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

Abstract

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.

Photochemical Generation and Reactivity of the Major Hydroxyl Radical Adduct of Thymidine

“…The carbocation ( 16 ) could form by direct heterolysis upon excitation and/or electron transfer between the radical pair (Scheme 4). 18 Varying the BME concentration had no effect on the yield of 13 . This indicates that 13 arises from heterolysis and/or electron transfer between the neutral intermediates within the initially formed radical cage.…”

Photochemical Control of DNA Structure through Radical Disproportionation

“…[21,23] The ratio of thymidine to 11 was 3:1, and the mass balance ranged from 73-78 %. Mass spectral analysis of 11 formed from 1 photolyzed in H 2 18 O contained 100 % 16 O, consistent with radical trapping by O 2 . Hydroperoxide 12 was also detected by LC/MS, further supporting the radical source of 11, which was presumably formed by thiol reduction of the hydroperoxide.…”

“…There is ample precedent for photochemical radical generation from aryl sulfides. [15][16][17][18][19] Dimethoxy substitution of the aromatic rings helps to shift absorption to the red, reducing the direct excitation and potential damage of the DNA.…”

Synthesis of 2‐silaoxane via 1,3‐ photocycloaddition

“…The alkoxy substituent, in particular, Synthesis of 2-Silaoxane Via 1,3-Photocycloaddition is ideally suited for stabilizing a partial positive charge at the ipso position that is presumably generated upon excitation. We have previously reported the results of MOPAC calculations that support such an intermediate for the irradiation of anisole [13]. More recently, Pincock has convincing evidence for an analogous species from excitation of substituted benzonitriles [14].…”