Design of an organocatalyst for ion–molecule S N2 reactions: A new solvent effect on the reaction rate predicted by ab initio calculations

Abstract

Using ab initio calculations coupled with a continuum solvation model (PCM), it was shown that an organic molecule, 1,4-benzenedimethanol, is able to make two hydrogen bonds with anion–molecule S N2 transition states. We have investigated the catalytic properties of this species for the S N2 reaction between acetate ion and ethyl chloride in DMSO solution. Our calculations predicts an activation free energy barrier of 26.1 kcal mol −1 for the uncatalyzed mechanism and 20.2 kcal mol −1 for the catalyzed mechanism, a drop in the activation barrier in relation to free reactants by 5.9 kcal mol −1. This organocatalyst is also able to catalyze E2 reactions and for the present system, the free energy barrier for the E2 mechanism drops by 5.3 kcal mol −1 due the action of the catalyst. Based on our theoretical data and even considering the formation of a complex between the acetate ion and the catalyst, we have estimated that the 1,4-benzenedimethanol mixed with the DMSO solvent should result in a powerful new solvent system, able to accelerate S N2 reactions by a factor as large as 10 3 in relation to pure DMSO.