REACTIONS OF METAL-ION COMPLEXES WITH LIGNIN MODEL COMPOUNDS .3. RH(TSPP) CATALYZED FORMATION OF GUAIACOL FROM BETA-ARYL ETHERS IN EXCEPTIONALLY HIGH-YIELD

Trisodium meso-tetra-4-sulfonatophenylporphinerhodium(III) (Rh(TSPP)) was found to be a very effective catalyst for cleaving the beta-aryl e ther bonds of the lignin model compounds guaiacylglycol beta-guaiacyl ether (1) and guaiacylglycerol beta-guaiacyl ether (11). In both cases very large amounts of guaiacol (2) were formed regardless of whether the reducing sugar glucose was present or not. With the substituted ph enylethane model 1 the other major product was acetovanillone (5); wit h the substituted phenylpropane model 11 considerable amounts of vanil lin (4) were formed. The products obtained with the rhodium catalyst w ere very different to those formed in the Co(TSPP) or anthraquinone (A Q) catalyst reactions. They were, however, similar to those obtained w ith Fe(TSPc), although the rhodium catalyst was much more active and f ormed guaiacol at a much faster rate. In addition to these reactions, the rhodium complex also catalyzes the partial demethylation of the 3, 4-dimethoxyphenyl model compound 10. Mechanisms are proposed for these reactions in which the key steps involve single electron transfer bet ween the catalyst and the substrate.