Chiral Synthesis via Organoboranes. 28. Reaction of α-Chiral Organyldichloroboranes with Organyl Azides Providing a Synthesis of Secondary Amines with Exceptionally High Enantiomeric Purities

Article abstract of DOI:10.1021/jo00003a046

2-Alkyl-1,3,2-dioxaborinanes R*BO2(CH2)3 of essentially 100percent enantiomeric purity were prepared by the asymmetric hydroboration of readily available prochiral olefins with mono- or diisopinocampheylboranes, followed by removal of the chiral auxiliary (α-pinene).The intermediate R*BO2(CH2)3 reacts readily with lithium aluminum hydride at 0 deg C to give the corresponding lithium monoalkylborohydrides stereospecifically in very good yields and in very high enantiomeric purities.The lithium monoalkylborohydrides, on treatment with hydrogen chloride in dimethyl sulfide, give the corresponding monoalkyldichloroboranes in very high enantiomeric purity.The intermediate monoalkyldichloroboranes react readily with organic azides in 1,2-dichloroethane with evolution of gaseous nitrogen and transfer of the organic group from boron to nitrogen with complete retention of configuration to provide the corresponding secondary amines, either (+)- or (-)-, in very high yields and exceptionally high enantiomeric purities.The procedure was applied to the synthesis of representative optically active amines of high enantiomeric purities (ee or de >99percent), including (2S,2'S)-di-2-butylamine, N-<(2S)-2-methyl-1-butyl>-(1S,2R)-trans-2-phenylcyclopentylamine, N<(3S)-3,7-dimethyloct-6-enyl>(1S,2S)-trans-2-methylcyclohexylamine, and the meso-di-2-butylamine.