Selective cleavage of C-alpha-C-beta bonds in lignin models using a bifunctional pyridinium photocatalyst via a PCET process

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 beta-O-4 and beta-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 beta-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.