COBALT-SCHIFF BASE COMPLEX-CATALYZED OXIDATION OF PARASUBSTITUTED PHENOLICS - PREPARATION OF BENZOQUINONES

Para-substituted phenolics, serving as models for lignin (a renewable source of carbon), are oxidized to the corresponding benzoquinone with oxygen in the presence of catalytic amounts of Co-Schiff base complexes; The reaction products observed depend on the structure of the catalyst. The 5-coordinate catalysts (pyridine)[bis(salicylidene)ethylenediamine)cobalt [(pyr)Co(salen)] and [bis[(salicylideneamino)ethyl]amine]cobalt [Co(N-Me salpr)] convert syringyl alcohol (3,5-dimethoxy-4-hydroxybenzyl alcohol) to 2,6-dimethoxybenzoquinone in high yield. In contrast, syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) is unreactive toward these catalysts. However, the 4-coordinate Co(salen) converts syringaldehyde to 2,6-dimethoxybenzoquinone in 72% isolated yield. Phenols bearing a single methoxy group on the ring are unreactive toward any catalyst in MeOH. However, vanillyl alcohol (3-methoxy-4-hydroxybenzyl alcohol) is converted to 2-methoxybenzoquinone with Co(N-Me salpr) and oxygen in 43% yield in CH2Cl2, and 58% yield in CH2Cl2 in the presence of 1% CuCl2. The success of the oxidations appears to be related to the ease of removal of the phenolic hydrogen by the Co/O-2 complex. Competitive deactivation of the catalyst occurs with substrates of lower reactivity.