Role of Substituents in the Solid Acid-Catalyzed Cleavage of the β-O-4 Linkage in Lignin Models

The role of substituents in the acidolysis of several lignin models bearing a beta-O-4 linkage was studied through experimental (by using Nafion SAC-13 as a heterogeneous catalyst) and theoretical data (including transition state calculations). An equilibrium involving the protonation of either alpha-OH or beta-OH groups as well as the dehydration of the alpha-OH protonated form to yield a benzylic carbocation is assumed. Phenylacetaldehyde and phenol derivatives are the major reaction products for a number of lignin models, though other products have also been identified (such as enol ethers and coumaran derivatives). The high reactivity observed for lignin models bearing an ortho-methoxy group on the O-bound phenyl ring has been attributed to the methoxy anchimeric assistance to the benzylic position. On the other hand, the low formaldehyde yields observed in the acidolysis of some lignin models have been attributed to the hydrogen bond formation between the hydroxymethyl group and the base abstracting the beta-hydrogen of the carbocationic intermediate.