Metal Triflates for the Production of Aromatics from Lignin

The depolymerization of lignin into valuable aromatic chemicals is one of the key goals towards establishing economically viable biorefineries. In this contribution we present a simple approach for converting lignin to aromatic monomers in high yields under mild reaction conditions. The methodology relies on the use of catalytic amounts of easy‐to‐handle metal triflates (M(OTf)x). Initially, we evaluated the reactivity of a broad range of metal triflates using simple lignin model compounds. More advanced lignin model compounds were also used to study the reactivity of different lignin linkages. The product aromatic monomers were either phenolic C2‐acetals obtained by stabilization of the aldehyde cleavage products by reaction with ethylene glycol or methyl aromatics obtained by catalytic decarbonylation. Notably, when the method was ultimately tested on lignin, especially Fe(OTf)3 proved very effective and the phenolic C2‐acetal products were obtained in an excellent, 19.3±3.2 wt % yield. Nutty triflates: Metal triflates are introduced as an easy‐to‐handle alternative to triflic acid for the cleavage of lignin β‐O‐4 linkages in conjunction with stabilization of reactive intermediates. The reactivity of several model compounds and lignin is studied. In particular, iron(III) triflate proved effective, providing as much as 19.3±3.2 wt % yield of a set of three aromatic C2‐acetals from lignin.