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

Abstract

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 77Se NMR spectroscopic measurements were carried out.

Imidazolium Dicyanomethylides as N‐Ylide Precursors of Anionic N‐Heterocyclic Carbenes

“…The formation of a double bond from the methanide carbon of 12A to the nitrogen of the imidazole framework yields a carbene position with a doubly negative charge according to the rules of resonance ( 12B ). As mentioned before, this behavior is typical for conjugated anionic carbenes [13,14,21] . In addition to the most common representation 12C , where the stabilizing effect of the nitrile groups located in the α‐position is evident, the negative charge may also be localized on the two nitrogen atoms of the nitrile groups ( 12D ).…”

“…Although this leads to a mesomeric structure of low relevance, which shows both a double negatively charged carbene position and an electron sextet structure at the oxygen, this is permitted according to the rules of mesomerism. Due to the increasing importance of anionic heterocyclic carbenes [10][11][12][13] and in continuation of our interest in mesomeric betaines and Nheterocyclic carbenes, [5,13,21] we became aware of ylides composed of a diazolium unit and an anionic dicyano moiety since literature regarding anionic NHCs derived from nitrogen ylides is scarce. [22] The synthesis of such ylides was first described almost simultaneously by Linn, Webster and Benson [23] and by Rieche and Dietrich.…”

“…As mentioned before, this behavior is typical for conjugated anionic carbenes. [13,14,21] In addition to the most common representation 12C, where the stabilizing effect of the nitrile groups located in the α-position is evident, the negative charge may also be localized on the two nitrogen atoms of the nitrile groups (12D). Overall, the negative charge of the anionic carbene is delocalized both in the substituent and at the carbene position, resulting in analogous behavior to the carbene derived from CMB 3 (see Scheme 1).…”