Ferrocenyl pyrazolines: Preparation, structure, redox properties and DFT study on regioselective ring-closure

Cyclocondensation of 1-phenyl-3-ferrocenyl-2-propen-1-one (1) with RNHNH2 hydrazines and the substituent-dependent product distribution were investigated. With methylhydrazine, formation of two regioisomeric pairs of pyrazolines and pyrazoles was observed. The ratio of the products varied with the solvent and temperature. Transformation of 5-ferrocenyl-N-substituted pyrazolines into pyrazoles was systematically studied and DDQ was found to be the most suitable reagent. Mechanism of the cyclization reactions taking place under kinetic- and thermodynamic controls was supported with DFT calculations. The energy-dependence of the transformation of pyrazoline to pyrazole was investigated also by EI MS. Structure determination of the new compounds was performed by IR, MS and NMR methods including 2D-HMQC, 2D-HMBC, DEPT and DIFFNOE measurements. For two compounds structures were also proved by X-ray diffraction.

Graphical abstract3-Ferrocenyl-1-phenyl-prop-2-en-1-ones with hydrazines gave 5-ferrocenyl pyrazolines and/or pyrazoles. With methylhydrazine, formation of regioisomeric pyrazolines and pyrazoles was also observed. Oxidation of isolated pyrazolines was systematically studied, reaction mechanism was supported by DFT. Structures of the new compounds were determined with complex spectroscopic methods. X-ray of two compounds is also presented.Figure optionsDownload full-size imageDownload as PowerPoint slide