Imidazolidin-1-oles, N-2-aminoethyl nitrones and 1,2,5-oxadiazinanes. A novel ring-chain tautomerism

Imidazolin-3-oxides 1 were reduced with NaBH₄ in THF at reflux to give the corresponding 2,3,5-triarylimidazolidin-1-oles 2, which are proved to be in a ring-chain-ring tautomeric equilibrium with N-2-aminoethyl nitrones 3 and 3,5,6-triphenyl-1,2,5-oxadiazinanes 4. The ratios of the ring and chain form are determined by the substituent at the reaction centre and can be described by the equation logK_X = ρσ⁺ + logK_X=H. These are the first examples of a novel three-component ring-chain-ring tautomeric equilibrium characterized by a Hammett-type equation. The stability of the ring form was favoured by electron-withdrawing substituents. Treatment of the equilibrium mixture of 2, 3 and 4 with phenylisocyanate in refluxing toluene gives selectively the corresponding O-carbamoylated imidazolidines 5; cis-5 was shown to isomerize to trans-5 on heating.