Mirroring Enzymes: The Role of Hydrogen Bonding in an Asymmetric Organocatalyzed Aza-Henry Reaction—a DFT Study

Abstract

The mechanism of the HQuin-BAM (1) catalyzed aza-Henry reaction between nitromethane and N-Boc-phenylaldimine was studied using density functional theory (DFT). Deprotonation of nitromethane by the catalyst was revealed to be the rate-limiting step, and C···C bond formation was found to be enantio-determining. The catalyst, in addition to acting as a Brønsted base, served to simultaneously activate both the electrophile and the nucleophile through hydrogen bonding during C···C bond formation and is thus essential for both reaction rate and selectivity. Analysis of the hydrogen bonding interactions revealed that there was a strong preference for the formation of a homonuclear positive-charge-assisted hydrogen bond (homonuclear (+)CAHB), which in turn governed the relative orientation of substrate binding. Furthermore, a direct correlation between the dihedral angle (θNCCN) of the reacting substrates and facial selectivity was found. This relationship between θ and facial selectivity was found to be a consequence of optimal secondary interactions and orbital overlap.