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 al
lylic stabilization of the radical formed by cleavage does not influence th
e direction of cleavage. The source of this kinetic preference is identifie
d as arising from two factors: (i) through-bond interactions significantly
slow the rate of bond cleavage in fused four-membered rings relative to exo
cyclic cleavage of four-membered rings, and (ii) allylic stabilization is n
ot effective in the early transition state of alkoxy radical cleavage in th
ese strained systems. The relationship between activation energies of cleav
age 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 ra
dical formed (Int. J. Chem. Kinet. 1981, 13, 833) is confirmed and extended
to more examples.