Direct Organocatalytic Chemoselective Synthesis of Pharmaceutically Active 1,2,3-Triazoles and 4,5'-Bitriazoles

Carbonyl-containing 1,4,5-trisubstituted- and 1,4-disubstituted-1,2,3-triazoles are well-known for their wide range of applications in pharmaceutical and medicinal chemistry, but their high-yielding metal-free synthesis has always remained challenging, as no comprehensive protocol has been outlined to date. Owing to their structural and medicinal importance, herein, we synthesized various carbonyl-containing 1,4,5-trisubstituted- and 1,4-disubstituted-1,2,3-triazoles and unsymmetrical 4,5'-bitriazoles with high yields and chemo-/regioselectivity from the library of 2,4-diketoesters and azides in a sequential one-pot manner through the combination of organocatalytic enolization, in situ [3 + 2]-cycloaddition, and hydrolysis reactions. The commercial availability of the starting materials/catalysts, diverse substrate scope, performance in a one-pot manner, chemo-/regioselectivity of organo-click reaction, quick synthesis of unsymmetrical 4,5'-bitriazoles, a large number of synthetic applications, and numerous medicinal applications of carbonyl-containing 1,2,3-triazoles are the key attractions of this metal-free organo-click work.

Metal-Free Cyanation of gem-Difluoroalkenes via Azide-Mediated C-C Double Bond Fragmentation

Activation and cleavage of C-C double bonds are long-standing challenges in synthetic chemistry. Herein, we report an unprecedented azide-mediated C-C double bond fragmentation of gem-difluoroalkenes under mild and metal-free conditions, enabling the efficient synthesis of structurally diverse aromatic nitriles in moderate to good yields. This protocol is also amenable to the cyanation of gem-dichloro and dibromo alkenes. This reaction features simple operation and good functional group compatibility and can be implemented at a gram scale.

Regitz Diazo Transfer Reaction for the Synthesis of 1,4,5-Trisubstituted 1,2,3-Triazoles and Subsequent Regiospecific Construction of 1,4-Disubstituted 1,2,3-Triazoles via C-C Bond Cleavage

A straightforward and efficient methodology has been developed for the synthesis of 1,4,5-trisubstituted dicarbonyl 1,2,3-triazoles and 1,4-disubstituted sole-carbonyl 1,2,3-triazoles via a C-C bond cleavage process. The Regitz diazo transfer and C-C bond cleavage were the key steps of this transformation, which provided diverse carbonyl-substituted structural 1,2,3-triazoles. This reaction featured with excellent regioselectivity, wide functional group tolerance, and mild conditions.

Metal-free syntheses of N-functionalized and NH-1,2,3-triazoles: an update on recent developments

An overview of the latest developments in the metal-free synthesis of non-benzo-fused N-functionalized and NH-1,2,3-triazoles is provided in this feature article. Synthetic studies that appeared from 2016 until August 2020 are organized according to a wide-ranging classification, comprising oxidative and eliminative azide-dipolarophile cycloadditions, diazo transfer reactions and N-tosylhydrazone-mediated syntheses. The newly developed methods constitute a significant contribution to the field of 1,2,3-triazole synthesis in terms of structural variation via either the exploration of novel reactions, or the exploitation of existing methodologies.

Organocatalytic enone-azide [3 + 2]-cycloaddition: synthesis of functionally rich C/N-double vinyl 1,2,3-triazoles

An enolate-mediated organocatalytic [3 + 2]-cycloaddition of enones with less reactive vinyl/alkyl/aryl azides is reported at room temperature for short reaction times. The metal-free amine-mediated catalytic conditions of this [3 + 2]-cycloaddition allowed us to synthesize a collection of C/N-double vinyl-1,2,3-triazoles and C-vinyl-1,2,3-triazoles through functionalized enones as quality azidophiles with various azides. It is an efficient catalytic [3 + 2]-cycloaddition for the synthesis of biologically important fully decorated C/N-double vinyl-1,2,3-triazoles with excellent outcomes with reference to the reaction rate, selectivity, operation simplicity, substrate scope, yields, and synthetic applications as demonstrated in the paper. Herein, we illustrated the importance of enolate reactivity with azides compared to enamines by correlation with previous enamine-mediated click reactions in the reaction mechanism section.

Catalyst-free facile synthesis of polycyclic indole/pyrrole substituted-1,2,3-triazoles

A general and catalyst-free access to the fused polycyclic N-heterocycles via an intramolecular azide-alkene cascade reaction under mild reaction conditions has been developed. The reaction is applicable to both indole and pyrrole substrates, and a variety of substituents are tolerated. The entire sequence can be carried out in a one-pot operation. This methodology provides a sustainable and efficient access to a variety of novel polycyclic indole/pyrrole substituted-1,2,3-triazoles.

Nucleophilic Iron Complexes in Proton-Transfer Catalysis: An Iron-Catalyzed Dimroth Cyclocondensation

The nucleophilic iron complex Bu₄ N[Fe(CO)₃ (NO)] (TBA[Fe]) is an active catalyst in C-H-amination but also in proton-transfer catalysis. Herein, we describe the successful use of this complex as a proton-transfer catalyst in the cyclocondensation reaction between azides and ketones to the corresponding 1,2,3-triazoles. Cross-experiments indicate that the proton-transfer catalysis is significantly faster than the nitrene-transfer catalysis, which would lead to the C-H amination product. An example of a successful sequential Dimroth triazole-indoline synthesis to the corresponding triazole-substituted indolines is presented.

CoFe2O4/Cu(OH)2 magnetic nanocomposite: an efficient and reusable heterogeneous catalyst for one-pot synthesis of β-hydroxy-1,4-disubstituted-1,2,3-triazoles from epoxides

A magnetically separable CoFe₂O₄/Cu(OH)₂ nanocomposite was prepared and characterized by various techniques such as FESEM, EDS, TEM, XRD, VSM and FT-IR. This novel composite was used as a heterogeneous catalyst for the regioselective synthesis of β-hydroxy-1,4-disubstituted-1,2,3-triazoles from sodium azide, terminal alkynes and structurally different epoxides in water at 60 °C. The formation of the product proceeds in one pot through a mechanism that involves an in situ generated organic azide intermediate, followed by rapid ring closure with the alkyne component. The simple procedure, short reaction times, perfect regioselectivity, high product yields, and use of a benign solvent and nontoxic catalyst are among the considerable advantages of this protocol. Furthermore, the catalyst was easily separated using an external magnet and reused several times without any significant loss of catalytic activity or magnetic properties.

Magnetically separable CoFe₂O₄/Cu(OH)₂ nanocomposite was prepared and used as a novel heterogeneous catalyst for synthesis of β-hydroxy-1,4-disubstituted-1,2,3-triazoles from epoxides.

Reaction engineering and photophysical studies of fully enriched C-vinyl-1,2,3-triazoles

A library of fluorogenic C-vinyl-1,2,3-triazoles were synthesized in very good yields with excellent selectivity by using an organocatalytic formal [3 + 2]-cycloaddition. One of the coumarin-triazoles 4ba has shown excellent fluorescence properties (λ_em = 533 nm).

Cycloadduct formation of α,β-unsaturated esters with azides catalyzed by NHC systems

NHC-catalyzed cycloadduct formation of α,β-unsaturated esters with azides has been developed. This strategy could generate 1,2,3-triazoles and dihydropyrazoles with high yields and regioselectivities. The broad substrate scope and mechanistic survey of this process are also presented.