Nickel-mediated hydrogenolysis of C-O bonds of aryl ethers: what is the source of the hydrogen?

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

Oriented External Electric-Field Effects on the Activation of Aryl CO Bond in Anisole using Rh(PEP) (E=Al, B, Ga) Catalysts

The present work reports a DFT-based mechanistic investigation of aryl C-O bond activation in anisole catalysed by a Rh-Al pincer-type complex at room temperature. The study is extended to analogues Rh-E complexes based on Group 13 elements (E=B/Ga). Our results show a preference for the heterolytic cleavage pathway over oxidative addition in the C-O bond activation. The calculated barriers range from 16 to 36 kcal/mol, following the order: E=Al Collapse

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Grants

• Planning and Budgeting Committee of the Council for Higher Education of Israel
• USF grant 520/18 Israel Science Foundation

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Affiliation(s)

Nishamol Kuriakose Institute of Chemistry, The Edmond J. Safra Campus, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel Present Address: Hylleraas Centre for Quantum Molecular Sciences, University of Oslo, Oslo, Norway
Sason Shaik Institute of Chemistry, The Edmond J. Safra Campus, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel

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Ni-catalyzed mild hydrogenolysis and oxidations of C-O bonds via carbonate redox tags

Oxygenated molecules are omnipresent in natural as well as artificial settings making the redox transformation of the present C-O bonds a central tool for their processing. However, the required (super)stoichiometric redox agents which traditionally include highly reactive and hazardous reagents pose multiple practical challenges including process safety hazards or special waste management requirements. Here, we report a mild Ni-catalyzed fragmentation strategy based on carbonate redox tags for redox transformations of oxygenated hydrocarbons in the absence of any external redox equivalents or other additives. The purely catalytic process enables the hydrogenolysis of strong C(sp2)-O bonds including that of enol carbonates as well as the catalytic oxidation of C-O bonds under mild conditions down to room temperature. Additionally, we investigated the underlying mechanism and showcased the benefits of carbonate redox tags in multiple applications. More broadly, the work herein demonstrates the potential of redox tags for organic synthesis.

Cobalt-Catalyzed Formation of Grignard Reagents via C-O or C-S Bond Activation

C(aryl)-OMe bond functionalization catalyzed by cobalt(II) chloride in combination with a nacnac-type ligand and magnesium as a reductant is reported. Borylation and benzoylation of aryl methoxides are demonstrated, and C(aryl)-SMe bond borylation can be achieved under similar conditions. This is the first example of achieving these transformations using cobalt catalysis. Mechanistic studies suggest that a Grignard reagent is generated as an intermediate in a rare example of a magnesiation via a C-O bond activation reaction. Indeed, an organomagnesium species could be directly observed by electrospray ionization mass spectroscopic analysis. Kinetic experiments indicate that a heterogeneous cobalt catalyst performs the C-O bond activation.

Mechanisms of the Nickel-Catalysed Hydrogenolysis and Cross-Coupling of Aryl Ethers

The Ni-catalysed hydrogenolysis and cross-coupling of aryl ethers has emerged as a powerful synthetic tool to transform inert phenol-derived electrophiles into functionalised aromatic molecules. This has attracted significant interest due to its potential to convert the lignin fraction of biomass into chemical feedstocks, or to enable orthogonal reactivity and late-stage synthetic modification. Although the scope of nucleophiles employed, and hence the C–C and C–heteroatom bonds that can be forged, has expanded significantly since Wenkert’s seminal work in 1979, mechanistic understanding on how these reactions operate is still uncertain since the comparatively inert Caryl–O bond of aryl ethers challenge the involvement of classical mechanisms involving direct oxidative addition to Ni(0). In this review, we document the different mechanisms that have been proposed in the Ni-catalysed hydrogenolysis and cross-coupling of aryl ethers. These include: (i) direct oxidative addition; (ii) Lewis acid assisted C–O bond cleavage; (iii) anionic nickelates, and; (iv) Ni(I) intermediates. Experimental and theoretical investigations by numerous research groups have generated a pool of knowledge that will undoubtedly facilitate future discoveries in the development of novel Ni-catalysed transformations of aryl ethers.

1 Introduction

2 Direct Oxidative Addition

3 Hydrogenolysis of Aryl Ethers

4 Lewis Acid Assisted C–O Bond Cleavage

5 Anionic Nickelates

6 Ni(I) Intermediates

7 The ‘Naphthalene Problem’

8 Conclusions and Outlook

Skeletal Ni electrode-catalyzed C-O cleavage of diaryl ethers entails direct elimination via benzyne intermediates

Diaryl ethers undergo electrocatalytic hydrogenolysis (ECH) over skeletal Ni cathodes in a mild, aqueous process that achieves direct C-O cleavage without initial benzene ring saturation. Mechanistic studies find that aryl phenyl ethers with a single para or meta functional group (methyl, methoxy, or hydroxy) are selectively cleaved to the substituted benzene and phenol, in contrast to recently reported homogeneous catalytic cleavage processes. Ortho positioning of substituents reverses this C-O bond selectivity, except for the 2-phenoxyphenol case. Together with isotope labeling and co-solvent studies, these results point to two distinct cleavage mechanisms: (a) dual-ring coordination and C-H activation, leading to vicinal elimination to form phenol and a surface-bound aryne intermediate which is then hydrogenated and released as the arene; and (b) surface binding in keto form by the phenolic ring of the hydroxy-substituted substrates, followed by direct displacement of the departing phenol. Notably, acetone inhibits the well-known reduction of phenol to cyclohexanol, affording control of product ring saturation. A byproduct of this work is the discovery that the ECH treatment completely defluorinates substrates bearing aromatic C-F and C-CF₃ groupings.

Biomass conversion holds promise as a more sustainable source of platform chemicals, but limitations in the ways in which lignin can be broken down is a current bottleneck. Here the authors report an electrocatalytic hydrogenolysis over skeletal Ni that cleaves diaryl ethers, chemically resistant moieties in both renewable carbon sources and persistent organic pollutants.

The Anionic Pathway in the Nickel-Catalysed Cross-Coupling of Aryl Ethers

The Ni‐catalysed cross‐coupling of aryl ethers is a powerful method to forge new C−C and C−heteroatom bonds. However, the inert C(sp²)−O bond means that a canonical mechanism that relies on the oxidative addition of the aryl ether to a Ni⁰ centre is thermodynamically and kinetically unfavourable, which suggests that alternative mechanisms may be involved. Here, we provide spectroscopic and structural insights into the anionic pathway, which relies on the formation of electron‐rich hetero‐bimetallic nickelates by adding organometallic nucleophiles to a Ni⁰ centre. Assessing the rich co‐complexation chemistry between Ni(COD)₂ and PhLi has led to the structures and solution‐state chemistry of a diverse family of catalytically competent lithium nickelates being unveiled. In addition, we demonstrate dramatic solvent and donor effects, which suggest that the cooperative activation of the aryl ether substrate by Ni⁰‐ate complexes plays a key role in the catalytic cycle.

Nickel-Mediated Alkoxycarbonylation for Complete Carbon Isotope Replacement

Many commercial drugs, as well as upcoming pharmaceutically active compounds in the pipeline, display aliphatic carboxylic acids or derivatives thereof as key structural entities. Synthetic methods for rapidly accessing isotopologues of such compounds are highly relevant for undertaking critical pharmacological studies. In this paper, we disclose a direct synthetic route allowing for full carbon isotope replacement via a nickel-mediated alkoxycarbonylation. Employing a nickel^II pincer complex ([(N₂N)Ni-Cl]) in combination with carbon-13 labeled CO, alkyl iodide, sodium methoxide, photocatalyst, and blue LED light, it was possible to generate the corresponding isotopically labeled aliphatic carboxylates in good yields. Furthermore, the developed methodology was applied to the carbon isotope substitution of several pharmaceutically active compounds, whereby complete carbon-13 labeling was successfully accomplished. It was initially proposed that the carboxylation step would proceed via the in situ formation of a nickellacarboxylate, generated by CO insertion into the Ni-alkoxide bond. However, preliminary mechanistic investigations suggest an alternative pathway involving attack of an open shell species generated from the alkyl halide to a metal ligated CO to generate an acyl Ni^III species. Subsequent reductive elimination involving the alkoxide eventually leads to carboxylate formation. An excess of the alkoxide was essential for obtaining a high yield of the product. In general, the presented methodology provides a simple and convenient setup for the synthesis and carbon isotope labeling of aliphatic carboxylates, while providing new insights about the reactivity of the N₂N nickel pincer complex applied.

Ni(0)-promoted activation of Csp2 -H and Csp2 -O bonds

A dinickel(0)-N2 complex, stabilized with a rigid acridane-based PNP pincer ligand, was studied for its ability to activate C(sp2)-H and C(sp2)-O bonds. Stabilized by a Ni-μ-N2-Na+ interaction, it activates C-H bonds of unfunctionalized arenes, affording nickel-aryl and nickel-hydride products. Concomitantly, two sodium cations get reduced to Na(0), which was identified and quantified by several methods. Our experimental results, including product analysis and kinetic measurements, strongly suggest that this C(sp2)-H activation does not follow the typical oxidative addition mechanism occurring at a low-valent single metal centre. Instead, via a bimolecular pathway, two powerfully reducing nickel ions cooperatively activate an arene C-H bond and concomitantly reduce two Lewis acidic alkali metals under ambient conditions. As a novel synthetic protocol, nickel(ii)-aryl species were directly synthesized from nickel(ii) precursors in benzene or toluene with excess Na under ambient conditions. Furthermore, when the dinickel(0)-N2 complex is accessed via reduction of the nickel(ii)-phenyl species, the resulting phenyl anion deprotonates a C-H bond of glyme or 15-crown-5 leading to C-O bond cleavage, which produces vinyl ether. The dinickel(0)-N2 species then cleaves the C(sp2)-O bond of vinyl ether to produce a nickel(ii)-vinyl complex. These results may provide a new strategy for the activation of C-H and C-O bonds mediated by a low valent nickel ion supported by a structurally rigidified ligand scaffold.

Selective C-O Bond Reduction and Borylation of Aryl Ethers Catalyzed by a Rhodium-Aluminum Heterobimetallic Complex

We report the catalytic reduction of a C-O bond and the borylation by a rhodium complex bearing an X-type PAlP pincer ligand. We have revealed the reaction mechanism based on the characterization of the reaction intermediate and deuterium-labeling experiments. Notably, this novel catalytic system shows steric-hindrance-dependent chemoselectivity that is distinct from conventional Ni-based catalysts and suggests a new strategy for selective C-O bond activation by heterobimetallic catalysis.

Mechanism and Selectivity Control in Ni- and Pd-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bond Activation

Transition-metal-catalyzed C-O bond activation provides a useful strategy for utilizing alcohol- and phenol-derived electrophiles in cross-coupling reactions, which has become a research field of active and growing interest in organic chemistry. The synergy between computation and experiment elucidated the mechanistic model and controlling factors of selectivities in these transformations, leading to advances in innovative C-O bond activation and functionalization methods.Toward the rational design of C-O bond activation, our collaborations with the Jarvo group bridged the mechanistic models of C(sp²)-O and C(sp³)-O bond activations. We found that the nickel catalyst cleaves the benzylic and allylic C(sp³)-O bonds via two general mechanisms: the stereoinvertive S_N2 back-side attack model and the stereoretentive chelation-assisted model. These two models control the stereochemistry in a wide array of stereospecific Ni-catalyzed cross-coupling reactions with benzylic or allylic alcohol derivatives. Because of the catalyst distortion, the ligands can differentiate the competing stereospecific C(sp³)-O bond activations. The PCy₃ ligand interacts with nickel mainly through σ-donation, and the Ni(PCy₃) catalyst can undergo facile bending of the substrate-nickel-ligand angle, which favors the stereoretentive benzylic C-O bond activation. The N-heterocyclic carbene SIMes ligand has additional d(metal)-p(ligand) back-donation with nickel, which leads to an extra energy penalty for the same angle bending. This results in the preference of stereoinvertive benzylic C-O bond activation under Ni/SIMes catalysis. In addition to ligand control, a Lewis acid can increase the selectivity for stereoinvertive C(sp³)-O activation by stabilizing the S_N2 back-side attack transition state. The oxygen leaving group complexes with the MgI₂ Lewis acid in the stereoinvertive activation, leading to the exclusive stereoinvertive Kumada coupling of benzylic ethers. We also identified that the competing C(sp³)-O bond activation models have noticeable differences in charge separation. This leads to the solvent polarity control of the stereospecificity in C(sp³)-O activations. Low-polarity solvents favor the neutral stereoretentive C-O bond activation, while high-polarity solvents favor the zwitterionic stereoinvertive cleavage.In sharp contrast to the nickel catalysts, the C(sp²)-O bond activation under palladium catalysis mainly proceeds via the classic three-membered ring oxidative addition mechanism instead of the chelation-assisted mechanism. This is due to the lower oxophilicity of palladium, which disfavors the oxygen coordination in the chelation-assisted-type activation. The three-membered ring activation model selectively cleaves the weak C-O bond, resulting in the exclusive chemoselectivity of acyl C-O bond activation in Pd-catalyzed cross-coupling reactions with aryl carboxylic acid derivatives. This explains the overall acylation in the Pd-catalyzed Suzuki-Miyaura coupling with aryl esters. In collaboration with the Szostak group, we revealed that the three-membered ring model applies in the Pd-catalyzed C-O bond activation of carboxylic acid anhydride, which stimulated the development of a series of Pd-catalyzed decarbonylative functionalizations of aryl carboxylic acids.

Zero-valent ML2 complexes of group 10 metals supported by terphenyl phosphanes

Bulky terphenyl phosphane ligands PMe₂Ar' (Ar' = terphenyl group) facilitate the isolation of zero-valent bis-phosphane complexes of nickel, palladium and platinum. The former show coordination numbers greater than two in the solid state due to the existence of Ni-C_arene interactions with the terphenyl fragment.

Hydrogenolysis of Polysilanes Catalyzed by Low-Valent Nickel Complexes

The dehydrogenation of organosilanes (R_x SiH_4-x ) under the formation of Si-Si bonds is an intensively investigated process leading to oligo- or polysilanes. The reverse reaction is little studied. To date, the hydrogenolysis of Si-Si bonds requires very harsh conditions and is very unselective, leading to multiple side products. Herein, we describe a new catalytic hydrogenation of oligo- and polysilanes that is highly selective and proceeds under mild conditions. New low-valent nickel hydride complexes are used as catalysts and secondary silanes, RR'SiH₂ , are obtained as products in high purity.

Exhaustive Reduction of Esters Enabled by Nickel Catalysis

We report a one-step procedure to directly reduce unactivated aryl esters into their corresponding tolyl derivatives. This is achieved by an organosilane-mediated ester hydrosilylation reaction and subsequent Ni/NHC-catalyzed hydrogenolysis. The resulting conditions provide a direct and efficient alternative to multi-step procedures for this transformation that often require the use of hazardous metal hydrides. Applications in the synthesis of -CD₃-containing products, derivatization of bioactive molecules, and chemoselective reduction in the presence of other C-O bonds are demonstrated.

The Critical Role of Reductive Steps in the Nickel-Catalyzed Hydrogenolysis and Hydrolysis of Aryl Ether C-O Bonds

The hydrogenolysis of the aromatic C-O bond in aryl ethers catalyzed by Ni was studied in decalin and water. Observations of a significant kinetic isotope effect (kH /kD =5.7) for the reactions of diphenyl ether under H2 and D2 atmosphere and a positive dependence of the rate on H2 chemical potential in decalin indicate that addition of H to the aromatic ring is involved in the rate-limiting step. All kinetic evidence points to the fact that H addition occurs concerted with C-O bond scission. DFT calculations also suggest a route consistent with these observations involving hydrogen atom addition to the ipso position of the phenyl ring concerted with C-O scission. Hydrogenolysis initiated by H addition in water is more selective (ca. 75 %) than reactions in decalin (ca. 30 %).

Nickel-catalyzed reductive defunctionalization of esters in the absence of an external reductant: activation of C–O bonds

The nickel-catalyzed reductive cleavage of esters in the absence of an external reductant, which involves the cleavage of an inert acyl C–O bond in O-alkyl esters is reported.

Dismantling of Vinyl Ethers by Pentanuclear [(iPr3 P)Ni]5 H6 : Facile Cooperative C-O, C-C and C-H Activation Pathways

The [(iPr₃ P)Ni]₅ H₆ cluster (1) and H₂ C=CHOtBu react at room temperature to form the new pentanuclear NiH carbide [(iPr₃ P)Ni]₅ H₄ (C)(CO) (3), along with an equivalent of isobutylene. This transformation requires the activation of multiple unreactive bonds, including C-H, C-C, and C(sp³ )-O bond cleavage. Analysis of the reaction mixture by ¹ H NMR revealed the production of two additional paramagnetic species, assigned as [(iPr₃ P)Ni]₄ H₄ (C-CH₃ )NiOtBu (4 a) and [(iPr₃ P)Ni]₄ H₄ (C-CH₂ OtBu)NiOtBu (5 a), which arise from C(sp² )-O bond cleavage and CH bond rearrangements. The reaction of 1 with H₂ C=CHOSiMe₂ CH₂ Ph produced the isolable 4 a analogue [(iPr₃ P)₄ Ni₅ ]H₄ (CCH₃ )(OSiMe₂ CH₂ Ph) (4 c). An isolable analogue of 5 a was obtained from the reaction of 1 with H₂ C=CHOAd (Ad=1-admantyl), which produced [(iPr₃ P)₄ Ni₅ ]H₄ (CCH₂ OAd)(OAd) (5 d). The utilization of both cluster faces and vertices for bonding substrate fragments in these transformations demonstrates the remarkable flexibility of the robust Ni₅ H₄ core in the cooperative activation of multiple C-O, C-C and C-H bonds.

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

π-Conjugated polymers are widely used in optoelectronics for fabrication of organic photovoltaic devices, organic light-emitting diodes, organic field effect transistors, and so on. Here we describe the protocol for polycondensation of bifunctional aryl ethers or aryl ammonium salts with aromatic dimetallic compounds through cleavage of inert C-O/C-N bonds. This reaction proceeds smoothly in the presence of commercially available Ni/Pd catalyst under mild conditions, affording the corresponding π-conjugated polymers with high molecular weight. The method is applicable to monomers that are unreactive in other currently employed polymerization procedures, and opens up the possibility of transforming a range of naturally abundant chemicals into useful functional compounds/polymers.

Hydride & dihydrogen complexes of earth abundant metals: structure, reactivity, and applications to catalysis

Recent developments in the chemistry of hydride and dihydrogen complexes of iron, cobalt, and nickel are summarized. Applications in homogeneous catalysis are emphasized.

Mild sp2Carbon-Oxygen Bond Activation by an Isolable Ruthenium(II) Bis(dinitrogen) Complex: Experiment and Theory

The isolable ruthenium(II) bis(dinitrogen) complex [Ru(H)₂(N₂)₂(PCy₃)₂] (1) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp²C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru(II)/Ru(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp²C–H bond activation is shown to be more facile than sp²C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway.

Ethereal C-O Bond Cleavage Mediated by Ni(0)-Ate Complex: A DFT Study

Density functional theory calculations were performed to explore the mechanism of Ni-catalyzed cross-coupling reactions involving organo-lithium and -zinc reagents through ethereal C-O bond cleavage. Based on this work, together with our previous mechanistic study on etheric Kumada-Tamao reaction, we identify and characterize a novel catalytic cycle for cross-coupling mediated by Ni(0)-ate complex.

Transition-metal catalyzed valorization of lignin: the key to a sustainable carbon-neutral future

The development of a sustainable, carbon-neutral biorefinery has emerged as a prominent scientific and engineering goal of the 21st century. As petroleum has become less accessible, biomass-based carbon sources have been investigated for utility in fuel production and commodity chemical manufacturing. One underutilized biomaterial is lignin; however, its highly crosslinked and randomly polymerized composition have rendered this biopolymer recalcitrant to existing chemical processing. More recently, insight into lignin's molecular structure has reinvigorated chemists to develop catalytic methods for lignin depolymerization. This review examines the development of transition-metal catalyzed reactions and the insights shared between the homogeneous and heterogeneous catalytic systems towards the ultimate goal of valorizing lignin to produce value-added products.

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

Nickel-catalyzed cross-coupling reactions of aryl esters, carbamates, carbonates, ethers and arenols are reviewed. Carbon-oxygen bonds in these phenol derivatives cannot be activated by palladium, a typical cross-coupling catalyst, but a low valent nickel species in conjunction with a strong σ-donor ligand is uniquely effective for achieving this. The review is organized primarily by substrate class and secondarily by coupling partners, encompassing organometallics, heteroatom nucleophiles, C-H bonds and many others. Although the reactions in this category are covered thoroughly, each reaction is described only briefly, so that it is possible to quickly overview the spectrum of nickel-catalyzed cross-coupling reactions of inert phenol derivatives. The robustness of inert phenol derivatives under typically used catalytic conditions as well as their utility as a directing group allow unique synthetic applications of these new C-O cross-coupling reactions, which is also included in cases where appropriate. Mechanistic aspects of C-O bond activation by nickel are also summarized, highlighting their diversity compared with the C-X bond activation involved in conventional cross-coupling processes.

Metal catalyzed defunctionalization reactions

The chronological development of metal assisted defunctionalization reactions is discussed from the stoichiometric to the catalytic stage with their application in synthetic organic chemistry.