Hydroboration. 61. Diisopinocampheylborane of High Optical Purity. Improved Preparation and Asymmetric Hydroboration of Representative Cis-Disubstituted Alkenes

Article abstract of DOI:10.1021/jo00147a004

The convenient preparation of diisopinocampheylborane of high enentiomeric purity (99.1percent) utilizing the commercially available relatively stable borane-methyl sulfide and α-pinene of 92percent enantiomeric purity is described.Methyl sulfide liberated in the hydroboration step interferes with the equilibration needed to improve the optical purity of the reagent.However, this difficulty is readily overcome by removal of methyl sulfide under vacuum following hydroboration of the α-pinene.The raegent is then equilibrated in THF with 15percent excess α-pinene at 0 deg C for3 days.During this equilibration period, the major isomer becomes incorporated selectively into the reagent.This high optical purity diisopinocampheylborane has been utilized for asymmetric hydroboration of representative cis-disubstituted alkenes such as cis-2-butene, cis-3-hexene, cis-2-pentene, norbornene, norbornadiene, cis-4,4-dimethyl-2-pentene, and cis-propenylbenzene.Oxidation of the intermediate organoboranes provides the coprresponding alcohols in enantiomeric purities of 60-98percent.