Accessing monomers from lignin through carbon-carbon bond cleavage

Lignin, the heterogeneous aromatic macromolecule found in the cell walls of vascular plants, is an abundant feedstock for the production of biochemicals and biofuels. Many valorization schemes rely on lignin depolymerization, with decades of research focused on accessing monomers through C-O bond cleavage, given the abundance of beta-O-4 bonds in lignin and the large number of available C-O bond cleavage strategies. Monomer yields are, however, invariably lower than desired, owing to the presence of recalcitrant C-C bonds whose selective cleavage remains a major challenge in catalysis. In this Review, we highlight lignin C-C cleavage reactions, including those of linkages arising from biosynthesis (beta-1, beta-5, beta-beta and 5-5) and industrial processing (5-CH2-5 and alpha-5). We examine multiple approaches to C-C cleavage, including homogeneous and heterogeneous catalysis, photocatalysis and biocatalysis, to identify promising strategies for further research and provide guidelines for definitive measurements of lignin C-C bond cleavage. To date, monomer yields from lignin are limited to those attainable through C-O bond cleavage. Cleaving C-C bonds often leads to deleterious product degradation and low monomer yields. Herein we review lignin C-C cleavage reports and advocate for a standardized reporting of yields.