation, 3 exhibits decarbonylation, a 1,3-acyl shift (1,3-AS) and an oxa-di-?-methane (ODPM) rearrangement.Under the same conditions, 4, on the other hand, affords only the ODPM product whereas 5 gives the decarboxylation, 1,3-AS and ODPM products.The formation of the ODPM product of 5 is efficiently quenched by triplet quenchers in contrast to the ODPM products of 3 and 4, the quantum yields of which are not reduced upon the addition of typical triplet quenchers.Triplet-sensitized irradiation of 5 leads to the ODPM rearrangement, whereas surprisingly 3 and 4 are unreactive.It is suggested that the ODPM rearrangement, upon direct irradiation of 3 and 4, proceeds from the S²(??*) state.The ODPM product formed upon direct irradiation of 5 is thought to be formed from the T¹(??*) state populated by ISC from the S¹(n?*) state.Upon direct irradiation, 6 exhibits a reversible 1,3-AS and in addition ketene formation via an intramolecular 1,5-H shift.Under conditions of triplet photosensitization, compound 6 appears to be stable.Given the apparently different photochemical behaviour of these constrained compounds, a general reactivity pattern for the direct and sensitized irradiation cannot be defined.