Cycloalkylmethyl radicals. Part 8. A conformational study of dioxa- and dithia-cyclohexylmethyl radicals by EPR spectroscopy

The conformations of some six-membered oxygen- and sulphur-containing heterocyclic rings have been investigated by EPR spectroscopy using the methylenyl group, CH₂^•, directly attached to a ring carbon atom as a 'spin probe'. For the 2-oxacyclohexylmethyl radical the CH₂^• group has a 'conformational free energy' preference for the equatorial position, -ΔG°₂₇₃ = 1.4 kcal mol^-1, which is about twice as large as the 0.7 kcal mol^-1 found previously for cyclohexylmethyl. The equatorial preference of the CH₂^• group is still greater in (1,3-dioxan-2-yl)methyl radicals; indeed, even with the cis-(5-tert-butyl-1,3-dioxan-2-yl)methyl radical the CH₂^• group was equatorial and the tert-butyl group axial. The CH₂^• group in (1,3-dioxan-5-yl)methyl also exhibits a strong preference for the equatorial position (ΔG° > ca. 1.5 kcal mol^-1), but with cis-(2-methyl-1,3-dioxan-5-yl)methyl it is the methyl group which is equatorial and the CH₂^• group axial. These and other axial/equatorial conformational preferences and the rotational conformational preference of the plane of the CH₂^• group with respect to the C_β-H_β bond are rationalized in terms of subtle steric factors which involve 1,3-axial/axial interactions, or lack thereof, and the variation in the lengths of C-C, C-O and C-S bonds.