Effect of alkyl substituents and ring size on alkoxy radical cleavage reactions

The alpha-cleavage ring-opening reactions of a series of acyclic and cyclic alkoxy radicals are examined computationally with CASSCF/6-31G*, UHF/6-31G*, and UB3LYP/6-31G* methods, to explain the anomalous results obtained by Zhang and Dowd (Tetrahedron 1993, 49, 1965): tricyclic alkoxy radicals were found to cleave to give the less-stable products in several cases; even allylic stabilization of the radical formed by cleavage does not influence the direction of cleavage. The source of this kinetic preference is identified as arising from two factors: (i) through-bond interactions significantly slow the rate of bond cleavage in fused four-membered rings relative to exocyclic cleavage of four-membered rings, and (ii) allylic stabilization is not effective in the early transition state of alkoxy radical cleavage in these strained systems. The relationship between activation energies of cleavage and the energy of the reaction is explored for a variety of cyclic and acyclic alkoxy radicals. Benson's observation that the ease of cleavage is related to both the heat of reaction and the ionization potential of the radical formed (Int. J. Chem. Kinet. 1981, 13, 833) is confirmed and extended to more examples.