Betaine–Carbene Interconversions. From N‑Ylides to Zwitterionic N‑Heterocyclic Carbene–Borane Adducts

In the presence of NBS 3-methylindole reacted with various imidazoles to give the (indol-2-yl)­imidazolium salts 21a–f, which were converted in aqueous solution into the 2-(imidazolium-3-yl)-3-methylindolates 22a–f by base. These conjugated ylideswhich represent a subclass of mesomeric betainesare the exclusively detectable form in the NMR spectra taken in DMSO-d6. A DFT calculation revealed that the betaine 22a is −9.3 kJ/mol more stable than the tautomeric N-heterocyclic carbene 23a and that the energy for the betaine–carbene interconversion is ΔG⧧ = 66.4 kJ/mol. The N-heterocyclic carbenes (3-methyl-indol-2-yl)­imidazol-2-ylidenes, however, can be trapped by sulfur, triethylborane, and triphenylborane. Whereas the first trapping reaction yielded the expected imidazolethiones, the borates gave the first representatives of new zwitterionic borane adducts, imidazo­[2′,1′:3,4]­[1,4,2]­diazaborolo­[1,5-a]­indolium-11-ides 26a–h. We performed DFT calculations on the structures of mesomeric betaine 22a, the carbene 23a, and the mechanisms of the borane adduct formation to 26a–h, NMR spectroscopic investigations including 15N, 7Li, and 11B NMR spectroscopy, and an X-ray single-crystal analysis of one of the borane adducts.