Mechanism of alkaline hydrolysis of cyclic and acyclic sulfates: An ab initio study with solvation correction

The large difference in the activation barriers for the alkaline hydrolysis of cyclic and acyclic sulfate esters (the activation barrier of the cyclic sulfate is 10.3 kcal/mol lower than that for the acyclic sulfate, leading to 10(6)-10(7)-fold rate acceleration) is investigated. Ab initio studies o f the reaction paths for the basic hydrolysis of ethylene sulfate (ES) and dimethyl sulfate (DMS) have been performed in vacuum and in solution. The c alculations focus on the attack of the hydroxyl ion on sulfur, the rate-det ermining step for these reactions. In analogy with the calculated results f or the isoelectronic phosphate esters (EP and DMP), differential solvation is the dominant factor that leads to the faster rate of hydrolysis of the c yclic ester. In both systems, the preferential solvation of the cyclic este r transition states is due primarily to the greater exposure and resulting stabilization of the attacking hydroxide ion. Although the differential sol vation effects are the same, the overall effect of solvent is in opposite d irections for the sulfate and the phosphate esters due to the fundamental d ifference in the electrostatic interactions involved (charge-charge for pho sphates, charge-dipole for sulfates); i.e., the transition states for phosp hate esters are stabilized by solvation, while they are destabilized for su lfate esters.