Evidence for the formation of chromium (III) diphenoquinone complexes during oxidation of guaiacol and 2,6-dimethoxyphenol with chromic acid

The reaction of guaiacol and 2,6-dimethoxyphenol with metal oxidants has been used as a model in order to better understand the chromium mediated reaction and photostabilization of lignin on wood surfaces. Even with a simple lignin model such as guaiacol the reaction is complex and some of the reaction products are not easily characterized. The reaction of aqueous ferric chloride with guaiacol in a 1:2 molar ratio yields an organic product with the isomeric diphenoquinones 5-(4-hydroxy-3-methoxy-phenyl)-3,3′-dimethoxy-bicyclohexylidene-2,5, 2′,5′,-tetraene-4,4′-dione (A) and 3-(4-hydroxy-3-methoxy-phenyl)-5,3′-dimethoxy-bicyclohexylidene-2,5, 2′,5′,-tetraene-4,4′-dione (B) as major components. By comparison the reaction of chromic acid with guaiacol at 1:2 and 1:5 molar ratios affords amorphous chromium (III) complexes in good yield. Reductive degradation of these complexes with sodium borohydride in tetrahydrofuran yields the coupled guaiacol trimer 3,5′,3″-trimethoxy-[1,1′;3′,1″]terphenyl-4, 4′,4″-triol as the major component. The formation of this trimer is consistent with the presence of bound diphenoquinones (A) and (B) in the original chromium (III) complex. Oxidation of 2,6-dimethoxyphenol is less complicated and clarifies the nature of chromium (III) complexes formed during these reactions. Oxidation of 2,6-dimethoxyphenol with aqueous ferric chloride yields the symmetrical diphenoquinone coerulignone. Oxidation of 2,6-dimethoxyphenol with chromic acid unambiguously yields an amorphous chromium (III) coerulignone complex which was characterized by solid state ¹³C MAS NMR and IR spectroscopy as well as by reductive degradation. It is postulated that chromic acid oxidizes lignin phenols in wood affording related chromium (III) quinone complexes that confer weather stability to the treated wood surface. Crown