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Acetylenic iminium triflates with the general formula [R–C≡ C–C(Ar)=N + R 2  TfO − ] were found to be excellent dipolarophiles in [3+ 2] cycloaddition reactions with diazoacetates leading to (1 H -pyrazol-3(5)-yl)methanaminium triflates in high yields. The terminal acetylenic iminium salt (propyne iminium salt) [HC≡C–C(Ph)=N + Me 2  TfO − ] reacted with an equimolar amount of methyl diazoacetate instantaneously at 20°C to form the expected pyrazole in almost quantitative yield. When a 2:1 stoichiometry was applied, subsequent Michael addition of the pyrazole at the alkyne occurred and the bis(iminium) ditriflate  4  was obtained in high yield. By hydride reduction or hydrolysis of the iminium group, some of the highly hygroscopic pyrazole iminium salts were converted into neutral, twofold functionalized, di- and tri- C -substitued 1 H -pyrazoles.

Cycloaddition reactions of acetylenic iminium salts and diazoacetates leading to pyrazole iminium salts