Selective Cα Alcohol Oxidation of Lignin Substrates Featuring a β‐O‐4 Linkage by a Dinuclear Oxovanadium Catalyst via Two‐Electron Redox Processes

Developing highly efficient catalyst systems to transform lignin biomass into value‐added chemical feedstocks is imperative for utilizing lignin as renewable alternatives to fossil fuels. Recently, the pre‐activated strategy involving the selective oxidation of Cα alcohol of lignin substrates containing (β‐O‐4 linkage mode has been demonstrated to significantly increase the depolymerization efficiency of native aspen lignin from 10–20 to 60 wt.‐%. In this study, we reported the synthesis of a dinuclear oxovanadium complex 2 that is capable of selectively oxidizing the Cα alcohol (80 – 100% selectivity) of various dimeric lignin substrates under a mild condition. Further investigation of catalytic mechanism has revealed that two V=O motifs of complex 2 could serve as proton ion sites for both Cα and Cγ alcohol of dimeric lignin substrates, respectively. Interestingly, the dinuclear vanadium intermediate 4 demonstrates the ability to uptake two electrons resulting from the oxidation of Cα alcohol and yields two corresponding mononuclear VIV intermediate 5. The mononuclear VIV intermediate 5 exhibits a characteristic 8‐line EPR spectrum and possesses one unpaired electron determined by the Evans method. The established structure‐reactivity relationships will be able to shed light on the future directions for rational design of highly efficient catalysts for selective oxidation of lignin biomass. The dinuclear oxovanadium molecular catalyst reported exhibits a highly selective secondary Cα aromatic alcohol oxidation (> 85% selectivity) of lignin substrates using air (1 atm) as an oxidant, with pyridine as additive. The excellent selectivity is attributed to the dinuclear nature of the catalyst facilitating two‐electron redox shuffling between VV‐VV and VIV‐VIV redox couples.