Catalysts for aromatic C-O bond activation can potentially be used for the lignin degradation process. We investigated the mechanisms of C-O bond hydrogenolysis of diphenyl ether (PhOPh) by the nickel N-heterocyclic carbene (Ni-SIPr) complex to produce benzene and phenol as products. Our calculations revealed that diphenyl ether is not only a substrate, but also serves as a ligand to stabilize the Ni-SIPr complex. The Ni(SIPr)(η(6)-PhOPh) complex is initially formed before rearranging to Ni(SIPr)(η(2)-PhOPh), the active species for C-O bond activation. The catalytic reaction has three steps: (i) oxidative addition of Ni(SIPr)(η(2)-PhOPh) to form [Ni(SIPr)(OPh)(Ph)](0), (ii) σ-complex-assisted metathesis, in which H2 binds to the nickel to form [Ni(SIPr)(OPh)(Ph)(H2)](0), and then benzene (or phenol) is eliminated, and (iii) reductive elimination of phenol (or benzene) and the binding of PhOPh to regenerate Ni(SIPr)(η(2)-PhOPh). As the rate determining step is the oxidative addition step (+24 kcal mol(-1)), we also calculated the free energy barriers for the oxidative addition of diaryl ether containing a trifluoromethyl electron withdrawing group (PhOC6H4CF3) and found that C-O bond activation at the carbon adjacent to the aryl ring that contains the electron withdrawing substituent is preferred. This is in agreement with the experimental results, in that the major products are phenol and trifluoromethylbenzene. Moreover, the hydrogenation of benzene via Ni(SIPr)(η(2)-C6H6) requires a high energy barrier (+39 kcal mol(-1)); correspondingly, the hydrogenation products, e.g. cyclohexane and cyclohexadiene, were not observed in the experiment. Understanding the reaction mechanisms of the nickel catalysts for C-O bond hydrogenolysis of diphenyl ether will guide the development of catalytic systems for aromatic C-O bond activation to achieve the highest possible selectivity and efficiency.

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The NH 2 group of 2-guanidinobenzimidazole (GBI) can be replaced by (R C R C )-NHCH(CH 2 ) 4 CHNMe 2 and elaborated to the enantiopure chelate salts (− . These catalyze highly enantioselective additions of 1,3-dicarbonyl compounds to nitroalkenes. The mechanism and basis for enantioselection are probed by DFT calculations. First, the parent GBI complex [(η 5 -C 5 H 5 )Ru(CO)-(GBI)] + PF 6 − (1 + PF 6 − ) is examined. This species has only ruthenium-centered chirality and must be used with a trialkylamine, as it lacks the internal base of 2 + PF 6 − . The dicarbonyl compound initially hydrogen bonds to the NH triad of the GBI ligand, but the transition states leading to each product enantiomer are essentially equal in energy. In contrast, after similar bonding of the dicarbonyl compound to (− , a proton is transferred to the :NMe 2 moiety, giving an enolate and a HNMe 2 + group. The latter mediates the introduction of trans-β-nitrostyrene such that one enolate π face attacks the C si C re Ph face to give an addition product with an R configuration, in agreement with experiment. Thus, the configurations of the catalyst carbon stereocenters control the product stereochemistry. Interactions in competing transition states are analyzed.

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)(η 2 -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)(η 2 -CH 2 O), (iii) H 2 binding to form Ni(SIPr)(H 2 )(η 2 -CH 2 O) prior to H-transfer from H 2 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)(η 2 -PhOMe). The tert-butoxide base could play a role to assist with the formation of Ni(SIPr)(η 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 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 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)(η 2 -CH 2 O) tends to undergo C−H bond activation and α-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 2methoxynaphthalene 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.