A Modified and Practical Synthetic Route to Indazoles and Pyrazoles Using Tungstate Sulfuric Acid

Abstract

Tungstate sulfuric acid-catalyzed Knorr reaction have been used as a simple, rapid, atom economic and green method for the synthesis of indazole and pyrazole derivatives based on the condensation of hydrazine derivatives and b-dicarbonyl compounds under solvent-free conditions. It was found that the catalyst could be recovered and reused without significant loss of its activity. The use of this method provides a novel and improved modification of Knorr synthesis in terms of clean reaction profile, use of a safe catalyst and solvent-free conditions.

Efficient MW-Assisted Synthesis, Spectroscopic Characterization, X-ray and Antioxidant Properties of Indazole Derivatives

“…(10). Preparation of compound 3a and other derivatives with a yield of 77% using cyclic (alkyl) (amino) carbene (CAAC)-gold(I) catalysts, has been described previously [39], and the use of tungstate sulfuric acid, under solvent-free conditions has been reported to give a similar yield [40].…”

“…Compound 3b and other derivatives have been previously prepared by the condensation reaction of 1,3-diones and hydrazines under layered zirconium sulfophenyl phosphonate catalysis, under solvent-free conditions [41]. The use of tungstate sulfuric acid at 80˝C under solvent-free conditions has been reported to give a yield of 80% [40]. …”

Unveiling a versatile heterocycle: pyrazoline – a review

“…A new catalytic application of tungstate sulfuric acid (TSA) as a highly efficient and green solid acid catalyst to synthesize pyrazoles was proposed. 67 Cellulose sulfonic acid (CSA), was also found to be able to cyclize a,b-unsaturated hydrazones to pyrazolines in excellent yields under green reaction conditions. 68 The examples of "on water" synthesis of N-unsubstituted pyrazoles, 2-pyrazoline derivatives under solvent-free conditions, using the grinding technique, ultrasonic irradiation to yield uorinated pyrazoles and the microwave (MW) irradiation technique with short workup procedure, direct us towards a positive path to green alternative methods, over the conventional synthesis.…”

The Structure ofN‐phenyl‐pyrazoles and Indazoles: Mononitro, Dinitro, and Trinitro Derivatives

“…Both classical methods to prepare the pyrazoles of the present publication, reaction of nitroarylhydrazines with β‐dicarbonyl compounds and N ‐arylation of pyrazoles, have been the subject of many studies trying to improve the yields or to make them cleaner (green). For instance, in the case of compounds 3 and 36 (4‐nitrophenyl derivatives), the following procedures have been reported for the first method: potassium aluminum sulfate , Sc(OTf) 3 , Zn[(L)proline] 2 , Mg(HSO 4 ) 2 , P 2 O 5 ·SiO 2 , nano‐silica sulfuric acid , and tungstate sulfuric acid as catalysts; hydrotropic solution and glycerol‐water as solvent , under catalyst‐free conditions; and in aqueous medium using Saccharomyces cerevisae biocatalyst .…”