Quantitative stereochemical analysis of a reagent that exhibits asymmetricamplification, B-chlorodiisopinocampheylborane (Dip-Cl)

We show that complexation of B-chlorodiisopinocampheylborane (Dip-Cl) with 8-hydroxyquinoline results in an air- and moisture-stable complex. Using en antiomerically pure (+)-alpha-pinene, the (+,+)-Dip-quinoline complex, [(+) -C10H17](2)B(eta(2)-N,O-C10H6NO), was isolated and characterized by spectro scopic and crystallographic methods. When Dip-Cl is prepared from enantiome rically impure (+)-alpha-pinene a mixture of heterochiral, (+,-)-Dip-Cl, an d homochiral, (+,+)-Dip-Cl and (-,-)-Dip-Cl, stereoisomers are formed. We h ave developed the 8-hydroxyquinoline complexation method for quantification of these stereoisomers by chiral HPLC. Since this is the first quantitativ e analysis of a reagent that exhibits asymmetric amplification, it enables us to verify part of Kagan's model for this phenomenon and evaluate the ter ms beta and K which are measures of the relative amounts of stereoisomers. Our analysis shows that there is a preference for the formation of the hete rochiral (+,-)-Dip-Cl isomer; therefore, the stereoisomers are not statisti cally distributed. This is beneficial for the asymmetric amplification proc ess because it causes the heterochiral diastereomer to absorb the minor (-) -alpha-pinene enantiomer, thereby increasing the effective concentration of (+,+)-Dip-Cl that is formed from (+)-alpha-pinene. We also studied the dis tribution of stereoisomers as a function of the preparation temperature of the Dip-Cl reagent (0, 10, 20 degrees C). Increasing the preparation temper ature increases the relative amounts of the homochiral stereoisomers, sugge sting that the activation energy for the formation of the homochiral isomer s is greater than for the heterochiral isomer. Thus, at higher preparation temperatures greater amounts of (-,-)-Dip-Cl are formed from (-)-alpha-pine ne. However, there is a surprising benefit as higher levels of asymmetric i nduction are observed, especially when low enantiomeric purity a-pinene is used. In addition, the reduction reactions proceed slightly faster when Dip -Cl is prepared at higher temperature. In sum, the complexation of Dip-Cl w ith 8-hydroxyquinoline and subsequent analysis by chiral HPLC provides cons iderable insight into the asymmetric amplification process observed with th is reagent. Moreover, we have shown how the conditions used for the prepara tion of the reagent affect the asymmetric amplification process.