Amine-Mediated Bond Cleavage in Oxidized Lignin Models

Introducing amines/ammonia into lignin cracking will allow novel bond cleavage pathways. Herein, a method of amines/ammonia-mediated bond cleavage in oxidized lignin beta-O-4 models was studied using a copper catalyst at room temperature, demonstrating the effect of the amine source on the selectivity of products. For primary and secondary aliphatic amines, lignin ketone models underwent oxidative C-alpha-C(beta)bond cleavage and C-alpha-N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between C-alpha-N and C-beta-N bond formation, generating amides and alpha-keto amides, respectively. For tertiary amines, the lignin models underwent oxidative C-alpha-C(beta)bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the C(alpha)or C(beta)position of the oxidized beta-O-4 linkage to be cleaved. This study represents a novel example that the breakage of oxidized lignin model can be regulated by amines with a copper catalyst.