Electron spin resonance detection of the intermediate radicals occurring in homolytic aromatic substitutions of furan and thiophen

Abstract
A number of radicals generated by photolysis add to furan and thiophen giving radicals involved in homolytic aromatic substitution: the corresponding e.s.r. spectra were observed. Photolysis of (PhCOO)2 in furan or thiophen gives the spectra of both (A) and (B). On the other hand photolysis of (MeCOO)2 in furan gave the spectrum [graphic omitted] of (C) at –110° but that of (D) at higher temperature (–10°). The interpretation of the aH hyperfine splitting constants suggests that the structure of these radicals is not planar. They seem rather to prefer envelope-like conformations with the substituents either in pseudoaxial or in pseudoequatorial positions or in equilibrium between the two, depending on the substituent.

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