Ionization kinetics of beta-substituted radicals in solution: A nanosecond laser flash photolysis study.

摘要

β-Substituted radicals are transient species with a potential leaving group attached to the carbon adjacent to the unpaired electron. In. nonpolar solutions, such intermediates exhibit familiar radical chemistry and react via radical-radical coupling and homolysis. However, when generated in polar media, β-substituted radicals undergo heterolytic cleavage of the β-group to form the corresponding olefin radical cation. This novel, open shell ionization mechanism has been implicated in a growing number of biochemical transformations, ranging from healthy enzyme mediated processes to the critical steps of radiation-induced DNA damage. Yet, in spite of its biological relevance, a survey of the literature unearths surprisingly little about the dynamics and kinetics of this β-heterolysis reaction that has been shown to occur unassisted, but that can be susceptible to acid catalysis.; This thesis presents the results of a mechanistic investigation into the ionization of β-mesylate, β-hydroxy, and β-diethylphosphate para-substituted phenethyl radicals in polar solvents such as HFIP, TFE and acetonitrile. The radicals of interest were generated upon nanosecond laser flash photolysis of the relevant precursor compound and detected by absorption spectroscopy. Time-resolved growth traces of the radical cation provides unambiguous evidence that the β-heterolysis mechanism is an energetically favorable reaction pathway for β-substituted radicals in nitrogen-saturated polar solvents. Rate constants calculated for the uncatalyzed ionization of the β-mesylate radicals are found to be fast, within the range of 106–107 s−1. As anticipated, the β-hydroxy and β-diethylphosphate radicals required acid catalysis to undergo heterolysis. Typical bimolecular rate constants calculated for the acid-dependent ionization of these radicals are of the order of 108 M−1 s−1.; The dynamics of the β-heterolysis mechanism were probed by varying the aromatic group on the radical and investigating how the electronic nature of the substituent influences the calculated rate constant for the ionization process. A linear correlation obtained between k_het and the σ + scale suggests the existence of some overlap between the SOMO and the cationic centre developing in the transition state. (Abstract shortened by UMI.)