Substituent Effects on the Photocleavage of Benzyl-Sulfur Bonds. Observation of the "Meta Effect"

Benzyl phenyl sulfide has been used to investigate the photocleavage mechanism for benzyl-sulfur bonds. Four experiments have shown that the reaction goes through a radical intermediate. First, the photoproducts observed can all be justified by radical mechanisms. Second, the radical intermediate was trapped with a five hexenyl tether. Third, UV analysis of analogs for the 4-NO(2) derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between sigma values and the quatum yields for loss of substituted benzyl phenyl sulide. The effect of oxygen on quantum yields is best observed after samples are thoroughly outgassed with consecutive freeze-pump-thaw cycles. It is shown that oxygen diminishes the substituent effect. Upon photolysis of the outgassed samples, the meta-substituted derivatives showed more significant variances than the para derivatives. The meta derivatives are most efficiently cleaved in the following order: 3-CN > 3-NO(2) > 3-CF(3) > 3-CH(3) > 3-OCH(3). These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH(3) derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.