| Research Abstract |
Cobalt(II) Schiff base complexes, Co(II)(SB),which show dioxygenase-like activities in aprotic solvents, have been found to catalyze also monooxygenation and dehydrogenation depending on the nature of reaction medium,reaction temperature, substrate,and structure of the cobalt catalyst. In dioxygenation reactions,it has been demonstrated that the ortho regioselectivity observed in the dioxygn incorporation into phenolate substrates is accomplished only when no phenoxy radical intermediate is involved in the dioxygen incorporation step. Coordinately saturated (HO)Co(III)(SB) have been found to promote the oxygenation of aliphatic nitro compounds to corresponding carbonyl products, providing an additional example fpr the dioxygenase-type reactions. On the other hand, it has been found that coordinately unsaturated (HO)Co(III)(SB) lead to monooxygenation reactions. For example, the oxygenations of aromatic hydrazones and aliphatic nitro compounds in methanol result in the oxidative C-C bon
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d cleavage to give carboxylic acid methyl esters, providing new oxidation reactions. Furthermore, styrenes are oxidized in alcohols at 60 ゜C to give acetophenones and l-phenylethanols. The oxygenation of styrene with coordinately unsaturated nonplanar (HO)Co(III)(SB) in 2-propanol results in predominantly formation of l-phenylethanol, in this case the use of optically active complexes leads to the asymmetric induction in the alcohol product. Newly developed synthetically useful dehydrogenation rections using Co(SB) include (1) the oxygenation of aromatic hydrazones to diazo copounds using coordinately saturated (HO)Co(III)(SB) in acetonitrile, (2) the oxygenation of aliphatic hydrazones to l-alkenylcobalt complexes using coordinately unsaturated (HO)Co(III)(SB) in dichloromethane, and (3) the oxidation of alcohols to carbonyl compounds with t-BuOOH in the presence of coordinately saturated (HO)Co(III)(SB). As mentioned above, hydroxo complexes (HO)Co(III)(SB) play important roles in the oxidation reactions. The possibility that (HO)Co(III)(SB) may display superacid and superbase catalyses is also suggested by some preliminary experiments, which will be further investigated. Less
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