Enantioselective Synthesis of Disubstituted Alkynes via Organoboranes

Iodine-induced rearrangement of the “ate” complex derived from diisopinocampheylalkylborane and various lithium acetylides furnishes optically active disubstituted alkynes with an enantiomeric excess comparable to that of the diisopinocampheylborane used in the initial asymmetric hydroboration. In the present investigation sec-butyldiisopinocampheylborane, prepared by assymetric hydroboration of cis-2-butene with chiral diisopinocampheylborane of high enantiomeric excess, is used to yield sec-butyl-substituted alkynes in excellent (≥95%) enantiomeric excess. Despite the statistical disadvantage of a 1:2 ratio of sec-butyl groups, the desired alkyne is formed in 58% yield by GLPC, compared to 38% for the product derived from the migration of the isopinocampheyl group, thus suggesting low migratory aptitude for the latter. The iodine-induced rearrangement is highly stereospecific in nature and proceeds with complete retention of configuration at the migrating terminus. The method is capable of producing both enantiomers. © 1986, American Chemical Society. All rights reserved.