REARRANGEMENT REACTIONS OF OXAZIRIDINES TO NITRONES - X-RAY CRYSTAL AND MOLECULAR-STRUCTURE OF N-TERT-BUTYL-ALPHA(O-HYDROXYPHENYL)NITRONE

Substituted N-benzylidene-t-butylamines are oxidized with m-chloroperoxybenzoic acid to oxaziridines which can rearrange to the corresponding nitrones when electron-donating groups are present in the phenyl ring. The oxaziridine-to-nitrone rearrangement, which has been considered as a 'pseudo-abnormal' reaction, can also be catalysed by Lewis acids. It has been found that the rearrangement of 2-t-butyl-3-(o-hydroxyphenyl)oxaziridine to the corresponding nitrone is of first order. The rearrangement has been investigated for different substituents in the phenyl ring, and in the case of 2-t-butyl-3-phenyloxaziridine substituted in the ortho position with electron-donating groups it has been found that the presence of protons or a Lewis acid is necessary. An X-ray structure of α-(o-hydroxyphenyl)-N-t- butylnitrone shows strong hydrogen bonding between the nitrone oxygen and the hydrogen in the hydroxy group. The oxaziridine-to-nitrone rearrangement is also analysed from a theoretical point of view using ab initio calculations. A Mulliken-population analysis of the C-O and N-O bonds in the oxaziridine ring for para-substituted 2-t-butyl-3-phenyloxaziridines shows a reduction of the C-O bond population when an electron-donating group is present in the para position of the phenyl ring compared with an electron-withdrawing group; the N-O bond populations show the reverse picture. A state-correlation diagram for the oxaziridine-to-nitrone rearrangement is also presented and the experimental and theoretical results support each other.