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The Reaction of Phenylhydrazine with Sterically Congested Stabilized Phosphorus Ylides in the Presence of Silica Gel Powder in Solvent-Free Conditions: A Novel Synthesis of Fully Substituted Pyrazole Derivatives

Reactions of sterically congested stabilized phosphorus ylides with phenylhydrazine in the presence of silica gel powder in solvent-free conditions proceed smoothly at 100°C to afford dialkyl 2-(3,5-dimethyl-1-phenyl-1H-pyrazol-4-yl)-2-butenedioates in good yields. The structures of the products were deduced from their IR, 1 H NMR, and 13 C NMR spectra. The mass spectra of these compounds displayed molecular ion peaks at the appropriate z values. The 1 H NMR (CDCl 3 ) spectra of the compounds show the presence of two estereoisomers (E and Z) for each pyrazoles. The relative population of E and Z isomers were determined via their 1 H NMR spectra The reaction is fairly stereoselective.