Betaine–N‐Heterocyclic Carbene Interconversions of Quinazolin‐4‐One Imidazolium Mesomeric Betaines. Sulfur, Selenium, and Borane Adduct Formation

Reaction of N‐alkylated imidazoles with 2‐chloro‐4‐quinazolinone gave mesomeric betaines, 2‐(1‐alkyl‐1H‐imidazolium‐3‐yl)quinazolin‐4‐olates, for which three tautomeric forms of N‐heterocyclic carbenes (NHCs) can be formulated, in addition to an anionic NHC after deprotonation. The NHC tautomers were trapped with sulfur, selenium, triethylborane, and triphenylborane as thiones, selenones and borane adducts, respectively. We obtained two isomers of the cyclic borane adducts, diazaboroloquinazolinones with [1,5‐a] and [5,1‐b]‐type fusion between the quinazolinone and the diazaborole rings. They correspond to two different NHC tautomers and to the anionic NHC derived thereof. The third NHC tautomer was trapped as a non‐cyclic adduct with tris(pentafluorophenyl)borane by coordination to the quinazoline oxygen atom. 2D 1H‐15N HMBC experiments of 15N‐labeled quinazolinone fragments, quantitative measurements of long‐range 1H‐15N coupling constants (JHN), and five X‐ray single crystal analyses have been carried out for the structure elucidations and to gain insight into the NMR spectroscopic properties of these compounds.