Homolytic reactions of ligated boranes. Part 19. Relationships between structure, reactivity and enantioselectivity for hydrogen-atom abstraction by chiral amine–boryl radicals

The molecular structures of optically active quinuclidine–isopinocampheylborane and of the polycyclic amine–borane formed by cyclisation of N-nopylpyrrolidine–borane have been determined by X-ray crystallography. These and related amine–borane complexes have been used previously as polarity-reversal catalysts to bring about kinetic resolution of racemic esters and ketones. The key step in these resolutions is enantioselective H-atom abstraction from an α-C–H group in the carbonyl compound by the chiral amine–boryl radical derived from the catalyst. Ab initio and semi-empirical molecular orbital calculations have been carried out for representative transition states involved in H-atom transfer to amine–boryl radicals and the roles of dipole–dipole interactions, stereoelectronic effects and hydrogen-bonding have been investigated. The steric demands of a variety of amine–boryl radicals in H-atom transfer reactions have been assessed by determining the relative rates of abstraction from the α-C–H bonds in diethyl malonate and diethyl methylmalonate.