Pulping Catalysts From Lignin (6). Nitrogen Dioxide Oxidation Of 5-Substituted Guaiacyl Compounds

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Abstract
Bromination was investigated as a potential approach to improve the yield of benzoquinones from lignins by blocking the C5-position of guaiacyl-type units. The 5-position is a reactive position, often leading to undesirable byproducts when guaiacyl units are oxidized to radical intermediates. The NO2 reaction of 5-bromovanillin in methanol in the presence of N-hydroxysuccinimde (NHS) provided ∼50% yield of 2,6-dimethoxy-p-benzoquinone (DMBQ), together with ∼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. The halovanillins gave no benzoquinones in the absence of NHS. A mechanism involving generation of phenolate radicals from NHS radicals can account for these results. During the oxidation, most of the 5- halo groups were converted to 5-methoxy groups by reaction with the solvent; experimental results suggested that the solvolysis occurred with the radical intermediates, before halobenzoquinone production. A high yield of DMBQ was obtained from a mixture of vanillin and syringaldehyde by treating first with bromine and then with NO2/NHS. This sequence could be a potential way for improving the yields of benzoquinones from lignins.