Density Functional Theory Study of the Mechanisms of Iron-Catalyzed Cross-Coupling Reactions of Alkyl Grignard Reagents
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- 作者：Qinghua Ren ; Shuhui Guan ; Feng Jiang ; Jianhui Fang
- 刊名：The Journal of Physical Chemistry A
- 出版年：2013
- 出版时间：January 31, 2013
- 年：2013
- 卷：117
- 期：4
- 页码：756-764
- 全文大小：489K
- 年卷期：v.117,no.4(January 31, 2013)
- ISSN：1520-5215
文摘
When compared with the established palladium and nickel catalyst systems, simple iron salts turn out to be highly efficient, cheap, toxicologically benign, and environmentally friendly precatalysts for a host of cross-coupling reactions of alkyl or aryl Grignard reagents. The inorganic Grignard reagent [Fe(MgX)2], where X corresponds to Br or I, is a good catalyst for cross-coupling reactions. The present study reports a thorough theoretical analysis of the mechanisms of the [Fe(MgBr)2] catalyzed cross-coupling reaction between 4-chlorobenzoic acid methyl ester and n-hexylicmagnesium bromide using density functional theory (DFT) calculations. Our calculations show that the overall catalytic cycle includes three basic steps: oxidation of [Fe(MgBr)2] to obtain [Ar鈥揊e(MgBr)], addition to yield [Ar鈥?n-hexyl)鈥揊e(MgBr)2], and reductive elimination to return to [Fe(MgBr)2]. The energy barrier is lower if n-hexylicmagnesium bromide attacks the intermediate of the oxidative addition directly before [Cl鈥揗g鈥揃r] dissociates to form the middle product [Ar鈥揊e(MgBr)] than if the attack occurs after the dissociation of [Cl鈥揗g鈥揃r]. The solvation effect in this step clearly leads to a lowering of the energy barrier. The rate-limiting step in the whole catalytic cycle is the reductive elimination of [Ar鈥?n-hexyl)鈥揊e(MgBr)2] to regenerate the catalyst [Fe(MgBr)2], where the electronic energy barrier 螖E is 29.74 kcal/mol in the gas phase and the Gibb鈥檚 free energy in solvent THF 螖Gsol is 28.13 kcal/mol computed using the C-PCM method.