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

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 complex es; The reaction products observed depend on the structure of the cata lyst. The 5-coordinate catalysts (pyridine)[bis(salicylidene)ethylened iamine)cobalt [(pyr)Co(salen)] and [bis[(salicylideneamino)ethyl]amine ]cobalt [Co(N-Me salpr)] convert syringyl alcohol (3,5-dimethoxy-4-hyd roxybenzyl alcohol) to 2,6-dimethoxybenzoquinone in high yield. In con trast, syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) is unreact ive toward these catalysts. However, the 4-coordinate Co(salen) conver ts syringaldehyde to 2,6-dimethoxybenzoquinone in 72% isolated yield. Phenols bearing a single methoxy group on the ring are unreactive towa rd any catalyst in MeOH. However, vanillyl alcohol (3-methoxy-4-hydrox ybenzyl alcohol) is converted to 2-methoxybenzoquinone with Co(N-Me sa lpr) 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 rel ated to the ease of removal of the phenolic hydrogen by the Co/O-2 com plex. Competitive deactivation of the catalyst occurs with substrates of lower reactivity.