Copper(I)-catalysed reaction of hydrazonyl chlorides with homopropargylic alcohols : regioselective synthesis of 5-substituted pyrazoles

Fully regioselective synthesis of 5-hydroxyethylpyrazoles was exploited by reacting hydrazonoyl chlorides with homopropargylic alcohols in the presence of catalytic amounts of copper(I) chloride. Good yields of pyrazolic products and mild reaction conditions were experienced notwithstanding the known, poor reactivity of homopropargylic alcohols towards hydrazonoyl chlorides. The role of copper(I) ion and some mechanistic insights for the formation of reaction products are also discussed.