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

Abstract

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.

Cross-coupling polycondensation via C–O or C–N bond cleavage

“…Further investigations of the reaction scope (e.g., organoboron, organozinc, and/or other C–O/C–N electrophiles) and synthetic applications of the present method to photoelectrical materials, are in progress. It should be noted that previous mechanistic studies on transition metal-catalyzed C–O/C–N bond cleavage reactions from our group 27 , 35 , 69 and others 35 , 43 , 46 , 47 , 49 have found that the oxidative addition of ethereal C–O bond to Ni generally suffers from high activation barriers. However, it can be greatly facilitated either by formation of the Ni(0)-ate complex with Grignard reagent or by NHC ligand.…”

Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel‐Catalyzed Direct Decarbonylative Borylation

“…[24] In addition, concerted b-hydrogen elimination and reductive hydrogen-vinyl elimination/coupling pathways were found by an umber of computational studies on nickel-catalyzed reactions. [25] Rebound of the open phosphine arm in IM9 converts it into IM10,w hich extrudes CO via the dissociative transition state TS8 to form IM11.S ubstitution of COD by IM11 releases the borylation product PhBcat, regenerates the active species IM1,a nd closes the catalytic cycle.T hus, adetailed plausible mechanism for the title reaction has now been established computationally.I tr eveals ac omplex yet well-defined reaction system consisting of two portions:( a) the activation of aryl carboxylic acids by B 2 cat 2 (Figure 1) and (b) the catalytic cycle beginning with the active species IM1 (Figure 2, Figure 3, and Figure 4). Thebalanced equations for the two portions and the overall reaction are the following:…”

Nickel-Catalyzed Cross-Coupling Reactions of Unreactive Phenolic Electrophiles via C–O Bond Activation

“…7), and C-O cleavage via oxidative addition would be most likely. Surawatanawong [118,119] as well as Chung [120] reported that the classical oxidative addition pathway is also viable for the Ni(0)/ NHC-catalyzed hydrogenolysis of aryl ethers (Fig. 14).…”