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.