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

Charles W. Moeder, Mark Whitener, John R. Sowa

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13 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)7218-7225
Number of pages8
JournalJournal of the American Chemical Society
Volume122
Issue number30
DOIs
StatePublished - 2 Aug 2000

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Stereoisomerism
Amplification
Complexation
Isomers
Oxyquinoline
Chemical analysis
Temperature
Enantiomers
High Pressure Liquid Chromatography
Moisture
Activation energy
Air

Cite this

@article{88e21ff778ff49b8ac9545e7197f6c4d,
title = "Quantitative stereochemical analysis of a reagent that exhibits asymmetric amplification, B-chlorodiisopinocampheylborane (Dip-Cl)",
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.",
author = "Moeder, {Charles W.} and Mark Whitener and Sowa, {John R.}",
year = "2000",
month = "8",
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doi = "10.1021/ja000158j",
language = "English",
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pages = "7218--7225",
journal = "Journal of the American Chemical Society",
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Quantitative stereochemical analysis of a reagent that exhibits asymmetric amplification, B-chlorodiisopinocampheylborane (Dip-Cl). / Moeder, Charles W.; Whitener, Mark; Sowa, John R.

In: Journal of the American Chemical Society, Vol. 122, No. 30, 02.08.2000, p. 7218-7225.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Quantitative stereochemical analysis of a reagent that exhibits asymmetric amplification, B-chlorodiisopinocampheylborane (Dip-Cl)

AU - Moeder, Charles W.

AU - Whitener, Mark

AU - Sowa, John R.

PY - 2000/8/2

Y1 - 2000/8/2

N2 - 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.

AB - 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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U2 - 10.1021/ja000158j

DO - 10.1021/ja000158j

M3 - Article

VL - 122

SP - 7218

EP - 7225

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 30

ER -