Kinetic analysis of the asymmetric amplification exhibited by B‐chlorodiisopinocampheylborane

Abstract

AbstractA quantitative investigation was undertaken to determine experimentally the relative reaction rates of the heterochiral and homochiral species of B‐chlorodiisopinocampheylborane (Dip‐Cl), a reagent that exhibits asymmetric amplification. Using the method of flooding to reduce the apparent second‐order reaction to pseudo‐first‐order conditions, rate constants of 3.8 (±1.0) × 10−4 and 1.7 (±0.8) × 10−5 m−1 s−1 (at −15°C) were found for the homochiral and heterochiral species, respectively. The resulting relative reaction rate, the value of g in Kagan and co‐workers' model, was 0.04 ± 0.03. Additional experiments were conducted to confirm this value. The deterioration of the chiral purity of the final product was simulated throughout reaction conversion and compared with actual values. Through minimization, a relative reaction rate (g) of 0.08 ± 0.04 was determined. Finally, using 8‐hydroxyquinoline complexation, the change in isomeric ratio of the Dip‐Cl remaining in the reaction solution was measured. The relative reaction rate (g) required to consume the isomers at the observed rate was determined as 0.08 ± 0.01. Thus, three independent methods have been used to determine the relative reaction rate (g) of the heterochiral to homochiral species with good agreement to give an average value of 0.07 ± 0.03. Copyright © 2004 John Wiley & Sons, Ltd.