Cyclization of arylhydrazones of cross-conjugated enynones: synthesis of luminescent styryl-1H-pyrazoles and propenyl-1H-pyrazoles?

Organic & Biomolecular Chemistry Pub Date: 2022-10-18 DOI: 10.1039/D2OB01427K

Abstract

Condensation of 1,5-disubstituted pent-1-en-4-yn-1-ones with arylhydrazines in acidified alcohol results mainly in the formation of the corresponding arylhydrazones with traces of the side products of cyclization at the double bond – 1,5-diaryl-3-(arylethynyl)-4,5-dihydro-1H-pyrazoles (pyrazolines). Arylhydrazones are cyclized only by refluxing in high-boiling polar solvents (DMF and ethylene glycol), with the selective formation of 1,5-disubstituted 3-styrylpyrazoles in up to 77–95% yields. Thermodynamically, the cyclization of arylhydrazones at the triple bond is the most preferable pathway, as shown by DFT calculations and preparative synthesis experiments. Thus, we demonstrate that the reactions of arylhydrazines with 1,5-disubstituted pent-1-en-4-yn-1-ones lead to the formation of arylhydrazones and side pyrazoline impurities in a parallel (not consecutive) manner. 2-Hydrazinylpyridine interacts with 1,5-disubstituted pent-1-en-4-yn-1-ones in some other way, giving not pyridinylhydrazones but 2-(5-styryl-3-phenyl-1H-pyrazol-1-yl)pyridines (despite the acidity of the medium). The authors have developed a gram-scale synthesis method for these compounds, which were obtained in up to 60–82% yields. Besides, we have developed the synthesis method for certain styrylpyrazoles, which are quite promising substances for use as fluorescent probes. Their spectral–luminescence characteristics were examined as well as their complexing with Hg2+, Cd2+, and Pb2+ ions.

Graphical abstract: Cyclization of arylhydrazones of cross-conjugated enynones: synthesis of luminescent styryl-1H-pyrazoles and propenyl-1H-pyrazoles
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