Ketenimines are an important class of reactive species and useful synthetic intermediates. During the last two decades several practical and versatile approaches to ketenimines have been developed, leading to exhaustive investigations on ketenimine chemistry and the discovery of a variety of highly efficient reactions. Five types of reactions for ketenimines have been reported, including nucleophilic additions (ketenimines can be used as both electrophiles and nucleophiles), radical additions, cycloaddition reactions, electrocyclic ring closure reactions, and σ rearrangements. Furthermore, numerous complex organic compounds, particularly the biologically interesting heterocycles, have been constructed using these methodologies. The review of these accomplishments is presented here.

The thriving chemistry of ketenimines

A detailed study of amidine synthesis from N-allyl-N-sulfonyl ynamides is described here. Mechanistically, this is a fascinating reaction consisting of diverging pathways that could lead to deallylation or allyl transfer depending upon the oxidation state of palladium catalysts, the nucleophilicity of amines, and the nature of the ligands. It essentially constitutes a Pd(0)-catalyzed aza-Claisen rearrangement of N-allyl ynamides, which can also be accomplished thermally. An observation of N-to-C 1,3-sulfonyl shift was made when examining these aza-Claisen rearrangements thermally. This represents a useful approach to nitrile synthesis. While attempts to render this 1,3-sulfonyl shift stereoselective failed, we uncovered another set of tandem sigmatropic rearrangements, leading to vinyl imidate formation. Collectively, this work showcases the rich array of chemistry one can discover using these ynamides.

N-allyl-N-sulfonyl ynamides as synthetic precursors to amidines and vinylogous amidines. An unexpected N-to-C 1,3-sulfonyl shift in nitrile synthesis.

Sustainable metal catalysis in CH activation

Epihalohydrins in organic synthesis.

Cu-catalyzed aerobic oxidative three-component coupling of a terminal alkyne, secondary amine, and sulfonamide enables efficient synthesis of amidines. The use of Cu(OTf)2 (5 mol %) produces amidines selectively without Glaser–Hay alkyne homocoupling products. Preliminary studies suggest that the reaction pathway involves initial oxidative coupling of the terminal alkyne with the secondary amine, followed by hydroamidation of the ynamine intermediate with the sulfonamide.

Cu-catalyzed aerobic oxidative three-component coupling route to N-sulfonyl amidines via an ynamine intermediate.

Formation of N-sulfonylamidines by copper-catalyzed coupling of sulfonyl azides, terminal alkynes, and trialkylamines

The Mitsonubo reagent (triphenylphosphine and dialkyl azodicarboxylates) was employed as a potential anionic nucleophile in a reaction involving aryl halides to produce aryl hydrazides. Aryl iodides and bromides, with electron-withdrawing as well as electron-releasing groups on the aromatic ring, undergo coupling reactions in good yields. The optimized conditions are developed for aryl iodides at room temperature and for aryl bromides at 60―75 °C.

Copper(I) Iodide Catalyzed Formation of Aryl Hydrazides from a Mitsunobo Reagent and Aryl Halides

An efficient synthesis of functionalized 2-(1,3-oxathiolan-2-ylidene)malononitriles containing a push-pull CC bond via a simple reaction between malononitrile, CS2, and oxiranes in the presence of Et3N is described (Scheme 1).

Triethylamine‐Catalyzed Efficient Synthesis of Oxathiolanes Containing a Highly Polarized Carbon‐Carbon Double Bond from Reaction of Malononitrile, Carbon Disulfide (CS2), and Oxiranes

A room-temperature, copper-catalyzed synthesis of symmetrical diarylsulfanes has been developed. The reaction proceeds from aryl iodides and elemental sulfur (S 8 ) by the action of copper(I) salts in the presence of N -ethyl- N -isopropylpropan-2-amine to afford the corresponding diarylsulfanes in good yields.

A Copper-Catalyzed Synthesis of Symmetrical Diarylsulfanes

An efficient protocol for direct arylsulfonamidation of aryl iodides and aryl bromides using arylsulfonylazides catalyzed by copper nanoparticles is described.

Yazdan Solgi

Papers

Formation of N-sulfonylamidines by copper-catalyzed coupling of sulfonyl azides, terminal alkynes, and trialkylamines

Copper(I) Iodide Catalyzed Formation of Aryl Hydrazides from a Mitsunobo Reagent and Aryl Halides

Triethylamine‐Catalyzed Efficient Synthesis of Oxathiolanes Containing a Highly Polarized Carbon‐Carbon Double Bond from Reaction of Malononitrile, Carbon Disulfide (CS2), and Oxiranes

A Copper-Catalyzed Synthesis of Symmetrical Diarylsulfanes

Nanocopper-mediated direct arylsulfonamidation of aryl halides with arylsulfonyl azides