NMR investigations on oxaziridines and diaziridines-, configurational stability at nitrogen

1 NMR investigations on oxaziridines and diaziridines, configurational stability at nitrogen 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

Z Arrangement was also ascribed to the isomer absorbing at higher field in the case of the ethyl compounds. CH and CH2 protons near the ring nitrogen are shielded by the aromatic ring in the Z compound. The protons at the ring carbon absorb at lower field (near 5.2 p.p.m.) in the Z compounds than in the E compounds (4.50-4.70 p.p.m.). The chemical shift of this proton may be used for E-Z discrimination in further substances. 

Also due to the high barrier of inversion, optically active oxaziridines are stable and were prepared repeatedly. To avoid additional centres of asymmetry in the molecule, symmetrical ketones were used as starting materials and converted to oxaziridines by optically active peroxyacids via their ketimines (69CC1086, 69JCS(C)2648). In optically active oxaziridines, made from benzophenone, cyclohexanone and adamantanone, the order of magnitude of the inversion barriers was determined by racemization experiments and was found to be identical with former results of NMR study. Inversion barriers of 128-132 kJ moF were found in the A-isopropyl compounds of the ketones mentioned inversion barriers of the A-t-butyl compounds lie markedly lower (104-110 kJ moF ). Thus, the A-t-butyloxaziridine derived from adamantanone loses half of its chirality within 2.3 days at 20 C (73JCS(P2)1575). 

Nitrogen chirality may also be produced by the action of an achiral peroxyacid on a Schiff base containing a chiral amine (75JOC3878). In this case the oxaziridine contains a configurationally known centre of chirality relative to this, absolute configurations of the centres of chirality at nitrogen and carbon, and thus the complete absolute configuration of the molecule, can be determined (see Section 5.08.2.2). 

---