Biomimetic oxidation of nonphenolic lignin models by Mn(III): new observations on the oxidizability of guaiacyl and syringyl substructures.

Mn(III) is a one-electron oxidant, produced in vivo by the Mn peroxidases of white-rot fungi, and thought to be involved in lignin degradation by these organisms. However, Mn(III) has not been shown to oxidize the major nonphenolic substructures of lignin under mild conditions. We have used Mn(III) acetate as a biomimetic model for enzymatically generated Mn(III), and report that low concentrations of this oxidant suffice to oxidize nonphenolic lignin models at physiological temperatures and pH values. Under these conditions, the monomeric lignin model veratryl alcohol was oxidized to veratraldehyde, and the diarylpropane model 1-(3,4-dimethoxyphenyl)-2-phenylpropanol was oxidatively cleaved to veratraldehyde, 1-phenylethanol, and acetophenone. In an attempt to identify other lignin models that might be oxidized by Mn(III) more rapidly, we compared the rates at which Mn(III) was reduced by two guaiacyl models, veratryl alcohol and 1-(3-methoxy-4-isopropoxyphenyl)ethanol, vs two syringyl models, 3,4,5-trimethoxybenzyl alcohol and 1-(3,5-dimethoxy-4-isopropoxyphenyl)ethanol. The results were the opposite of those predicted: the syringyl models were oxidized slower than the guaiacyl models by Mn(III). To investigate the basis for this unexpected result, we recorded the visible absorption spectra of charge-transfer complexes prepared between each of the lignin models and an electron acceptor, tetracyanoethylene or p-chloranil. The results, in general agreement with the kinetic findings, showed that the nonphenolic syringyl lignin models had higher ionization potentials than the guaiacyl models.