Computational study of Cu-free allylic alkylation mechanism with Grignard reagents: the role of NHC ligand.

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.