Homolytic reactions of ligated boranes. Part 18. The scope of enantioselective hydrogen-atom abstraction by chiral amine–boryl radicals for kinetic resolution under conditions of polarity reversal catalysis

A variety of new and previously-known optically active amine–borane complexes have been used as polarity reversal catalysts for the kinetic resolution of representative racemic carbonyl-containing compounds. The key step involves enantioselective abstraction of hydrogen from a C–H bond α to the carbonyl function by optically active amine–boryl radicals derived from the catalyst by hydrogen-atom transfer to tert-butoxyl radicals generated by photolysis of di-tert-butyl peroxide. Chiral discrimination is generally not large, although enantioselectivity factors up to 8.8 were obtained at –74 °C in oxirane as solvent. The more reactive substrate enantiomer can generally be predicted by consideration of the steric interactions between the substituents attached to the boron atom and to the α-carbon atom in the diastereoisomeric transition states. However, hydrogen bonding and dipole–dipole interactions, together with stereoelectronic effects, may also play a part in determining enantioselectivity particularly when there is not marked steric asymmetry around the reacting centres.