Ag-Catalyzed Synthesis of Pyrazoles from Propargyl N-Sulfonylhydrazones

Significance

The reported reaction procures 1,3,5-trisubstituted pyrazoles in good to excellent yields from propargyl N-sulfonylhydrazones, which are easily accessible from propargyl bromides, aldehydes and N-tosyl hydrazine. Silver hexafluoroantimonate was found to be the best catalyst, in comparison to other metals; for example, a gold salt did not furnish any products and with a platinum salt the reaction was much slower (55% yield after 18 h at 80 ˚C). Interestingly, the reaction shows an unprecedented 1,3-tosyl group migration. The same migration was observed with an N-mesyl derivative; however, N-phenyl or -benzoyl derivatives showed no reaction. A mechanism was proposed based on deuterium-labeling and cross-over experiments.