Quantitative Stereochemical Analysis of a Reagent That Exhibits Asymmetric Amplification, B-Chlorodiisopinocampheylborane (Dip-Cl)

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Abstract

We show that complexation of B-chlorodiisopinocampheylborane (Dip-Cl) with 8-hydroxyquinoline results in an air- and moisture-stable complex. Using enantiomerically pure (+)-α-pinene, the (+,+)-Dip-quinoline complex, [(+)-C10H17]2B(η2-N,O-C10H6NO), was isolated and characterized by spectroscopic and crystallographic methods. When Dip-Cl is prepared from enantiomerically impure (+)-α-pinene a mixture of heterochiral, (+,−)-Dip-Cl, and homochiral, (+,+)-Dip-Cl and (−,−)-Dip-Cl, stereoisomers are formed. We have developed the 8-hydroxyquinoline complexation method for quantification of these stereoisomers by chiral HPLC. Since this is the first quantitative analysis of a reagent that exhibits asymmetric amplification, it enables us to verify part of Kagan's model for this phenomenon and evaluate the terms β and K which are measures of the relative amounts of stereoisomers. Our analysis shows that there is a preference for the formation of the heterochiral (+,−)-Dip-Cl isomer; therefore, the stereoisomers are not statistically distributed. This is beneficial for the asymmetric amplification process because it causes the heterochiral diastereomer to absorb the minor (−)-α-pinene enantiomer, thereby increasing the effective concentration of (+,+)-Dip-Cl that is formed from (+)-α-pinene. We also studied the distribution of stereoisomers as a function of the preparation temperature of the Dip-Cl reagent (0, 10, 20 °C). Increasing the preparation temperature increases the relative amounts of the homochiral stereoisomers, suggesting that the activation energy for the formation of the homochiral isomers is greater than for the heterochiral isomer. Thus, at higher preparation temperatures greater amounts of (−,−)-Dip-Cl are formed from (−)-α-pinene. However, there is a surprising benefit as higher levels of asymmetric induction are observed, especially when low enantiomeric purity α-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 with 8-hydroxyquinoline and subsequent analysis by chiral HPLC provides considerable insight into the asymmetric amplification process observed with this reagent. Moreover, we have shown how the conditions used for the preparation of the reagent affect the asymmetric amplification process.

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