Copper-catalyzed C–C bond cleavage coupling with CN bond formation toward mild synthesis of lignin-based benzonitriles

Abstract

N-participated lignin depolymerization is of great importance for the transformation of waste lignin into value-added chemicals. The vast majority of developed strategies employ organic amines as nitrogen source, and considerable methods rely on excessive use of strong base, which suffers severe environmental issues. Herein, benzonitrile derivatives are synthesized from oxidized lignin β-O-4 model compounds in the presence of solid nitrogen source (NH₄)₂CO₃ under mild, base-free conditions over commercially available copper catalyst. Mechanism studies suggest the transformation undergoes a one-pot, highly coupled cascade reaction path involving oxidative C-C bond cleavage and in-situ formation of CN bond. Of which, Cu(OAc)₂ catalyzes the transfer of hydrogen from C_β (C_β-H) to C_α, leading to the cleavage of C_α-C_β bonds to offer benzaldehyde derivative, this intermediate then reacts in-situ with (NH₄)₂CO₃ to afford the targeted aromatic nitrile product. Tetrabutylammonium iodide (TBAI), acting as a promoter, plays a key role in breaking the C_α-C_β bonds to form the intermediate benzaldehyde derivative. With this protocol, the feasibility of the production of value-added syringonitrile from birchwood lignin has been demonstrated. This transformation provides a sustainable approach to benzonitrile chemicals from renewable source of lignin.