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

Abstract

The convenient preparation of diisopinocampheylborane of high enantiomeric purity (99.1%) utilizing the commercially available relatively stable borane-methyl sulfide and α-pinene of 92% 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 reagent is then equilibrated in THF with 15% excess α-pinene at 0 °C for 3 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 corresponding alcohols in enantiomeric purities of 60–98%. © 1982, American Chemical Society. All rights reserved.