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

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journal contribution
posted on 19.01.2016, 00:00 by Taveechai Wititsuwannakul, Yuthana Tantirungrotechai, Panida Surawatanawong
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 H2, the catalytic cycle involves the following: (i) aromatic C–O bond oxidative addition of Ni­(SIPr)­(η2-PhOMe) to form Ni­(SIPr)­(OMe)­(Ph), (ii) β-H transfer from the methoxy to phenyl group in Ni­(SIPr)­(OMe)­(Ph) via σ-complex-assisted metathesis (σ-CAM), which eliminates benzene and forms Ni­(SIPr)­(η2-CH2O), (iii) H2 binding to form Ni­(SIPr)­(H2)­(η2-CH2O) prior to H-transfer from H2 to the formaldehyde carbon via σ-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)­(η2-PhOMe). The tert-butoxide base could play a role to assist with the formation of Ni­(SIPr)­(η2-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 β-H transfer from the methoxy to phenyl group on Ni-SIPr is not a stepwise β-H elimination as generally perceived, but rather a concerted process that occurs via σ-CAM. This leads to benzene elimination before H2 binding, in accordance with the results of the isotope labeling experiment of C–O bond hydrogenolysis of 2-methoxynaphthalene. In the absence of H2, Ni­(SIPr)­(η2-CH2O) tends to undergo C–H bond activation and α-H elimination to release H2 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 H2. Thus, H2 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.