| Résumé | 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 Cβ-Hβ 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. |
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