Redox-neutral photocatalytic strategy for selective C–C bond cleavage of lignin and lignin models via PCET process

Abstract

It remains challenging to achieve the selective cleavage of C–C bonds in lignin or lignin model compounds to produce aromatic products in high yield and selectivity. We have developed a redox-neutral photocatalytic strategy to accomplish this goal in both β-O-4 and β-1 lignin models at room temperature (RT) via proton-coupled electron transfer (PCET) process without any pretreatments of substrate, by adjusting the alkalinity of base to obtain a lignin models/base PCET pair with a bond dissociation free energy close to 102 kcal/mol. Without breaking down Cβ–Cγ bond and any C–O bonds, this PCET method is 100% atom economy and produces exclusive Cα–Cβ bond cleavage products, such as benzaldehydes (up to 97%) and phenyl ethers (up to 96%), in high to excellent yields and selectivities. Preliminary studies indicated that the PCET strategy is also effective for the depolymerization of native lignin at RT, thus providing significantly important foundation to the depolymerization of lignin.