HOMOLYTIC REACTIONS OF LIGATED BORANES .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 alpha-C-H group in the carbonyl compound by the chiral amine-boryl radical derived from the catalyst. Ab initio and semiempirical 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 alpha-C-H bonds in diethyl malonate and diethyl methylmalonate.