Electrospray ionization mass spectrometric studies of nucleophilic carbenes derived from pyrazolium and indazolium carboxylates

Abstract

Pyrazolium-3-carboxylate and indazolium-3-carboxylate, which belong to the class of pseudo-cross-conjugated mesomeric betaines and which represent the electronically relevant partial structures of the betaine alkaloid Nigellicin, were examined by electrospray ionization mass spectrometry. These compounds decarboxylate to pyrazol-3-ylidene and indazol-3-ylidene. The formation of adducts of these new nucleophilic carbenes under the measurement conditions was examined.

N‐Heterocyclic Carbenes of 5‐Haloindazoles Generated by Decarboxylation of 5‐Haloindazolium‐3‐carboxylates

“…Known pseudo‐cross‐conjugated heterocyclic mesomeric betaines, which form N‐heterocyclic carbenes by in situ decarboxylation, are presented in Figure 2. The alkaloid Homarine ( III , R 1 = R 2 = R 3 = H),25 and quinolinium ( III , R 1 = H, R 2 –R 3 = –CH=CH–CH=CH–), as well as isochinolinium derivatives ( III , R 1 –R 2 = –CH=CH–CH=CH–,R 3 = H),26 imidazolium‐2‐carboxylates IV ,27 pyrazolium‐3‐carboxylates V ,28 as well as indazolium‐3‐carboxylates V (R 1 –R 2 = –CH=CH–CH=CH–)29 were found to undergo thermal fragmentation to CO 2 and the correspondinghetarene ylidenes. Extrusions of isocyanates and isothiocyanates have been reported as well 29.…”

Synthesis of a Pyrazol-3-ylidene Palladium Complex, Pyrazolium Salts and Mesomeric Betaines of Pyrazole as N-Heterocyclic Carbene Precursors

“…This NHC can be detected as the sodium adduct in electrospray ionization mass spectrometric measurements at m/z = 195.1, [M+Na] + [11,12]. When KOtBu is employed as base, and toluene is used as the solvent to remove the stabilizing water of crystallization by azeotropic distillation, a sequence of ring cleavage, 6π-electrocyclization of 3 and subsequent tautomerization occurs to result in the formation of 4-aminoquinoline 4 [10].…”

Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole and Indazole

“…55 This is also reflected in the lower C-H acidity of pyrazolium salts in comparison to imidazolium salts. 56 The formation of free pyrazol-3-ylidene and its adducts 57 and pyrazol-3-ylidene complexes of chromium 60 and iron 61 have been studied by means of mass spectrometry. 58 In addition, charge densities of 1,2-dimethylpyrazol-3-ylidene and its chromium complex were calculated.…”