Chemistry of beta-ester radicals: Evidence supporting ion pair processes

Rate constants for heterolysic fragmentation, rearrangement and deprotonation of several beta-ester radicals were measured in various solvents by direct laser flash photolysis kinetic studies.; The rate constants measured indicated that rearrangements occurred through a rate-limiting heterolysis in high polarity solvent mixtures. Furthermore, consistent entropy of activation terms for heterolysis and/or rearrangement of beta-phosphate and beta-acetate radicals suggested a dissociative pathway in all solvents for migration of these latter leaving groups.; An ion pair was detected during reaction of a beta-phosphate radical. The kinetic values indicated that for this latter system the rearrangement occurred by a heterolysis pathway in all solvents. Kinetic modeling permitted development of a complex ion pair model describing the chemistry of beta-phosphate radicals. This ion pair model might be valid for all beta-ester radicals. The outcome of the ion pair processes, collapse versus free ion formation, was contingent on the stability of the ions formed and on the nature of the media.; Kinetic studies of different beta-(ester)alkyl radicals permitted development of kinetic scales predicting the solvent polarity effect on the rate constants for heterolysic cleavage of beta-mesylate, beta-diphenylphosphate and beta-trifluoroacetate radicals. beta-Methoxy and beta-mesyloxy radicals were found to undergo general Bronsted acid-catalyzed heterolysis. Heterolysis of beta-methoxy radicals also was catalyzed by boron trifluoride.; Deprotonation of radical canons was observed in the presence of 2,2,2-trifluoroethanol (TFE) and in THF. The results suggested that extremely weak bases such as TFE can deprotonate the transient. Also, the studies indicated that THF solvent should be avoided when one wishes to generate diffusively free radical cations.