Synthesis of aromatic amides from lignin and its derivatives.

Abstract

Benzamides constitute an important class of bulk and fine chemicals as well as essential parts of many life science molecules. Currently, all these compounds are majorly produced from petrochemical-based feedstocks. Notably the selective aerobic oxidative conversion of lignin and lignin-derived compounds to primary, secondary, and tertiary amides and phenols offers the potential for a more sustainable synthesis of valuable building blocks for fine chemicals, monomers for polymers, biologically active molecules, and diverse consumer products. Here we present the concept of "lignin to amides" which is demonstrated by a one-pot, multi-step oxidation process utilizing molecular oxygen and a 3d-metal catalyst with highly dispersed and stable cobalt species (Co-SACs) supported on nitrogen-doped carbon in water as solvent. Moreover, our cobalt-based methodology allows for the cost-efficient transformation of a lignin and its variety of derivates simply using O₂ and organic amines. Mechanistic investigations and control experiments suggest that the process involves an initial dehydrogenation of C_α-OH, cleavage of the C_β-O as well as C(O)-C bond and condensation of the resulting carboxylic acids with amines. Spectroscopic studies indicate that the formation of superoxide species (O₂^●-) and specific Co-nitrogen sites anchored on mesoporous carbon sheets are key for the success of this transformation.