6 ニトロシルルテニウムサレン錯体を触媒に用いるガラクトールオキシダーゼ型高選択的酸化反応の開発とその反応機構の研究(口頭発表の部)
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Aerobic oxidation of alcohols, especially chemo- and enantioselective ones, is a current topic, because alcohol oxidation is of tremendous importance in organic synthesis. Therefore, many methodologies for oxidations using dioxygen as the stoichiometric oxidant have been developed, but most of the oxidations need forced reaction conditions or addition of some mediators to perform the oxidations in a catalytic manner. On the other hand, alcohol oxidizing enzymes like Galactose Oxidase (GOase) catalyze the oxidation at a bodily temperature. Recent studies have disclosed that GOase-catalyzed oxidation proceeds via single electron transfer (SET) and intramolecular hydrogen atom transfer (Scheme 1). We recently found that (ON)Ru(salen) complexes catalyzed aerobic oxidation of alcohols at ambient temperature under irradiation of visible light without any additives. In this study, we disclosed that suitable (ON)Ru(salen) complexes catalyzed chemoselective oxidation of primary alcohols in the presence of secondary alcohols or regioselective oxidation of terminal hydroxy group of polyols (Scheme 2). Furthermore, it was found that desymmetrization of meso-diols could be effected with high selectivity by using well-designed (ON)Ru(salen) complexes (Figure 1). In the course of the studies, it was also found that the stereochemistry and reaction rate were affected by the apical ligand of ruthenium complexes (Table 1). To understand this apical ligand effect, we examined kinetics and kinetic isotope effect (KIE) in the aerobic oxidations using 4 and 5 (Scheme 2 and Table 2). The rate raw for the oxidation with 4 was rate=k_<Cl>[alcohol][cat.][O_2], while that for the oxidation with 5 was rate=k_<OH>[cat.][O_2]^<0.33-0.42>. KIE studies also indicated that ligand exchange step contributes to rate determining step (RDS) for the oxidation with 4, while hydrogen atom transfer step contributes to RDS for the oxidation with 5. Based on these results, we proposed that the mechanism of the present oxidation includes SET, hydrogen atom transfer and ligand exchange (Scheme 3).^<3c> It is noteworthy that the oxidation of mono-ols with (ON)Ru(salen) complexes involves intramolecular hydrogen atom transfer via a phenoxy radical intermediate (Scheme 3, A and B) in accord with the oxidation catalyzed by Goase, while the oxidation of diols proceeds via alkoxy radical intermediate (Scheme 3, C and D). Chemoselective oxidation of primary alcohols with complex 1 or 2 was also demonstrated to proceed via an intramolecular hydrogen atom transfer.^<5b> In conclusion, we achieved enantio- and chemoselective aerobic oxidation of meso-diols and primary alcohols using (ON)Ru(salen) complexes, respectively, and demonstrated that its mechanism is intrinsically similar to that of the Goase-catalyzed oxidation, though SET needs visible light irradiation.
- 天然有機化合物討論会の論文
- 2005-09-15
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