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Monomeric C 6 -C 2 -type lignin model compounds with a p -hydroxyphenyl ( H ), guaiacyl ( G ), syringyl ( S ), or p -ethylphenyl ( E ) nucleus (1-phenylethanol derivatives) were individually oxidized by MnO 2 at a pH of 1.5 and room temperature. The results were compared with those of the corresponding C 6 -C 1 -type benzyl alcohol derivatives obtained in our recent report to examine the effect of the presence of the β -methyl group on the oxidation. The presence decelerated the oxidation regardless of the type of aromatic nucleus, although it did not change the order of the oxidation rates: G  >  S  >>  H  >  E . This deceleration results from the steric factor of the β -methyl group in the C 6 -C 2 -type compounds. The MnO 2 oxidations of the corresponding C 6 -C 2 -type compounds deuterated at their α -(benzyl)positions showed that the magnitudes of the kinetic isotope effects are smaller than those observed in the oxidations of the corresponding C 6 -C 1 -type compounds, regardless of the type of aromatic nucleus. These smaller magnitudes suggest that the presence of the β -methyl group shifts the initial oxidation mode of MnO 2 from direct oxidation of the benzyl position to one-electron oxidation of the aromatic nucleus. Only the S -type compounds afforded products via degradation of the aromatic nuclei.

Effect of side-chain length in lignin model compound on MnO2 oxidation: comparison of oxidations between C6-C2- and C6-C1-type compounds