Enantioselective synthesis of D-lactic acid via chemocatalysis using MgO: Experimental and molecular-based rationalization of the triose's reactivity and preliminary insights with raw biomass

Abstract

The widespread applications of d-lactic acid (D-LaA) significantly prompted the research of its synthesis from renewable biomass via chemo-catalysis. Herein, we developed a robust but simple MgO-d-LaA system without using extra chiral ligand, achieving the highest LaA yield ∼90 % with ∼75 % enantiomeric excess (ee) value to d-LaA from dihydroxyacetone, glyceraldehyde, and pyruvaldehyde. The experimental and quantum chemistry computational results revealed that MgO and d-LaA presented a pronounced synergetic effect, where pre-added d-LaA provided suitable pH to promote [Mg(OH)(H2O)3]+ formation, acting as the active species to facilitate the production of pyruvaldehyde, from dihydroxyacetone and glyceraldehyde. d-LaA interacted with MgII species forming a chiral pocket-like complex, MgII-2(D-Lactate), which combined to pyruvaldehyde giving the transition state with lower strain energy due to the smaller steric repulsion, thus enabling 1, 2-H shift of hydrated-pyruvaldehyde preferentially via si-face attack, leading to d-LaA. The catalytic system could also be applied for the conversion of raw corn stover.