Cycloalkylmethyl radicals. Part 3. Dynamic stereochemistry of axial and equatorial cyclohexylmethyl and 4-alkylcyclohexylmethyl radicals

Abstract

For cyclohexylmethyl and 4-alkylcyclohexylmethyl radicals the conformer in which the CH2˙ group adopts the axial position and that in which the CH2˙ group adopts the equatorial position can both be observed by e.s.r. spectroscopy. At 140 K the axial conformers have a(Hβ)ca. 42–43 G; the equatorial conformers have a(Hβ)ca. 30–31 G. For cis-4-methylcyclohexylmethyl radicals the ratio of the concentrations of the two conformers was studied as a function of temperature and shown to depend on the rate of radical ring inversion vs. the radical lifetime; the rate constant for ring inversion was obtained. As a check on the e.s.r. results the conformational equilibrium of cis-4-methylcyclohexylmethyl bromide was studied by 1H n.m.r. spectroscopy, which gave –ΔG°300(CH2Br)= 1.91 kcal mol–1. The relative conformer concentrations were also measured as a function of temperature for cyclohexylmethyl radicals and the conformational free energy difference of the CH2˙ group (–ΔG°300) was found to be 0.71 kcal mol–1. The preponderance of the conformer of the cis-4-methylcyclohexylmethyl radical with the CH3 group axial at T < ca. 175 K was attributed to the fact that the axial non-rotating CH3 group can adopt a staggered, minimum-energy conformation, whereas the axial non-rotating CH2˙ group cannot because of its planarity. The barriers to rotation about the Cα–Cβ bonds in the axial radicals were found to be ca. 1.0 kcal mol–1 greater than those of the equatorial radicals; this is responsible for the greater a(Hβ) values of the axial radicals. The axial and equatorial conformers of cyclohexylmethyl radicals were investigated by semi-empirical SCF MO methods.