The chemo-catalytic racemization of lactic acid enantiomer derived from biomass via keto-enol tautomerization

Abstract

Biomass valorization towards chiral lactic acid (LaA) production via chemocatalysis is of great significance for meeting its huge demand in widespread applications, but still confronts a long-standing obstacle of low enantioselectivity. Herein, we first shed light on the reason for the low enantioselectivity, and discover that configuration inversion of pure lactic acid enantiomer occurs with the assistance of alkaline earth metal oxide, so far it has only been reported by enzymatic catalysis. The racemization mechanism passing through keto-enol tautomerization is proposed. Alkaline earth metal oxide successfully promotes H-removal at C2, and the remaining electron transfers to –COO-. This changed C1-C2 from σ to σ-π bond, and produced enolate intermediate with plane resonance structure. Due to the same steric hindrance of H-return to C2, racemic LaA was obtained.