Intramolecular abstraction of aldehydic hydrogen by ketone triplets: formation of 2-hydroxy-2-phenylcycloalkanones

Abstract

Irradiation of 4-oxo-4-phenylbutanal (1) leads, via intramolecular abstraction of the aldehydic γ-hydrogen atom, to mixtures of acetophenone and 2-hydroxy-2-phenylcyclobutanone (2), and an abstraction rate constant of 8 ± 1 × 10⁸ s^–1, equal to the highest rate constant so far measured for a γ-substituted butyrophenone derivative has been obtained in steady state triplet quenching studies; the high reactivity of aldehydic hydrogen atoms toward abstraction is indicated further by an approximately 2 : 1 δ(aldehydic): γ(methylene) abstraction preference in the case of the homologous compound 5-oxo-5-phenylpentanal (3), which affords 2-hydroxy-2-phenylcyclopentanone (4) as the major photoproduct.