Dynamic ۱H NMR Study Around the Carbon–Carbon Double Bonds and Carbon–Carbon Single Bonds in a Particular Phosphorous Ylide and ۲,۵-Dihydro-۵,۵-Diaryl-۲-Thio-۱H-Imidazoles

Stable crystalline phosphorus ylides are obtained in excellent yields from the ۱:۱:۱ 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)-۲-(۲,۵-dihydro-۵,۵-diaryl-۲-thioxo-۱H-imidazol-۴-yl)fumarates and ۲,۵-Dihydro-۵,۵-diaryl-۲-thio-۱H-imidazoles in good yields. Dynamiceffects were observed in the ۱H 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 (۶۸.۲ and ۷۱.۷) ± ۲ kJ mol-۱.

Keywords:

Dynamic ۱H NMR , Acetylenic ester , Restricted rotation , Triphenylphosphine