Guaiacylglycerol as an Intermediate in Vanillin Production during Oxidative Conversion of Guaiacyl Lignin under Alkaline Conditions

Abstract

Depolymerizing lignin into hydroxybenzaldehydes like vanillin via oxidative degradation is a promising approach for lignin valorization. Alkaline nitrobenzene (AN) oxidation is a highly selective method for oxidative lignin conversion and serves as a benchmark in lignin chemistry, yet its reaction mechanisms are not fully understood. Previously, we proposed a pathway converting a phenolic β‐O‐4 model compound, erythro‐guaiacylglycerol‐β‐guaiacyl ether (GG), into vanillin through enol ether intermediates, E/Z‐2‐methoxy‐4‐[2‐(2‐methoxyphenoxy)‐ethenyl]‐phenol EE. This study extends that work by exploring alternative AN oxidation pathways of GG that bypass EE. Through experiments assessing the impact of OH⁻ concentration and studying deuterium incorporation at the α‐position of vanillin produced from both GG and isolated EE in NaOD/D₂O, we identified guaiacylglycerol (GGL) as a new intermediate in a distinct pathway. GGL forms via β‐ether cleavage of a dianion arising from dual deprotonation of GG′s phenolic and α‐OH groups, a process favored under high OH⁻ concentrations conducive to dianion formation. Subsequent oxidation of GGL yields vanillin, with significant exchange of the α‐hydrogen with solvent molecules. This study elucidates the complex mechanisms of AN oxidation and introduces novel intermediates and pathways, enhancing strategies for efficient lignin conversion into valuable chemicals.