An Easily Accessed Nickel Nanoparticle Catalyst for Alkene Hydrosilylation with Tertiary Silanes

Hydrosilylation of Arynes with Silanes and Silicon-Based Polymer

Benzyne derived from hexadehydrogenated Diels Alder (HDDA) reactions was found to be an efficient hydrosilylation acceptors. Various silanes can react smoothly with HDDA-derived benzyne to give the arylation products. Lewis acid such as boron trifluoride etherate can accelerate these hydrosilylation reactions. Polyhydromethylsiloxane (PHMS), a widely used organosilicon polymer, was also successfully modified using our method. About 5 % of Si-H bonds in the polymer were inserted by benzynes, giving a functional PHMS with much more solubility in methanol and with a blue-emitting fluorescence behavior. Mechanism research shows that the insertion of benzyne into the Si-H bond probably undergoes a synergistic pathway, which is quite different from the traditional radical-initiated hydrosilylation or transition-metal-catalyzed hydrosilylation.

Mechanisms of Photoredox Catalysis Featuring Nickel-Bipyridine Complexes

Metallaphotoredox catalysis can unlock useful pathways for transforming organic reactants into desirable products, largely due to the conversion of photon energy into chemical potential to drive redox and bond transformation processes. Despite the importance of these processes for cross-coupling reactions and other transformations, their mechanistic details are only superficially understood. In this review, we have provided a detailed summary of various photoredox mechanisms that have been proposed to date for Ni-bipyridine (bpy) complexes, focusing separately on photosensitized and direct excitation reaction processes. By highlighting multiple bond transformation pathways and key findings, we depict how photoredox reaction mechanisms, which ultimately define substrate scope, are themselves defined by the ground- and excited-state geometric and electronic structures of key Ni-based intermediates. We further identify knowledge gaps to motivate future mechanistic studies and the development of synergistic research approaches spanning the physical, organic, and inorganic chemistry communities.

Diphosphine Ligand-Enabled Nickel-Catalyzed Chelate-Assisted Inner-Selective Migratory Hydroarylation of Alkenes

The precise control of the regioselectivity in the transition metal-catalyzed migratory hydrofunctionalization of alkenes remains a big challenge. With a transient ketimine directing group, the nickel-catalyzed migratory β-selective hydroarylation and hydroalkenylation of alkenyl ketones has been realized with aryl boronic acids using alkyl halide as the mild hydride source for the first time. The key to this success is the use of a diphosphine ligand, which is capable of the generation of a Ni(II)-H species in the presence of alkyl bromide, and enabling the efficient migratory insertion of alkene into Ni(II)-H species and the sequent rapid chain walking process. The present approach diminishes organosilanes reductant, tolerates a wide array of complex functionalities with excellent regioselective control. Moreover, this catalytic system could also be applied to the migratory hydroarylation of alkenyl azahetereoarenes, thus providing a general approach for the preparation of 1,2-aryl heteroaryl motifs with wide potential applications in pharmaceutical discovery.

Recent Progress in NiH-Catalyzed Linear or Branch Hydrofunctionalization of Terminal or Internal Alkenes

The construction of C-C and C-X (X = N, O, Si, etc.) bonds is an important field in organic synthesis and methodology. In recent decades, studies on transition metal-catalyzed functionalization of alkenes have been on the rise. The individual properties of different transition metals determine the type of reaction that can be applied. Generally, post-transition metals with a large number of electrons in the d-orbit such as Mn, Fe, Co, Ni, Cu and Zn, etc., can be applied to more reaction types than pre-transition metals with a small number of electrons (e.g., Ti, Zr, etc.). Alkyl nickel intermediates formed by oxidative addition could couple with various of nucleophiles or electrophiles. Moreover, nickel has several oxidation valence states, which can flexibly realize a variety of catalytic cycles. These characteristics make nickel favored by researchers in the field of functionalization of alkenes, especially for the hydrofunctionalization of alkenes. Both terminal and internal alkenes could be converted, and the strategies of synthesizing linear and branched compounds have been expanded. Moreover, the guiding groups in alkenes played an almost decisive role in the regional selectivity, and the ligand or temperature also had regulating effects. Herein, we will give a comprehensive and timely overview of the works about the Ni-catalyzed hydrofunctionalization of alkenes and some insights on regional selectivity.

Enantioselective Nickel-Catalyzed Hydrosilylation of 1,1-Disubstituted Allenes

Here, we report the first example of Ni-catalyzed asymmetric hydrosilylation of 1,1-disubstituted allenes with high level of regioselectivities and enantioselectivities. The key to achieve this stereoselective hydrosilylation reaction was the development of the SPSiOL-derived bisphosphite ligands (SPSiPO). This protocol features broad substrate scope, excellent functional group, and heterocycle tolerance, thus provides a versatile method for the construction of enantioenriched tertiary allylsilanes in a straightforward and atom-economic manner. DFT calculations were performed to reveal the reaction mechanism and the origins of the enantioselectivity.

Synthesis of vinyl-substituted alcohols using acetylene as a C2 building block

Vinyl-substituted alcohols represent a highly useful class of molecular skeletons. The current method typically requires either stoichiometric metallic reagents or preformed precursors. Herein, we report a nickel catalysis-enabled synthesis of vinyl-substituted alcohols via a 5-membered oxa-metallacycle. In this protocol, acetylene, the simplest alkyne and abundant feedstock, is employed as an ideal C2 synthon. The reaction features mild conditions, good functional group tolerance and broad substrate scope. Mechanistic exploration implies that the oxa-metallacycle originated from the cyclometallation of aldehyde and acetylene is the key intermediate for this transformation, which is then terminated by a silane-mediated σ-bond metathesis and subsequent reductive elimination.

Remote Hydroamination of Disubstituted Alkenes by a Combination of Isomerization and Regioselective N-H Addition

Remote hydrofunctionalizations of alkenes incorporate functional groups distal to existing carbon-carbon double bonds. While remote carbonylations are well-known, remote hydrofunctionalizations are most common for addition of relatively nonpolar B-H, Si-H, and C-H bonds with alkenes. We report a system for the remote hydroamination of disubstituted alkenes to functionalize an alkyl chain selectively at the subterminal, unactivated, methylene position. Critical to the high regioselectivity and reaction rates are the electronic properties of the substituent on the amine and the development of the ligand DIP-Ad-SEGPHOS by evaluating the steric and electronic effects of ligand modules on reactivity and selectivity. The remote hydroamination is compatible with a broad scope of alkenes and aminopyridines and enables the regioconvergent synthesis of amines from an isomeric mixture of alkenes. The products can be derivatized by nucleophilic aromatic substitution on the amino substituent with a variety of nucleophiles.

Ligand-Controlled NiH-Catalyzed Regiodivergent Chain-Walking Hydroalkylation of Alkenes

A NiH-catalyzed migratory hydroalkylation of alkenyl amines with predictable and switchable regioselectivity is reported. By utilizing a ligand-controlled, directing group-assisted strategy, various alkyl units are site-selectively installed at inert sp³ C-H sites far away from the original C=C bonds. A range of structurally diverse α- and β-branched protected amines are conveniently synthesized via stabilization of 5- and 6-membered nickelacycles respectively. This method exhibits broad scope and high functional group tolerance, and can be applied to late-stage modification of medicinally relevant molecules.

Remote Site-Selective Asymmetric Protoboration of Unactivated Alkenes Enabled by Bimetallic Relay Catalysis

A remote C(sp³ )-H bond asymmetric borylation of unactivated alkenes was achieved by bimetallic relay catalysis. The reaction proceeded through reversible and consecutive β-H elimination/olefin insertion promoted by CoH species generated in situ, followed by copper-catalyzed asymmetric protoboration. The use of this synergistic Co/Cu catalysis protocol allowed the enantioselective protoboration of various unactivated terminal alkenes and internal alkenes, as well as an unrefined mixture of olefin isomers, at the distal less-reactive β-position to a functional group, leading to chiral organoboronates.

γ-Selective C(sp3)-H amination via controlled migratory hydroamination

Remote functionalization of alkenes via chain walking has generally been limited to C(sp3)-H bonds α and β to polar-functional units, while γ-C(sp3)-H functionalization through controlled alkene transposition is a longstanding challenge. Herein, we describe NiH-catalyzed migratory formal hydroamination of alkenyl amides achieved via chelation-assisted control, whereby various amino groups are installed at the γ-position of aliphatic chains. By tuning olefin isomerization and migratory hydroamination through ligand and directing group optimization, γ-selective amination can be achieved via stabilization of a 6-membered nickellacycle by an 8-aminoquinoline directing group and subsequent interception by an aminating reagent. A range of amines can be installed at the γ-C(sp3)-H bond of unactivated alkenes with varying alkyl chain lengths, enabling late-stage access to value-added γ-aminated products. Moreover, by employing picolinamide-coupled alkene substrates, this approach is further extended to δ-selective amination. The chain-walking mechanism and pathway selectivity are investigated by experimental and computational methods.

Thioether-Directed NiH-Catalyzed Remote γ-C(sp3)-H Hydroamidation of Alkenes by 1,4,2-Dioxazol-5-ones

A NiH-catalyzed thioether-directed cyclometalation strategy is developed to enable remote methylene C-H bond amidation of unactivated alkenes. Due to the preference for five-membered nickelacycle formation, the chain-walking isomerization initiated by the NiH insertion to an alkene can be terminated at the γ-methylene site remote from the alkene moiety. By employing 2,9-dibutyl-1,10-phenanthroline (L4) as the ligand and dioxazolones as the reagent, the amidation occurs at the γ-C(sp³)-H bonds to afford the amide products in up to 90% yield (>40 examples) with remarkable regioselectivity (up to 24:1 rr).

Nickel-Catalyzed Multicomponent Coupling: Synthesis of α-Chiral Ketones by Reductive Hydrocarbonylation of Alkenes

A nickel-catalyzed, multicomponent regio- and enantioselective coupling via sequential hydroformylation and carbonylation from readily available starting materials has been developed. This modular multicomponent hydrofunctionalization strategy enables the straightforward reductive hydrocarbonylation of a broad range of unactivated alkenes to produce a wide variety of unsymmetrical dialkyl ketones bearing a functionalized α-stereocenter, including enantioenriched chiral α-aryl ketones and α-amino ketones. It uses chiral bisoxazoline as a ligand, silane as a reductant, chloroformate as a safe CO source, and a racemic secondary benzyl chloride or an N-hydroxyphthalimide (NHP) ester of a protected α-amino acid as the alkylation reagent. The benign nature of this process renders this method suitable for late-stage functionalization of complex molecules.

H-BPin/KOtBu Promoted Activation of Cobalt Salt to a Heterotopic Catalyst for Highly Selective Cyclotrimerization of Alkynes

A mixture of HBPin with KO^tBu was found to activate cobalt salt to form a heterotopic cobalt species that is highly active for catalytic intermolecular trimerization of alkynes. This protocol affords 1,2,4-regioisomers in good yields with high regioselectivities under mild conditions. These salient features, together with the operational simplicity and high efficiency, as well as obviating the use of any costly and/or air sensitive ligands, renders the protocol promising for practical applications.

14-Electron Rh and Ir silylphosphine complexes and their catalytic activity in alkene functionalization with hydrosilanes

Herein we report an experimental and computational study of a family of four coordinated 14-electron complexes of Rh(iii) devoid of agostic interactions. The complexes [X-Rh(κ³(P,Si,Si)PhP(o-C₆H₄CH₂SiⁱPr₂)₂], where X = Cl (Rh-1), Br (Rh-2), I (Rh-3), OTf (Rh-4), Cl·GaCl₃ (Rh-5); derive from a bis(silyl)-o-tolylphosphine with isopropyl substituents on the Si atoms. All five complexes display a sawhorse geometry around Rh and exhibit similar spectroscopic and structural properties. The catalytic activity of these complexes and [Cl-Ir(κ³(P,Si,Si)PhP(o-C₆H₄CH₂SiⁱPr₂)₂], Ir-1, in styrene and aliphatic alkene functionalizations with hydrosilanes is disclosed. We show that Rh-1 catalyzes effectively the dehydrogenative silylation of styrene with Et₃SiH in toluene while it leads to hydrosilylation products in acetonitrile. Rh-1 is an excellent catalyst in the sequential isomerization/hydrosilylation of terminal and remote aliphatic alkenes with Et₃SiH including hexene isomers, leading efficiently and selectively to the terminal anti-Markonikov hydrosilylation product in all cases. With aliphatic alkenes, no hydrogenation products are observed. Conversely, catalysis of the same hexene isomers by Ir-1 renders allyl silanes, the tandem isomerization/dehydrogenative silylation products. A mechanistic proposal is made to explain the catalysis with these M(iii) complexes.

Selective hydroboration of unsaturated bonds by an easily accessible heterotopic cobalt catalyst

Homogeneous earth-abundant metal catalysis based on well-defined molecular complexes has achieved great advance in synthetic methodologies. However, sophisticated ligand, hazardous activator and multistep synthesis starting from base metal salts are generally required for the generation of active molecular catalysts, which may hinder their broad application in large scale organic synthesis. Therefore, the development of metal cluster catalysts formed in situ from simple earth-abundant metal salts is of importance for the practical utilization of base metal resource, yet it is still in its infancy. Herein, a mixture of catalytic amounts of cobalt (II) iodide and potassium tert-butoxide is discovered to be highly active for selective hydroboration of vinylarenes and dihydroboration of nitriles, affording a good yield of diversified hydroboration products that without isolation can readily undergo further one pot transformations. It should be highlighted that the alkoxide-pinacolborane combination acts as an efficient activation strategy to activate cobalt (II) iodide for the generation of metastable heterotopic cobalt catalysts in situ, which is proposed to be catalytically active species.

Homogeneous earth-abundant metal catalysis based on well-defined metal complexes is of interest for organic synthesis, but typically employs expensive catalysts, air sensitive or synthetically challenging chemicals. Here, the authors report an efficient and regio-selective catalytic system for hydroboration of vinylarenes and organic nitriles with HBPin, using commercially available CoI₂ and KO^tBu under ligand-free conditions.

Palladium-Catalyzed Direct Arylation of 2-Pyridylmethyl Silanes with Aryl Bromides

The first palladium-catalyzed direct arylation of 2-pyridylmethyl silanes with aryl bromides to generate a diverse array of aryl(2-pyridyl)-methyl silane derivatives has been developed. This protocol facilitates access to various kinds of heterocycle-containing silanes in good to excellent yields (40 examples, 66-97% yield) with good functional group tolerance. The scalability of this transformation is demonstrated.

Nickel-Catalyzed Asymmetric Hydroarylation of Vinylarenes: Direct Enantioselective Synthesis of Chiral 1,1-Diarylethanes

The enantioselective hydroarylation of vinylarenes catalyzed by a chiral, non-racemic nickel catalyst is presented as a facile method to generate chiral 1,1-diarylethanes. These reactions proceed via formation of a chiral, non-racemic nickel benzyl intermediate. Transmetalation with arylboron nucleophiles and subsequent reductive elimination enable the formation of a variety of chiral 1,1-diarylethanes. The 1,1-diarylethane products from reactions of arylboronic acids containing electron-donating substituents are formed with typically greater than 90% ee, while the 1,1-diarylethanes generated from reactions of arylboronic acids containing electron-withdrawing groups are generated with typically less than 80% ee. These results are consistent with the rate of transmetalation with an arylboron nucleophile playing a key role in the enantioselectivity of these hydroarylation reactions. This mechanistic insight has led to the development of reactions of neo-pentylglycolate esters of arylboronic acids with vinylarenes that occur with higher enantioselectivities based on increased rates of transmetalation.

Nickel-Catalyzed Hydrosilylation of Terminal Alkenes with Primary Silanes via Electrophilic Silicon-Hydrogen Bond Activation

We report a simple and effective nickel-based catalytic system, NiCl₂·6H₂O/^tBuOK, for the electrophilically activated hydrosilylation of terminal alkenes with primary silanes. This protocol provides excellent performance under mild reaction conditions: exclusive anti-Markovnikov selectivity, broad functional group tolerance (36 examples), and good scalability (TON = 5500). However, the secondary and tertiary silanes are not suitable. Mechanistic studies revealed that this homogeneous catalytic hydrosilylation includes an electrophilically activated Si-H bond process without the generation of nickel hydrides.

Recent Advances in Catalytic Hydrosilylations: Developments beyond Traditional Platinum Catalysts

Hydrosilylation reactions, which allow the addition of Si-H to C=C/C≡C bonds, are typically catalyzed by homogeneous noble metal catalysts (Pt, Rh, Ir, and Ru). Although excellent activity and selectivity can be obtained, the price, purification, and metal residues of these precious catalysts are problems in the silicone industry. Thus, a strong interest in more sustainable catalysts and for more economic processes exists. In this respect, recently disclosed hydrosilylations using catalysts based on earth-abundant transition metals, for example, Fe, Co, Ni, and Mn, and heterogeneous catalysts (supported nanoparticles and single-atom sites) are noteworthy. This minireview describes the recent advances in this field.

Nickel-catalyzed electrochemical reductive relay cross-coupling of alkyl halides with alkyl carboxylic acids

A highly regioselective Ni-catalyzed electrochemical (undivided cell) reductive relay cross-coupling between alkyl carboxylic acids and alkyl bromides has been developed.

Nickel-Catalyzed, Regio- and Enantioselective Benzylic Alkenylation of Olefins with Alkenyl Bromide

A NiH-catalyzed migratory hydroalkenylation reaction of olefins with alkenyl bromides has been developed, affording benzylic alkenylation products with high yields and excellent chemoselectivity. The mild conditions of the reaction preclude olefinic products from undergoing further isomerization or subsequent alkenylation. Catalytic enantioselective hydroalkenylation of styrenes was achieved by using a chiral bisoxazoline ligand.

A family of rhodium(i) NHC chelates featuring O-containing tethers for catalytic tandem alkene isomerization-hydrosilylation

The rhodium complex Rh(HL)(COD)Cl, 1, L being a functionalized N-heterocyclic carbene (NHC) ligand with an oxygen-containing pendant arm, has been used as the entry point to synthesize a series of neutral and cationic Rh(i) O,C chelates. While the Rh-carbene interaction is similar in all these 16-electron complexes, structural analysis reveals that the strength of the Rh-O bond is greatly affected by the nature of the O-donor: R-O^- > R-OH > R-OBF₃. These subtle changes in the nature of the O-containing tether are found to be responsible for large differences in the alkene hydrosilylation catalytic activity of these compounds: the stronger the Rh-O interaction, the better the catalytic performances. The most active catalyst, [Rh(L)(COD)], 2, demonstrated good catalytic activity under mild reaction conditions for the hydrosilylation of a range of alkene substrates with the industrially relevant non-activated tertiary silane, 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD^HM). Furthermore, this complex is an effective catalyst for the selective remote functionalization of internal olefins at room temperature via tandem alkene isomerization-hydrosilylation.

Enantio- and Regioselective NiH-Catalyzed Reductive Hydroarylation of Vinylarenes with Aryl Iodides

A highly enantio- and regioselective hydroarylation process of vinylarenes with aryl halides has been developed using a NiH catalyst and a new chiral bis imidazoline ligand. A broad range of structurally diverse, enantioenriched 1,1-diarylalkanes, a structure found in a number of biologically active molecules, have been obtained with excellent yields and enantioselectivities under extremely mild conditions.

Enantioselective Nickel-Catalyzed Migratory Hydrocyanation of Nonconjugated Dienes

Metal-catalyzed chain-walking reactions have recently emerged as a powerful strategy to functionalize remote positions in organic molecules. However, a chain-walking protocol for nonconjugated dienes remains scarcely reported, and developments are currently ongoing. In this Communication, a nickel-catalyzed asymmetric hydrocyanation of nonconjugated dienes involving a chain-walking process is demonstrated. The reaction exhibits excellent regio- and chemoselectivity, and a wide range of substrates were tolerated, delivering the products in high yields and enantioselectivities. Deuterium-labeling experiments support the chain-walking process, which involves an iterative β-H elimination and reinsertion processes. Gram-scale synthesis, regioconvergent experiments, and downstream transformations gave further insights into the high potential of this transformation.

Efficient and selective alkene hydrosilation promoted by weak, double Si-H activation at an iron center

Cationic iron complexes [Cp*(ⁱPr₂MeP)FeH₂SiHR]⁺, generated and characterized in solution, are very efficient catalysts for the hydrosilation of terminal alkenes and internal alkynes by primary silanes at low catalyst loading (0.1 mol%) and ambient temperature.

Cationic iron complexes [Cp*(ⁱPr₂MeP)FeH₂SiHR]⁺, generated and characterized in solution, are very efficient catalysts for the hydrosilation of terminal alkenes and internal alkynes by primary silanes at low catalyst loading (0.1 mol%) and ambient temperature. These reactions yield only the corresponding secondary silane product, even with SiH₄ as the substrate. Mechanistic experiments and DFT calculations indicate that the high rate of hydrosilation is associated with an inherently low barrier for dissociative silane exchange (product release).

Ru-catalyzed isomerization of ω-alkenylboronates towards stereoselective synthesis of vinylboronates with subsequent in situ functionalization

The stereoselective preparation of synthetically versatile vinylboronates from ω-alkenylboronates is achieved through a ruthenium-catalyzed isomerization reaction. A variety of di- and trisubstituted vinylboronates were conveniently produced and could be used as a new starting point for subsequent in situ remote functionalization through either a sequential Ru/Pd or Ru/Cu double catalytic system.

A regio- and stereoselective ruthenium-catalyzed isomerization of ω-alkenyl boronates into stereodefined di- and trisubstituted alkenylboronate derivatives is reported.

Nickel-Catalyzed Cross-Electrophile Coupling of Aryl Chlorides with Primary Alkyl Chlorides

Alkyl chlorides and aryl chlorides are among the most abundant and stable carbon electrophiles. Although their coupling with carbon nucleophiles is well developed, the cross-electrophile coupling of aryl chlorides with alkyl chlorides has remained a challenge. We report here the first general approach to this transformation. The key to productive, selective cross-coupling is the use of a small amount of iodide or bromide along with a recently reported ligand, pyridine-2,6-bis(N-cyanocarboxamidine) (PyBCam^CN). The scope of the reaction is demonstrated with 35 examples (63 ± 16% average yield), and we show that the Br^- and I^- additives act as cocatalysts, generating a low, steady-state concentration of more-reactive alkyl bromide/iodide.

Efficient alkene hydrosilation with bis(8-quinolyl)phosphine (NPN) nickel catalysts. The dominant role of silyl-over hydrido-nickel catalytic intermediates

A cationic nickel complex of the bis(8-quinolyl)(3,5-di-tert-butylphenoxy)phosphine (NPN) ligand, [(NPN)NiCl]+, is a precursor to efficient catalysts for the hydrosilation of alkenes with a variety of hydrosilanes under mild conditions and low catalyst loadings. DFT studies reveal the presence of two coupled catalytic cycles based on [(NPN)NiH]+ and [(NPN)NiSiR3]+ active species, with the latter being more efficient for producing the product. The preferred silyl-based catalysis is not due to a more facile insertion of alkene into the Ni-Si (vs. Ni-H) bond, but by consistent and efficient conversions of the hydride to the silyl complex.

Ligand-Enabled Nickel-Catalyzed Redox-Relay Migratory Hydroarylation of Alkenes with Arylborons

A redox-relay migratory hydroarylation of isomeric mixtures of olefins with arylboronic acids catalyzed by nickel complexes bearing diamine ligands is described. A range of structurally diverse 1,1-diarylalkanes, including those containing a 1,1-diarylated quaternary carbon, were obtained in excellent yields and with high regioselectivity. Preliminary experimental evidence supports the proposed non-dissociated chainwalking of aryl-nickel(II)-hydride species along the alkyl chain of alkenes before selective reductive elimination at a benzylic position. A catalyst loading as low as 0.5 mol % proved to be sufficient in large-scale synthesis while retaining high reactivity, highlighting the practical value of this transformation.