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

Hydrogen-transfer strategy in lignin refinery: Towards sustainable and versatile value-added biochemicals

Lignin, the most prevalent natural source of polyphenols on Earth, offers substantial possibilities for the conversion into aromatic compounds, which is critical for attaining sustainability and carbon neutrality. The hydrogen-transfer method has garnered significant interest owing to its environmental compatibility and economic viability. The efficacy of this approach is contingent upon the careful selection of catalytic and hydrogen-donating systems that decisively affect the yield and selectivity of the monomeric products resulting from lignin degradation. This paper highlights the hydrogen-transfer technique in lignin refinery, with a specific focus on the influence of hydrogen donors on the depolymerization pathways of lignin. It delineates the correlation between the structure and activity of catalytic hydrogen-transfer arrangements and the gamut of lignin-derived biochemicals, utilizing data from lignin model compounds, separated lignin, and lignocellulosic biomass. Additionally, the paper delves into the advantages and future directions of employing the hydrogen-transfer approach for lignin conversion. In essence, this concept investigation illuminates the efficacy of the hydrogen-transfer paradigm in lignin valorization, offering key insights and strategic directives to maximize lignin's value sustainably.

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

The Ni-catalyzed decarbonylative borylation of (hetero)aryl carboxylic acids with B₂ cat₂ has been achieved without recourse to any additives. This Ni-catalyzed method exhibits a broad substrate scope covering poorly reactive non-ortho-substituted (hetero)aryl carboxylic acids, and tolerates diverse functional groups including some of the groups active to Ni⁰ catalysts. The key to achieve this decarbonylative borylation reaction is the choice of B₂ cat₂ as a coupling partner that not only acts as a borylating reagent, but also chemoselectively activates aryl carboxylic acids towards oxidative addition of their C(acyl)-O bond to Ni⁰ catalyst via the formation of acyloxyboron compounds. A combination of experimental and computational studies reveals a detailed plausible mechanism for this reaction system, which involves a hitherto unknown concerted decarbonylation and reductive elimination step that generates the aryl boronic ester product. This mode of boron-promoted carboxylic acid activation is also applicable to other types of reactions.

Nickel-Catalyzed Ipso-Borylation of Silyloxyarenes via C-O Bond Activation

The conversion of silyloxyarenes to boronic acid pinacol esters via nickel catalysis is described. In contrast to other borylation protocols of inert C-O bonds, the method is competent in activating the carbon-oxygen bond of silyloxyarenes in isolated aromatic systems lacking a directing group. The catalytic functionalization of benzyl silyl ethers was also achieved under these conditions. Sequential cross-coupling reactions were achieved by leveraging the orthogonal reactivity of silyloxyarenes, which could then be functionalized subsequently.

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.

Aryl C–O oxidative addition of phenol derivatives to nickel supported by an N-heterocyclic carbene via a Ni0 five-centered complex

An aryl ether with the assistance of organoaluminum, an aryl sulfonate/sulfamate and an ester/carbamate proceeds towards C–O bond cleavage via a Ni⁰ five-centered complex.

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.

Mechanistic insight into Ni-mediated decarbonylation of unstrained ketones: the origin of decarbonylation catalytic activity

The origin of the Ni-mediated decarbonylation catalytic cycle of unstrained ketones was explored using the DFT calculation method.

Less hindered ligands give improved catalysts for the nickel catalysed Grignard cross-coupling of aromatic ethers

The challenging reaction of unactivated ortho-substituted aromatic ethers with Grignard reagents has been found to be most effectively catalysed using nickel complexes of less sterically hindered ligands.

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.