Insights into the mechanism of selective olefin epoxidation catalyzed by [γ-(SiO4)W10O32H4]4-.: A computational study

被引:45
作者
Prabhakar, Rajeev
Morokuma, Keiji
Hill, Craig L.
Musaev, Djamaladdin G. [1 ]
机构
[1] Emory Univ, Cherry L Emerson Ctr Sci Computat, Atlanta, GA 30322 USA
[2] Emory Univ, Dept Chem, Atlanta, GA 30322 USA
关键词
D O I
10.1021/ic060725p
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
A mechanism for the H2O2-based epoxidation of olefins catalyzed by the lacunary polyoxometalate (POM)[gamma-(SiO4)-W10O32H4](4-) (1) has been investigated at the DFT level. In this study, for the first time a "hydroperoxy" mechanism for this important process has been proposed. It is divided into two steps and investigated using the whole lacunary compound as a model. In the first step, a hydroperoxy (W-OOH) species and a water molecule are generated. The formation of this nonradical oxidant (W-OOH), consistent with the experimental suggestions, occurs with a barrier of 4.4 (7.2) kcal/mol (the number without parenthesis includes solvent effects in benzene, while the one with parenthesis is in the gas phase). In the second step, the O-O bond of the W-OOH species is cleaved, and an epoxide is formed. This step has a barrier of 38.7 (40.0) kcal/mol. It was found that the presence of one and two (CH3)(4)N+ countercations significantly reduces the rate-limiting barrier by 7.6 (8.3) and 11.9 (12.6) kcal/mol, respectively, and makes this lacunary POM a very efficient catalyst for epoxidation of olefins by hydrogen peroxide. It was demonstrated that the lacunary polyoxometalate basically acts as a mononuclear W( VI) complex in activating the oxidant, a conceptually noteworthy finding.
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页码:5703 / 5709
页数:7
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