Insights into Iminium Catalysis

Significance The Mayr group prepared various cinnamaldehyde-derived iminium triflates 2 ×OTf. The kinetic analysis of the addition of silyl ketene acetals 3 to 2 ×OTf allowed for the determination of electrophilicities E of these iminium ions, which are comparable to those of highly electron-deficient Michael acceptors such as benzylidene ­malononitriles. Iminium ion 2c derived from MacMillan’s first generation imidazolidinone followed by iminium ion 2b derived from α,α-diarylprolinol trimethylsilyl ether were the most active compounds of this ­series reflecting the high catalytic potential of the corresponding secondary amines (see 1 ) in iminium catalysis. The iminium triflates 2 ×OTf were investigated by 2D-NMR spectroscopy indicating that 2b and 2c exist with ( E )-configuration in solution.