Electrophotochemical metal-catalyzed synthesis of alkylnitriles from simple aliphatic carboxylic acids

We report a practical and sustainable electrophotochemical metal-catalyzed protocol for decarboxylative cyanation of simple aliphatic carboxylic acids. This environmentally friendly method features easy availability of substrates, broad functional group compatibility, and directly converts a diverse range of aliphatic carboxylic acids including primary and tertiary alkyl acids into synthetically versatile alkylnitriles without using chemical oxidants or costly cyanating reagents under mild reaction conditions.

Cyanation with isocyanides: recent advances and perspectives

Cyanation has attracted considerable attention in organic synthesis because nitriles are key structural motifs in numerous important dyes, agrochemicals, natural products and drug molecules. As the fourth generation of cyanating reagents, isocyanides occupy a prominent place in the synthesis of nitriles due to their favorable stability, easy operability and high reactivity. In recent years, three types of cyanation with isocyanides have been established: the cleavage of the C-NC bond of tertiary alkyl isocyanides (Type I), the rearrangement of aryl isocyanides with azides (Type II), and the reductive cyanation of ketones with α-acidic isocyanides (Type III). This review focuses on advances in cyanation with isocyanides with an emphasis on reaction scope, limitations and mechanisms, which could reveal their remarkable value and superiority for accessing various nitriles. In addition, the future development prospects of this specific field are also introduced. We believe that this feature article will serve as a comprehensive tool to navigate cyanation with isocyanides across the vast area of synthetic chemistry.

Direct Carbon Isotope Exchange of Pharmaceuticals via Reversible Decyanation

The incorporation of carbon-14 allows tracking of organic molecules and provides vital knowledge on their fate. This information is critical in pharmaceutical development, crop science, and human food safety evaluation. Herein, a transition-metal-catalyzed procedure enabling carbon isotope exchange on aromatic nitriles is described. By utilizing the radiolabeled precursor Zn([¹⁴C]CN)₂, this protocol allows the insertion of the desired carbon tag without the need for structural modifications, in a single step. By reducing synthetic costs and limiting the generation of radioactive waste, this procedure will facilitate the labeling of nitrile containing drugs and accelerate ¹⁴C-based ADME studies supporting drug development.

Lewis acid-promoted site-selective cyanation of phenols

An efficient Lewis acid-promoted site-selective electrophilic cyanation of 3-substituted and 3,4-disubstituted phenols has been developed. The cyanation reactions using MeSCN as the cyanating reagent proceeded efficiently to afford a wide range of 2-hydroxybenzonitriles with high efficiency and excellent regioselectivity. This protocol could provide a practical method for the synthesis and modification of biologically active molecules.

Mechanistic study of the cooperative palladium/Lewis acid-catalyzed transfer hydrocyanation reaction: the origin of the regioselectivity

Density functional theory (DFT) calculations have been performed to gain insights into the catalytic mechanism of the palladium/Lewis acid-catalyzed transfer hydrocyanation of terminal alkenes to reach the linear alkyl nitrile with excellent anti-Markovnikov selectivity. The study reveals that the whole catalysis can be characterized via three stages: (i) oxidative addition generates the π-allyl complex IM2, followed by β-hydride elimination leading to the intermediate IM4, (ii) ligand exchange followed by Pd-H migratory alkene insertion gives the anti-Markovnikov intermediate IM6 and (iii) IM6 undergoes a reductive elimination step to form the linear terminal nitrile 3a and regenerates the active species for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible. The oxidative addition step, with a barrier of 30.9 kcal mol-1, should be the rate-determining step (RDS) in the whole catalysis, which agrees with the experimental high temperature of 110 °C. Furthermore, the origin of the high regioselectivity of the product with excellent anti-Markovnikov selectivity is discussed.

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.

Origins of Lewis acid acceleration in nickel-catalysed C–H, C–C and C–O bond cleavage

The effects of charge transfer, Pauli repulsion and electrostatics/polarization are identified as dominant factors for Lewis acid accelerations in Ni-catalyzed C–X (X = H, C and O) bond cleavages.

Ni-catalyzed reductive decyanation of nitriles with ethanol as the reductant

Ethanol, as an environmentally friendly and renewable reagent, was applied as a hydride donor to convert C–CN to C–H bonds in this nickel-catalyzed protocol.

Directing group strategies in catalytic sp2 C–H cyanations: scope, mechanism and limitations

Directing group strategy in transition metal catalyzed sp² C–H bond cyanation has contributed to the direct conversion of hydrocarbons to cyano-containing compounds. Recent developments in transition metal-mediated sp² C–H bond cyanation using this strategy are reviewed.

Nickel-catalyzed C-O/N-H, C-S/N-H, and C-CN/N-H annulation of aromatic amides with alkynes: C-O, C-S, and C-CN activation

The Ni-catalyzed reaction of ortho-phenoxy-substituted aromatic amides with alkynes in the presence of LiO^tBu as a base results in C–O/N–H annulation with the formation of 1(2H)-isoquinolinones. The use of a base is essential for the reaction to proceed. The reaction proceeds, even in the absence of a ligand, and under mild reaction conditions (40 °C). An electron-donating group on the aromatic ring facilitates the reaction. The reaction was also applicable to carbamate (C–O bond activation), methylthio (C–S bond activation), and cyano (C–CN bond activation) groups as leaving groups.

The Ni-catalyzed reaction of ortho-phenoxy-substituted aromatic amides with alkynes in the presence of LiO^tBu as a base results in C–O/N–H annulation with the formation of 1(2H)-isoquinolinones.

Conversion of anilines to chiral benzylic amines via formal one-carbon insertion into aromatic C-N bonds

Insertion of atoms into aromatic carbon-nitrogen bonds is an appealing method for the synthesis of nitrogen-containing molecules and it has the advantage of the availability and abundance of anilines. However, the direct cleavage of aromatic carbon-nitrogen bonds is challenging due to the particularly inert and stable nature of these bonds. Here we report a formal, enantioselective one-carbon insertion into an aromatic carbon-nitrogen bond via an aromaticity dissembly-reconstruction process to directly convert anilines to chiral α-branched benzylic amines. The process involves oxidative dearomatization of para-substituted anilines, chiral sulfur ylide-mediated asymmetric aziridination, and subsequent rearrangement. Chiral sulfur ylides serve as one-carbon insertion units.

Atom insertion into aromatic carbon-nitrogen (C-N) bonds is useful for the synthesis of nitrogen-containing molecules, but challenging due to the inert nature of these bonds. Here, the authors report one-carbon insertion into aromatic C-N bonds to directly convert anilines to chiral benzylic amines.

Pd-Senphos Catalyzed trans-Selective Cyanoboration of 1,3-Enynes

The first trans-selective cyanoboration reaction of an alkyne, specifically a 1,3-enyne, is described. The reported palladium-catalyzed cyanoboration of 1,3-enynes is site-, regio-, and diastereoselective, and is uniquely enabled by the 1,4-azaborine-based Senphos ligand structure. Tetra-substituted alkenyl nitriles are obtained providing useful boron-dienenitrile building blocks that can be further functionalized. The utility of our method has been demonstrated with the synthesis of Satigrel, an anti-platelet aggregating agent.

Lewis Acid Mediated Electrophilic Cyanation of 2,2'-Biphenols

A Lewis acid mediated electrophilic cyanation of 2,2'-biphenols with a trifluoromethanesulfonyl (Tf) protecting group is reported. The cyanation reactions with less toxic, commercially available MeSCN as a cyanating reagent afforded a range of 3-cyan-2,2'-biphenols in moderate to high yields. The use of trifluoromethanesulfonyl (Tf) as a protecting group is crucial to the success of this transformation. Moreover, the cyanated products were readily transformed into various synthetically useful molecules. This protocol features high efficiency, excellent regioselectivity, and good functional group compatibility and may provide a practical tool for the synthesis and modification of biologically active compounds, catalysts, and ligands.

A methylation platform of unconventional inert aryl electrophiles: trimethylboroxine as a universal methylating reagent

Methylation is one of the most fundamental conversions in medicinal and material chemistry. Extension of substrate types from aromatic halides to other unconventional aromatic electrophiles is a highly important yet challenging task in catalytic methylation. Disclosed herein is a series of transition metal-catalyzed methylations of unconventional inert aryl electrophiles using trimethylboroxine (TMB) as the methylating reagent. This transformation features a broad substrate type, including nitroarenes, benzoic amides, benzoic esters, aryl cyanides, phenol ethers, aryl pivalates and aryl fluorides. Another important merit of this work is that these widespread “inert” functionalities are capable of serving as directing or activating groups for selective functionalization of aromatic rings before methylation, which greatly expands the connotation of methylation chemistry.

A universal platform for catalytic methylation of a series of unconventional inert aryl electrophiles with trimethylboroxine (TMB) is established, which provides a synthetic chemist's toolbox for the construction of methylated arenes.

Asymmetric Total Syntheses of Di- and Sesquiterpenoids by Catalytic C-C Activation of Cyclopentanones

To show the synthetic utility of the catalytic C-C activation of less strained substrates, described here are the collective and concise syntheses of the natural products (-)-microthecaline A, (-)-leubehanol, (+)-pseudopteroxazole, (+)-seco-pseudopteroxazole, pseudopterosin A-F and G-J aglycones, and (+)-heritonin. The key step in these syntheses involve a Rh-catalyzed C-C/C-H activation cascade of 3-arylcyclopentanones, which provides a rapid and enantioselective route to access the polysubstituted tetrahydronaphthalene cores presented in these natural products. Other important features include 1) the direct C-H amination of the tetralone substrate in the synthesis of (-)-microthecaline A, 2) the use of phosphoric acid to enhance efficiency and regioselectivity for problematic cyclopentanone substrates in the C-C activation reactions, and 3) the direct conversion of serrulatane into amphilectane diterpenes by an allylic cyclodehydrogenation coupling.

Chiral Aluminum Complex Controls Enantioselective Nickel-Catalyzed Synthesis of Indenes: C-CN Bond Activation

A chiral aluminum complex controlled, enantioselective nickel-catalyzed domino reaction of aryl nitriles and alkynes proceeding by C-CN bond activation was developed. The reaction provides various indenes, bearing chiral all-carbon quaternary centers, under mild reaction conditions in yields of 32 to 91 % and ee values within the 73-98 % range. The reaction mechanism and aspects of stereocontrol were investigated by DFT calculations.

Mechanistic Study of Unprecedented Highly Regioselective Hydrocyanation of Terminal Alkynes: Insight into the Origins of the Regioselectivity and Ligand Effects

Density functional theory (DFT) calculations were performed to gain insight into the mechanism of the nickel-catalyzed hydrocyanation of terminal alkynes with Zn(CN)₂ and water to exclusively generate the branched nitrile with excellent Markovnikov selectivity. After precatalyst activation to give the LNi(0) active species, the transformation proceeds via the following steps: (1) oxidative addition of H₂ O to the LNi(0) provides the intermediate LNi(II)H(OH); (2) ligand exchange of LNi(II)H(OH) with Zn(CN)₂ gives the intermediate LNi(II)H(CN); (3) alkyne insertion to the LNi(II)H(CN) forms the alkenyl nickel complex, followed by the reductive elimination step reaching the final product. This mechanism is kinetically and thermodynamically more favorable than that of the experimental proposed ones. On the basis of the experimental observations, more water molecules cannot further improve the reaction as it has also been rationalized. Furthermore, the origin of the high regioselectivity of the product, the variable effectiveness of the metal mediator as function of ligands, as well as the high yield of the alkyl-substituted alkynes substrates, is analyzed in detail. © 2019 Wiley Periodicals, Inc.

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.

Titanium(III)-Catalyzed Reductive Decyanation of Geminal Dinitriles by a Non-Free-Radical Mechanism

A titanium-catalyzed mono-decyanation of geminal dinitriles is reported. The reaction proceeds under mild conditions, tolerates numerous functional groups, and can be applied to quaternary malononitriles. A corresponding desulfonylation is demonstrated as well. Mechanistic experiments support a catalyst-controlled cleavage without the formation of free radicals, which is in sharp contrast to traditional stoichiometric radical decyanations. The involvement of two TiIII species in the C-C cleavage is proposed, and the beneficial role of added ZnCl2 and 2,4,6-collidine hydrochloride is investigated.

Ruthenium-Catalyzed Reductive Cleavage of Unstrained Aryl-Aryl Bonds: Reaction Development and Mechanistic Study

Cleavage of carbon-carbon bonds has been found in some important industrial processes, for example, petroleum cracking, and has inspired development of numerous synthetic methods. However, nonpolar unstrained C(aryl)-C(aryl) bonds remain one of the toughest bonds to be activated. As a detailed study of a fundamental reaction mode, here a full story is described about our development of a Ru-catalyzed reductive cleavage of unstrained C(aryl)-C(aryl) bonds. A wide range of biaryl compounds that contain directing groups (DGs) at 2,2' positions can serve as effective substrates. Various heterocycles, such as pyridine, quinoline, pyrimidine, and pyrazole, can be employed as DGs. Besides hydrogen gas, other reagents, such as Hantzsch ester, silanes, and alcohols, can be employed as terminal reductants. The reaction is pH neutral and free of oxidants; thus a number of functional groups are tolerated. Notably, a one-pot C-C activation/C-C coupling has been realized. Computational and experimental mechanistic studies indicate that the reaction involves a ruthenium(II) monohydride-mediated C(aryl)-C(aryl) activation and the resting state of the catalyst is a η4-coordinated ruthenium(II) dichloride complex, which could inspire development of other transformations based on this reaction mode.

Non-toxic cyanide sources and cyanating agents

The present review gives an overview over non-toxic cyanation agents and cyanide sources used in the synthesis of structurally diverse products containing the nitrile function. Nucleophilic as well as electrophilic agents/systems that transfer the entire CN-group were taken in consideration. Reactions in which a preexisting carbon functionality is transformed into a nitrile function by addition of nitrogen are however not covered here.

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

A catalyst system of [Ni(MeCN)₆](BF₄)₂, 1,10-phenanthroline, and 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (Si–Me₄-DHP) assisted cyanation of aryl halides in acetonitrile to give the corresponding aryl nitriles.

We developed a non-toxic cyanation reaction of various aryl halides and triflates in acetonitrile using a catalyst system of [Ni(MeCN)₆](BF₄)₂, 1,10-phenanthroline, and 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (Si–Me₄-DHP). Si–Me₄-DHP was found to function as a reductant for generating nickel(0) species and a silylation reagent to achieve the catalytic cyanation via C–CN bond cleavage.

Formal group insertion into aryl C‒N bonds through an aromaticity destruction-reconstruction process

Given the abundance and the ready availability of anilines, the selective insertion of atoms into the aryl carbon–nitrogen bonds will be an appealing route for the synthesis of nitrogen-containing aromatic molecules. However, because aryl carbon–nitrogen bonds are particularly inert, anilines are normally activated by conversion to more reactive intermediates such as aryldiazonium salts to achieve functionalization of the aryl carbon–nitrogen bonds, but the nitrogen atom is usually not incorporated into products, instead being discarded. The selective insertion of groups into aryl carbon–nitrogen bonds remains an elusive challenge and an unmet need in reaction design. Here we show an aromaticity destruction-reconstruction process that selectively inserts a trimethylenemethane (TMM) group into the aromatic carbon–nitrogen bond of anilines concomitant with a benzylic carbon–hydrogen bond functionalization. This process provides a transformative mode for anilines, and the insertion products are versatile precursor to various nitrogen-containing aromatic molecules through simple conversions.

Activation of C–N bonds of anilines requires transformation of the amino group into a more reactive functionality. Here, the authors report an aromaticity destruction-reconstruction process that converts abundant anilines into valuable amines through group insertion into the C–N bond and benzylic C–H functionalization.

Palladium and Lewis-Acid-Catalyzed Intramolecular Aminocyanation of Alkenes: Scope, Mechanism, and Stereoselective Alkene Difunctionalizations

An expansion of methodologies aimed at the formation of versatile organonitriles, via the intramolecular aminocyanation of unactivated alkenes, is herein reported. Importantly, the need for a rigid tether in these reactions has been obviated. The ease-of-synthesis and viability of substrates bearing flexible backbones has permitted for diastereoselective variants as well. We demonstrated the utility of this methodology with the formation of pyrrolidones, piperidinones, isoindolinones, and sultams. Furthermore, subsequent transformation of these motifs into medicinally relevant molecules is also demonstrated. A double crossover ¹³C-labeling experiment is consistent with a fully intramolecular cyclization mechanism. Deuterium labeling experiments support a mechanism involving syn-addition across the alkene.

Transition-metal-free arylation of benzoxazoles with aryl nitriles

Transition-metal-free arylation of benzoxazoles with aryl nitriles has been developed to afford important 2-aryl benzoxazoles under simple reaction conditions.

Nickel(II)-Catalyzed Site-Selective C-H Bond Trifluoromethylation of Arylamine in Water through a Coordinating Activation Strategy

The first example of nickel(II)-catalyzed site-selective C-H bond trifluoromethylation of arylamine in water is established. In this transformation, a coordinating activation strategy is performed by the utilization of picolinamide as a directing group, and target products are obtained in moderate to good yields. In addition, the catalyst-in-water system can be reutilized eight times with a slight loss of catalytic activity and applied in the green, concise synthesis of acid red 266. Furthermore, a series of control experiments verify that a single-electron transfer mechanism is responsible for this reaction.

Nickel-Catalyzed Coupling of Azoles with Aromatic Nitriles

This manuscript describes the Ni-catalyzed coupling of azoles with aromatic nitriles. The use of BPh3 promotes these arylations with electronically diverse azoles and benzonitriles. While the nickel catalyst is necessary for the arylations of phenyl oxazoles, arylation of benzoxazoles with some nitriles affords the arylated products even in the absence of the Ni catalyst albeit in lower yield than the catalyzed process. The Ni-catalyzed process exhibits higher rates and a broader scope than the uncatalyzed transformation.

Development and Mechanistic Study of Quinoline-Directed Acyl C-O Bond Activation and Alkene Oxyacylation Reactions

The intramolecular addition of both an alkoxy and acyl substituent across an alkene, oxyacylation of alkenes, using rhodium catalyzed C-O bond activation of an 8-quinolinyl ester is described. Our unsuccessful attempts at intramolecular carboacylation of ketones via C-C bond activation ultimately informed our choice to pursue and develop the intramolecular oxyacylation of alkenes via quinoline-directed C-O bond activation. We provide a full account of our catalyst discovery, substrate scope, and mechanistic experiments for quinoline-directed alkene oxyacylation.