Pyrazoles in the Intersection of Mesomeric Betaines and N-Heterocyclic Carbenes: Formation of NHC Selenium Adducts of Pyrazolium-4-aminides

Starting from 4-nitropyrazole, eight mesoionic pyrazolium-4-aminides were prepared by a six-step reaction sequence. The deprotonation of 1,2-disubstituted 4-amido-1H-pyrazolium salts by an anion exchange resin in its hydroxide form is the final step of the synthesis. A tautomeric equilibrium between the mesoionic compounds (pyrazolium-4-aminides) and N-heterocyclic carbenes (pyrazol-3-ylidenes) can be formulated; however, the NHC tautomers were not detected by means of NMR spectroscopy in polar aprotic solvents such as DMSO-d(6) or MeCN-d(3). Apart from tautomerism, anionic N-heterocyclic carbenes can be formulated as a result of a deprotonation of the mesoionic compounds. Trapping reactions were performed with selenium, which resulted in the formation of pyrazole-3-selenones. Methylation at the selenium atom gave the corresponding 3-(methylselanyl)-4-amido-1H-pyrazolium salts, which were deprotonated to give new mesomeric betaines, 3-(methylselanyl)-1H-pyrazolium-4-aminides as unique compounds. DFT-calculations as well as Se-77 NMR spectroscopic measurements were carried out.

Automated summary

In this study, eight mesoionic pyrazolium-4-aminides were synthesized from 4-nitropyrazole through a six-step reaction sequence. The final step involved the deprotonation of 1,2-disubstituted 4-amido-1H-pyrazolium salts using an anion exchange resin in its hydroxide form. The study also revealed the tautomeric equilibrium between the mesoionic compounds and N-heterocyclic carbenes, as well as the possibility of anionic N-heterocyclic carbenes formation through deprotonation.