Strong Directed Electrostatic Activation

Significance

The authors report their investigation on the kinetics of the reactions between iminium ions 4a-f and sulfur ylide 2. A good agreement between experimental and calculated (by the equation in the scheme) rate constants was observed with the exception of 4d, indicating a strong electrostatic interaction between the zwitterion 4d and 2, an effect that is not taken into account in the equation. In contrast to MacMillan’s statement (J. Am. Chem. Soc. 2005, 127, 3240), the reaction of iminium triflate 4a-OTf with ylide 2 gives the cyclopropanation product in good yield. Therefore, MacMillan’s observation that 0% conversion of the reaction between cinnamaldehyde 1 and ylide 2, catalyzed with imidazolidinone trifluoroacetic acid salt, cannot be ascribed to the low rate of the iminium 4a and 2, but the high basicity of 2, which inhibits the formation of the iminium.