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Insights into Iminium Catalysis
S. Lakhdar, T. Tokuyasu, H. Mayr*
Ludwig-Maximilians-Universität München, Germany
22 January 2009 (online)
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.