The role of benzoic acid in proline-catalyzed asymmetric michael addition: A density functional theory study

In asymmetric Michael addition between ketones and nitroolefins catalyzed by L-proline, we observed that it was benzoic acid or its derivatives rather than other proton acid that could accelerate the reaction greatly, and different benzoic acid derivatives brought different yields. To explain the experimental phenomena, a density functional theory study was performed to elucidate the mechanism of proline-catalyzed asymmetric Michael addition with benzoic acid. The results of the theoretical calculation at the level of B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) demonstrated that benzoic acid played two major roles in the formation of nitroalkane: assisting proton transfer and activating the nitro group. In the stage of enamine formation from imine, the energy profiles of benzoic acid derivatives were also calculated to investigate the reasons why different benzoic acid derivatives caused different yields. The results demonstrated that the pKa value was the major factor for p-substituted benzoic acid derivatives to improve the yields, whereas for m/o-substituted benzoic acid derivatives, both pKa value and electronic and steric effects could significantly increase the yields. The calculated results would be very helpful for understanding the reaction mechanism of Michael addition and provide some insights into the selection of efficient additives for similar experiments. (c) 2012 Wiley Periodicals, Inc.