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Paper | Regular issue | Vol 96, No. 7, 2018, pp.1203-1215
Published online, 26th June, 2018
DOI: 10.3987/COM-18-13916
Deprotonation of 4-Ethynylpyrazolium Salts

Nils Lennart Ahlburg, Olivia Doppleb, Kai Hillrichs, Jan C. Namyslo, Eike G. Hübner, and Andreas Schmidt*

*Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany

Abstract

4-Ethynyl-1,2-dimethylpyrazolium salts were prepared by methylation of the corresponding 4-ethynyl-1-methylpyrazoles with trimethyloxonium tetrafluoroborate and were deprotonated to give the corresponding pyrazolium-4-acetylenides, which are mesomeric betaines. These can be represented as alkynyl- or mesoionic allenylidene-type resonance forms. Calculations and spectroscopic investigations were performed to determine the contribution of each canonical form to the overall structure. Ylides and N-heterocyclic carbenes are tautomers of the betaines. Their relative stabilities have been compared.