Chiral Synthesis via Organoboranes. 33. The Controlled Reaction of B- Alkydiisopinocampheylboranes with Aldehydes Providing a Convenient Procedure for the Enantiomeric Enrichment of the Boronic Ester Products Through Kinetic Resolution
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Controlled treatment of B-alkyldiisipinocampheylborane, Ipc2BR, obtained by asymmetric hydroboration of appropriate olefin, with aldehydes produces chiral boronate esters having enantiomeric purities markedly higher than those of the substrate. A systematic study of the reaction revealed that the intermediate borinic esters are being kinetically resolved. Since asymmetric hydroboration of alkenes with diisopinocampheylborane provides predominantly the diastereomer that reacts faster with aldehydes, the reaction furnishes in situ enantiomeric enrichment of the products. Thus, B-alkyldiisopinocampheylboranes possessing 81-96 ee are readily converted into boronic esters including 2- butyl, 3-hexyl and exo-norbornyl derivatives. Successful efforts were also made to extend the scope of asymmetric hydroboration-kinetic resolution to representative cyclic dienes making available pure enantiomers of exo-5- norbornenyl- and -3-cyclohexenyl-boronic esters.
- Inorganic Chemistry