N-Heterocyclic carbenes from ylides of indolyl-imidazolium, azaindolyl-imidazolium, and indolyl-triazolium salts, and their borane adducts

Indol-2-yl-imidazolium salts were deprotonated at N1 of the indole ring to give ylides. Their tautomeric N-heterocyclic carbenes (NHCs) were trapped by sulfur to give imidazole-2-thiones. Treatment of the ylides with triethylborane resulted in the formation of zwitterionic borane adducts. An analogous sequence of reactions was performed with 8-azaindol-2-yl-imidazolium salts, which served as precursor to prepare first representatives of a new heterocyclic ring system on reaction of their NHC-tautomers with triethylborane. Similarly, an indol-2-yl-1,2,4-triazolium salt was examined with respect to ylide-NHC tautomerism and trapping reactions. A nucleophilic ring transformation of indol-3-amine with a 1,3,4-oxadiazolium salt gave an indol-3-yl-triazolium salt, which was converted into a triazolethione by trapping of the tautomeric N-heterocyclic carbene of its ylide. (C) 2014 Elsevier Ltd. All rights reserved.