Asymmetric Reactions Using Optically Active (Salen)ruthenium Complexes as Catalysts
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33; Hakozaki, Higashi-ku, Fukuoka-shi 812-8581 Japan
Second generation metallosalen complexes bearing binaphthyl units as their chiral auxiliaries serve as efficient catalysts for a variety of asymmetric reactions. To date, however, only the complexes of the first transition series of metals have been used as asymmetric catalysts and asymmetric catalysis of the complexes of the second transition series of metals still remains unexplored. Recently, we synthesized second generation (R, S)- and (R, R)-(chloro)(nitrosyl)(salen)ruthenium complexes and examined their asymmetric catalysis, which was found to be considerably different from that by the salen complexes of the first transition metals. The most salient feature of these new (salen)ruthenium complexes is that they are activated upon photo-irradiation and are inert in the dark. Furthermore, they show novel catalysis that can not be achieved by (salen)metal complexes of the first transition series of metals. For example, (R, S)-(nitrosyl)(salen)ruthenium complex could be successfully used as the catalyst for asymmetric and stereospecific epoxidation of cis- and trans-olefins, and high cis- and enantioselective cyclopropanation was for the first time achieved by using (R, R)-(nitrosyl)(salen)ruthenium complex as the catalyst. Furthermore, aerobic asymmetric oxidation of secondary alcohols was also achieved with the same complex as the catalyst.
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