Synthesis and dynamic NMR studies of novel hydantoin and thiohydantoin derivatives. Crystal structure of diethyl 2-(4,4-diaryl-2,5-dioxoimidazolidin-1-yl) fumarate and diethyl 2-(4,4-diaryl-2-mercapto-5-oxo-4,5-dihydro-1H-imidazol-1-yl)fumarate

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 PPh 3 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 (?G#) for their interconversion process of rotational isomersequals to (53.6 and 17.2) ? 2 kcal mol-1.