Photochemistry of acyclic βγ-unsaturated ketones. Determination of the electronically excited state responsible for α-cleavage by chemically induced dynamic nuclear polarisation

Abstract

The CIDNP effects during irradiation of six acyclic βγ-unsaturated ketones have been studied. The α-cleavage is mainly a reaction from the electronically excited singlet n–π* state if the alkene part of the βγ-enone is substituted with a phenyl group at the γ-position, but the α-cleavage occurs mainly from the triplet excited state if the alkene system is substituted with methyl groups at that position. The difference in multiplicity is rationalized in terms of whether intramolecular quenching is possible or not.