Electron spin resonance studies of cycles and bicycles

Abstract

The e.s.r. spectra of cyclohexylmethyl radicals have shown that two conformations, one with the CH2˙ group equatorial and one with the CH2˙ group axial, can be distinguished. The Hβ h.f.s. of the axial conformer is large because rotation about the Cβ—Cα˙ bond is hindered by axial hydrogens at C(3) and C(5) in the ring. This proved to be a very useful property enabling the conformations and ring-inversion barriers of cyclohexanes, cyclohexenes and related radicals to be studied by e.s.r. spectroscopy. In addition the various conformers of larger rings (up to 15-membered) also show different spectra. Their preferred conformations and the dynamics of ‘corner migration’ in the medium rings have been investigated. The e.s.r. spectra of cycloheptenylmethyl radicals showed the presence of a minor conformer which suggested that transannular cyclisation might be important. Product analysis confirmed that bicyclo[3.2.1]octane can be obtained in good yield.