title: Electrophilicities of benzaldehyde-derived iminium ions: quantification of the electrophilic activation of aldehydes by iminium formation.
abstract: 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 k(20°C) = 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-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 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.
authors.affiliation: Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13, 81377 München, Germany.
authors.forename: Roland
authors.initials: R
authors.lastname: Appel
authors.forename: Saloua
authors.initials: S
authors.lastname: Chelli
authors.forename: Takahiro
authors.initials: T
authors.lastname: Tokuyasu
authors.forename: Konstantin
authors.initials: K
authors.lastname: Troshin
authors.forename: Herbert
authors.initials: H
authors.lastname: Mayr