Structure–reactivity studies and catalytic effects in the photosolvolysis of methoxy-substituted benzyl alcohols

Abstract

The photosolvolysis of several methoxy-, dimethoxy-, and hydroxy-substituted benzyl alcohols has been studied in aqueous solution. The primary photochemical event is photodehydroxylation, to give a benzyl cation intermediate, which can be trapped by added external nucleophiles. The reaction is via the singlet excited state, based on observation of fluorescence quenching by hydronium ion in a complementary manner with acid catalysis of reaction observed for several derivatives. Solvent isotope effects on fluorescence efficiency and reaction for (7) and (8) provide additional support of singlet-state reactivity for these compounds. Dimethoxy-substituted alcohols are more reactive than monosubstituted compounds, with quantum yields of methanolysis of up to 0.31 for the most reactive compound, 2,6-dimethoxybenzyl alcohol (8). Using a kinetic argument, the quantum yields of the primary photodehydroxylation process has been estimated to be 1.0 ± 0.1 for this compound. The results observed for the dimethoxy-substituted derivatives suggest the existence of additivity of substituent effects in these photodehydroxylation reactions.