Catalytic oxidation of para-substituted phenols with cobalt-Schiff base complexes/O2-selective conversion of syringyl and guaiacyl lignin models to benzoquinones

Models of guaiacyl (G) and syringyl (S) subunits in lignin have been catalytically oxidized to their corresponding p-quinones in the presence of molecular oxygen. The oxidation of syringyl-like phenols readily occurred with 5-coordinate cobalt catalysts on which one of the ligands is a monodentate pyridine or imidazole base that coordinates axially to the metal. Formation of p-quinones with this system depends on the coordination of the axial base to the metal as influenced by its pKa and its size. The yield of p-quinones from guaiacyl models was markedly improved by the addition of a sterically hindered aliphatic nitrogen base that does not coordinate to the catalyst. A mechanism involving deprotonation of the phenol substrate by the bulky base is proposed.