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Cycloaddition reactions of acetylenic iminium salts and diazoacetates leading to pyrazole iminium salts
- Gerster, Holger, Keim, Michael, Maas, Gerhard
- Zeitschrift für Naturforschung 2019 v.74 no.4 pp. 347-355
- alkynes, cycloaddition reactions, hydrides, hydrolysis, salts, stoichiometry
- Acetylenic iminium triflates with the general formula [R–C≡ C–C(Ar)=N⁺R₂ TfO⁻] were found to be excellent dipolarophiles in [3+ 2] cycloaddition reactions with diazoacetates leading to (1H-pyrazol-3(5)-yl)methanaminium triflates in high yields. The terminal acetylenic iminium salt (propyne iminium salt) [HC≡C–C(Ph)=N⁺Me₂ 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 1H-pyrazoles.