Nature and Kinetic Analysis of Carbon−Carbon Bond Fragmentation Reactions of Cation Radicals Derived from SET-Oxidation of Lignin Model Compounds

Features of the oxidative cleavage reactions of diastereomers
of dimeric lignin model compounds, which are models of the major types
of structural units found in the lignin backbone, were examined. Cation
radicals of these substances were generated by using SET-sensitized
photochemical and Ce(IV) and lignin peroxidase promoted oxidative
processes, and the nature and kinetics of their C−C bond cleavage
reactions were determined. The results show that significant differences
exist between the rates of cation radical C1−C2 bond cleavage
reactions of 1,2-diaryl-(β-1) and 1-aryl-2-aryloxy-(β-O-4)
propan-1,3-diol structural units found in lignins. Specifically, under
all conditions C1−C2 bond cleavage reactions of cation radicals
of the β-1 models take place more rapidly than those of the
β-O-4 counterparts. The results of DFT calculations on cation
radicals of the model compounds show that the C1−C2 bond dissociation
energies of the β-1 lignin model compounds are significantly
lower than those of the β-O-4 models, providing clear evidence
for the source of the rate differences.