Strain Release in Hydrogen Atom Transfer from 1,4-Disubstituted Cyclohexanes to the Cumyloxy Radical

A kinetic, product and computational study on the reactions of the cumyloxyl radical (CumO•) with 1,4-dimethyl- and 1,4-diphenylcyclohexanes is reported. The rate constants for hydrogen atom transfer (HAT) from the C-H bonds of these substrates to CumO•, together with the corresponding oxygenation product distributions reveal the role of strain release on reaction site-selectivity. Transition structures and activation barriers obtained by DFT calculations are in excellent agreement with the experimental results. Tertiary/secondary ratios of oxygenation products of 0.6, 1.0 and 3.3 were observed, for trans-1,4-dimethyl-, cis-1,4-dimethyl- and trans-1,4-diphenylcyclohexane, respectively. With cis-1,4-diphenylcyclohexane, exclusive formation of the diastereomeric tertiary alcohol products was observed. Within the two diastereomeric couples, the tertiary equatorial C-H bond in the cis- isomer is ∼6 and 27 times more reactive, respectively, than the tertiary axial ones, a behavior that reflects the release of 1,3-diaxial strain in the HAT transition state. The tertiary axial C-H bonds of the four substrates show remarkably similar reactivities in spite of the much greater stabilization of the benzyl radicals resulting from HAT from the 1,4-diphenylcyclohexanes. The lack of benzylic acceleration is discussed in the framework of Bernasconi’s “principle of non-perfect synchronization”.