N-Heterocyclic carbenes by tautomerization of mesomeric betaines. Thione, selenone, and borane adduct formations of imidazolium-isocytosinates

Abstract 2-Amino-6-chloropyrimidin-4(3 H )-one (6-chloroisocytosine) reacted with methyl-, vinyl-, phenyl-, and benzylimidazole, respectively, to give isocytosinate-imidazolium salts which were deprotonated with an anion exchange resin to yield the corresponding cross-conjugated mesomeric betaines (imidazolium-isocytosinates). The vinyl derivative, however, polymerized under these conditions. The betaines are in equilibrium with their N -heterocyclic carbenes, isocytosine-imidazol-2-ylidenes. Results of DFT calculations reveal that the betaine tautomer is the most stable tautomer in polar solvents, whereas the carbene is more stable in vacuo. Trapping reactions of the carbene tautomers, including the vinyl derivative, were performed with sulfur and selenium which resulted in the formation of the corresponding thiones and selenones, respectively. The in situ generation of the betaine/carbene tautomers in the presence of triethylborane in a Schlenk tube at elevated temperatures and pressures gave novel zwitterionic 8-amino-10,10-diethyl-6-oxo-6,10-dihydro-1 H -imidazo[2′,1′:3,4][1,4,2]diaza-borolo[1,5-c]pyrimidin-4-ium-10-ides, in which the boron atom bears the negative charge whereas the positive charge is delocalized in the imidazolium ring. This ring system is a formal trapping product of the carbene as well as the betaine tautomer.