Chiral Synthesis via Organoboranes. 5. Asymmetric Allylboration via Chiral Allyldialkylboranes. Synthesis of Homoallylic Alcohols with Exceptionally High Enantiomeric Excess

Article abstract of DOI:10.1021/jo00354a003

Allyldiisopinocampheylborane, prepared readily by treatment of methoxydiisopinocampheylborane with allylmagnesium bromide, adds smoothly to aldehydes with remarkable enantioselectivity, transferring an allyl group to the carbonyl carbon with boron going to the oxygen.The enantioselectivity in allylboration varies with the reaction temperature, increasing considerably by decreasing the temperature from 0 deg C to -78 deg C.However, the enantioselectivity achieved does not vary significantly with the structure of the aldehyde.The enantioselectivity in the condensation of the reagent with representative ketones is less favorable, but in selected cases the results are promising (as high as 75percent ee).Condensation of methallyldiisopinocampheylborane and (3,3-dimethyl-allyl)diisopinocampheylborane with aldehydes proceeds with equally high asymmetric induction, indicating that wide variation in the structure of the allylic moiety can be accommodated.The effect of changes in the chiral ligand on boron has also been studied.