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摘要
The catalytic hydrosilylation of carbonyl compounds by the high-valent mono-oxo rhenium(V) complex Re(O)Cl-3(PPh3)(2) (1) was theoretically investigated to determine the underlying reaction mechanism. According to our results, an ionic mechanistic pathway featuring an S(N)2-Si structure of the transition state is the most favored. The ionic mechanistic catalytic cycle is initiated by the nucleophilic attack of the eta(1)-silane metal adduct by carbonyl substrates. This attack results in the heterolytic cleavage of the Si-H bond and generation of a silycarbenium ion paired with an anionic rhenium hydride, [Re(O)Cl-3(H)(PPh3)(-)[R3SiOCR'R ''](+). Then by transferring the hydride from the metal center to the silycarbenium ion yields the silyl ether product. The activation energy of the turnover-limiting step was calculated as similar to 24.1 kcal/mol with diphenylketone. This value is energetically more favorable than the sigma-bond metathesis pathway that involves the initial exchange of silane hydrogen to chloride ligand generating the hydride intermediate, Re(O)HCl2(PPh3)(2) by 7.4kcal/mol. Furthermore, the [2 + 2] addition pathway involving the Si-H bond that adds across Re=bond requires an activation energy that is similar to 11.1 kcal/mol higher than that of the ionic mechanistic pathway. The strikingly different behavior of the mono-oxo-rhenium(V) complex and di-oxo-rhenium(V) complex - that favors a [2 + 2] addition mechanism - to activate the Si-H bond of silanes is attributed to the absence of an oxo ligand in the mono-oxo-rhenium(V) complex. This decreases the electron density on the rhenium atom, enhances its electrophilicity, thus makes it favorable to prompt the heterolytic cleavage of the Si-H bond upon the nucleophilic attack of carbonyl compounds.
- 出版日期2015-5-1
- 单位南京师范大学
