Selective cleavage of Cα–Cβ bonds in lignin models using a bifunctional pyridinium photocatalyst via a PCET process

Abstract

Cleaving the C–C bonds in lignin to afford value-added low-molecular-weight aromatic products is promising but a challenging project. In this contribution, a mild and efficient strategy for one-step selective C–C bond cleavage in β-O-4 and β-1 lignin models has been developed over pyridinium-based photocatalysts. The strong electron-accepting ability of the pyridinium core unit and the neutral nitrogen atom of the pyridazine substituent group provide a proton coupled electron transfer (PCET) channel to promote hydrogen dissociation from the OH or CH groups to form the corresponding free radical intermediates, thus directly triggering the cleavage of C–C bonds via a β-scission process. Benzaldehyde and phenyl formate are eventually formed as the main products. This work not only enriches the application of the pyridinium family as a low-cost, eco-friendly, and sustainable alternative for producing value-added chemicals from renewable biomass resources, but also provides a new idea for designing novel organic catalysts with PCET features to decrease the consumption of precious metals.