title: Nucleophilicity parameters of enamides and their implications for organocatalytic transformations.
abstract: 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 k(2) (20 °C)=s(N)(E+N), which allowed us to derive reactivity parameters N and s(N). 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 s(N) 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.
authors.affiliation: Department Chemie, Ludwig-Maximilians-Universiät München, Germany.
authors.forename: Biplab
authors.initials: B
authors.lastname: Maji
authors.forename: Sami
authors.initials: S
authors.lastname: Lakhdar
authors.forename: Herbert
authors.initials: H
authors.lastname: Mayr