Pd/C promotes C–H bond activation and oxidation of p-hydroxybenzoate during hydrogenolysis of poplar

Abstract

Hydrogenolysis of lignin generates a portfolio of products, the yields of which are generally calculated using a subset of phenolic monomers that are dependent on the lignin composition, product distribution, and analytical technique. Some lignins are naturally γ-acylated; poplar lignins, for example, have p-hydroxybenzoate groups on 1–15% of their syringyl subunits. Upon hydrogenolysis, it is generally assumed that the p-hydroxybenzoate is cleaved before the deacylated lignin is depolymerized. Hydrogenolysis of model γ-p-hydroxybenzoylated β-aryl ethers do not, however, produce the deacylated β-aryl ether intermediates, as was previously conjectured; products instead derive from palladium-assisted reactions on the cinnamyl p-hydroxybenzoates resulting in initial β-ether cleavage. The p-hydroxybenzoate moiety itself also undergoes carboxylate-assisted palladium-catalyzed C–H bond activation to form the 2,4-dihydroxybenzoate, that subsequently converts to the 2,4-dihydroxycyclohex-1-enoate. These details underscore previously unrecognized pathways and products that are key to understanding the different hydrogenolysis product distributions from naturally acylated lignins that are prevalent biomass-conversion feedstocks. Hydrogenolysis of lignin produces a complex mixture of products, including small lignin-derived monomers, dimers, and higher oligomers. Using poplar and lignin model compounds, the authors demonstrate that the sequential demand for surface-bound hydrogen during hydrogenolysis creates temporal windows that allow catalytic oxidation events to take place, even within an overall hydrogen-rich, reductive environment.

Topics & Concepts