Pseudo-Cross-Conjugated Mesomeric Betaines and N-Heterocyclic Carbenes of Indazole

Abstract

1,2-Dimethylindazolium-3-carboxylates are pseudocross-conjugated mesomeric betaines (PCCMB) and derivatives of the indazole alkaloid Nigellicin. They decarboxylate on heating to give intermediary N-heterocyclic carbenes of indazole that can be trapped with iso(thio)cyanates to amidates. Alternatively, 1,2-dimethylindazolium-3-amidates can be prepared starting from the corresponding 1H-indazol-3-carboxylic acid which is converted into its chloride, reacted with anilines and deprotonated on anion exchange resin. The heterocumulene moieties of these amidates, which are new representatives of pseudo-cross-conjugated mesomeric betaines, can be exchanged to 3,5-dichlorophenyl isocyanate via the N-heterocyclic carbene.

N‐Heterocyclic Carbene Induced Cycloaddition Reactions of Indazoles with Acetylenes To Form a New Ring System

“…Thus, the multiplet at δ = 6.01 ppm and the signal at δ = 75.8 ppm were assigned to 2-H and C-2, respectively. The signal of the C-3-H 2 group appears at δ = 68.6 ppm in the 13 C NMR spectrum and displays all the expected couplings. Conclusions concerning the regiochemistry of this reaction were drawn from the HMBC spectra, which revealed strong interactions of the C-2-H and C-3-H 2 groups with the phenyl ring, but only weak interactions between C-3-H 2 and the carbonyl group.…”

“…Signals of the C-3-H 2 group appear at δ = 4.62 and 4.30 ppm, and for C-3 at δ = 65.9 ppm. The 2,5-dihydropyrrole moiety is proved by couplings to C-2, C-1, The signal of the quaternary atom of 16 appears at δ = 70.3 ppm in the 13 C NMR spectrum, and displays couplings to 4-H and to a CH 2 group of an ester in the HMBC measurements. The methyl group attached to N-2 shows a small coupling to one of the acetylenic carbon atoms and a strong coupling to C-3.…”

“…The methyl group attached to N-2 shows a small coupling to one of the acetylenic carbon atoms and a strong coupling to C-3. The signals of the sp carbon atoms appear at δ = 80.3 and 80.8 ppm in the 13 C NMR spectrum, and an absorption band can be seen at 2232 cm -1 in the IR spectrum.…”

“…[11] These can be generated by decarboxylation of indazolium-3-carboxylates, which belong to the class of pseudo-cross-conjugated heterocyclic mesomeric betaines. [12,13] The N-heterocyclic carbenes of indazole undergo typical trapping reactions with sulfur to give indazolethione 3, [13] with isocyanates to give indazol-3-amidates 4 (X = O), with isothiocyanates to give indazole-3-thioamidates 4 (X = S), and with protons to give indazolium salts 5 (Scheme 1). Ketones form stable 1:1 adducts 6.…”

Betaine Adducts of N‐Heterocyclic Carbenes: Synthesis, Properties, and Reactivity

“…Although natural alkaloid betaines and their derivatives display a very rich chemistry and have been shown to generate new NHCs upon extrusion of CO 2 , [62,84] their study is beyond the scope of the present survey. Interested readers are referred to the recent review of Schmidt et al for the latest advances in the field of indazole-based NHCs and betaines.…”