Comparison of Photochemical Reactions of Aromatic Carbonyl Compounds with a Silyl Ketene Thioacetal and a Silyl Ketene Acetal

Photoaddition reactions of carbonyl compounds with silyl ketene thioacetals have been explored and the results are compared to those arising from investigations of analogus reactions with silyl ketene acetals. Observations made in this study show that photoirradiation of benzaldehyde (8) and benzophenone (9) and with the dimethyl substituted silyl ketene thioacetal (13) promotes reactions that take place predominantly via Paterno-Buchi type [2+2]-cycloaddition pathways to produce oxetanes. In addition, photoreactions between acetophenone (10) and p-cyanoacetophenone (11) and 13 occur via competitive sequential single electron transfer (SET)-desilylation or [2+2]-cycloaddition modes. Lastly, photochemical reaction of the highly electron deficient carbonyl compound, p-trifiuoromethylacetophenone (12), and 13 gives rise to exclusive formation of beta-hydroxythioester arising via the SET pathway. In contrast, photochemical reactions of all of these aromatic carbonyl compounds with the analogous dimethyl substituted silyl ketene acetal 14 take place predominantly via a sequential SET-desilylation pathway to form beta-hydroxyesters. A comparison of these results reveals that replacement of an alkoxy by alkylthio group in the ketene derivatives brings about dramatic changes in chemoselectivities of the photoreactions with aryl-substituted ketones.