The kinetics and mechanism of the organo-iridium catalysed racemisation of amines

Matthew J. Stirling, Joseph M. Mwansa, Gemma Sweeney, A. John Blacker, Michael I. Page

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

The dimeric iodo-iridium complex [IrCp∗I2]2 (Cp∗ = pentamethylcyclopentadiene) is an efficient catalyst for the racemisation of secondary and tertiary amines at ambient and higher temperatures with a low catalyst loading. The racemisation occurs with pseudo-first-order kinetics and the corresponding four rate constants were obtained by monitoring the time dependence of the concentrations of the (R) and (S) enantiomers starting with either pure (R) or (S) and show a first-order dependence on catalyst concentration. Low temperature 1H NMR data is consistent with the formation of a 1 : 1 complex with the amine coordinated to the iridium and with both iodide anions still bound to the metal-ion, but at the higher temperatures used for kinetic studies binding is weak and so no saturation zero-order kinetics are observed. A cross-over experiment with isotopically labelled amines demonstrates the intermediate formation of an imine which can dissociate from the iridium complex. Replacing the iodides in the catalyst by other ligands or having an amide substituent in Cp∗ results in a much less effective catalysts for the racemisation of amines. The rate constants for a deuterated amine yield a significant primary kinetic isotope effect kH/kD = 3.24 indicating that hydride transfer is involved in the rate-limiting step. © The Royal Society of Chemistry 2016.
Original languageEnglish
Pages (from-to)7092-7098
Number of pages7
JournalOrganic and Biomolecular Chemistry
Volume14
Issue number29
Early online date28 Jun 2016
DOIs
Publication statusPublished - 2016

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Iridium
iridium
Amines
amines
catalysts
Catalysts
Kinetics
kinetics
Iodides
iodides
Temperature
Rate constants
Imines
Enantiomers
enantiomers
Amides
Hydrides
Isotopes
isotope effect
imines

Cite this

Stirling, M. J., Mwansa, J. M., Sweeney, G., Blacker, A. J., & Page, M. I. (2016). The kinetics and mechanism of the organo-iridium catalysed racemisation of amines. Organic and Biomolecular Chemistry, 14(29), 7092-7098. https://doi.org/10.1039/C6OB00884D
Stirling, Matthew J. ; Mwansa, Joseph M. ; Sweeney, Gemma ; Blacker, A. John ; Page, Michael I. / The kinetics and mechanism of the organo-iridium catalysed racemisation of amines. In: Organic and Biomolecular Chemistry. 2016 ; Vol. 14, No. 29. pp. 7092-7098.
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abstract = "The dimeric iodo-iridium complex [IrCp∗I2]2 (Cp∗ = pentamethylcyclopentadiene) is an efficient catalyst for the racemisation of secondary and tertiary amines at ambient and higher temperatures with a low catalyst loading. The racemisation occurs with pseudo-first-order kinetics and the corresponding four rate constants were obtained by monitoring the time dependence of the concentrations of the (R) and (S) enantiomers starting with either pure (R) or (S) and show a first-order dependence on catalyst concentration. Low temperature 1H NMR data is consistent with the formation of a 1 : 1 complex with the amine coordinated to the iridium and with both iodide anions still bound to the metal-ion, but at the higher temperatures used for kinetic studies binding is weak and so no saturation zero-order kinetics are observed. A cross-over experiment with isotopically labelled amines demonstrates the intermediate formation of an imine which can dissociate from the iridium complex. Replacing the iodides in the catalyst by other ligands or having an amide substituent in Cp∗ results in a much less effective catalysts for the racemisation of amines. The rate constants for a deuterated amine yield a significant primary kinetic isotope effect kH/kD = 3.24 indicating that hydride transfer is involved in the rate-limiting step. {\circledC} The Royal Society of Chemistry 2016.",
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Stirling, MJ, Mwansa, JM, Sweeney, G, Blacker, AJ & Page, MI 2016, 'The kinetics and mechanism of the organo-iridium catalysed racemisation of amines', Organic and Biomolecular Chemistry, vol. 14, no. 29, pp. 7092-7098. https://doi.org/10.1039/C6OB00884D

The kinetics and mechanism of the organo-iridium catalysed racemisation of amines. / Stirling, Matthew J.; Mwansa, Joseph M.; Sweeney, Gemma; Blacker, A. John; Page, Michael I.

In: Organic and Biomolecular Chemistry, Vol. 14, No. 29, 2016, p. 7092-7098.

Research output: Contribution to journalArticle

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T1 - The kinetics and mechanism of the organo-iridium catalysed racemisation of amines

AU - Stirling, Matthew J.

AU - Mwansa, Joseph M.

AU - Sweeney, Gemma

AU - Blacker, A. John

AU - Page, Michael I.

PY - 2016

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N2 - The dimeric iodo-iridium complex [IrCp∗I2]2 (Cp∗ = pentamethylcyclopentadiene) is an efficient catalyst for the racemisation of secondary and tertiary amines at ambient and higher temperatures with a low catalyst loading. The racemisation occurs with pseudo-first-order kinetics and the corresponding four rate constants were obtained by monitoring the time dependence of the concentrations of the (R) and (S) enantiomers starting with either pure (R) or (S) and show a first-order dependence on catalyst concentration. Low temperature 1H NMR data is consistent with the formation of a 1 : 1 complex with the amine coordinated to the iridium and with both iodide anions still bound to the metal-ion, but at the higher temperatures used for kinetic studies binding is weak and so no saturation zero-order kinetics are observed. A cross-over experiment with isotopically labelled amines demonstrates the intermediate formation of an imine which can dissociate from the iridium complex. Replacing the iodides in the catalyst by other ligands or having an amide substituent in Cp∗ results in a much less effective catalysts for the racemisation of amines. The rate constants for a deuterated amine yield a significant primary kinetic isotope effect kH/kD = 3.24 indicating that hydride transfer is involved in the rate-limiting step. © The Royal Society of Chemistry 2016.

AB - The dimeric iodo-iridium complex [IrCp∗I2]2 (Cp∗ = pentamethylcyclopentadiene) is an efficient catalyst for the racemisation of secondary and tertiary amines at ambient and higher temperatures with a low catalyst loading. The racemisation occurs with pseudo-first-order kinetics and the corresponding four rate constants were obtained by monitoring the time dependence of the concentrations of the (R) and (S) enantiomers starting with either pure (R) or (S) and show a first-order dependence on catalyst concentration. Low temperature 1H NMR data is consistent with the formation of a 1 : 1 complex with the amine coordinated to the iridium and with both iodide anions still bound to the metal-ion, but at the higher temperatures used for kinetic studies binding is weak and so no saturation zero-order kinetics are observed. A cross-over experiment with isotopically labelled amines demonstrates the intermediate formation of an imine which can dissociate from the iridium complex. Replacing the iodides in the catalyst by other ligands or having an amide substituent in Cp∗ results in a much less effective catalysts for the racemisation of amines. The rate constants for a deuterated amine yield a significant primary kinetic isotope effect kH/kD = 3.24 indicating that hydride transfer is involved in the rate-limiting step. © The Royal Society of Chemistry 2016.

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DO - 10.1039/C6OB00884D

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SN - 1477-0520

IS - 29

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