Light-driven site-selective O–H activation in lignin by triplet excited alkylanthraquinone at simulated natural conditions

Abstract

Harnessing photocatalyzed hydrogen atom transfer (HAT) for the precise activation of C–H/O–H bonds is a pivotal yet challenging strategy to selectively drive oxidative C–C bond scission in renewable lignin, yielding value-added chemicals with exceptional selectivity. Herein, we present a metal-free photochemical strategy that enables selective C–C bond scission in lignin via a unique synergistic HAT pathway driven by triplet-excited 2-ethylanthraquinone (EAQ*) and hydroxyl radicals (^•OH) generated in situ from EAQH₂ and O₂. Under simulated natural conditions, this process achieves a benzaldehyde yield of 146.6 mol% from a lignin-derived phenolic dimer. Mechanistic investigations reveal that preferential activation of the C_α-OH in lignin facilitates a tandem HAT process, forming alkoxy radical intermediates that undergo β–scission to produce benzaldehyde, as corroborated by extensive control reactions and density functional theory calculations. Furthermore, this straightforward protocol efficiently cleaves the C–C bonds of technical kraft lignins, providing a rapid, scalable, and metal-free protocol for lignin valorization under mild conditions.