Nucleophilicity Parameters of Enamides and Their Implications for Organocatalytic Transformations
Chem.Eur.J., 2012, 18, 5732-5740 published on 27.03.2012
The kinetics of the reactions of eleven substituted enamides with benzhydrylium ions (diarylcarbenium ions) were determined in acetonitrile solution. The second-order rate constants follow the correlation log k2(20°C)=sN(E+N), which allowed us to derive reactivity parameters N and sN. With 4.6<N<7.1, enamides had similar nucleophilicities to enol ethers and non- or weakly activated indoles and pyrroles. The combination of the N and sN parameters with the previously reported E parameters of typical Michael acceptors, α,β-unsaturated iminium ions, and the chlorinating agent hexachlorocyclohexa-2,4-dienone allowed us to reliably reproduce the experimental rate constants of the reactions with these electrophiles. The reactions of enamides with α,β-unsaturated iminium ions only proceeded in the presence of bases (e.g., 2,6-lutidine), which was explained by the low Lewis acidities of the iminium ions. The consequences of these results for the use of enamides in organocatalytic reactions are discussed.