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Levens, Alison; An, Feng; Fernando, Jared E. M.; Ofial, Armin R.; Lupton, David W.; Mayr, Herbert (2018): Quantification of the Michael-Acceptor Reactivity of alpha,beta-Unsaturated Acyl Azolium Ions. In: Topics in Catalysis, Vol. 61, No. 7-8: pp. 585-590
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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 degrees C) = s(N) (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 s (N) 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 approximate to 7 will react with 4 at 20 degrees 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.