Electrochemically Mediated Synthesis of Cyanated Heterocycles from α-amino Esters, Pyridine-2-carbaldehydes and NH4SCN as Cyano Group Source

The electrochemically mediated cyanation/annulation process with in situ cyanide ion generation from NH4SCN and multi-step oxidative construction of CN-functionalized heterocycles from easily available α-amino esters and pyridine-2-carbaldehydes has been discovered. Depending on the nature of the α-amino ester, 1-cyano-imidazo[1,5-a]pyridine-3-carboxylates, 3-alkyl- and 3-aryl-imidazo[1,5-a]pyridines-1-carbonitriles, and the first reported 4-oxo-4H-pyrido[1,2-a]pyrazine-1-carbonitriles were obtained. The electrosynthesis is carried out in an undivided electrochemical cell under constant current conditions. The success of the discovered electrochemical synthesis is based on the combination of two anodic processes: oxidation of SCN anion to CN anion and oxidation of C-N bonds to C=N bonds during heterocycle construction. Mechanistic studies based on CV measurements, and control experiments confirm the generation of [CN] species from NH4SCN with subsequent addition to an imine formed from α-amino esters and pyridine-2-carbaldehyde. Computational analysis suggests that for reactive intermediates from glycine esters, the subsequent 5-endo-trig cyclization leading to 1-cyano-imidazo[1,5-a]pyridine-3-carboxylates is more favourable and the 6-exo-trig cyclization leading to 4-oxo-4H-pyrido[1,2-a]pyrazine-1-carbonitriles is less favourable. For α-amino esters with alkyl or aryl substituents, both cyclization pathways are relatively thermodynamically possible. The leading 4-oxo-4H-pyrido[1,2-a]pyrazine-1-carbonitrile showed high fungicidal activity against phytopathogenic fungi.

Formation of a diiron-(μ-η1:η1-CN) complex from acetonitrile solution

The activation of C-C bonds by transition-metal complexes is of continuing interest and acetonitrile (MeCN) has attracted attention as a cyanide source with comparatively low toxicity for organic cyanation reactions. A diiron end-on μ-η1:η1-CN-bridged complex was obtained from a crystallization experiment of an open-chain iron-NHC complex, namely, μ-cyanido-κ2C:N-bis{[(acetonitrile-κN)[3,3'-bis(pyridin-2-yl)-1,1'-(methylidene)bis(benzimidazol-2-ylidene)]iron(II)} tris(hexafluorophosphate), [Fe2(CN)(C2H3N)2(C25H18N6)2](PF6)3. The cyanide appears to originate from the MeCN solvent by C-C bond cleavage or through carbon-hydrogen oxidation.

Reaction mechanism - explored with the unified reaction valley approach

One of the ultimate goals of chemistry is to understand and manipulate chemical reactions, which implies the ability to monitor the reaction and its underlying mechanism at an atomic scale. In this article, we introduce the Unified Reaction Valley Approach (URVA) as a tool for elucidating reaction mechanisms, complementing existing computational procedures. URVA combines the concept of the potential energy surface with vibrational spectroscopy and describes a chemical reaction via the reaction path and the surrounding reaction valley traced out by the reacting species on the potential energy surface on their way from the entrance to the exit channel, where the products are located. The key feature of URVA is the focus on the curving of the reaction path. Moving along the reaction path, any electronic structure change of the reacting species is registered by a change in the normal vibrational modes spanning the reaction valley and their coupling with the path, which recovers the curvature of the reaction path. This leads to a unique curvature profile for each chemical reaction, with curvature minima reflecting minimal change and curvature maxima indicating the location of important chemical events such as bond breaking/formation, charge polarization and transfer, rehybridization, etc. A decomposition of the path curvature into internal coordinate components or other coordinates of relevance for the reaction under consideration, provides comprehensive insight into the origin of the chemical changes taking place. After giving an overview of current experimental and computational efforts to gain insight into the mechanism of a chemical reaction and presenting the theoretical background of URVA, we illustrate how URVA works for three diverse processes, (i) [1,3] hydrogen transfer reactions; (ii) α-keto-amino inhibitor for SARS-CoV-2 Mpro; (iii) Rh-catalyzed cyanation. We hope that this article will inspire our computational colleagues to add URVA to their repertoire and will serve as an incubator for new reaction mechanisms to be studied in collaboration with our experimental experts in the field.

Catalytic Cyanation of C-N Bonds with CO2/NH3

Cyanation of benzylic C-N bonds is useful in the preparation of important α-aryl nitriles. The first general catalytic cyanation of α-(hetero)aryl amines, analogous to the Sandmeyer reaction of anilines, was developed using reductive cyanation with CO₂/NH₃. A broad array of α-aryl nitriles was obtained in high yields and regioselectivity by C-N cleavage of intermediates as ammonium salts. Good tolerance of functional groups such as ethers, CF₃, F, Cl, esters, indoles, and benzothiophenes was achieved. Using ¹³CO₂, a ¹³C-labeled tryptamine homologue (five steps, 31% yield) and Cysmethynil (six steps, 37% yield) were synthesized. Both electronic and steric effects of ligands influence the reactivity of alkyl nickel species with electrophilic silyl isocyanates and thus determine the reactivity and selectivity of the cyanation reaction. This work contributes to the understanding of the controllable activation of CO₂/NH₃ and provides the promising potential of the amine cyanation reaction in the synthesis of bio-relevant molecules.

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.

Rapid and Simple Access to α-(Hetero)arylacetonitriles from Gem-Difluoroalkenes

A scalable cyanation of gem-difluoroalkenes to (hetero)arylacetonitrile derivatives was developed. This strategy features mild reaction conditions, excellent yields, wide substrate scope, and broad functional group tolerance. Significantly, in this reaction, aqueous ammonia offers a "N" source for the "CN" reagent and entirely avoids the use of toxic cyanating reagents or metal catalysis. Hence, we provide a green and alternative method for the synthesis of arylacetonitriles.

Cyanide Anions as Nucleophilic Catalysts in Organic Synthesis

The nucleophilic addition of a cyanide anion to a carbonyl group is the basis for several cyanide-catalyzed organic reactions, which are summarized in this review. Since cyanide is also a good leaving group, it is an excellent catalyst for transacylation reactions. As an electron-withdrawing group, it also stabilizes a negative charge in its α-position, thus allowing the umpolung of aldehydes to formyl anion equivalents. The two leading examples are the benzoin condensation and the Michael–Stetter reaction furnishing α-hydroxy ketones and 1,4-dicarbonyl compounds, which are both catalyzed by cyanides. The review also covers variants like the silyl-benzoin coupling, the aldimine coupling and the imino-Stetter reaction. Moreover, some cyanide-catalyzed heterocyclic syntheses are reviewed.

1 Introduction

2 Nucleophilic Additions

2.1 Cyanohydrin Formation

2.2 Corey–Gilman–Ganem and Related Oxidation Reactions

2.3 Conjugate Addition

2.4 Intramolecular Carbocyanation

3 Transacylation Reactions

3.1 Ester Hydrolysis and Transesterification

3.2 Formation of Amides

3.3 Ketones from Esters

3.4 Esters from Ketones

4 Transformations Involving an Umpolung

4.1 Benzoin Condensation

4.2 Aldimine Coupling

4.3 Michael–Stetter Reaction

4.4 Imino-Stetter Reaction

5 Formation of Heterocycles

5.1 Oxazolines from Isocyanoacetates

5.2 Imidazoles from TosMIC via Oxazolines

5.3 Bargellini Reaction

6 Conclusion

Cu-catalyzed cyanomethylation of imines and α,β-alkenes with acetonitrile and its derivatives

We describe copper-catalyzed cyanomethylation of imines and α,β-alkenes with a methylnitrile source and provide an efficient route to synthesize arylacrylonitriles and β,γ-unsaturated nitriles. This method tolerates aliphatic and aromatic alkenes substituted with a variety of functional groups such as F, Cl, Br, Me, OMe, tert-Bu, NO2, NH2 and CO2H with good to excellent yields (69-98%). These systems consist of inexpensive, simple copper catalyst and acetonitrile with its derivatives (α-bromo/α-iodo-acetonitrile) and are highly applicable in the industrial production of acrylonitriles.

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.