Synthesis of 3,4,5-Trisubstituted 1,2,4-Triazoles via I2-Catalyzed Cycloaddition of Amidines with Hydrazones

A general and practical method for the construction of various 3,4,5-trisubstituted 1,2,4-triazoles via I2-catalyzed cycloaddition of N-functionalized amidines with hydrazones is reported. This strategy features cheap and readily available catalyst and starting materials, broader substrate scope, and moderate-to-good yields. The mechanism study shows that the existence of hydrogen on the nitrogen of hydrazones is crucial for this transformation.

Synthesis of 3,5-Disubstituted-1,2,4-thiadiazoles via NaH-DMF-Promoted Dehydrogenative Intramolecular N-S Bond Formation

A facile transition-metal-free synthesis of 3,5-bis(het)aryl/arylaminothiadiazoles has been reported. The overall protocol involves base-mediated tandem thioacylation of amidines with dithioesters or aryl isothiocyanates in DMF solvent and subsequent in situ intramolecular dehydrogenative N-S bond formation of thioacylamidine intermediates under an inert atmosphere. A probable mechanism involving a carbamoyl anion, generated by deprotonation of DMF, acting as a radical initiator has been suggested.

Metal free cross-dehydrogenative N-N coupling of primary amides with Lewis basic amines

Hydrazides, N-N containing structural motifs, are important due to their presence in a wide variety of biologically significant compounds. While the homo N-N coupling of two NH moieties to form the hydrazide N-N bond is well developed, the cross-dehydrogenative hetero N-N coupling remains very unevolved. Here we present an efficient intermolecular N-N cross-coupling of a series of primary benzamides with broad range of Lewis basic primary and secondary amines using PhI(OAc)2 as both a terminal oxidant and a cross-coupling mediator, without the need for metal catalysts, high temperatures, and inert atmospheres, and with substantial potential for use in the late-stage functionalization of drugs.

Synthesis of 1,2-dihydro-1,3,5-triazine derivatives via Cu(II)-catalyzed C(sp3)-H activation of N,N-dimethylethanolamine with amidines

1,2-Dihydro-1,3,5-triazines and symmetrical 1,3,5-triazines were obtained in up to 81% yields from amidines and N,N-dimethylethanolamine catalyzed by CuCl2. The reaction involves three C-N bond formations during the oxidative annulation process and the mechanism was proposed. This efficient synthesis of 1,2-dihydro-1,3,5-triazines was developed for the first time.

Mechanistic insights into copper-catalyzed aerobic oxidative coupling of N-N bonds

Catalytic N–N coupling is a valuable transformation for chemical synthesis and energy conversion. Here, mechanistic studies are presented for two related copper-catalyzed oxidative aerobic N–N coupling reactions, one involving the synthesis of a pharmaceutically relevant triazole and the other relevant to the oxidative conversion of ammonia to hydrazine. Analysis of catalytic and stoichiometric N–N coupling reactions support an “oxidase”-type catalytic mechanism with two redox half-reactions: (1) aerobic oxidation of a Cu^I catalyst and (2) Cu^II-promoted N–N coupling. Both reactions feature turnover-limiting oxidation of Cu^I by O₂, and this step is inhibited by the N–H substrate(s). The results highlight the unexpected facility of the N–N coupling step and establish a foundation for development of improved catalysts for these transformations.

Mechanistic studies provide valuable insights into Cu-catalyzed N–N coupling reactions relevant to energy conversion and pharmaceutical synthesis.

Developments towards synthesis of N-heterocycles from amidines via C–N/C–C bond formation

This review focuses on the synthesis of N-heterocycles using amidines as starting materials, with an emphasis on the mechanisms of these reactions via C–N/C–C bond formation.

[3 + 2] Cycloaddition of Nitrile Ylides with Diazonium Salts: Copper-Catalyzed One-Pot Synthesis of Fully Substituted 1,2,4-Triazoles

A novel [3 + 2] cycloaddition between nitrile ylides and diazonium salts was well-established. This copper-catalyzed three-component reaction was distinguished by mild conditions, ready availability, and operational simplicity, thus opening access to 1,2,4-triazoles with a diverse set of substitution patterns.

Regio- and stereoselective 1,3-dipolar cycloaddition reactions of C-aryl (or hetaryl)-N-phenylnitrones to monosubstituted ylidene malononitriles and 4-benzylidene-2-phenyloxazol-5(4H)-one

The first example of 1,3-dipolar cycloaddition reactions of C-aryl (or hetaryl)-N-phenylnitrones to monosubstituted ylidene malononitriles and 4-benzylidene-2-phenyl-oxazol-5(4H)-one is described. The reaction of C-(4-(dimethylamino)phenyl)-N-phenyl-nitrone (1a) with 2-(4-substituted-benzylidene)malononitriles 2a, b in dry toluene, in the absence of catalyst, at reflux temperature furnished the novel cycloadducts 2-(3-aryl-2-phenyl-2,3-dihydro-1,2,4-oxadiazol-5-yl)-3-(4-methoxyphenyl)acrylonitriles 3a, b. Refluxing C-aryl (or hetaryl)-N-phenylnitrones 1b–i with 2-(4-methoxy-benzylidene)malononitrile (2a) in dry toluene, in the absence of catalyst, gave the unexpected 2-cyano-3-(4-methoxyphenyl)-acrylamide (5), as the sole product. On the other hand, refluxing 4-benzylidene-2-phenyloxazol-5(4H)-one (7) with an equimolar amount of C-aryl (or hetaryl)-N-phenyl-nitrones 1a–c, f–i in absolute EtOH afforded the previously unknown 5-anilino-4-benzoyl-2-phenyloxazole (10), as the only isolable product. The resulting products were formed with a high degree of regio- and stereoselectivity. Quantum chemical calculations were performed to verify stereoselectivity of the studied reaction. A mechanistic proposal is presented to rationalize the formation of these products.

Cu-Catalyzed intermolecular [3 + 3] annulation involving oxidative activation of an unreactive C(sp3)–H bond: access to pyrimidine derivatives from amidines and ketones

An efficient approach to pyrimidines through a copper-catalyzed oxidative unreactive C(sp³)–H bond and intermolecular [3 + 3] annulation of amidines and ketones is described.