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-NO₂ derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between a 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-NO₂ > 3-CF₃ > 3-CH₃ > 3-OCH₃. These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH₃ derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.