Structure and mechanism in complex general acid-base catalyzed reactions

Abstract

The mechanisms of acid and base catalyzed carbonyl and acyl group reactions are determined largely by the lifetime of the initial addition intermeidate. Catalysis through diffusion-controlled trapping by relatively strong acids or bases must occur if reversion of the intermediate to reactants is faster than proton transfer involving solvent and product formation. If breakdown to reactants is faster than separation of the intermediate and catalyst, the intermediate is formed within a solvent cage containing the catalyst through a pre-association mechanism. If the “intermediate” is still more unstable or if there is no barrier for proton transfer, a stepwise reaction is impossible and the reaction must be concerted. Thus, changes in structure of the reactants can be correlated with changes in the lifetime of the intermediate and the mechanism of catalysis.