Quantification of the Michael-Acceptor Reactivity of α,β-Unsaturated Acyl Azolium Ions

Original Paper
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Abstract

2-Cinnamoylimidazolium ions 4 have been synthesized by treatment of 2-cinnamoylimidazoles 8 with methyl triflate. They were characterised by NMR and mass spectroscopy, in one case (4f) also by X-ray analysis. The kinetics of their reactions [and also those of cinnamoyl fluoride (1)] with stabilised carbanions 9a–e and silyl ketene acetal 9f (reference nucleophiles) were measured photometrically. The correlation log k(20 °C) = sN (E + N) was used to calculate the electrophilicity parameters E of the cinnamoyl azolium ions 4 from the resulting second-order rate constants k and the previously reported N and sN parameters of the reference nucleophiles 9. All 2-cinnamoylimidazolium ions 4 were found to be 2–4 orders of magnitude more electrophilic than cinnamoyl fluoride (1) showing that the direct attack of nucleophiles at 1 can be avoided if sufficient concentrations of 4 are produced in the NHC-catalysed reactions of 1 with nucleophiles. From the range of electrophilicity(–12 < E < − 10) for the cinnamoylimidazolium ions 4 one can derive that only nucleophiles stronger than N ≈ 7 will react with 4 at 20 °C in reasonable time, suggesting that in NHC-catalysed reactions of cinnamoyl fluoride (1) with silyl enol ethers (typically 4 < N < 7), enolate ions, produced by fluoride-induced desilylation of silyl enol ethers, are the active nucleophiles.

Keywords

Kinetics Organocatalysis Nucleophilic carbenes Reactivity Electrophilicity 

Notes

Acknowledgements

The authors thank the Australian Research Council (Discovery Program DP120101315) and the Deutsche Forschungsgemeinschaft (SFB 749, Project B1) for financial support and Dr. Peter Mayer for the X-ray analysis of 4f. DWL is grateful to the Alexander von Humboldt Foundation for the Ludwig-Leichardt Award.

Supplementary material

11244_2018_914_MOESM1_ESM.docx (17.4 mb)
Supplementary material 1 (DOCX 17796 KB)

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.School of ChemistryMonash UniversityClaytonAustralia
  2. 2.Department ChemieLudwig-Maximilians-Universität MünchenMunichGermany

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