Cobalt-Catalyzed C–H Cyanation of (Hetero)arenes and 6-Arylpurines with N-Cyanosuccinimide as a New Cyanating Agent

Ligand-Enabled C6-Selective C-H Arylation of Pyrrolo[2,3-d] Pyrimidine Derivatives with Pd Catalysts: An Approach to the Synthesis of EGFR Inhibitor AEE-788

Herein, we report the Pd(II)-catalyzed direct C-H arylation of pyrrolo[2,3-d]pyrimidine derivatives with aryl iodides, which is enabled by bidentate pyridine-pyridine ligands. A range of aryl iodides proved to be suitable coupling partners affording the desired products in good yields with high levels of C6 selectivity. This protocol features good tolerance of reactive functional groups, mild reaction conditions, and a simple reaction system, which provides an expeditious route to an essential class of 6-arylpyrrolo[2,3-d]pyrimidines frequently found in bioactive compounds, and provides a step-economical access to the second-generation EGFR inhibitor AEE-788.

Cobalt-Catalyzed Enantio- and Regioselective C(sp3 )-H Alkenylation of Thioamides

An enantioselective cobalt-catalyzed C(sp3 )-H alkenylation of thioamides with but-2-ynoate ester coupling partners employing thioamide directing groups is presented. The method is operationally simple and requires only mild reaction conditions, while providing alkenylated products as single regioisomers in excellent yields (up to 85 %) and high enantiomeric excess [up to 91 : 9 enantiomeric ratio (er), or up to >99 : 1 er after a single recrystallization]. Diverse downstream derivatizations of the products are demonstrated, delivering a range of enantioenriched constructs. Extensive computational studies using density functional theory provide insight into the detailed reaction mechanism, origin of enantiocontrol, and the unusual regioselectivity of the alkenylation reaction.

N-Cyano-2,2'-biphenyldicarboimide as a Cyanation Reagent for Co(III)-Catalyzed C-H Cyanation of Indoles in Ionic Liquids

A mild strategy for Co(III)-catalyzed C(sp²)-H cyanation of indoles was developed by using NCBLD as an electrophilic cyanation reagent and 1-butyl-3-acetylimidazole ditrifluoromethylsulfonimide ([BAIM]NTf₂) as an environmentally friendly and recyclable solvent, and a series of 2-cyano products were obtained at room temperature. Adopting this strategy, the unnatural nucleotide fragment precursor of Remdesivir, which was a drug for COVID-19, was synthesized through cyano transformation, further proving the practicability of this cyanation method.

Direct C-H Cyanation by ICN Formed In Situ: Nannozinone B

A novel method for C-H cyanation of different pyrans, pyrroles, indoles, and acyclic nucleophilic double bonds using TMSCN, NIS, and Zn(OTf)₂ as a catalyst is described. The transformation is conducted under mild conditions tolerating a variety of functional groups. Zn(OTf)₂ is likely to serve a dual catalytic role as an activator for TMSCN and for the cyanogen iodide generated in situ. Optimization, the substrate scope, and mechanistic observations are reported. Furthermore, this method is applied in the first total synthesis of the natural product nannozinone B.

Regiodivergent Cu-Promoted, AcOH-Switchable Distal Versus Proximal Direct Cyanation of 1-Aryl-1H-indazoles and 2-Aryl-2H-indazoles via Aerobic Oxidative C-H Bond Activation

A copper-promoted regiodivergent, AcOH-switchable, distal and proximal direct cyanation of N-aryl-(1H/2H)-indazoles via aerobic oxidative C(sp²)-H bond activation has been developed. The inclusion or exclusion of AcOH as an additive is the foremost cause for the positional switch in the C-CN bond formation method that results in (C-2')-cyanated 2-aryl-2H-indazoles 3a-j, (C-2')-cyanated 1-aryl-1H-indazoles 4a-j [distal], or C-3 cyanated 2-aryl-2H-indazoles 5a-i [proximal] products in good to excellent yields and showed various functional group tolerance. The cyanide (CN^-) ion surrogate was generated via the unification of dimethylformamide and ammonium iodide (NH₄I). The utilization of molecular oxygen (aerobic oxidative strategy) as a clean and safe oxidant is liable for generous value addition. The further pertinence of the developed protocol has been demonstrated by transforming the synthesized cyanated product into numerous other functional groups, which will, undoubtedly, accomplish utilization in the synthetic area of biologically important compounds and medicinal chemistry.

The stable "F-SO2 +" donor provides a mild and efficient approach to nitriles and amides

In this update, we developed a mild, efficient and practical method using fluorosulfuryl imidazolium salt A as an environment friendly promoter for conversion of oximes to nitriles or amides via β-elimination or Beckmann rearrangement in almost quantitative yield in 10 minutes. The target products were generated in gram-scale and could be collected through crystallization without silica gel column purification in excellent yield.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.

Copper-Mediated Direct Aromatic ortho-C–H Cyanation by AIBN

We have developed a copper-mediated chelation-assisted direct aromatic ortho-C–H cyanation that uses AIBN as a safe cyanation reagent. The substrate scope included indoles, pyrroles, a carbazole, and a thiophene.

Recent advances and perspectives in ruthenium-catalyzed cyanation reactions

The cyanation reaction has achieved rapid progress in recent times. The ability to exhibit multiple oxidation states increased the demand of ruthenium in the field of catalysis. These cyanation reactions have wide application in pharmacological and biological fields. This review gives an overview of the ruthenium-catalyzed cyanation reactions covering literature up to 2021.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

Photo-Promoted Decarboxylative Alkylation of α,β-Unsaturated Carboxylic Acids with ICH2CN for the Synthesis of β,γ-Unsaturated Nitriles

An efficient, catalyst/photocatalyst-free, and cost-effective methodology for the decarboxylative alkylation of α,β-unsaturated carboxylic acids to synthesize β,γ-unsaturated nitriles has been developed. The reaction proceeded in an environmentally benign atmosphere of blue light-emitting diode irradiation with K₂CO₃ and water at room temperature. The methodology worked for a wide range of substrates (22 examples) with up to 83% yield. The protocol is also compatible for gram-scale synthesis.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

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.

Revisiting the synthesis of aryl nitriles: a pivotal role of CAN

Facilitated by the dual role of Ceric Ammonium Nitrate (CAN), herein we report a cost-effective approach for the cyanation of aryl iodides/bromides with CAN-DMF as an addition to the existing pool of combined cyanation sources. In addition to being an oxidant, CAN acts as a source of nitrogen in our protocol. The reaction is catalyzed by a readily available Cu(ii) salt and the ability of CAN to generate ammonia in the reaction medium is utilized to eliminate the additional requirement of a nitrogen source, ligand, additive or toxic reagents. The mechanistic study suggests an evolution of CN- leading to the synthesis of a variety of aryl nitriles in moderate to good yields. The proposed mechanism is supported by a series of control reactions and labeling experiments.

Synthesis of Nitrile Bearing Acyclic Quaternary Centers through Co(III)-Catalyzed Sequential C-H Bond Addition to Dienes and N-Cyanosuccinimide

Herein we disclose a three-component strategy to access quaternary centers bearing nitriles by cobalt-catalyzed C-H bond activation and sequential addition to internally substituted 1,3-dienes and an electrophilic cyanating reagent with high regio and stereocontrol. 2-Aryl and alkyl monosubstituted dienes provide α-aryl and α-alkyl α-methyl-substituted nitriles, respectively. An even wider variety of functionality can be installed at the quaternary carbon by using 1,2-disubstituted dienes. The synthetic utility of the nitrile products was successfully demonstrated by various transformations, including conversions to γ-lactones and tetrazoles. The observed connectivity in the products along with studies with deuterium labeled reactants provide insight into the mechanism. Formation of a 7-membered cobaltacycle by C-H activation and migratory insertion of the diene is followed by β-hydride elimination and hydride reinsertion to give a 6-membered cobaltacycle that then reacts with the cyanating agent.

C–CN bond formation: an overview of diverse strategies

Aim for cyanation: a comprehensive overview on various approaches on C–CN bond formation in arenes/heteroarenes by activated halides/pseudohalide, directed, non-directed, electro-catalyzed, photoredox-catalyzed, and radical approaches.

Stereospecific cyanation of the olefinic C–H bond enabled by 1,4-rhodium migration

Rhodium-catalyzed stereospecific cyanation of the olefinic C–H bond for the synthesis of β,β-disubstituted acrylonitriles has been developed.

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.

Rh(III)-Catalyzed C-H Cyanation of 2H-Indazole with N-Cyano-N-phenyl-p-toluenesulfonamide

A Rh(III)-catalyzed direct cyanation of 2H-indazoles with N-cyano-N-phenyl-p-toluenesulfonamide has been realized via a chelation-assisted strategy. The methodology enables regioselective access to various ortho-cyanated phenylindazoles in good yields with a broad substrate scope and good functional group compatibility. The obtained cyanated indazoles could further be converted into other value-added chemicals. Importantly, the current protocol is featured with several characteristics, including a novel cyanating agent, good regioselectivity, and operational convenience.

Cobalt-Catalyzed Enantioselective Hydroarylation of 1,6-Enynes

An asymmetric hydroarylative cyclization of enynes involving a C-H bond cleavage is reported. The cobalt-catalyzed cascade generates three new bonds in an atom-economical fashion. The products were obtained in excellent yields and excellent enantioselectivities as single diastereo- and regioisomers. Preliminary mechanistic studies indicate that the reaction shows no intermolecular C-H crossover. This work highlights the potential of cobalt catalysis in C-H bond functionalization and enantioselective domino reactivity.

Direct cyanation, hydrocyanation, dicyanation and cyanofunctionalization of alkynes

In this review, direct cyanation, hydrocyanation, dicyanation, cyanofunctionalization and other cyanation reactions of alkynes were highlighted. Firstly, the use of nitriles and development of cyanation was simply introduced. After presenting the natural properties of alkynes, cyanation reactions of alkynes were classified and introduced in detail. Transition metal catalysed direct cyanation and hydrocyanation of alkynes gave alkynyl cyanides and alkenyl nitriles in good yields. Dicyanation of alkynes produced 1,2-dicyano adducts. Cyanofunctionalization of alkynes afforded functional cyanated compounds. Thiocyanation and selenocyanation yielded the expected functional vinylthiocyanates and vinylselenocyanates. A plausible reaction mechanism is presented if available.

Synthesis of Benzofulvenes via Cp*Co(III)-Catalyzed C-H Activation and Carbocyclization of Aromatic Ketones with Internal Alkynes

A highly efficient and practical synthesis of benzofulvenes was developed via ketone-directed Cp*Co(III)-catalyzed sequential C-H activation, addition, cyclization, and dehydration cascade processes between simple aromatic ketones and alkynes. The reaction tolerates a variety of functional groups, and various benzofulvenes were efficiently synthesized in 42-92% yields.

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.

Rhodium(iii)-catalyzed aromatic C–H cyanation with dimethylmalononitrile as a cyanating agent

Safe, bench-stable, and commercially available DMMN was used as a cyanating reagent in Rh(iii)-catalyzed aromatic C–H bond direct cyanation in the presence of copper oxide as a promotor.