1419209-72-5Relevant articles and documents
Betaine-carbene interconversions. from N-ylides to zwitterionic n-heterocyclic carbene-borane adducts
Pidlypnyi, Nazar,Namyslo, Jan C.,Drafz, Martin H. H.,Nieger, Martin,Schmidt, Andreas
, p. 1070 - 1079 (2013/04/10)
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 ΔGa?§§ = 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.
