Ring-chain tautomerism has been studied in 2-substituted and 1, 3-disubstituted diazolidines. In trifluoroacetic acid, these heterocycles are in equilibrium with the open-chain iminium ion. For materials lacking 2 substitution, &lo% of the chain form is present at room temperature. Because of resonance stabilization of the positive charge, 2-aryl-substituted systems have about 30% of the chain form. All of these ring and chain forms undergo ...