Cycloalkylmethyl Radicals. 5. 6- to 15-Membered Rings: EPR Studies of Ring Conformations and Stereodynamics

Cycloalkylmethyl radicals having 7- to 15-membered rings (excluding cyclotetradecylmethyl) have been generated and examined by EPR spectroscopy. All the cycloalkylmethyl radicals from the 9-membered ring upward show two distinguishable conformers, though there may be additional conformers present with cyclodecylmethyl and cyclopentadecylmethyl radicals. All the cycloalkylmethyl radicals from the 10-membered ring upward have one conformer, the quasi-equatorial, QE, with an Ho hfs in the range 27.7-32.0 G at 140 K and one conformer, the quasi-axial, QA, with an Ho hfs in the range 38.3-40.4 G at 140 K. the QE and QA conformers have been assigned to species in which the CH₂' group occupies “outer-edge” and “corner” sites, respectively, in the preferred conformations of the cycloalkanes. the EPR results indicated that the preferred conformations of the C₉, C₁₀, C_n, C₁₂, C₁₃, and C₁₅rings were [333], [2323], [335], [3333], [346], and [33333], respectively. Arrhenius parameters for ring-atom site exchange in the cycloalkylmethyl radicals have been determined from the exchange broadening in the EPR spectra. These EPR barriers are compared with literature data on cycloalkane free energy barriers measured by NMR and enthalpic barriers estimated by force field and related methods.