A THEORETICAL INVESTIGATION OF THE 1,3-MIGRATION IN ALLYLPEROXYL RADICALS

Allylperoxyl radicals undergo a 1,3-transfer of oxygen, and both a concerted mechanism and a mechanism involving fragmentation of the peroxyl and readdition of oxygen have been previously proposed. In our theoretical study of this rearrangement, calculations were performed at the unrestricted Hartree-Fock, MP2, and MP3 levels: standard 6-31G(d) basis sets were used for geometry optimizations and pathway studies; single point energies for all optimized structures were obtained at the MP2/6-31G(d,p)//H-F/6-31G(d) level and for some structures at the MP3/6-31G(d,p)//HF/6-31g(d) level. A conformational energy map of the allylperoxyl radical was calculated, and a relatively flat energy surface was found; all of the conformations are accessible and populated under normal conditions. Attempts to find a transition-state structure for the rearrangement revealed a bound cyclic radical that is close in energy to the allylperoxyl. The lowest energy pathway from the acyclic allylperoxyl to the cyclic radical proceeds over a barrier of almost 170 kJ/mol. Fragmentation of allylperoxyl to give the allyl radical and oxygen requires only 90 kJ/mol, and the calculations therefore are interpreted to suggest that the rearrangement proceeds by this lower energy fragmentation-readdition mechanism. Calculations were also carried out on allyl and substituted allyl radicals. All of the radicals investigated were planar, so that the experimentally observed stereoselectivity of the rearrangement cannot be explained by arguing that the allyl radicals retain a pyramidal conformation after fragmentation of the allylperoxyl radical. The selectivity of the rearrangement thus must result from a cage effect wherein t he diffusion of the allyl radical-oxygen pair is prevented due to caging by the solvent.