Ab initio quantum chemical and ReaxFF-based study of the intramolecular iminium-enamine conversion in a proline-catalyzed reaction

Among all strategies used by organic chemists to control the stereoselectivity of reactions, organocatalysis, which consists in using the chirality of a small organic molecule, is an increasingly popular method. The prolinecatalyzed aldol reaction was one of the first reported cases that demonstrated the power of organocatalysis in the field of asymmetric synthesis. Previous theoretical contributions focused on the reaction mechanism using quantum mechanics (QM) methods. We here present a theoretical study about one specific step of the proline-catalyzed aldol reaction, namely, the conversion of the iminium intermediate into the corresponding enamine. It consists of an intramolecular rearrangement that involves the transfer of a hydrogen atom. First, we investigate this transfer using modern QM models, that is, density functional theory calculations with the M06-2X functional. On the basis of these QM results, we then assess the performance of a reactive force field, ReaxFF, used in combination with molecular dynamics simulations in order to provide a complementary light on this reaction.