The reaction of stoichiometric amounts of dialkyl acetylenedicarboxylates with triphenylphosphine in the presence of hydantoins or thiohydantoins afforded stable crystalline phosphorus ylides. These compounds undergo smooth elimination of PPh3 to produce dialkyl 2-(4,4-diaryl-2,5-dioxoimidazolidin-1-yl)fumarate, 4 or dialkyl 2-(4,4-diaryl-2-mercapto-5-oxo-4,5-dihydro-1H-imidazol-1-yl)fumarate, 7. Single crystal X-ray diffraction study on 4b and 7b proved the structures unambiguously with C=O and SH functionality at the 2-position of the imidazole ring, respectively. Dynamic effects were observed in the NMR spectra of these compounds and were attributed to restricted rotation around the carbon-nitrogen single bonds. Rotational energy barrier (ΔG#) for their interconversion process of rotational isomers equals to (53.6 and 17.2) ± 2 kcal mol-1.
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