1,3-Dicyclohexylimidazol-2-ylidene as a superior ligand for the nickel-catalyzed cross-couplings of aryl and benzyl methyl ethers with organoboron reagents

Substrate switchable Suzuki–Miyaura coupling for benzyl ester vs. benzyl halide

Two reaction conditions were developed to accomplish the substrate switchable (benzyl esters vs. benzyl halides) Suzuki–Miyaura coupling.

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.

Selective Synthesis of (Benzyl)biphenyls by Successive Suzuki-Miyaura Coupling of Phenylboronic Acids with 4-Bromobenzyl Acetate under Air Atmosphere

An efficient Pd-catalyzed cross-coupling reaction of phenylboronic acids and benzyl carbonates was developed, producing diarylmethanes. Benzyl acetates could also be used as coupling partners instead of benzyl carbonates, affording diarylmethanes in comparable yields. This reaction can be conducted under air atmosphere without any care for moisture and oxygen. The ester function showed an intermediate reactivity between chloro and bromo groups. This property facilitated the selective synthesis of diverse (benzyl)biphenyls by successive Suzuki-Miyaura coupling reactions using bromo- and chloro-substituted benzyl esters with two types of boronic acids.

Mechanism and Origins of Ligand-Controlled Stereoselectivity of Ni-Catalyzed Suzuki-Miyaura Coupling with Benzylic Esters: A Computational Study

Nickel catalysts have shown unique ligand control of stereoselectivity in the Suzuki-Miyaura cross-coupling of boronates with benzylic pivalates and derivatives involving C(sp3)-O cleavage. The SIMes ligand (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) produces the stereochemically inverted C-C coupling product, while the tricyclohexylphosphine (PCy3) ligand delivers the retained stereochemistry. We have explored the mechanism and origins of the ligand-controlled stereoselectivity with density functional theory (DFT) calculations. The oxidative addition determines the stereoselectivity with two competing transition states, an SN2 back-side attack type transition state that inverts the benzylic stereogenic center and a concerted oxidative addition through a cyclic transition state, which provides stereoretention. The key difference between the two transition states is the substrate-nickel-ligand angle distortion; the ligand controls the selectivity by differentiating the ease of this angle distortion. For the PCy3 ligand, the nickel-ligand interaction involves mainly σ-donation, which does not require a significant energy penalty for the angle distortion. The facile angle distortion with PCy3 ligand allows the favorable cyclic oxidative addition transition state, leading to the stereoretention. For the SIMes ligand, the extra d-p back-donation from nickel to the coordinating carbene increases the rigidity of the nickel-ligand bond, and the corresponding angle distortion is more difficult. This makes the concerted cyclic oxidative addition unfavorable with SIMes ligand, and the back-side SN2-type oxidative addition delivers the stereoinversion.

Parameterization of phosphine ligands demonstrates enhancement of nickel catalysis via remote steric effects

The field of Ni-catalysed cross-coupling has seen rapid recent growth because of the low cost of Ni, its earth abundance, and its ability to promote unique cross-coupling reactions. Whereas advances in the related field of Pd-catalysed cross-coupling have been driven by ligand design, the development of ligands specifically for Ni has received minimal attention. Here, we disclose a class of phosphines that enable the Ni-catalysed Csp³ Suzuki coupling of acetals with boronic acids to generate benzylic ethers, a reaction that failed with known ligands for Ni and designer phosphines for Pd. Using parameters to quantify phosphine steric and electronic properties together with regression statistical analysis, we identify a model for ligand success. The study suggests that effective phosphines feature remote steric hindrance, a concept that could guide future ligand design tailored to Ni. Our analysis also reveals that two classic descriptors for ligand steric environment-cone angle and % buried volume-are not equivalent, despite their treatment in the literature.

A Mild and Ligand-Free Ni-Catalyzed Silylation via C-OMe Cleavage

Metal-catalyzed transformations that forge carbon-heteroatom bonds are of central importance in organic synthesis. Despite the formidable potential of aryl methyl ethers as coupling partners, the scarcity of metal-catalyzed C-heteroatom bond formations via C-OMe cleavage is striking, with isolated precedents requiring specialized, yet expensive, ligands, high temperatures, and π-extended backbones. We report an unprecedented catalytic ipso-silylation of aryl methyl ethers under mild conditions and without recourse to external ligands. The method is distinguished by its wide scope, which includes the use of benzyl methyl ethers, vinyl methyl ethers, and unbiased anisole derivatives, thus representing a significant step forward for designing new C-heteroatom bond formations via C-OMe scission. Applications of this transformation in orthogonal silylation techniques as well as in further derivatizations are also described. Preliminary mechanistic experiments suggest the intermediacy of Ni(0)-ate complexes, leaving some doubt that a canonical catalytic cycle consisting of an initial oxidative addition of the C-OMe bond to Ni(0) species comes into play.

Phosphine-catalyzed Friedel–Crafts reaction of naphthols with para-quinone methides: expedient access to triarylmethanes

A novel phosphine-catalyzed Friedel–Crafts reaction of naphthols with para-quinone methides has been developed.

Alcohols as Latent Coupling Fragments for Metallaphotoredox Catalysis: sp3-sp2 Cross-Coupling of Oxalates with Aryl Halides

Alkyl oxalates, prepared from their corresponding alcohols, are engaged for the first time as carbon radical fragments in metallaphotoredox catalysis. In this report, we demonstrate that alcohols, native organic functional groups, can be readily activated with simple oxalyl chloride to become radical precursors in a net redox-neutral Csp3-Csp2 cross-coupling with a broad range of aryl halides. This alcohol-activation coupling is successfully applied to the functionalization of a naturally occurring steroid and the expedient synthesis of a medicinally relevant drug lead.

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.

Iridium/N-heterocyclic carbene-catalyzed C-H borylation of arenes by diisopropylaminoborane

Catalytic C-H borylation of arenes has been widely used in organic synthesis because it allows the introduction of a versatile boron functionality directly onto simple, unfunctionalized arenes. We report herein the use of diisopropylaminoborane as a boron source in C-H borylation of arenes. An iridium(I) complex with 1,3-dicyclohexylimidazol-2-ylidene is found to efficiently catalyze the borylation of arenes and heteroarenes. The resulting aminoborylated products can be converted to the corresponding boronic acid derivatives simply by treatment with suitable diols or diamines.

Nickel catalyzed reduction of arenols under mild conditions

The mild Ni-catalyzed reduction of arenols features a broad substrate scope, non-sensitivity to steric hindrance and no over-reduction of the aryl ring.

Half-sandwich nickel(II) complexes bearing 1,3-di(cycloalkyl)imidazol-2-ylidene ligands

Two new nickel catalysts have been prepared using a convenient procedure where nickelocene, the NHC·HBF₄ salts, and [Et₄N]Cl were heated in THF using microwave irradiation. The resulting [NiCl(Cp)(NHC)] complexes are air- and moisture stable in the solid state, and represent two new members of this valuable and practical class of nickel catalysts. The new species were fully characterised using methods including NMR spectroscopy and X-ray crystallography. When tested in model Suzuki–Miyaura cross-coupling reactions, these complexes were found to be active for the cross-coupling of aryl bromides and aryl chlorides.

Stereospecific nickel-catalyzed cross-coupling reactions of benzylic ethers and esters

This Account presents the development of a suite of stereospecific alkyl-alkyl cross-coupling reactions employing nickel catalysts. Our reactions complement related nickel-catalyzed stereoconvergent cross-coupling reactions from a stereochemical and mechanistic perspective. Most reactions of alkyl electrophiles with low-valent nickel complexes proceed through alkyl radicals and thus are stereoablative; the correct enantioselective catalyst can favor the formation of one enantiomer. Our reactions, in contrast, are stereospecific. Enantioenriched ethers and esters are cleanly converted to cross-coupled products with high stereochemical fidelity. While mechanistic details are still to be refined, our results are consistent with a polar, two-electron oxidative addition that avoids the formation of radical intermediates. This reactivity is unusual for a first-row transition metal. The cross-coupling reactions engage a range of benzylic ethers and esters, including methyl ethers, tetrahydropyrans, tetrahydrofurans, esters, and lactones. Coordination of the arene substituent to the nickel catalyst accelerates the reactions. Arenes with low aromatic stabilization energies, such as naphthalene, benzothiophene, and furan, serve as the best ligands and provide the highest reactivity. Traceless directing groups that accelerate reactions of sluggish substrates are described, providing partial compensation for arene coordination. Kumada, Negishi, and Suzuki reactions provide incorporation of a broad range of transmetalating agents. In Kumada coupling reactions, a full complement of Grigard reagents, including methyl, n-alkyl, and aryl Grignard reagents, are employed. In reactions employing methylmagnesium iodide, ligation of the nickel catalyst by rac-BINAP or DPEphos provides the highest yield and stereospecificity. For all other Grignard reagents, Ni(dppe)Cl2 has emerged as the best catalyst. Negishi cross-coupling reactions employing dimethylzinc are reported as a strategy to increase the functional group tolerance of the reaction. We also describe Suzuki reactions using arylboronic esters. These reactions provided the first example in the series of a switch in stereochemical outcome. The reactions maintain stereospecificity, but reactions employing different achiral ligands provide opposite enantiomers of the product. Use of an N-heterocyclic carbene ligand, SIMes, provides inversion, consistent with our prior work in Kumada and Negishi coupling reactions. Use of the electron-rich phosphine PCy3, however, provides retention with stereospecificity, signaling a change in the mechanistic details. Potential applications of the reported cross-coupling reactions include the synthesis of medicinal agents containing the 2-arylalkane and 1,1-diarylalkane moieties, which are pharmacophores in medicinal chemistry. These moieties are found in compounds with activity against a broad range of indications, including cancer, heart disease, diabetes, osteoporosis, smallpox, tuberculosis, and insomnia. We highlight representative examples of bioactive compounds that we have prepared with high enantioselectivity employing our methods, as well as the discovery of a new anti-cancer agent.

Directed Amination of Aryl Methyl Ethers Mediated by Ti(NMe2)4 at Room Temperature

An efficient C-O amination of aryl methyl ethers has been achieved. This transformation proceeds via imine-directed Ti(IV)-mediated cross-coupling reactions between aryl methyl ethers and Ti(NR2)4 at room temperature, straightforwardly leading to a series of arylamines. This protocol features a wide substrate scope, exclusive regioselectivity, and mild reaction conditions.

Cross-Couplings Using Aryl Ethers via C-O Bond Activation Enabled by Nickel Catalysts

Arene synthesis has been revolutionized by the invention of catalytic cross-coupling reactions, wherein aryl halides can be coupled with organometallic and organic nucleophiles. Although the replacement of aryl halides with phenol derivatives would lead to more economical and ecological methods, success has been primarily limited to activated phenol derivatives such as triflates. Aryl ethers arguably represent one of the most ideal substrates in terms of availability, cost, safety, and atom efficiency. However, the robust nature of the C(aryl)-O bonds of aryl ethers renders it extremely difficult to use them in catalytic reactions among the phenol derivatives. In 1979, Wenkert reported a seminal work on the nickel-catalyzed cross-coupling of aryl ethers with Grignard reagents. However, it was not until 2004 that the unique ability of a low-valent nickel species to activate otherwise unreactive C(aryl)-O bonds was appreciated with Dankwardt's identification of the Ni(0)/PCy3 system, which significantly expanded the efficiency of the Wenkert reaction. Application of the nickel catalyst to cross-couplings with other nucleophiles was first accomplished in 2008 by our group using organoboron reagents. Later on, several other nucleophiles, including organozinc reagents, amines, hydrosilane, and hydrogen were shown to be coupled with aryl ethers under nickel catalysis. Despite these advances, progress in this field is relatively slow because of the low reactivity of benzene derivatives (e.g., anisole) compared with polyaromatic substrates (e.g., methoxynaphthalene), particularly when less reactive and synthetically useful nucleophiles are used. The "naphthalene problem" has been overcome by the use of N-heterocyclic carbene (NHC) ligands bearing bulky N-alkyl substituents, which enables a wide range of aryl ethers to be coupled with organoboron nucleophiles. Moreover, the use of N-alkyl-substituted NHC ligands allows the use of alkynylmagnesium reagents, thereby realizing the first Sonogashira-type reaction of anisoles. From a mechanistic perspective, nickel-catalyzed cross-couplings of aryl ethers are at a nascent stage, in particular regarding the mode of activation of C(aryl)-O bonds. Oxidative addition is one plausible pathway, although such a process has not been fully verified experimentally. Nickel-catalyzed reductive cleavage of aryl ethers in the absence of an external reducing agent provides strong support for this oxidative addition process. Several other mechanisms have also been proposed. For example, Martin demonstrated a new possibility of the involvement of a Ni(I) species, which could mediate the cleavage of the C(aryl)-O bond via a redox-neutral pathway. The tolerance of aryl ethers under commonly used synthetic conditions enables alkoxy groups to serve as a platform for late-stage elaboration of complex molecules without any tedious protecting group manipulations. Aryl ethers are therefore not mere economical alternatives to aryl halides but also enable nonclassical synthetic strategies.

Nickel-catalyzed reductive cleavage of aryl alkyl ethers to arenes in absence of external reductant

The reductive cleavage of the C-O bonds of aryl ethers has great potential in organic synthesis. Although several catalysts that can promote the reductive cleavage of aryl ethers have been reported, all such systems require the use of an external reductant, e.g., hydrosilane or hydrogen. Here, we report the development of a new nickel-based catalytic system that can cleave the C-O bonds of ethers in the absence of an external reductant. The hydrogen atom required in this new reductive cleavage reaction is provided by the alkoxy group of the substrate, which serves as an internal reductant. The absence of an external reductant enables the unique chemoselectivity, i.e., the selective reduction of an alkoxy group over alkenes and ketones.

Ipso-Borylation of Aryl Ethers via Ni-Catalyzed C-OMe Cleavage

A Ni-catalyzed ipso-borylation of aryl ethers via C(sp(2))-OMe and C(sp(3))-OMe cleavage is described. The transformation is characterized by its wide substrate scope under mild conditions and an exquisite divergence in site selectivity that can be easily switched by selecting the appropriate boron reagent.