Thermolysis of Highly Congested Tri-tert-alkylmethanols: Strain Energies of Bridgehead Alkyl Radicals

Article abstract of DOI:10.1021/jo00222a019

The rate constants and the products of the thermolysis of tertiary alcohols, R¹R²R³COH, where Rⁱ is tert-butyl, 1-adamantyl, 1-bicyclo<2.2.2>octyl, or 1-norbornyl, have been determined.Apart from the usual secondary alcohols and ketones, the products include new ketones, formed by ring opening of the 1-norbornyl group, and solvent-incorporated alcohols.The activation energies for (t-Bu)-C, Ad-C, Oc-C, and, in one case, Nor-C cleavage are compared with the molecular mechanics calculated strain-energy changes, Δstrain.These latter are based on a simple model of the reaction intermediate where the bridgehead radicals are represented by the corresponding alkanes.Taking the ΔG(excit.)(200 deg C)/Δstrain correlation for t-Bu-radical formation as a reference, one can show that the deviations, ΔΔstrain, of the data for the formation of the other radicals (Ad, 2.4; Oc, 4.0; Nor, 7.7 kcal mol^-1) indicate the real difference between the strain energies of the radicals and the alkanes.Analogous ΔΔ(excit.) data for other reactions considered to involve rate-determining radical formation correlate with ΔΔstrain, the slopes ranging from 0.16 to 1.13.The ΔΔstrain values also correlate with solvolysis rate constants of tert-alkyl tosylates, with a slope (ΔΔstrain/ΔG(excit.)(70 deg C)) of 0.36; tentative values for the strain energies of other bridgehead radicals are proposed and compared with the predictions of a radical force field.