The structure of hydantoins in solution and in the solid state

The results of NMR spectroscopic and X-ray crystallographic studies are critically discussed with respect to the structure of hydantoins, their tautomerism, and their acidity. The imide NH proton of the preferred, nearly planar 2,4-imidazolidine-dione tautomer proved to be more acidic than the corresponding amide NH proton. Phenyl substituents at the ring nitrogen atoms and at C-5 are twisted from the plane of the hydantoin ring; in case of ortho substituents restricted rotation about the N-aryl bond was found and the barrier to rotation determined by dynamic NMR spectroscopy. For 5-benzyl substituents, a folded conformation of the two rings, due to intramolecular interactions, was found and for 5-exo-methylene substituted hydantoins the relevant E/Z isomerism at the exo-cyclic C, C double bond was studied. In addition, the H-1 and C-13 chemical shifts of the hydantoins proved to excellently indicate the electronic distribution along the hydantoin ring moiety. Finally, the mass spectrometric fragmentation of the hydantoins is critically discussed.