31. Light-Driven Depolymerization of Native Lignin Enabled by Proton-Coupled Electron Transfer
Suong T. Nguyen, Philip R. D. Murray, Robert R. Knowles:
ACS Catal. 2020, 10, 800–805. DOI: 10.1021/acscatal.9b04813
ABSTRACT: Here we report a catalytic, light-driven method for the redox-neutral depolymerization of native lignin biomass at ambient temperature. This transformation proceeds via a proton-coupled electron-transfer (PCET) activation of an alcohol O–H bond to generate a key alkoxy radical intermediate, which then facilitates the β-scission of a vicinal C–C bond. Notably, this single-step depolymerization is driven solely by visible-light irradiation, requires no stoichiometric chemical reagents and produces no stoichiometric waste. This method exhibits good efficiency and excellent selectivity for the activation and fragmentation of the β-O-4 linkage in the polymer backbone, even in the presence of numerous other PCET-active functional groups. The feasibility of this protocol in enabling the cleavage of the β-1 linkage in model lignin dimers was also demonstrated. These results provide further evidence that visible-light photocatalysis can serve as a viable method for the direct conversion of lignin biomass into valuable arene feedstocks.