Metal-free synthesis of 3,5-disubstituted 1H- and 1-aryl-1H-pyrazoles from 1,3-diyne-indole derivatives employing two successive hydroaminations

A robust and efficient atom-economic one-pot synthesis of 3,5-disubstituted 1H- and 1-aryl-1H-pyrazoles under base, acid and metal-free reaction conditions, is reported. The transformation conveniently takes place between 1,4-disubstituted 1,3-diynes and hydrazines in PEG-400 as an eco-friendly solvent, and involves two successive hydroaminations. The reaction was optimized for both, symmetric and non-symmetric 1,3-diyne-indole derivatives, as well as for hydrazine and substituted phenylhydrazines. The scope and limitations of the transformation were examined, observing that it is not sensitive to moisture or atmospheric oxygen, and that it tolerates a variety of functional groups. Even sterically hindered substrates afforded the expected pyrazoles in good to excellent yields, under mild conditions. A detailed reaction mechanism, which explains its regioselectivity, was also proposed.