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.
