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 reactionbetween hydantoins or thiohydantoins and dialkyl acetylenedicarboxylates in the presence oftriphenylphosphine. These phosphoranes undergo smooth intramolecular Wittig reaction followedby 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. Dynamiceffects were observed in the 1H NMR spectra of these compounds and were attributed to restrictedrotation around the Carbon-Carbon single bonds. Rotational energy barrier (ΔG#) for their interconversionprocess of rotational isomers equals to (68.2 and 71.7) ± 2 kJ mol-1.