Keywords: Kinetics / Linear free energy relationships / Solvent effects / Nucleophilicity / AminesThe kinetics of the reactions of primary and secondary amines with benzhydrylium ions and quinone methides in acetonitrile have been studied under pseudo-first-order conditions (high excess of amines) by UV/Vis spectrophotometry. Generally, the plots of k obs versus amine concentration were linear, in line with a second-order rate law. However, for some reactions of secondary amines with quinone methides, the plots of k obs versus amine concentration showed an upward curvature, which indicates rate-limiting deprotonation of the initially formed adduct by a second molecule of amine.

The kinetics of the reactions of the azodicarboxylates 1 with the enamines 2 have been studied in CH(3)CN at 20 °C. The reactions follow a second-order rate law and can be described by the linear free energy relationship log k(2) (20 °C) = s(N+E) (E = electrophilicity parameter, N = nucleophilicity parameter, and s = nucleophile-specific slope parameter). With E parameters from -12.2 to -8.9, the electrophilic reactivities of 1 turned out to be comparable to those of α,β-unsaturated iminium ions, amino-substituted benzhydrylium ions, and ordinary Michael acceptors. While the E parameters of the azodicarboxylates 1 determined in this work also hold for their reactions with triarylphosphines, they cannot be used for estimating rate constants for their reactions with amines. Comparison of experimental and calculated rate constants for cycloadditions and ene reactions of azodicarboxylates provides information on the concertedness of these reactions.

Acid‐free Friedel–Crafts chemistry: A paradox? Nucleophilicity scales, based on reactions with benzhydrylium ions, show that many π systems are more nucleophilic than aqueous or alcoholic solutions that are generally employed as solvents for SN1 reactions. Solvolytically generated carbocations can, therefore, be trapped by donor‐substituted arenes and alkenes to form products of Friedel–Crafts‐type reactions in neutral aqueous solutions (see scheme).

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The kinetics of the reactions of 1,2-diaza-1,3-dienes 1 with acceptor-substituted carbanions 2 have been studied at 20 °C. The reactions follow a second-order rate law, and can be described by the linear free energy relationship log k(20 °C)=s(N+E) [Eq. (1)]. With Equation (1) and the known nucleophile-specific parameters N and s for the carbanions, the electrophilicity parameters E of the 1,2-diaza-1,3-dienes 1 were determined. With E parameters in the range of -13.3 to -15.4, the electrophilic reactivities of 1a-d are comparable to those of benzylidenemalononitriles, 2-benzylideneindan-1,3-diones, and benzylidenebarbituric acids. The experimental second-order rate constants for the reactions of 1a-d with amines 3 and triarylphosphines 4 agreed with those calculated from E, N, and s, indicating the applicability of the linear free energy relationship [Eq. (1)] for predicting potential nucleophilic reaction partners of 1,2-diaza-1,3-dienes 1. Enamines 5 react up to 10(2) to 10(3) times faster with compounds 1 than predicted by Equation (1), indicating a change of mechanism, which becomes obvious in the reactions of 1 with enol ethers.