Synthesis of Pyrazoles via Electrophilic Cyclization

Electrophilic cyclizations of alpha,beta-alkynic hydrazones by molecular iodine were investigated for the synthesis of 4-iodopyrazoles. alpha,beta-Alkynic hydrazones were readily prepared by the reactions of hydrazines with propargyl aldehydes and ketones. When treated with molecular iodine in the presence of sodium bicarbonate, alpha,beta-alkynic hydrazones underwent electrophilic cyclization to afford 4-iodopyrazoles in good to high yields. Iodocyclization was general for a wide range of alpha,beta-alkynic hydrazones and tolerated the presence of aliphatic, aromatic, heteroaromatic, and ferrocenyl moieties with electron-withdrawing and electron-donating substituents.