Long-wave photochemistry of di-methoxylated compounds related to lignin

Abstract

Four methoxylated compounds which resemble moieties either found in lignin or produced in lignin, i.e. poly(3,4-dimethoxyacrylophenone) (Poly 34), guaiacylacetoveratrone (34Keto), 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propan-1,3-diol (34Hydroxy) and 3,3′,5,5′-tetramethoxy-4-hydroxytilbene (ST2), were irradiated in the form of thin films under vacuum, wavelengths being restricted to 300 nm and greater. In all cases new absorptions were formed in the long-wave UV and visible regions and the samples became coloured (yellow), ST2 being particularly reactive. In all cases methane and ethane were the only two low molecular weight products, and it was proposed that OCH 3 bond fission was occurring, all of the compounds having appreciable long-wave absorptions before irradiation. 13C NMR spectra showed that the depletion of the methoxy C-atoms was accompanied by the formation of new carbonyls. Further evidence for OCH 3 fission was obtained from the photolysis of 3-ethoxy-4-hydroxystilbene in which ethane, ethylene and n-butane were formed. Spectral observations indicated that the new carbonyls were quinones and o-quinones, these being formed from the phenoxy radicals in turn formed by OC fissions, and the coloration was attributed to quinonoid species. The work demonstrates that lignin-type compounds can undergo coloration reactions, which are independent of oxidation, and are totally photo-induced. Plausible reaction mechanisms are proposed.