Organoborane-catalysed reductive depolymerisation of catechyl lignin under ambient conditions

Abstract

Catalytic depolymerisation of naturally occurring catechyl lignins (C-lignins) with benzodioxane linkages represents a direct and sustainable protocol for producing catechols, but the configuration of this process typically involves operation at high temperature, high H₂ pressure, and high solvent-to-biomass ratios. Here, for the first time, we demonstrate that tris(pentafluorophenyl)borane (B(C₆F₅)₃) can efficiently catalyse reductive cleavage of vicinal C–O bonds in benzodioxane structures with Et₃SiH as a hydrogen donor. This homogeneous catalytic system can be generalized to C-lignin biopolymer derived from castor seed coats at room temperature and ambient pressure in highly concentrated form, thus giving silylated catechol derivatives in 85% yield. The as-depolymerized product is readily purified from the reaction mixture owing to high monomer selectivity. Reactivity screening of various C-lignin mimics, together with DFT calculations, offer an insight into the mechanistic understanding of the current depolymerisation of benzodioxane C-lignin. Overall, our results pave an economic and environmental pathway for producing catechols from biomass waste products.