Keywords: Kinetics / Ion pairs / Anions / Enols / Reaction mechanismsKinetics of the reactions of enolate anions derived from acyclic and cyclic β-keto esters with benzhydrylium ions and quinone methides have been determined photometrically in dimethyl sulfoxide solution at 20°C. The reactions follow second-order rate laws: first-order with respect to the electrophile and first-order with respect to the enolate. Reactions conducted in the presence of 18-crown-6 ether and in the presence of variable concentrations of K + allowed the influ-
The kinetics of the reactions of substituted ethyl arylacetates with quinone methides and structurally related diethyl benzylidenemalonates have been studied in DMSO. The second‐order rate constants (lg k2) correlated linearly with the electrophilicities E according to the linear free‐energy relationship lg k2 = sN(N + E), allowing us to determine the nucleophilicity parameters N and sN for these anions. The nucleophilic reactivities of the carbanions vary from N = 27.5 for the parent compound to N = 20.0 for the 4‐nitro‐substituted derivative.
Second-order rate constants for the reactions of acceptor-substituted phenacyl (PhCOCH(-) Acc) and benzyl anions (PhCH(-) Acc) with diarylcarbenium ions and quinone methides (reference electrophiles) have been determined in dimethylsulfoxide (DMSO) solution at 20 °C. By studying the kinetics in the presence of variable concentrations of potassium, sodium and lithium salts (up to 10(-2) mol L(-1) ), the influence of ion-pairing on the reaction rates was examined. As the concentration of K(+) did not have any influence on the rate constants at carbanion concentrations in the range of 10(-4) -10(-3) mol L(-1) , the acquired rate constants could be assigned to the reactivities of the free carbanions. The counter ion effects increase, however, in the series K(+)
The influence of the metal on the nucleophilic reactivities of indenyl metal compounds was quantitatively determined by kinetic investigations of their reactions with benzhydrylium ions (Ar2 CH(+) ) and structurally related quinone methides. With the correlation equation log k2 =sN (N+E), it can be derived that the ionic indenyl alkali compounds are 10(18) to 10(24) times more reactive (depending on the reference electrophile) than the corresponding indenyltrimethylsilane.
Kinetics of the reactions of 2‐substituted malonate anions and 5‐substituted Meldrum's acid anions with benzhydrylium ions and structurally related quinone methides have been monitored in dimethyl sulfoxide solution at 20 °C. The resulting second‐order rate constants followed the correlation lg k(20 °C) = sN(E + N), which allowed the nucleophile‐specific parameters N and sN to be calculated for these highly stabilized carbanions and to integrate them in our comprehensive nucleophilicity scale. Given that the reactions of the benzhydrylium ions with the anions derived from 5‐aryl‐substituted Meldrum's acids did not proceed to completion, the corresponding equilibrium constants could be determined. In combination with available data for pyridines and benzoate ions, these equilibrium constants provide a direct comparison of the strengths of C‐, N‐, and O‐centered Lewis bases with respect to C‐centered Lewis acids.
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