Cycloalkylmethyl radicals. Part 4. Electron spin resonance study of conformational equilibria in cyclohexenylmethyl and 4-alkylcyclohexenylmethyl radicals

Abstract

For cyclohex-2-enylmethyl and 4-alkylcyclohex-2-enylmethyl radicals the quasi-axial and the quasi-equatorial conformers can both be observed by e.s.r. spectroscopy. Similarly, the axial and equatorial conformers of cyclohex-3-enylmethyl radical can be distinguished by e.s.r. spectroscopy. The conformational free-energy difference of the CH₂˙ group in the 2-position, –ΔG°₃₀₀, was found to be 0.17 ± 0.03 kcal mol^–1 and in the 3-position –ΔG°₃₀₀= 0.0 ± 0.1 kcal mol^–1. The Arrhenius parameters for inversion of the half-chair conformation of cyclohex-2-enylmethyl radical were determined by line-shape analysis of the exchange-broadened spectra and found to be: log(k_f/s^–1)= 12.3 –(5.7 kcal mol^–1)/2.3RT and log (k_b/s^–1)= 12.0 –(5.5 kcal mol^–1)/2.3RT. The barrier to rotation about the Ċ_α–C_β bond in a cyclohexenylmethyl radical is much less than the barrier in a cyclohexylmethyl radical because the former radical has only one syn-axial hydrogen on C(5) to impede the rotation whereas the latter radical has two syn-axial hydrogens on C(3) and C(5).