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

J. Am. Chem. Soc., 2013, 135 (17), 6579-6587 published on 09.04.2013
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-acet
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