Copper Catalyzed Alcohol Oxidation and Cleavage of β-O-4 Lignin Model Systems: From Development to Mechanistic Examination

Lignin, an abundant natural polymer with high aromaticity, is a potential source of renewable chemicals and fuels among other biomaterials. Herein, a study on copper catalyzed alcohol oxidation and cleavage of lignin β-O-4 model substrates is reported. The potential of CuX/BiPy/TEMPO(/additive) systems (X=halide or triflate, BiPy=2,2’-bipyridine; TEMPO=2,2,6,6-tetramethylpiperidine-N-oxyl) was explored and the optimized protocol was studied with several type of model compounds. Mechanistic studies elucidated two types of cascade reactions: Both involve a cleavage of C_α–C_β bond, which is most likely activated by 1) primary alcohol oxidation in β-O-4 glycerolaryl ethers resulting in retro-aldol cleavage and 2) C_β–H oxidation after secondary alcohol oxidation in β-O-4 ethanolaryl ethers leading to oxidative radical cleavage. The first process affords aromatic aldehydes while the latter delivers aryl acids.