Cycloalkylmethyl radicals. Part 7. - Electron paramagnetic resonance characterisation of axial and equatorial centres in seven-membered alicyclic molecules

The EPR spectra of cycloheptylmethyl, cyclohept-4-enylmethyl and 1,3-dioxacyclohept-5-enylmethyl radicals have been observed. The cycloheptylmethyl radical spectrum shows only a single, average conformation, but for the cycloheptene and dioxacycloheptene rings the CH₂^. group proved to be a useful probe of the conformations. The preferred conformation of the 1,3-dioxacycloheptene ring was found to depend on the type of substitution at C(2). For the 2-CH₂^. radical, twist-boat and chair conformers were observed with an Arrhenius activation barrier to inversion of 5 kcal mol^-1. For the 2-CH₃, 2-CH₂^.2 radical the main conformation was the twist-boat. For the 2-Ph, 2-CH₂^. radical the main conformation was the chair; the rotation of the CH₂^. group was found to be restricted by a barrier of ca. 7.5 kcal mol^-1, probably because of steric interaction from two syn axial hydrogens on C(4) and C(7).