Effects of Alkoxy Groups on Arene Rings of Lignin β-O-4 Model Compounds on the Efficiencies of Single Electron Transfer-Promoted Photochemical and Enzymatic C-C Bond Cleavage Reactions

To gain information about how alkoxy substitution in arene rings of beta-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1-C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric beta-O-4 models undergo more rapid C1-C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1-C2 bond dissociation energies of beta-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two beta-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C-C bond cleavage was observed to occur in one case at the C1-C2 bond in the less alkoxy-substituted beta-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted beta-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of beta-O-4 units would be more efficiently cleaved by SET mechanisms.