A series of α,β-unsaturated iminium ions derived from substituted cinnamaldehydes and C2- and C5-substituted chiral imidazolidin-4-ones were isolated and characterized in solution and in the solid state. The kinetics of the reactions of the iminium ions with silyl ketene acetals were determined in dichloromethane at 20 °C. The resulting second-order rate constants were used to estimate the electrophilicity E of the iminium ions according to the linear free energy relationship log k2(20 °C)=sN(N+E). The kinetics for the reactions of two of the iminium ions with tributylphosphine were studied by laser flash spectroscopy and their second-order rate constants were found to agree within a factor of 2.2 with those calculated by using the linear free energy relationship above.