Origin and turnaround of enantioselectivity in a chiral organocatalysed Diels-Alder reaction: A mechanistic study

The present work comprises exploring the efficacies of the chiral organocatalysts employed in the classic Diels-Alder reaction between anthrone and maleimide in generating an enantioselective product. The complete mechanism of the reaction has been figured out by performing the electronic structure calculations at the density functional theory (DFT) level. Our calculations suggest that the reaction commences with the abstraction of acidic proton of anthrone instigated by organocatalysts, (S)-(-)-N-(methyl-2 '-pyridyl)-2-(alpha-hydroxyethyl)benzimidazole [Pyr-HEB] and (S)-(-)-N-(methyl-1 '-benzotriazolyl)-2-(alpha-hydroxyethyl)benzimidazole [Btz-HEB]. Subsequently, N-methyl maleimide interacts with the nucleophilic carbon of anthrone in a stepwise pathway, either from left-hand side or right-hand side, resulting in the generation of an enantioselective chiral product. A unique feature in the above reaction was that Pyr-HEB and Btz-HEB favoured the formation of SS-product and RR-product, respectively, with high enantioselectivity. The turnaround of enantioselectivity upon changing the catalyst has been attributed to the noncovalent interaction and steric orientation in the molecule during the intermediate reaction steps. An estimated value of degree of enantioselectivity, for both the catalysts, was obtained in accordance with the experimental findings.