Stereochemistry of iminoxy radicals

Iminoxy radicals of general structure ArC(X)=N+., where X = H, CH@H, n-Bu, t-Bu, SiEh, SiPh3, GePh3, SnMe3, SnBu3, SnPh3, SMe, SBu, SPh, C1, and Br, have been photolytically generated from the parent oximino compounds or from aromatic nitrile N-oxides in an aprotic solvent. Two configurational isomers, interconvertible in solution, have been detected by electron spin resonance spectroscopy for the majority of these radicals. The preferred geometry of iminoxyls derived from ortho-unsubstituted benzaldoximes is that which places the aryl ring and the oxygen atom on the same side of the C=N double bond (anti). Substitution of the azomethine proton leads to a stabilization of the syn configuration, the effect being larger the greater the atomic number of the leading atom of the substituent group. The relative stability of the syn isomer is ale0 increased by substitution of the aromatic ortho protons. INDO calculations have been carried out on several model systems in order to rationalize the experimental results. The effects responsible for the configurational preference of the different terms of this series of radicals are discussed in terms of a perturbation molecular orbital (PMO) approach.