Conformational equilibria and nitrogen inversion in tetrahydro-1,2,5-oxadiazines

Conformational equilibria and nitrogen inversion barriers in some tetrahydro-1,2,5-oxadiazines have been investigated by ¹H NMR spectroscopy and by chemical equilibration. The conformational free energy differences obtained are: 4-methyl, 1.2±0.2; 6-p-nitrophenyl, 1.4±0.2kcal mole^?1. Barriers to inversion of the N atom at position 2 are in the region 14.1–15.6 kcal mole^?1. The conformational behaviour of the tetrahydro-1,2,5-oxadiazine ring is shown to be composed of aspects of the behaviour of the tetrahydro-1,2-and 1,3-oxazine rings.