Organic Reducing Agents: Some Radical Chain Reactions of Ketyl and 1,3-Dioxolanyl Radicals with Activated Bromides

The radical chain reduction of primary and secondary alpha-bromo esters by 2-propanol and 2-methyldioxolane has been shown to be a kinetically viable process with average chain lengths of >10 at temperatures between 30 and 60 degrees C. For electron-deficient bromides, these simple organic reagents may effectively replace the more commonly used (and more expensive) organosilicon or tin hydides. Some mechanistic insights have been obtained from a combination of kinetic and thermodynamic measurements. The rate constants for the reaction of the 2-hydroxy-2-propyl and the 2-methyl-1,3-dioxolan-2-yl radicals with a primary bromo ester have been estimated to be 8 x 10(3) M(-1) s(-1) and 6 x 10(3) M(-1) s(-1), respectively, at 30 degrees C in acetonitrile. The electrochemical oxidation potentials of a number of electron-rich radicals derived from hydrogen atom abstraction from alcohols and dioxolanes, determined using photomodulation voltammetry, provide an assessment of the thermochemisty of the putative outer sphere electron-transfer reaction between the radical and the alpha-bromo ester (the value for 1,3-dioxolane has been redetermined and corrects an erroneous value previously published). From these data, it is shown that the order of reactivity of the organic reducing agents follows the same trend as the oxidation potentials of the corresponding radicals. Rate constants for the outer sphere electron-transfer reduction of the bromo esters by electrochemically generated radical anions were used to establish a Marcus-type relationship between the rate constants and the standard potential of the reducing agent. Comparison of these rate constants with those for the reactions of the electron-rich radicals suggest that the reactions of the methanol; and ethanol-derived radicals do not proceed via outer sphere electron transfer as had been previously suggested but have significant bonding (i.e., atom transfer) character at the transition state. Electron transfer in the reactions of the 2-propanol- and 2-methyl-1,3-dioxolane-derived radicals cannot be ruled out. Nevertheless, the order of reactivity of the organic reducing agents follows the same trend as the oxidation potentials of the corresponding radicals suggesting that these potentials can be used as a predictive tool for the design of new reagents.