Substituent Effects on the Photocleavage of Benzyl−Sulfur Bonds. Observation of the “Meta Effect”

• Published in: Journal of organic chemistry Vol. 61; no. 20; pp. 7040 - 7044
• Main Authors: Fleming, Steven A, Jensen, Anton W
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04-10-1996
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo9606923

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 σ 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.