Steric effect on photochemistry of benzyl ester derivatives 2. Effects of substituents and oxygen on photolysis of alpha-methyl-substituted benzyl alkanoates

The photochemistry of alpha-methyl-substituted benzyl acetates and pivalate s, la-e and 2a-c, respectively, has been examined in methanol (MeOH). Sensi tization and quenching experiments confirmed the predominance of the single t-state photolysis of these esters. The product distribution depended on me thyl groups, on the methylene carbon, and/or alkyl groups attached to the c arbonyl group. Eater conversion decreased with methyl substitution. These r esults are explicable on the basis of steric hindrance, or conformational r estriction, around the eater bond. We also found that the photolysis was ma rkedly affected by oxygen, particularly for the most hindered ester 2c. It is likely that the effect of oxygen is attributed to the formation of an ex cited charge-transfer complex between the esters and an oxygen molecule. (C )1999 Elsevier Science S.A. All rights reserved.