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Selective C α Alcohol Oxidation of Lignin Substrates Featuring a β‐O‐4 Linkage by a Dinuclear Oxovanadium Catalyst via Two‐Electron Redox Processes
Author(s) -
Tsai YanTing,
Chen ChihYao,
Hsieh YiJu,
Tsai MingLi
Publication year - 2019
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201900807
Subject(s) - chemistry , lignin , catalysis , depolymerization , redox , vanadium , alcohol oxidation , alcohol , organic chemistry , oxidizing agent , reactivity (psychology) , electron paramagnetic resonance , photochemistry , alternative medicine , pathology , nuclear magnetic resonance , medicine , physics
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 α 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 C α 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 α and C γ 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 α 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.