Computational Study of the Cu‐Free Allylic Alkylation Mechanism with Grignard Reagents: Role of the NHC Ligand

Abstract

The mechanism of Cu‐free allylic alkylation reactions catalyzed by an NHC ligand with Grignard reagents has been explored by using the hybrid B3PW91 DFT method. The nature of the bonding of the active Mg species, which was validated by reference to the 13C NMR chemical shift, was found to be highly ionic. In particular, a strong nucleophilic Mg–R (R = CH3) bond was explained by the activation of the NHC Lewis base ligand on Mg. The effect of ethereal solvent was examined by using explicit dimethyl ether solvent and a density‐based solvation model. An adequate representation of solvation is required to properly reproduce the formation of the supposed active species. The mechanism of the allylic alkylation emphasizes the bifunctional role of the Mg center that activates both the nucleophile and nucleofuge promoting an SN2‐type mechanism. The NHC ligand bonded to Mg is crucial and activates the Mg–R bond favoring C–C bond formation.

Transition-Metal-Free Enantioselective Reactions of Organo­magnesium Reagents Mediated by Chiral Ligands

“…The latter reacts with the allyl bromide substrate via a six-membered pseudochair transition state to yield the enantioenriched S N 2′ product and regenerate 49. This model, which was recently supported by DFT calculations, 56 differs from Hoveyda's proposition (vide supra) as the NHC-bound organomagnesium reagent directly alkylates the alkene. It is assumed that the association of the strong σ-donor NHC ligand with the Lewis acidic magnesium atom increases the nucleophilicity of the alkyl group of the Grignard reagent.…”

Theoretical Investigation on Ni-Catalyzed C(sp3)–F Activation and Ring Contraction of Tetrahydropyrans: Exploration of an SN2 Pathway

“…It is noteworthy that the solution of the Grignard reagent is in fact a complicated system (taking the Schlenk equilibrium as an example), in which a variety of organomagnesium species coexist. For the modeling of RMgX in computational studies, several forms of the Grignard reagent were applied, including both the monomeric and the dimeric forms . Meanwhile, it is also suggested to consider the explicit solvation effect on the Mg atom.…”