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

摘要

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 CH_3CN solution and used to determine the electrophilicity parameters E of the cations defined by the correlation log k_(20℃) = s_n(E + N) (Mayr, H.; et al. J. Am. Chem. Soc. 2001, 123, 9500-9512). With electrophilicity parameters from E = -10.69 (Ar = p-MeOC_6H_4) to E =-8.34 (Ar = p-CF_3), the iminium ions Ar-CH=NMe_2~+ 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 10~3-10~5 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.