Hydroxylation Reactions of Olefins with Cobalt(II) Complex-NaBH₄-O₂ Systems

Abstract

Hydroxylation reactions of olefins were carried out using a catalyst system, Cobalt(II) complex NaBH₄-O₂, as a model for monoxygenase enzymes. The catlyst system showed only a hydroxylation activity for styrene and its derivatives among the olefins tried. [Co^II(tpp)] was more active than a Sciff base complex of Cobalt(III), and the order of olefins to be hydroxylated was styrene> β-methylstyrene _??_α-methylstyrene. In the case of a standard reaction system including, for example, styrene, the main product was only 1-phenylethanol, but acetophenone and poly(styrene peroxide) were also formed when the porphyrin complex having a substituted phenyl group was used as a catalyst or when a ratio of NaBH₄/Co(II) was decreased. The use of the Schiff base complexes of Cobalt(II) having asymmetric ligands is found to induce an enantioexcess of the alcohol, depending on the ligand; e. e. of 8.1%was attained for (R)-(+)-1-phenylethanol with the [Co^II(sal₂-R-chxda)]-NaBH₄-O₂ system. Reaction mechanism including the asymmetric induction is estimated.