Radical-induced hydrogen exchange in the presence of reducing transition metal ions

Abstract

Fenton reagents (aqueous solutions of hydrogen peroxide and reducing metal ions) containing tracer tritium in the solvent are shown to effect a small amount of tritium exchange with a dissolved organic solute (dioxan or diethyl ether). The effectiveness of the metal ions (which are in stoicheiometric excess over hydrogen peroxide) follows the order Cr^II > V^II > Ti^III > Fe^II and is thought to reflect their reducing power towards the organic radicals (which are formed by hydrogen abstraction by hydroxyl radicals from the substrates). The reduction of radicals by Cr^II is known to proceed via the formation of isolable organochromium(III) complexes, and the trends in yields and rate constants suggest that the same sequence of steps may be involved with the other ions. The observation of exchange using the complex Cr(bipy)₃²⁺ in place of an ordinary metal ion of a Fenton reagent points to the additional possibility of outer-sphere reduction of radicals to carbanions as unstable intermediates. The low yield of the hydrogen isotope exchange in all cases is attributed to the combined operation of (i) a substantial kinetic isotope effect and (ii) reactions of radicals competing with their reduction.