Dynamic 1H NMR Study Around the Carbon–Carbon Double Bonds and Carbon–Carbon Single Bonds in a Particular Phosphorous Ylide and 2,5-Dihydro-5,5-Diaryl-2-Thio-1H-Imidazoles

Stable crystalline phosphorus ylides are obtained in excellent yields from the 1:1:1 addition reaction between hydantoins or thiohydantoins and dialkyl acetylenedicarboxylates in the presence of triphenylphosphine. These phosphoranes undergo smooth intramolecular Wittig reaction followed by an electrocyclic ring opening to produce dialkyl (E)-2-(2,5-dihydro-5,5-diaryl-2-thioxo-1H-imidazol- 4-yl)fumarates and 2,5-Dihydro-5,5-diaryl-2-thio-1H-imidazoles in good yields. Dynamic effects were observed in the 1H NMR spectra of these compounds and were attributed to restricted rotation around the Carbon-Carbon single bonds. Rotational energy barrier (?G#) for their interconversion process of rotational isomers equals to (68.2 and 71.7) ± 2 kJ mol-1.