Electrophilicities of Benzaldehyde-Derived Iminium Ions: Quantification of the Electrophilic Activation of Aldehydes by Iminium Formation

Rate
constants for the reactions of benzaldehyde-derived iminium
ions with C-nucleophiles (enamines, silylated ketene acetals, and
enol ethers) have been determined photometrically in CH3CN solution and used to determine the electrophilicity parameters E of the cations defined by the correlation log k20°C = sN(E + N) (Mayr, H.; et al. J. Am.
Chem. Soc. 2001, 123, 9500–9512).
With electrophilicity parameters from E = −10.69
(Ar = p-MeOC6H4) to E = −8.34 (Ar = p-CF3), the iminium ions Ar–CHNMe2+ have almost the same reactivities as analogously substituted arylidenemalononitriles
Ar–CHC­(CN)2 and are 10 orders of magnitude
more reactive than the corresponding aldehydes. The rate constants
for the reactions of iminium ions with amines and water in acetonitrile
are 103–105 times faster than predicted
by the quoted correlation, which is explained by the transition states
which already experience the anomeric stabilization of the resulting
N,N- and O,N-acetals.