Recent advances in radical thiocyanation cyclization or spirocyclization reactions

Organic thiocyanates are valuable biological moities and drug-building blocks. They can also transform effectively into thioethers, thiols, alkynyl thioethers, and thiocarbamates in synthetic chemistry. With respect to the merits of thiocyanates, many chemists and our research team have developed diverse strategies to access SCN-revised heterocycles/spirocycles via an effective radical cyclization process. Hence, this review article first describes the importance/application of thiocyanates. Subsequently, it summarizes the reaction conditions, substrate scopes, and plausible mechanism, respectively, of the excellent work stated above.

Photoinduced Tandem Cyanomethylation/Cyclization of Unsaturated Compounds: Access to Cyanomethylated 7- or 5-Membered N-Heterocycles

A cyanomethylation/cyclization of aryl acetylenes/ethylenes with bromoacetonitrile was finished in a photopromoted condition, which offers an efficient and mild protocol for the preparation of cyanomethylated 7- or 5-membered N-heterocycles with good yields. Meanwhile, trichloroacetonitrile was also compatible with this radical pathway. In addition, a variety of single-crystal X-ray diffraction measurements, scaled-up operations to 1 mmol, functional group transformations of final products, light on/off experiments, and even radial inhibition studies were smoothly performed in this tandem system.

Copper-Catalyzed Bromo-cyanomethylative Cyclization of Enynes

A copper-catalyzed bromo-cyanomethylative cyclization of 1,6-enynes is demonstrated. The treatment of 2-bromoacetonitrile with CuI enables the alkyl radical generation and triggers the radical addition/cyclization/bromination sequence, giving various vinyl C-Br bonds containing functionalized heterocycles in good yields.

Visible light photocatalysis in the synthesis of pharmaceutically relevant heterocyclic scaffolds

Visible light and photoredox catalysis have emerged as a powerful and long-lasting tool for organic synthesis, demonstrating the importance of a variety of chemical bond formation methods.

The visible-light-induced acylation/cyclization of alkynoates with acyl oximes for the construction of 3-acylcoumarins

A nitrogen-centered radical-mediated carbon-carbon bond cleavage strategy is described to synthesize functionalized 3-acylcoumarins. The strategy is enabled by the visible-light-induced acylation/cyclization of alkynoates with various acyl oxime compounds in acetonitrile. The difunctionalization of carbon-carbon triple bonds precedes the generation of iminyl radicals, which is followed by the formation of acyl radicals. The acyl radicals then attack the carbon-carbon triple bonds, followed by 5-exo-trig cyclization and 1,2-ester migration. This strategy has wide substrate adaptability and good substituent tolerance.

Visible-Light-Driven, Photocatalyst-Free Cascade to Access 3-Cyanoalkyl Coumarins from ortho-Hydroxycinnamic Esters

A visible-light-driven, photocatalyst-free route starting from easily accessed ortho-hydroxycinnamic esters and O-perfluoropyridin-4-yl oximes has been successfully developed to rapidly assemble a wide range of 3-cyanoalkyl coumarins. This process does not require addition of external photocatalysts, exhibiting beneficial features including mild reaction conditions, synthetic simplicity, and excellent substrate compatibility. Extensive mechanistic investigations revealed that the in situ generated phenolate anions served as photosensitizers to drive this photoinduced transformation.

Visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates leading to coumarins via SO2 insertion

A visible-light-mediated cascade cyanoalkylsulfonylation/cyclization of alkynoates with cycloketone oxime compounds for the preparation of 3-cyanoalkylsulfonylcoumarins via SO₂ insertion is reported.

A computational study for the reaction mechanism of metal-free cyanomethylation of aryl alkynoates with acetonitrile

A computational study of metal-free cyanomethylation and cyclization of aryl alkynoates with acetonitrile is carried out employing density functional theory and high-level coupled-cluster methods, such as coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. Our results indicate that the reaction of aryl alkynoates with acetonitrile in the presence of tert-butyl peroxybenzoate (TBPB) under metal-free conditions tends to proceed through cyanomethylation, spirocyclization and ester migration of the kinetically favoured coumarin derivatives. 1,2-Ester migration in the spiro-radical intermediate 10 does not proceed via the formation of the carboxyl radical 11 suggested by Sun and co-workers. Our results also demonstrate that the t-butoxy radical is substantially responsible the formation of the cyanomethyl radical by the abstraction of a hydrogen atom from acetonitrile.

A computational study of metal-free cyanomethylation and cyclization of aryl alkynoates with acetonitrile is carried out employing density functional theory and high-level coupled-cluster methods, such as [CCSD(T)].

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.

Cyanation of alkynes was highlighted. Direct cyanation and hydrocyanation gave alkynyl cyanides and alkenyl nitriles. Dicyanation produced 1,2-dicyano adducts. Cyanofunctionalization afforded functional cyano compounds.

Bis-Cyclometalated Indazole Chiral-at-Rhodium Catalyst for Asymmetric Photoredox Cyanoalkylations

A new class of bis‐cyclometalated rhodium(III) catalysts containing two inert cyclometalated 6‐tert‐butyl‐2‐phenyl‐2H‐indazole ligands and two labile acetonitriles is introduced. Single enantiomers (>99 % ee) were obtained through a chiral‐auxiliary‐mediated approach using a monofluorinated salicyloxazoline. The new chiral‐at‐metal complex is capable of catalyzing the visible‐light‐induced enantioselective α‐cyanoalkylation of 2‐acyl imidazoles in which it serves a dual function as the chiral Lewis acid catalyst for the asymmetric radical chemistry and at the same time as the photoredox catalyst for the visible‐light‐induced redox chemistry (up to 80 % yield, 4:1 d.r., and 95 % ee, 12 examples).

Ring-opening C(sp3)–C coupling of cyclobutanone oxime esters for the preparation of cyanoalkyl containing heterocycles enabled by photocatalysis

A ring-opening C(sp³)–C coupling of cyclobutanone oxime esters for the preparation of cyanoalkyl containing heterocycles was realized under visible-light or sunlight irradiation.

An efficient light on–off one-pot method for the synthesis of 3-styryl coumarins from aryl alkynoates

An efficient one-pot stepwise method to synthesize 3-styryl-4-arylcoumarins from simple alkynoates is demonstrated.