Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by Selenium-Containing Dinuclear Peroxotungstate and Kinetic, Spectroscopic, and Theoretical Investigation of the Mechanism.
英文:
Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by Selenium-Containing Dinuclear Peroxotungstate and Kinetic, Spectroscopic, and Theoretical Investigation of the Mechanism.
The dinuclear peroxotungstate with a SeO42- ligand, (TBA)2[SeO4{WO(O2)2}2] (I; TBA = [(n-C4H9)4N]+), could act as an efficient homogeneous catalyst for the selective oxidn. of various kinds of org. substances such as olefins, alcs., and amines with H2O2 as the sole oxidant. The turnover frequency (TOF) was as high as 210 h-1 for the epoxidn. of cyclohexene catalyzed by I with H2O2. The catalyst was easily recovered and reused with maintenance of the catalytic performance. The SeO42- ligand in I played an important role in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen-transfer reactivity. Several kinetic and spectroscopic results showed that the present catalytic epoxidn. included the following two steps: (i) formation of the subsequent peroxo species [SeWmOn]o- (II; m = 1 and 2) by the reaction of I with an olefin and (ii) regeneration of I by the reaction of II with H2O2. Compd. I was the dominant species under steady-state turnover conditions. The reaction rate for the catalytic epoxidn. showed a first-order dependence on the concns. of olefin and I and a zero-order dependence on the concn. of H2O2. The rate of the stoichiometric epoxidn. with I agreed well with that of the catalytic epoxidn. with H2O2 by I. All of these kinetic and spectroscopic results indicate that oxygen transfer from I to the C-C double bond is the rate-detg. step. The computational studies support that the oxygen-transfer step is the rate-detg. step. [on SciFinder(R)]
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