Regioselective oxidation of aliphatic acids by complexed hydroxyl radicals

Abstract

Radicals produced by the TiCl3/H2O2 oxidation of various saturated and unsaturated aliphatic acids have been identified by electron spin resonance, and their relative concentrations have been determined by computer simulation of the spectra. In acid solution saturated acids are oxidized principally at the β and ω-1 (penultimate) positions, and in basic solution at the α and ω-1 positions. A mechanism involving intermolecular oxidation of the terminal methylene group of one acid by OH complexed through titanium to the carboxyl group of another acid in a head to tail oriented dimer is proposed to explain the prevalence of w-1 oxidation. Explanations for other selective oxidations can be made in terms of inductive and mesomeric effects. The lower unsaturated acids react by addition of OH to the double bond. Failure to detect radicals other than from α-hydrogen abstraction in the larger acids is attributed to micelle formation. In some cases assignments have been confirmed by the oxidation of related hydroxy acids, and by reduction of halogenated acids.