PULPING CATALYSTS FROM LIGNIN .6. NITROGEN-DIOXIDE OXIDATION OF 5-SUBSTITUTED GUAIACYL COMPOUNDS

Bromination was investigated as a potential approach to improve the yi eld of benzoquinones from lignins by blocking the Cg-position of guaia cyl-type units. The 5-position is a reactive position, often leading t o undesirable byproducts when guaiacyl units are oxidized to radical i ntermediates. The NO2 reaction of 5-bromovanillin in methanol in the p resence of N-hydroxysuccinimde (NHS) provided similar to 50% yield of 2,6-dimethoxy-p-benzoquinone (DMBQ), together with similar to 4% yield of the expected product, 5-bromo-3-methoxy-p-benzoquinone (BMBQ). The NO2 reaction of 5-chlorovanillin gave a similar result, while 5-iodo- and 5-hydroxymethylvanillin provided poor yields of benzoquinones. Th e halovanillins gave no benzoquinones in the absence of NHS. A mechani sm involving generation of phenolate radicals from NHS radicals can ac count for these results. During the oxidation, most of the 5-halo grou ps were converted to 5-methoxy groups by reaction with the solvent; ex perimental results suggested that the solvolysis occurred with the rad ical intermediates, before halobenzoquinone production. A high yield o f DMBQ was obtained from a mixture of vanillin and syringaldehyde by t reating first with bromine and then with NO2/NHS. This sequence could be a potential way for improving the yields of benzoquinones from lign ins.