Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c–4e Bonding: A DFT Mechanistic Study

Abstract

A DFT study has been performed to gain insight into the formic-acid-catalyzed depolymerization of the oxidized lignin model (<b>1</b>^<b>ox</b>) to monoaromatics, developed by Stahl <i>et al.</i> (<i>Nature</i> <b>2014</b>, <i>515</i>, 249–252). The conversion proceeds sequentially via formylation, elimination, and hydrolysis. Intriguingly, the elimination process exploits an unconventional mechanism different from the known ones such as E2 and E1cb. The new mechanism is characterized by passing through an intermediate stabilized by a proton-shared 3c–4e bond (HCOO^⊖···H^⊕···^⊖OC^α) and by shifting the 3c–4e bond to the 3c–4e HCOO^⊖···H^⊕···^⊖OOCH bond in the joint leaving group that is originally a regular H-bond (HCOO–H···OOCH−). According to these characteristics, as well as the important role of the original HCOO–H···OOCH– bond, we term the mechanism as E1H-3c4e elimination. The root-cause of the E1H-3c4e elimination is that the poor leaving formate group is less competitive in stabilizing the negative charge resulted from H^β abstraction by the HCOO^– base than the nearby carbonyl group (C^αO) that can utilize the negative charge to form a stabilizing 3c–4e bond with a formic acid molecule. In addition, the study characterizes versatile roles of formic acid in achieving the whole transformation, which accounts for why the HCO₂H/NaCO₂H medium works so elegantly for <b>1</b>^<b>ox</b> depolymerizaion