Flash photolytic generation and study of 5-methoxy-o-quinone alpha-phenylethidet in aqueous solution: comparison of cis and trans isomer reactivity

Cis and trans isomeric 5-methoxy-o-quinone alpha-phenylethides were generated by flash photolysis of o-hydroxybenzyl alcohol and o-hydroxystyrene precursors and their rates of decay were measured in aqueous solutions of perchloric acid, sodium hydroxide, and biphosphate (dihydrogen phosphate) ion and carbonate ion buffers. The data so obtained gave two parallel rate profiles displaced from one another by about an order of magnitude, the faster of which could be assigned to the cis isomer and the slower to the trans isomer. Both rate profiles showed acid catalyzed, uncatalyzed, and hydroxide ion catalyzed portions, with acid catalysis becoming saturated at the high acidity end of the range investigated. This saturation implies a pre-equilibrium mechanism involving quinone ethide protonation on carbonyl oxygen for the acid catalyzed process, and analysis of the data shows the trans isomer to be a slightly stronger base than the cis isomer. The fact that two separate rate profiles are observed implies that the cationic intermediates formed by substrate protonation in these acid catalyzed processes do not interconvert on the time scale of the quinone ethide decay reactions, and that in turn indicates that the former ethide bonds of the cationic intermediates have substantial double-bond character; a lower limit of 10 kcal mol(-1) may be estimated for the barrier to rotation about this bond.