Reactivity parameters for rationalizing iminium-catalyzed reactions

The correlation equation (1), lg k(20°C)=s(E+N), where electrophiles are characterized by one (E) and nucleophiles are characterized by two parameters (N, s) was used to rationalize the scope of iminium-catalyzed reactions. Kinetics of the reactions of iminium triflates, pregenerated from cinnamaldehyde and secondary amines, with cyclic ketene acetals were studied by UV-Vis spectroscopy. From the second-order rate constants, electrophilicity parameters -10<<-7 have been derived for these iminium ions. Eqn (1) was found to correctly predict the rate constants for the reactions of the cinnamaldehyde-derived iminium ions with pyrroles, indoles, and sulfur ylides. The zwitterion obtained from cinnamaldehyde and indoline-2-carboxylic acid reacts more than 105 times faster with a sulfur ylide than predicted by Eqn (1), which is explained by MacMillan's 'electrostatic activation'. The failure of imidazolidinones to catalyze cyclopropanations of a,b-unsaturated carbonyl compounds by sulfur ylides is not due to the low nucleophilic reactivity of sulfur ylides but due to their high Brønsted basicity which inhibits the formation of the iminium ions.