Homolytic organometallic reactions. Part X. Kinetics of the homolytic photoreductions of aromatic ketones by trialkylboranes

Abstract

The kinetics of the homolytic photoreductions of a series of ring-substituted acetophenone triplets with trialkylboranes in benzene have been investigated by comparing the rate of the S_H2 process [equation (i)] with those of the quenching of the triplets by piperylene or of the sensitized isomerization of trans- to cis-piperylene. Rate constants, ArMeCO *+ BR₃→ ArMeĊOBR₂+ R·(i) for the S_H2 process are of the order of 10⁸ I mol^–1 s^–1 at 28° for both n→π* or π→π* triplets. Quantum yields vary from ca. 0·3 to 0·7 and decrease when t-butyl alcohol is present in the solvent. It is suggested that the ketone triplets from a polar complex with the borane, which can then decompose by loss of the alkyl radical R· or can revert to trialkylborane and a ground-state ketone.