Unexpected Formation of 6-(1H-Benzo[d]imidazol-2-yl)-1-phenyl-hexan-1-one and Its Structure in Solution and Solid State Analyzed in the Context of Tautomerism

The structure of the title compound (4d), unexpectedly obtained in the reaction between o-phenylenediamine and 2-benzoylcyclohexanone instead of the target 3H-benzo[b][1,4]diazepine derivative 3d, was determined spectroscopically in solution and by a single-crystal X-ray diffraction (XRD) study. It involves two enantiomeric rotamers, called forms D and U, of which the structure was elucidated based on NMR spectra measured and predicted in DFT-GIAO calculations. An averaging of δCs for all tautomeric positions in the benzimidazole part of the 4d hydrate studied in wet (probably slightly acidic) CDCl3 unambiguously indicates tautomeric exchange in its imidazole unit. An XRD analysis of this material confirms the existence of only one tautomer in the solid phase. The non-covalent interactions forming between molecules of water and benzimidazole derivative are shorter than the sum of van der Waals radii and create an infinite-chain hydrogen bond motif along the b-axis. A possible mechanism for the observed cyclocondensation is also proposed.