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 β-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 Cα -Cβ bond cleavage and Cα -N bond formation to generate aromatic amides. For ammonia, the competition between oxygen and ammonia determined the selectivity between Cα -N and Cβ -N bond formation, generating amides and α-keto amides, respectively. For tertiary amines, the lignin models underwent oxidative Cα -Cβ bond cleavage to benzoic acids. Control experiments indicated that amines act as nucleophiles attacking at the Cα or Cβ position of the oxidized β-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.