Reversal of the Stereochemical Course of 1-Methyl-1H-indole Addition to Cinnamaldehyde withcis-5-Benzyl-(2-fluoromethyl)-2,3-dimethylimidazolidin-4-ones as Catalysts - a Puzzling ‘Fluorine Effect’

Replacement of the cis-Me group by CH2F in the imidazolidinone organocatalyst specified in the title (so-called McMillan generation-I catalyst) leads to reversal of the product configuration in the title reaction. The topicity reversal in the nucleophilic addition step must arise either from cis-addition with respect to the benzylic substituent of an (E)-iminium ion intermediate or from trans-addition to the corresponding (Z)-iminium ion. Mechanistic investigations have not provided evidence for either one of these two possibilities, so far.