Density Functional Study of Nickel N-Heterocyclic Carbene Catalyzed C-O Bond Hydrogenolysis of Methyl Phenyl Ether: The Concerted β-H Transfer Mechanism

The catalytic C-O bond activation of aryl ethers attracts substantial interest as it is significant for the lignin degradation process. A nickel complex with N-heterocyclic carbene (Ni-SIPr) has been shown to selectively catalyze C-O bond hydrogenolysis of aryl methyl ether to obtain arene and alcohol as the only products. Here, the reaction mechanism of Ni-SIPr catalyzed C-O bond hydrogenolysis of methyl phenyl ether (PhOMe) was studied using density functional theory. In the presence of H-2, the catalytic cycle involves the following: (i) aromatic C-O bond oxidative addition of Ni(SIPr)(eta(2)-PhOMe) to form Ni(SIPr)-(OMe)(Ph), (ii) beta-H transfer from the methoxy to phenyl group in Ni(SIPr)(OMe)(Ph) via sigma-complex-assisted metathesis (sigma-CAM), which eliminates benzene and forms Ni(SIPr)(eta(2)-CH2O), (iii) H-2 binding to form Ni(SIPr)(H-2)(eta(2)-CH2O) prior to H-transfer from H-2 to the formaldehyde carbon via sigma-CAM to generate Ni(SIPr)(H)(OMe), and (iv) reductive elimination to form methanol and the binding of methyl phenyl ether to regenerate Ni(SIPr)(eta(2)-PhOMe). The tert-butoxide base could play a role to assist with the formation of Ni(SIPr)(eta(2)-PhOMe), the catalytically active species, and could bind to Ni(SIPr)(H)(OMe) before reductive elimination. A similar mechanism was found for the C-O bond hydrogenolysis of 2-methoxynaphthalene. Our study showed that the C-O bond oxidative addition is the rate-determining step and that the aromatic C-O bond cleavage to form Ni(SIPr)(OMe)(Ph) is more favorable than the aliphatic C-O bond cleavage to form Ni(SIPr)(OPh)(Me), consistent with the arene and alcohol products obtained from the experiment. Notably, the beta-H transfer from the methoxy to phenyl group on Ni-SIPr is not a stepwise beta-H elimination as generally perceived, but rather a concerted process that occurs via sigma-CAM. This leads to benzene elimination before H-2 binding, in accordance with the results of the isotope labeling experiment of C-O bond hydrogenolysis of 2-methoxynaphthalene. In the absence of H-2, Ni(SIPr)(eta(2)-CH2O) tends to undergo C-H bond activation and alpha-H elimination to release H-2 and generate a nickel carbonyl complex, the catalytically inactive species. This was reflected by experimental results which demonstrated low conversion of 2-methoxynaphthalene in the absence of H-2. Thus, H-2 is crucial to the catalytic reaction through its role in suppressing the formation of the inactive nickel carbonyl species. Insights into the mechanisms of Ni-SIPr catalyzed conversion of methyl phenyl ether should benefit the development of catalysts for C-O bond activation.