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

N‐Heterocyclic Carbene–Betaine Interconversions: Tautomeric Equilibria of Imidazolium‐Indolates and Indole‐Substituted Imidazol‐2‐ylidenes

“…13 -1-yl)pyridin-2-yl]-1-(3-methyl-1H-indol-2-yl) NMR spectra were obtained with a Bruker Avance 400 MHz and a Bruker Avance III 600 MHz spectrometer.…”

Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole and Indazole

“…It was prepared by condensation of phenylhydrazine and ethyl 2-acetyl-3-oxobutanoate (24) which underwent a condensation to pyrazole ester 25. 47 Methylation with dimethyl sulfate in the presence of catalytic amounts of nitrobenzene followed by saponification under acidic conditions gave the target betaine 26. Instead of decarboxylation, ester formation to 28 was observed on heating in mesitylene at reflux temperature.…”

“…Palladium complexes of pyrazol-3-ylidene were obtained in 51% yield upon deprotonation of the pyrazolium salt 74 with silver oxide in anhydrous acetonitrile at 20°C, followed by the addition of bis(triphenylphosphine)palladium(II) chloride (Scheme 22). 47 The carbene carbon atom was detected at δ = 165.6 ppm in the 13 C NMR spectrum, slightly more downfield than in the complexes of 1,4-diphenyl-2,3-dimethylpyrazol-5-ylidene and its 1,2,3,4-tetramethyl derivative (see below). The complex crystallized as solvate 75•CHCl 3 •1/6 H 2 O.…”

Sulfur, mercury, and boron adducts of sydnone imine derived anionic N-heterocyclic carbenes

“…By contrast, cross-conjugated mesomeric betaines (CCMB) such as pyrazolium-4-carboxylates 3 require harsh reaction conditions for the formation of remote Nheterocyclic carbenes 4 so that this approach is not useful from a synthetic point of view. 10 Some mesomeric betaines undergo reactions via their formal tautomers which are Nheterocyclic carbenes. The CMB 5 and the CCMB 7 are examples.…”