Nickel Hydroxo Complexes as Intermediates in Nickel-Catalyzed Suzuki–Miyaura Cross-Coupling

ProPhos: A Ligand for Promoting Nickel-Catalyzed Suzuki-Miyaura Coupling Inspired by Mechanistic Insights into Transmetalation

Nickel-catalyzed Suzuki-Miyaura coupling (Ni-SMC) offers the potential to reduce the cost of pharmaceutical process synthesis. However, its application has been restricted by challenges such as slow reaction rates, high catalyst loading, and a limited scope of heterocycles. Despite recent investigations, the mechanism of transmetalation in Ni-SMC, often viewed as the turnover-limiting step, remains insufficiently understood. We elucidate the "Ni-oxo" transmetalation pathway, applying PPh2Me as the ligand, and identify the formation of a nickel-oxo intermediate as the turnover-limiting step. Building on this insight, we develop a scaffolding ligand, ProPhos, featuring a pendant hydroxyl group connected to the phosphine via a linker. The design preorganizes both the nucleophile and the nickel catalyst, thereby facilitating transmetalation. This catalyst exhibits fast kinetics and robust activity across a wide range of heteroarenes, with a catalyst loading of 0.5-3 mol %. For arene substrates, the catalyst loading can be further reduced to 0.1 mol %.

Phenoxythiazoline (FTz)-Cobalt(II) Precatalysts Enable C(sp2 )-C(sp3 ) Bond-Formation for Key Intermediates in the Synthesis of Toll-like Receptor 7/8 Antagonists

Evaluation of the relative rates of the cobalt-catalyzed C(sp2 )-C(sp3 ) Suzuki-Miyaura cross-coupling between the neopentylglycol ester of 4-fluorophenylboronic acid and N-Boc-4-bromopiperidine established that smaller N-alkyl substituents on the phenoxyimine (FI) supporting ligand accelerated the overall rate of the reaction. This trend inspired the design of optimal cobalt catalysts with phenoxyoxazoline (FOx) and phenoxythiazoline (FTz) ligands. An air-stable cobalt(II) precatalyst, (FTz)CoBr(py)3 was synthesized and applied to the cross-coupling of an indole-5-boronic ester nucleophile with a piperidine-4-bromide electrophile that is relevant to the synthesis of reported toll-like receptor (TLR) 7/8 antagonist molecules including afimetoran. Addition of excess KOMe⋅B(Oi Pr)3 improved catalyst lifetime due to attenuation of alkoxide basicity that otherwise resulted in demetallation of the FI chelate. A first-order dependence on the cobalt precatalyst and a saturation regime in nucleophile were observed, supporting turnover-limiting transmetalation and the origin of the observed trends in N-imine substitution.

Hierarchical NiZn Solid Solution as a Highly Efficient Palladium Free Heterogeneous Catalyst for Suzuki-Miyaura Cross-Coupling Reaction

In this report, surfactant free solid solution of NiZn with a hierarchical architecture was synthesized via a one-pot colloidal approach. Evidence supporting hierarchical crystal growth and alloying of metals at the atomic level was obtained from field emission scanning electron microscopy and transmission electron microscopy-energy-dispersive X-ray data. Lattice sites of face-centered metallic Ni were found to be occupied by Zn as evident from powder X-ray diffraction where a gradual shift in the peak position and increase in the average lattice parameter upon reduction of the Ni content in the alloy samples can be observed. This well-alloyed, magnetically separable, non-noble metal-based solid solution has the potential to replace the palladium-based catalyst in the Suzuki-Miyaura cross-coupling of aryl halides (-Cl/-Br) and phenylboronic acid. The nanostructured catalyst was formed through the assembly of a triangular spiked and sheet-like structure and is magnetically well separable that is stable enough under the catalytic reaction condition. The developed heterogeneous catalyst and the designed economical catalytic model are the first ever reported work. The catalytic results outperformed most of the reported state-of-the-art works involving the transition metal-based catalyst.

Enabling Suzuki-Miyaura coupling of Lewis-basic arylboronic esters with a nonprecious metal catalyst

The high cost and negative environmental impact of precious metal catalysts has led to increased demand for nonprecious alternatives for widely practiced reactions such as the Suzuki-Miyaura coupling (SMC). Ni-catalyzed versions of this reaction have failed to achieve high reactivity with Lewis-basic arylboron nucleophiles, especially pinacolboron esters. We describe the development of (PPh2Me)2NiCl2 as an inexpensive and air-stable precatalyst that addresses this challenge. Under activation by n-BuMgCl, this complex can catalyze the coupling of synthetically important heteroaryl pinacolborons with heteroaryl halides. Mildly basic conditions (aqueous K3PO4) allow the reaction to tolerate sensitive functional groups that were incompatible with other Ni-SMC methods. Experimental and computational studies suggest that catalyst inhibition by substitution of PPh2Me from Ni(ii) intermediates by Lewis basic reactants and products is disfavored relative to more commonly employed ligands in the Ni-SMC, which allows it to operate efficiently in the presence of Lewis bases such as unhindered pyridines.

Structure-Reactivity Relationships of Buchwald-Type Phosphines in Nickel-Catalyzed Cross-Couplings

The dialkyl-ortho-biaryl class of phosphines, commonly known as Buchwald-type ligands, are among the most important phosphines in Pd-catalyzed cross-coupling. These ligands have also been successfully applied to several synthetically valuable Ni-catalyzed cross-coupling methodologies and, as demonstrated in this work, are top performing ligands in Ni-catalyzed Suzuki Miyaura Coupling (SMC) and C-N coupling reactions, even outperforming commonly employed bisphosphines like dppf in many circumstances. However, little is known about their structure-reactivity relationships (SRRs) with Ni, and limited examples of well-defined, catalytically relevant Ni complexes with Buchwald-type ligands exist. In this work, we report the analysis of Buchwald-type phosphine SRRs in four representative Ni-catalyzed cross-coupling reactions. Our study was guided by data-driven classification analysis, which together with mechanistic organometallic studies of structurally characterized Ni(0), Ni(I), and Ni(II) complexes allowed us to rationalize reactivity patterns in catalysis. Overall, we expect that this study will serve as a platform for further exploration of this ligand class in organonickel chemistry as well as in the development of new Ni-catalyzed cross-coupling methodologies.

Stereoconvergent and -divergent Synthesis of Tetrasubstituted Alkenes by Nickel-Catalyzed Cross-Couplings

We report the development of a method to diastereoselectively access tetrasubstituted alkenes via nickel-catalyzed Suzuki-Miyaura cross-couplings of enol tosylates and boronic acid esters. Either diastereomeric product was selectively accessed from a mixture of enol tosylate starting material diastereomers in a convergent reaction by judicious choice of the ligand and reaction conditions. A similar protocol also enabled a divergent synthesis of each product isomer from diastereomerically pure enol tosylates. Notably, high-throughput optimization of the monophosphine ligands was guided by chemical space analysis of the kraken library to ensure a diverse selection of ligands was examined. Stereoelectronic analysis of the results provided insight into the requirements for reactive and selective ligands in this transformation. The synthetic utility of the optimized catalytic system was then probed in the stereoselective synthesis of various tetrasubstituted alkenes, with yields up to 94% and diastereomeric ratios up to 99:1 Z/E and 93:7 E/Z observed. Moreover, a detailed computational analysis and experimental mechanistic studies provided key insights into the nature of the underlying isomerization process impacting selectivity in the cross-coupling.

Biradical o-iminobenzosemiquinonato(1-) complexes of nickel(ii): catalytic activity in three-component coupling of aldehydes, amines and alkynes

The six-coordinated bis-o-iminosemiquinone complex, NiL2 BIS, in which LBIS is the o-iminosemiquinone 1-electron oxidized form of the tridentate o-aminophenol benzoxazole-based ligand H2LBAP, was synthesized and characterized. The crystal structure of the complex reveals octahedral geometry with a NiN4O2 coordination sphere in which Ni(ii) has been surrounded by two tridentate LBIS ligands. This compound exhibits (S Ni = 1) with both spin and orbital contribution to the magnetic moment and antiferromagnetic coupling between two electrons on two LBIS ligands which results in a triplet spin ground state (S = 1). The electronic transitions and the electrochemical behavior of this open-shell molecule are presented here, based on experimental observations and theoretical calculations. The electrochemical behavior of NiL2 BIS was investigated by cyclic voltammetry and indicates ligand-centered redox processes. Three-component coupling of aldehydes, amines and alkynes (A3-coupling) was studied in the presence of the NiL2 BIS complex, and the previously reported four-coordinated bis-o-iminosemiquinone NiL2 NIS. Furthermore, among these two o-iminobenzosemiquinonato(1-) complexes of Ni(ii) (NiL2 NIS and NiL2 BIS), NiL2 NIS was found to be an efficient catalyst in A3-coupling at 85 °C under solvent-free conditions and can be recovered and reused for several cycles with a small decrease in activity.

Ancillary ligand electro-activity effects towards phenyl acetylene homocoupling reaction by a nickel(ii) complex of a non-innocent O-amino phenol ligand: a mechanistic insight

A new Ni(ii) complex, was synthesized from the reaction of a non-innocent o-aminophenol ligand, and Ni(OAc)₂. The crystal structure of Ni^IIL₂^NIS (in which, IS stands for iminosemiquinone radical ligand with cyanide (shown by N in NIS) substituent on phenolate rings) exhibits the square planar environment of Ni(ii). The complex has been crystalized in the monoclinic system and Ni(ii) was surrounded by two oxygen and two nitrogen atoms of two ligands. Variable-temperature magnetic susceptibility measurement for crystalline samples of complex shows the effective magnetic moment per molecule (μ_eff) of near zero and the diamagnetic nature of the complex (S = 0) which emphasize that strong antiferromagnetic coupling prevailed between the two unpaired electrons of L^NIS ligands and Ni(ii) high spin electrons. The complex is EPR silent which confirms the diamagnetic character of the Ni(ii) complex. Electrochemical measurement (CV) indicates the redox-active character of ligand and metal. Ni^IIL₂^NIS complex proved to be effective for free metal- or base counterpart homocoupling of phenyl acetylene at room temperature. To the best of our knowledge, this is the first example of using Ni(ii) complex without using any reducing agent due to the promotion ancillary effect of non-innocent o-aminophenol ligand which acts as an “electron reservoir” and can reversibly accept and donate electrons in the catalytic cycle. The theoretical calculation confirms the magnetostructure, electronic spectrum and confirmed the suggested mechanism of phenyl acetylene homocoupling with emphasis on the role of non-innocent ligand electro-activity and the effect of ligand substituent on the efficiency and stability of the complex.

A new Ni(ii) complex, was synthesized from the reaction of a non-innocent o-aminophenol ligand, and Ni(OAc)₂.

Enantioselective Assembly of Cycloenones with a Nitrile-Containing All-Carbon Quaternary Center from Malononitriles Enabled by Ni Catalysis

Chiral nitriles are valuable molecules in modern organic synthesis and drug discovery. Selectively differentiating the two nitrile groups of widely available malononitrile derivatives is a straightforward yet underdeveloped route to construct enantioenriched nitriles. Here we report an enantioselective nickel-catalyzed desymmetrization of malononitriles for the generation of nitrile-containing all-carbon quaternary stereocenters. This protocol involves a nickel-catalyzed addition of aryl boronic acids to alkynes, followed by a selective nitrile insertion, providing unprecedented access to enantioenriched 5-7-membered α-cyano-cycloenones with a fully substituted olefin from a broad range of substrates. The synthetic utility of these nitrile products is demonstrated by gram-scale synthesis and conversion to several useful functional groups.

Amination of Aryl Boronic Acids with Alkylnitrites: A Convenient Complement to Cu-Promoted Reductive Amination

Copper-catalyzed amination of aryl boronic acids with alkylnitrites leading to symmetrical diarylamines with a practical 50-80% yield was elaborated. Two C(sp²)-N bonds are formed in the one-pot process under mild conditions. This new approach to diarylamines is a complement to the Cu-assisted reductive amination of aryl boronic acids avoiding preliminary synthesis of nitrosoarenes. The possible reaction scheme based on quantum chemical calculations was suggested, clarifying key intermediates.

Ni vs. Pd in Suzuki-Miyaura sp2-sp2 cross-coupling: a head-to-head study in a comparable precatalyst/ligand system

The Suzuki-Miyaura reaction is a cornerstone method for sp²-sp² cross-coupling in industry. There has been a concerted effort to enable the use of Ni catalysis as an alternative to Pd in order to mitigate cost and improve sustainability. Despite significant advances, ligand development for Ni-catalyzed Suzuki-Miyaura cross-coupling remains underdeveloped when compared to Pd and, as a consequence, ligands for Ni-catalyzed processes are typically taken from the Pd arena. In this study we evaluate the effect of using a similar Ni and Pd precatalyst based on a common bidentate ligand (dppf) in a head-to-head format for the most common type of biaryl couplings, establishing the practical implications of direct replacement of Pd with Ni, and identifying the potential origins of these observations in a mechanistic context.

Modifications to the Aryl Group of dppf-Ligated Ni σ‑Aryl Precatalysts: Impact on Speciation and Catalytic Activity in Suzuki-Miyaura Coupling Reactions

There is currently significant interest in the development of efficient nickel precatalysts for cross-coupling. In this work, 14 nickel(II) precatalysts of the form (dppf)Ni(aryl)(X) (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) were synthesized. In particular, both the electronic and steric properties of the aryl group were modified to understand how this affects precatalyst activation. Using EPR spectroscopy, it was demonstrated that the amount of off-cycle nickel(I) species which are formed via comproportionation during precatalyst activation varies depending on the nature of the aryl group. For example, sterically bulky aryl groups reduce comproportionation. Additionally, the catalytic activity of the family of precatalysts was evaluated in five different Suzuki-Miyaura coupling reactions. The results from these catalytic studies provide information about how precatalyst structure affects catalytic efficiency, which may be useful for the rational design of improved nickel precatalysts for cross-coupling.

Nickel-Catalysed Synthesis of 17-Arylandrosta-5,16-Dienes

An efficient Ni-catalysed coupling reaction between arylboronic acids and 17-trifluoromethanesulfonyl- 3β-acetoxyandrosta-5,16-diene which was obtained by the sulfonylation of 3β-acetoxylandrost-5-ene-17-one was developed to afford 17-aryl- 3β-acetoxy-androsta-5,16-dienes in moderate to good yields (52–85%). The structure of the 3,4-dimethoxyaryl product was confirmed by X-ray crystallography.

Spectroscopic Studies of the Chan-Lam Amination: A Mechanism-Inspired Solution to Boronic Ester Reactivity

We report an investigation of the Chan-Lam amination reaction. A combination of spectroscopy, computational modeling, and crystallography has identified the structures of key intermediates and allowed a complete mechanistic description to be presented, including off-cycle inhibitory processes, the source of amine and organoboron reactivity issues, and the origin of competing oxidation/protodeboronation side reactions. Identification of key mechanistic events has allowed the development of a simple solution to these issues: manipulating Cu(I) → Cu(II) oxidation and exploiting three synergistic roles of boric acid has allowed the development of a general catalytic Chan-Lam amination, overcoming long-standing and unsolved amine and organoboron limitations of this valuable transformation.

A Transmetalation Reaction Enables the Synthesis of [18F]5-Fluorouracil from [18F]Fluoride for Human PET Imaging

Translation of new ¹⁸F-fluorination reactions to produce radiotracers for human positron emission tomography (PET) imaging is rare because the chemistry must have useful scope and the process for ¹⁸F-labeled tracer production must be robust and simple to execute. The application of transition metal mediators has enabled impactful ¹⁸F-fluorination methods, but to date none of these reactions have been applied to produce a human-injectable PET tracer. In this article we present chemistry and process innovations that culminate in the first production from [¹⁸F]fluoride of human doses of [¹⁸F]5-fluorouracil, a PET tracer for cancer imaging in humans. The first preparation of nickel σ-aryl complexes by transmetalation from arylboronic acids or esters was developed and enabled the synthesis of the [¹⁸F]5-fluorouracil precursor. Routine production of >10 mCi doses of [¹⁸F]5-fluorouracil was accomplished with a new instrument for azeotrope-free [¹⁸F]fluoride concentration in a process that leverages the tolerance of water in nickel-mediated ¹⁸F-fluorination.

How easy is CO2 fixation by M–C bond containing complexes (M = Cu, Ni, Co, Rh, Ir)?

A comparison between different M–C bonds (M = Cu(i), Ni(ii), Co(i), Rh(i) and Ir(i)) has been reported by using density functional theory (DFT) calculations to explore the role of the metal in the fixation or incorporation of CO₂ into such complexes.

Nickel–glycerol: an efficient, recyclable catalysis system for Suzuki cross coupling reactions using aryl diazonium salts

A palladium free, chemoselective and environmentally benign protocol for a C–C bond formation reaction.

A modular, air-stable nickel precatalyst

The synthesis and catalytic activity of [(TMEDA)Ni(o-tolyl)Cl], an air-stable, crystalline solid, is described. This complex is an effective precatalyst in a variety of nickel-catalyzed transformations. The lability of TMEDA allows a wide variety of ligands to be used, including mono- and bidentate phosphines, diimines, and N-heterocyclic carbenes. Preliminary mechanistic studies are also reported, which suggest that [(TMEDA)Ni(o-tolyl)Cl] can activate by either a Ni-B or Ni-Ni transmetalation event, depending on the reaction conditions.

Palladium-catalyzed chemoselective allylic substitution, Suzuki-Miyaura cross-coupling, and allene formation of bifunctional 2-B(pin)-substituted allylic acetate derivatives

A formidable challenge at the forefront of organic synthesis is the control of chemoselectivity to enable the selective formation of diverse structural motifs from a readily available substrate class. Presented herein is a detailed study of chemoselectivity with palladium-based phosphane catalysts and readily available 2-B(pin)-substituted allylic acetates, benzoates, and carbonates. Depending on the choice of reagents, catalysts, and reaction conditions, 2-B(pin)-substituted allylic acetates and derivatives can be steered into one of three reaction manifolds: allylic substitution, Suzuki-Miyaura cross-coupling, or elimination to form allenes, all with excellent chemoselectivity. Studies on the chemoselectivity of Pd catalysts in their reactivity with boron-bearing allylic acetate derivatives led to the development of diverse and practical reactions with potential utility in synthetic organic chemistry.