Kinetic analysis of the asymmetric amplification exhibited by B‐chlorodiisopinocampheylborane

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Abstract

Abstract A 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.

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