Enantioselectivity in a free-radical oxidation: measurement of small enantiomeric excesses

Abstract

Partial conversion of 2-methyl-1 -phenylpropan-1 -01 into isobutyrophenone by the acyl aminoxyl radicals formed on oxidation of enantiomerically pure N-fenchelylbenzohydroxamic acid or its paranitro derivative occurs with small but measurable enantioselectivity. This has been demonstrated using H PLC with diode-laser-based polarimetric detection to examine unchanged alcohol in the reaction mixtures.Enantioselectivity in intermolecular hydrogen-atom transfer has been demonstrated by allowing incomplete reaction of a dissymmetric radical with a racemic substrate. lP3 Recovered substrate from such a reaction has been found to be optically active, showing that selective removal of one enantiomer has taken place. Examples include the reaction of dissymmetric alkoxyl radicals with 2-phenylbutane,' and oxidation of racemic benzoin by the pinanecarbonyl aminoxyl 1. ' We have