Selective C-alpha Alcohol Oxidation of Lignin Substrates Featuring a beta-O-4 Linkage by a Dinuclear Oxovanadium Catalyst via Two-Electron Redox Processes

摘要

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 C-alpha alcohol of lignin substrates containing (beta-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 C-alpha 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 abstraction sites for both C-alpha and C-gamma alcohol of dimeric lignin substrates, respectively. Interestingly, the dinuclear vanadium intermediate 4 demonstrates the ability to uptake two electrons resulting from the oxidation of C-alpha alcohol and yields two corresponding mononuclear V-IV intermediate 5. The mononuclear V-IV 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.