- Olefin epoxidation with hydrogen peroxide catalyzed by lacunary polyoxometalate [γ-SiW10O34(H2O) 2]4-
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The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA]4[γ-SiW10O34-(H 2O)2] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30% aqueous hydrogen peroxide. The negative Hammett ρ+ (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), Xso = 0.04, for I-catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-spoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. 29Si and 183W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA]4[γ-SiW10O32(O2) 2] (II), with retention of a γ-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III.
- Kamata, Keigo,Kotani, Miyuki,Yamaguchi, Kazuya,Hikichi, Shiro,Mizuno, Noritaka
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p. 639 - 648
(2007/10/03)
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- Chemistry of (Glycidyloxy)propiolactones. An Intramolecular Transfer of Alkoxy Group in the Alcoholysis and Reduction Reactions
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An interesting intramolecular transfer of an acetal alkoxy group is observed in the alkaline alcoholysis and in reduction by LiAlH4 of α-methyl-α-((1-tert-butoxy-2-methyl-2,3-epoxypropyl)oxy)-β-propiolactone (3a).With either methanol or ethanol and NaOH at 30 deg C, the (glycidyloxy)propiolactone 3a cleaves to produce α-methylglycidaldehyde and either methyl or ethyl α-tert-butoxy-β-hydroxyisobutyrate.Reduction with LiAlH4 at 30 deg C also cleaves 3a, this time with partial reduction to give 2-methyl-2,3-epoxypropanol (6) and 2-methyl-2-tert-butoxypropane-1,3-diol (7).In each case the tert-butoxy group has been transferred to the α carbon of the β-lactone portion of 3a.
- Jedlinski, Zbigniew,Klimek-Slezak, Robert,Kowalczuk, Marek
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p. 2427 - 2429
(2007/10/02)
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