Recueil des Travaux Chimiques des Pays-Bas p. 549 - 562 (1988)
Update date:2022-08-03
Topics:
Koppes, Margareth J. C. M.
Cerfontain, Hans
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.
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