Selectivity in photohydroxylation of 4-nitroveratrole and nitroanisoles catalysed by cyclodextrins

摘要

Photohydroxylation of 4-nitroveratrole in the presence of cyclodextrins results in regioselective formation of 2-methoxy-4-nitrophenol as the exclusive product. This is in contrast to solution photolysis, wherein isomeric 2-methoxy-5-nitrophenol is the sole product. Similarly cyclodextrin complexation of 2-nitro- and 4-nitroanisoles, upon photohydroxylation, results in displacement of nitro group, in contrast to solution photolysis in which mixture of nitro- as well as methoxy-displaced products are obtained. The observed results are explained as due to an increase in the local concentration of the nucleophile, upon cyclodextrin encapsulation. This active participation by cyclodextrins in photohydroxylation causes a shift in reaction mechanism from a direct displacement route (S_N2Ar~*) to one involving electron-transfer (S_N(ER)Ar~*) pathway.