Role of solvent effects on nucleophilic substitution of 4H-pyran-4-one and its 2,6-dimethyl derivative with hydroxide ion in aqueous solution: ab initio and density functional theory studies on a supermolecular reaction model

Nucleophilic vinylic substitutions of 4H-pyran-4-one and 2,6-dimethyl-4H-pyran-4-one with hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G(d) basis sets. The aqueous solution was modelled by a supermolecular approach where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of hydroxide ion at the C2 vinylic carbon of the pyranone ring with activation barrier of 10-11 kcal mol−1 (B3LYP) was shown to be rate determing in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step, and in opposite, decrease the barrier of the elimination step.