Nickel-catalyzed cyanation of aryl halides and triflates using acetonitrile via C-CN bond cleavage assisted by 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine

Palladium-Catalyzed Denitrative Synthesis of Aryl Nitriles from Nitroarenes and Organocyanides

A denitrative cyanation of nitroarenes using organocyanides and a palladium catalyst was developed. The key for this reaction was the utilization of an aminoacetonitrile as a cyano source to avoid the generation of stoichiometric metal- and halogen-containing chemical waste. A wide range of nitroarenes, including heteroarenes and pharmaceutical molecules, can be converted into aryl nitriles.

Light-Triggered, Ni-Catalyzed Cyanation of Aryl Triflates with 1,4-Dicyanobenzene as the CN Source

A light-triggered, Ni-catalyzed cyanation of aryl triflates was herein reported, which provides a benign photochemical synthesis of aryl nitriles using 1,4-dicyanobenzene as the CN source instead of HCN or a metallic CN source. This mild method uses a readily available bisphosphine ligand and a soluble organosilicon reagent as the reductant and is carried out under purple light without an external photocatalyst. This cyanation was effective for aryl triflates derived from phenols and bisphenols as well as lignin-derived phenolic compounds, demonstrating its potential utility for the synthesis of aryl nitriles from biomass.

Recent advances in ring-opening of cyclobutanone oximes for capturing SO2, CO or O2via a radical process

Cyclobutanone oximes and their derivatives are pivotal core structural motifs in organic chemistry. Iminyl-radical-triggered C-C bond cleavage of cyclobutanone oximes delivers an efficient strategy to produce stable distal cyano-substituted alkyl radicals, which can capture SO2, CO or O2 to form cyanoalkylsulfonyl radicals, cyanoalkylcarbonyl radicals or cyanoalkoxyl radicals under mild conditions. In the past several years, cyanoalkylsulfonylation/cyanoalkylcarbonyaltion/cyanoalkoxylation has attracted a lot of interest. In this updated report, the strategies for trapping SO2, CO or O2via iminyl-radical-triggered ring-opening of cyclobutanone oximes are summarized.

Ligand-Promoted Rosenmund–von Braun Reaction

Two picolinamide ligands were found to have significant accelerating effect to classical Rosenmund–von Braun reaction, making the coupling of (hetero)aryl bromides with CuCN occur at 100–120 °C with good to excellent yields in most cases. A large number of functional groups and heterocycles were tolerated under these conditions, thereby providing a convenient and reliable approach for diverse synthesis of aryl nitriles.

Nickel-catalyzed asymmetric reductive arylcyanation of alkenes with acetonitrile as the cyano source

Chiral 3-cyanomethyl oxindoles were synthesized in high enantioselectivities and yields. The employment of acetonitrile as a cyano source via Zn(OTf)2-assisted β-carbon elimination is distinct from the common cyanation reaction modes.

A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [Ni^III(PaPy₃*)]²⁺ (1) in acetone/CH₃CN (v/v 19:1). 1 was synthesized by reacting [Ni^II(PaPy₃*)]⁺ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H₂O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H⁺ to the PaPy₃* ligand of 2; the one-electron reduction potential (E_red) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the E_red value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA^•+ to 2 due to the binding of H⁺ to the PaPy₃* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn⁰ or Mn⁰, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry. In principle, homogeneous organic reductants can address some of the challenges associated with heterogeneous reductants and also provide greater control of the reductant strength, which can lead to new reactivity. Nevertheless, homogeneous organic reductants have rarely been used in XEC. In this Perspective, we summarize recent progress in the use of homogeneous organic electron donors in Ni-catalyzed XEC and related reactions, discuss potential synthetic and mechanistic benefits, describe the limitations that inhibit their implementation, and outline challenges that need to be solved in order for homogeneous organic reductants to be widely utilized in synthetic chemistry. Although our focus is on XEC, our discussion of the strengths and weaknesses of different methods for introducing electrons is general to other reductive transformations.

Photochemically Enabled, Ni-Catalyzed Cyanation of Aryl Halides

A light-promoted Ni-catalyzed cyanation of aryl halides employing 1,4-dicyanobenzene as a cyanating agent is reported. A broad array of aryl bromides, chlorides, and druglike molecules could be converted into their corresponding nitriles (65 examples). Mechanistic studies suggest that upon irradiation, the oxidative addition product Ni(II)(dtbbpy)(p-C₆H₄CN)(CN) undergoes homolytic cleavage of the Ni-aryl bond to generate an aryl radical and a Ni(I)-CN species, the latter of which initiates subsequent cyanation reactions.

Ru(III)-Catalyzed C4-H Bond Cyanoalkoxylation of 1-Naphthylamine Derivatives with Azobisisobutyronitrile

A simple and efficient protocol for ruthenium-catalyzed C4-H bond cyanoalkoxylation of 1-naphthylamine derivatives with AIBN was developed under an oxygen atmosphere. This reaction provides an efficient method for preparing cyano...

Tunable and Practical Homogeneous Organic Reductants for Cross-Electrophile Coupling

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air stability compared to current homogeneous reductants for metal-mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of -0.85 V versus ferrocene, which is significantly milder than conventional reductants used in XEC. All of the new reductants can facilitate C(sp²)-C(sp³) Ni-catalyzed XEC reactions and are compatible with complex substrates that are relevant to medicinal chemistry. The reductants span a range of nearly 0.5 V in reduction potential, which allows for control over the rate of electron transfer events in XEC. Specifically, we report a new strategy for controlled alkyl radical generation in Ni-catalyzed C(sp²)-C(sp³) XEC. The key to our approach is to tune the rate of alkyl radical generation from Katritzky salts, which liberate alkyl radicals upon single electron reduction, by varying the redox potentials of the reductant and Katritzky salt utilized in catalysis. Using our method, we perform XEC reactions between benzylic Katritzky salts and aryl halides. The method tolerates a variety of functional groups, some of which are particularly challenging for most XEC transformations. Overall, we expect that our new reductants will both replace conventional homogeneous reductants in current reductive transformations due to their stability and relatively facile synthesis and lead to the development of novel synthetic methods due to their tunability.

Reductively disilylated N-heterocycles as versatile organosilicon reagents

The reductively disilylated N-heterocyclic systems 1,4-bis(trimethylsilyl)-1-aza-2,5-cyclohexadiene (1Si), 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2Si) and its methyl derivatives (3Si and 4Si), and 1,1'-bis(trimethylsilyl)-4,4'-bipyridinylidene (5Si) are proficient organosilicon reagents owing to their low first vertical ionization potentials and the heterophilicity of the polarized N-Si bonds. These have prompted their reactivity as two-electron reductants or reductive silylations. These reactions benefit from the concomitant rearomatization of the N-heterocycles and liberation of trimethylsilyl halides or (Me₃Si)₂O, which are mostly volatile or easily removable byproducts. In this perspective, we have discussed the utilization of these reductively disilylated N-heterocyclic systems as versatile reagents in the salt-free reduction of transition metals (A) and main-group halides (B), in organic transformations (C) and in materials syntheses (D).

Extensive Redox Non-Innocence in Iron Bipyridine-Diimine Complexes: a Combined Spectroscopic and Computational Study

Metal-ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]-1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

Visible-Light-Induced Multicomponent Cascade Cycloaddition of N-Propargyl Aromatic Amines, Cyclobutanone Oxime Esters, and K2S2O5: Access to Cyanoalkylsulfonylated Quinolines

A visible-light-induced cascade cyanoalkylsulfonylation/cyclization/aromatization of N-propargyl aromatic amines with K₂S₂O₅ and cyclobutanone oxime esters for the construction of cyanoalkylsulfonylated quinolines is developed. This cascade transformation features mild reaction conditions, a broad substrate scope, and excellent functional group compatibility, providing a convenient route toward cyanoalkylsulfonylated quinolines via the formation of a C-C bond and two C-S bonds in one step.

Ni-Catalyzed radical cyclization of vinyl azides with cyclobutanone oxime esters to access cyanoalkyl containing quinoxalin-2(1H)-ones

A nickel-catalyzed cascade addition/cyclization of 2-azido-N-arylacrylamides and cyclobutanone oxime esters for the construction of 3-cyanoalkylated quinoxalin-2(1H)-ones is developed. This reaction proceeds under mild conditions with good functional group tolerance and broad substrate scope. A preliminary mechanistic experiment indicated that the cyanoalkyl radical might be involved in this transformation.

An advancement in the synthesis of nano Pd@magnetic amine-Functionalized UiO-66-NH2 catalyst for cyanation and O-arylation reactions

The magnetic MOF-based catalytic system has been reported here to be an efficient catalyst for synthesis of benzonitriles and diarylethers of aryl halides under optimal conditions. The MOF catalyst was built based on magnetic nanoparticles and UiO-66-NH₂ which further modified with 2,4,6-trichloro-1,3,5-triazine and 5-phenyl tetrazole at the same time and the catalyst structure was confirmed by various techniques. This new modification has been applied to increase anchoring palladium into the support. Furthermore, the products' yields were obtained in good to excellent for all reactions under mild conditions which result from superior activity of the synthesized heterogeneous catalyst containing palladium. Also, the magnetic property of the MOF-based catalyst makes it easy to separate from reaction mediums and reuse in the next runs.

Hydroboration of Nitriles, Esters, and Carbonates Catalyzed by Simple Earth-Abundant Metal Triflate Salts

During the past decade earth-abundant metals have become increasingly important in homogeneous catalysis. One of the reactions in which earth-abundant metals have found important applications is the hydroboration of unsaturated C-C and C-X bonds (X=O or N). Within these set of transformations, the hydroboration of challenging substrates such as nitriles, carbonates and esters still remain difficult and often relies on elaborate ligand designs and highly reactive catalysts (e. g., metal alkyls/hydrides). Here we report an effective methodology for the hydroboration of challenging C≡N and C=O bonds that is simple and applicable to a wide set of substrates. The methodology is based on using a manganese(II) triflate salt that, in combination with commercially available potassium tert-butoxide and pinacolborane, catalyzes the hydroboration of nitriles, carbonates, and esters at room temperature and with near quantitative yields in less than three hours. Additional studies demonstrated that other earth-abundant metal triflate salts can facilitate this reaction as well, which is further discussed in this report.

Cyanation: a photochemical approach and applications in organic synthesis

This review summarises the photocatalytic cyanation strategies to construct C(sp²)–CN, C(sp³)–CN and X–CN (X = N, S) bonds.

Reductive cyanation of organic chlorides using CO2 and NH3 via Triphos-Ni(I) species

Cyano-containing compounds constitute important pharmaceuticals, agrochemicals and organic materials. Traditional cyanation methods often rely on the use of toxic metal cyanides which have serious disposal, storage and transportation issues. Therefore, there is an increasing need to develop general and efficient catalytic methods for cyanide-free production of nitriles. Here we report the reductive cyanation of organic chlorides using CO₂/NH₃ as the electrophilic CN source. The use of tridentate phosphine ligand Triphos allows for the nickel-catalyzed cyanation of a broad array of aryl and aliphatic chlorides to produce the desired nitrile products in good yields, and with excellent functional group tolerance. Cheap and bench-stable urea was also shown as suitable CN source, suggesting promising application potential. Mechanistic studies imply that Triphos-Ni(I) species are responsible for the reductive C-C coupling approach involving isocyanate intermediates. This method expands the application potential of reductive cyanation in the synthesis of functionalized nitrile compounds under cyanide-free conditions, which is valuable for safe synthesis of (isotope-labeled) drugs.

Nitriles are key intermediates in production of pharmaceuticals, agrochemicals and organic materials. Here, the authors report a nickel-catalyzed reductive cyanation of organic chlorides with CO₂/NH₃ and urea as cyanation reagents to afford a broad range of organic nitriles.

New Nanomagnetic Heterogeneous Cobalt Catalyst for the Synthesis of Aryl Nitriles and Biaryls

Cobalt nanoparticles immobilized on magnetic chitosan (Fe3O4@CS-Co) have been prepared. They were identified using various techniques such as Fourier-transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, thermogravimetric analysis, vibrating sample magnetometry, X-ray photoelectron spectroscopy, and inductively coupled plasma atomic emission spectroscopy analysis and applied efficiently as a cobalt catalyst in the cyanation and fluoride-/palladium-free Hiyama reactions of different types of aryl halides employing K4[Fe(CN)6]·3H2O and triethoxyphenylsilane, respectively. After each reaction, the catalyst was isolated and reused for the second run. The catalytic activity of the catalyst was not lost apparently even after five runs. No considerable changes in its chemical structure and morphology were observed. It is worth to note that in this paper, the cobalt catalyst has been used for the first time for the cyanation of aryl halides.

Overcoming Selectivity Issues in Reversible Catalysis: A Transfer Hydrocyanation Exhibiting High Kinetic Control

Reversible catalytic reactions operate under thermodynamic control, and thus, establishing a selective catalytic system poses a considerable challenge. Herein, we report a reversible transfer hydrocyanation protocol that exhibits high selectivity for the thermodynamically less favorable branched isomer. Selectivity is achieved by exploiting the lower barrier for C-CN oxidative addition and reductive elimination at benzylic positions in the absence of a cocatalytic Lewis acid. Through the design of a novel type of HCN donor, a practical, branched-selective, HCN-free transfer hydrocyanation was realized. The synthetically useful resolution of a mixture of branched and linear nitrile isomers was also demonstrated to underline the value of reversible and selective transfer reactions. In a broader context, this work demonstrates that high kinetic selectivity can be achieved in reversible transfer reactions, thus opening new horizons for their synthetic applications.

Chromium-catalyzed cyclopropanation of alkenes with bromoform in the presence of 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine

Chromium-catalyzed cyclopropanation of alkenes with bromoform was achieved to produce the corresponding bromocyclopropanes. In this catalytic cyclopropanation, an organosilicon reductant, 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (1a), was indispensable for reducing CrCl3(thf)3 to CrCl2(thf)3, as well as for in situ generation of (bromomethylidene)chromium(iii) species from (dibromomethyl)chromium(iii) species. The (bromomethylidene)chromium(iii) species are proposed to react spontaneously with alkenes to give the corresponding bromocyclopropanes. This catalytic cyclopropanation was applied to various olefinic substrates, such as allyl ethers, allyl esters, terminal alkenes, and cyclic alkenes.

Recent advances in C–CN and C–H bond activation of green nitrile (MeCN) for organo-complexation, cyanation and cyanomethylation

The use of green and inexpensive organic nitrile (MeCN) as a cyano and cyano-methyl source for organo-complexation, cyanation, and cyanomethylation is reviewed.

Visible-light photoredox-catalyzed dual C–C bond cleavage: synthesis of 2-cyanoalkylsulfonylated 3,4-dihydronaphthalenes through the insertion of sulfur dioxide

A novel visible-light photoredox-catalyzed dual C–C bond cleavage of methylenecyclopropanes and cycloketone oximes for accessing 2-cyanoalkylsulfonated 3,4-dihydronaphthalenes is established.

An aerobic and green C-H cyanation of terminal alkynes

This study describes a benign C-H cyanation of terminal alkynes with α-cyanoesters serving as a nontoxic cyanide source. In situ generation of the key copper cyanide intermediate is proposed by a sequence of α-C-H oxidation and copper-mediated β-carbon elimination of α-cyanoesters, releasing the α-ketoester byproduct observed experimentally. The ensuing reaction of copper cyanide with terminal alkynes delivers preferentially cyanoalkynes and surpasses the possible Glaser type dimerization of terminal alkynes or the undesired accumulation of HCN under protic conditions. The presence of the co-oxidant K2S2O8 is crucial to this selectivity, probably by promoting oxidative transmetalation and the resulting formation of the Cu(iii)(acetylide)(CN) intermediate. All the reagents and salts used are commercially available, cheap and nontoxic, avoiding the use of highly toxic cyanide salts typically required in cyanation studies. The scope of this reaction is demonstrated with a set of alkynes and α-cyanoesters. The application of this method to late-stage functionalization of the terminal alkyne group in an estrone derivative is also feasible, showing its practical value for drug design.

Access to cyano-substituted pyrazolines through copper-catalyzed cascade cyanation/cyclization of unactivated olefins

A mild copper-catalyzed cascade cyanation/cyclization of hydrazone-tethered unactivated olefins was developed for the efficient and practical synthesis of cyano-containing pyrazolines.

Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4'-bipyridinylidene Derivatives

Chemical reduction of transition metals provides the corresponding low-valent transition metal species as a key step for generating catalytically active species in metal-assisted organic transformations and is a fundamental unit reaction for preparing organometallic complexes. A variety of metal-based reductants, such as metal powders and organometallic reagents of alkali and alkaline-earth metals, have been developed to date to access low-valent metal species. During the reduction, however, reductant-derived metal salts are formed as reaction waste, some of which often interact with the reactive low-valent metal center, thereby disrupting the catalytic performance and hampering the isolation of organometallic complexes as a result of salt coordination to the coordinatively unsaturated vacant and active sites and the formation of thermally unstable ate complexes. In this Account, we emphasize the synthetic utility and versatility of organic reductants containing two trimethylsilyl groups, i.e., 1,4-bis(trimethylsilyl)cyclohexa-2,5-diene (1a) and its methyl derivative (1b), 1,4-bis(trimethylsilyl)dihydropyrazine (2a) and its dimethyl (2b) and tetramethyl (2c) derivatives, and 1,1'-bis(trimethylsilyl)-4,4'-bipyridinylidene (3), leading to the reduction of various kinds of metal compounds in a salt-free fashion by release of two electrons together with the coproduction of easily removable (hetero)aromatics and trimethylsilyl derivatives from these organic reductants 1-3. When homoleptic chlorides of group 5 and 6 metals are treated with 1a and 1b, in situ-generated highly reactive low-valent metal species react with redox-active molecules such as ethylene, α-diimines, and α-diketones to produce metallacyclopentane, (ene-diamido)metal, and (ene-diolato)metal complexes, respectively. The advantage of the salt-free protocol is further exemplified in the low-valent titanocene-catalyzed Reformatsky-type reaction when 2c is used as a reductant: the yield of the product using the organosilicon reductant is higher than that when manganese powder is used as the reductant for the catalytic Reformatsky-type reaction of ethyl 2-bromoisobutyrate and its derivatives with various aldehydes. Moreover, when halides, carboxylates, and acetylacetonate compounds of late transition metals and main-group elements are treated with the organosilicon reductant 2c, metal(0) particles are smoothly precipitated under mild conditions. Among them, metallic nickel(0) nanoparticles are applicable to reductive biaryl formation and reductive cross-coupling of aryl halides/aryl aldehydes. In addition, reduction of the heterogeneous catalysts on a solid supporting matrix was also achieved by this salt-free reduction method; volatile byproducts are easily removed from the catalyst surface without suppressing the catalytic performance. Thus, the salt-free reduction strategy is a very powerful synthetic method that can be extended to various metals throughout the periodic table.