Thermolysis of Highly Congested tert-Butyldialkylcarbinols with Bridgehead Substituents: Molecular Mechanics Treatment of Radical-Forming Processes

Article abstract of DOI:10.1021/jo00144a020

The termolysis rate constants of eight tertiary alcohols, R¹R²R³COH, bearing a tert-butyl group (R³) with various combinations of tert-butyl, tert-amyl, 1-adamantyl, 1-bicyclo<2.2.2>octyl, and 1-norbornyl substituents at the tertiary carbon have been measured in dodecane.The effects of these groups on the rate of tert-butyl cleavage are roughly additive and in quantitive agreement with their known steric requirements, exept for tert-amyl which appears to be smaller than E'_s would suggest.Activation energies can be interpreted in terms of strain energy changes, calculated by molecular mechanics; several different models are used to describe the reaction and, where possible, three different force fields.The most satisfactory description of the reaction is based upon Allinger's MM2 force field, with the assumption that the radical intermediate, R¹R²COH, can be represented by the corresponding dialkylcarbinol.Product analysis shows that the tert-butyl radical is formed in preference to any other, except tert-amyl, but small amounts of products arising from C-Ad fission are detected in certain cases.This result, in accordance with previous molecular mechanics calculations on thermolysis rates, indicates that the radical strain energies in bridgehead systems must be slightly greater than those of the corresponding alkanes but substantially less than those of the related carbocations.