Mechanistic insights of the copper(i)-catalysed reaction between chlorohydrazones and terminal alkynes

Deuterium incorporation in the 5-substituted pyrazoles arising from the copper(i)-catalysed reaction between chlorohydrazones and terminal acetylenes suggests the intermediacy of copper(i)-complexed alkynylhydrazones. Since the efficiency of such a complexation depends on both the chlorohydrazone and the solvent, the obtainment of the pyrazoles and/or the corresponding alkynylhydrazones is variable depending on the reaction conditions. Copper(i)-complexed alkynylhydrazone intermediates should play a pivotal role in the proposed catalytic cycle.