A comprehensive theoretical investigation of the transition states and a proposed kinetic model for the cinchoninium ion asymmetric phase-transfer catalyzed alkylation reaction

Abstract

The discovered that the cinchoninium ion could promote a highly enantioselective alkylation of an indanone enolate via phase-transfer catalysis was a breakthrough in the area asymmetric organocatalysis. However, only recently the mechanism of asymmetric induction was unraveled by theoretical calculations. It was found the process takes place through cooperative catalysis with a key role of the cinchoninum hydroxyl group. In this work, we have done a more detailed investigation of the possible transition states, reporting twenty-four structures. More reliable calculations at MP2 level was applied to the most important species and we have observed that dispersion forces play a very important role to selectively stabilize the critical transition state leading to the main enantiomer. The possible partition equilibria of several species have also been considered and a kinetic model was proposed. We have found the formation of a dimer of the cinchoninium ion takes place and this behavior leads to fractional kinetic order in the catalyst concentration. The theoretical overall kinetics is in very good agreement with the experimental data, with an effective activation free energy barrier deviating by only 1.2kcalmol−1 from the experimentally observed value. This study provides a very detailed picture at molecular level for this reaction system at first time.