β-Phosphatoxyalkyl Radical Reactions: Competing Phosphate Migration and Phosphoric Acid Elimination from a Radical Cation−Phosphate Anion Pair Formed by Heterolytic Fragmentation

β-Phosphatoxyalkyl radical reactions were studied experimentally and computationally. The 1,1-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical ( 1 ) reacted to give the migration product 2-benzyl-2-(diphenylphosphatoxy)-1,3-diphenylpropyl radical ( 2 ) and the elimination product 2-benzyl-1,3-diphenylallyl radical ( 3 ) in a variety of solvents. A modest kinetic solvent effect for reactions of 1 was found. Variable temperature studies in THF and acetonitrile gave Arrhenius functions with similar log A terms; the entropies of activation are ∼−5 eu. A deuterated analogue of radical 1 reacted in THF and acetonitrile with rate constants indistinguishable from those of 1, but the ratio of products 2: 3 increased for the deuterated radical requiring kinetic isotope effects (KIEs) in reactions following the rate-limiting step. In aqueous acetonitrile solutions, the β,β-dibenzylstyrene radical cation ( 4 ) was detected as a short-lived intermediate, and the rate constants for formation of 3 and 4 indicated that both species derived from a common intermediate. The 1,1-dimethyl-2-(dimethylphosphatoxy)ethyl radical ( C1 ) was studied computationally. Transition states for concerted phosphate migrations and phosphoric acid elimination were found with energies in the order [1,2]-migration < [1,3]-elimination < [3,2]-migration; each transition state was more polarized than radical C1 . A transition state for homolytic fragmentation of C1 to give 2-methylpropene and the dimethylphosphatoxyl radical could not be found, but the reaction from the ensemble of these two entities to give the 2-methylallyl radical and phosphoric acid was followed computationally. KIEs and solvent dielectric effects were computed for each concerted reaction of C1 . The results indicate that radical 1 reacts in all solvents studied by a common pathway involving initial heterolysis. The first-formed contact pair of radical cation 4 and diphenyl phosphate anion collapses to products 2 and 3 and, as a minor process, evolves to diffusively free radical cation 4 in aqueous acetonitrile solutions. A model for heterolytic fragmentation of β-ester radicals involving contact and solvent-separated ion pairs is presented.