Quantification of the Michael-Acceptor Reactivity of α,β-Unsaturated Acyl Azolium Ions
Topics in Catalysis, 2018, 61 (7–8), 585–590
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.