title: Density functional theory study of the mechanisms of iron-catalyzed cross-coupling reactions of alkyl grignard reagents.
abstract: 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-Fe(MgBr)], addition to yield [Ar-(n-hexyl)-Fe(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-Mg-Br] dissociates to form the middle product [Ar-Fe(MgBr)] than if the attack occurs after the dissociation of [Cl-Mg-Br]. 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-(n-hexyl)-Fe(MgBr)(2)] to regenerate the catalyst [Fe(MgBr)(2)], where the electronic energy barrier ΔE is 29.74 kcal/mol in the gas phase and the Gibb's free energy in solvent THF ΔG(sol) is 28.13 kcal/mol computed using the C-PCM method.
authors.affiliation: Department of Chemistry, Shanghai University, 99 Shangda Road, Shanghai 200444, China. qinghua.ren@shu.edu.cn
authors.forename: Qinghua
authors.initials: Q
authors.lastname: Ren
authors.forename: Shuhui
authors.initials: S
authors.lastname: Guan
authors.forename: Feng
authors.initials: F
authors.lastname: Jiang
authors.forename: Jianhui
authors.initials: J
authors.lastname: Fang