Configuration of arylchloromethaniminoxy radicals. A reassignment

In a previous article some of us1 reported that iminoxy radicals 1 obtained b'y oxidation of benzohydroxymoyl chlorides with lead tetraacetate in benzene solution exist in only one of the two possible configurations which, on the basis of the ESR spectral parameters, was assigned to be anti (E).2 In a more recent study on the stereochemistry of a variety of iminoxyl~w,~e have found that in aprotic solvents the preferred geometry of the radicals derived from ortho-unsubstituted benzaldoximes is anti (21, viz., that the aryl group and the oxygen are on the same side of the plane defined by the C=N bond, while substitution of the ortho positions of the aryl ring leads to a stabilization of the syn isomer. Stabilization of the syn configuration is also observed by substitution of the azomethine proton by an XR, group (X = Si, Sn, and Ge; R, are alkyls or aryls), the effect being that the larger the atomic number of X the greater the stabilization. In contradiction with these new findings, it appeared to be the assignment previously made for arylchloromethaniminoxyls. We therefore collected further experimental data on these radicals and on related iminoxyls with the aim of establishing on more firm grounds their preferred geometry.