Mechanism of Ni-NHC Catalyzed Hydrogenolysis of Aryl Ethers: Roles of the Excess Base

The transformation of aromatic carbon–oxygen (C_Ar–O) bonds in lignin to useful chemical building blocks has great potential in biomass conversion. A Ni-NHC (N-heterocyclic carbene) catalyzed selective hydrogenolysis of aryl ethers has recently been developed by Hartwig and co-worker, but the reaction mechanism, including the role of different additives found to accelerate the reaction and the origin of the selectivity, remains unclear. DFT calculations of several possible pathways for this useful and important transformation suggest a new mechanistic pathway which involves coordination of the excess base (^<i>t</i>BuO^–) to facilitate the rate-determining C–O activation step, dissociation of the ArO^– ligand, H₂ activation through a Ni–O^<i>t</i>Bu bond to give HO^<i>t</i>Bu, and finally reductive elimination to afford the arene product. Another new ion-pair (S_NAr-like) pathway for the base-assisted C–O bond activation could compete with the above base-assisted oxidative addition pathway for some diaryl ethers. The regioselective cleavage of different ethers and the effects of the Lewis acid were also examined and compared. The results demonstrate that bulkiness and strong donating ability of the NHC ligand and the presence of excess base are the keys to a Ni(0)/Ni­(II) catalytic cycle