A nucleophilicity scale has been developed that covers 30 orders of magnitude from simple arenes to stabilized carbanions (see picture). Amazingly, the addition of an electrophilicity parameter E and a nucleophilicity parameter N is sufficient to predict semiquantitatively the rates of a large variety of organic reactions.
The selectivities of stabilized carbanions towards electron‐deficient π‐electron systems are nearly independent of their reactivities, as shown by the rate constants of the reactions of nine carbanions 2 with four quinone methides 1. These constant selectivity relationships may constitute the basis of a nucleophilicity scale for carbanions.
Auf den folgenden Seiten wird gezeigt, warum es mit nur zwei Parametern E und N mˆglich ist, semiquantitativ die Geschwindigkeitskonstanten einer Vielzahl polarer organischer Reaktionen vorherzusagen.
Die Kinetik von Carbokation-Carbanion-Kombinationenwar die noch fehlende Schnittstelle f¸r ein umfassendes Modell polarer organischer Reaktivit‰t.
The second-order rate constants of the reactions of nine substituted diethyl benzylidenemalonates 1 a-i with the carbanions 2 a-e have been determined spectrophotometrically in dimethyl sulfoxide (DMSO). Product studies show that the nucleophiles attack regioselectively at the electrophilic C==C double bond of the Michael acceptors to form the carbanionic adducts 4. The correlation log k(20 degrees C)=s(N+E) allows the determination of the electrophilicity parameters E for the electrophiles 1 a-i from the rate constants determined in this work and the previously published N and s parameters for the nucleophiles 2 a-e. The electrophilicities E for compounds 1 a-i cover a range of six units (-17.7>E>-23.8) and correlate excellently with Hammett's substituent constants sigma(p). The title compounds are roughly ten orders of magnitude less reactive than analogously substituted benzylidene Meldrum's acids, their cyclic analogues. Due to their low reactivities, compounds 1 a-i are suitable reference electrophiles for determining the reactivities of highly reactive nucleophiles, such as carbanions with 16
Second-harmonic alternating current voltammetry has been used to determine one-electron reduction potentials of 15 diarylcarbenium ions and 5 structurally analogous quinone methides, which have been employed as reference electrophiles for the development of nucleophilicity scales. A linear correlation (r(2) = 0.993) between the empirical electrophilicity parameters E and the reduction potentials in acetonitrile (E = 14.091E degrees (red) - 0.279) covering a range of 1.64 V (or 158 kJ mol(-)(1)) has been observed. For a large number of nucleophiles, it has been demonstrated that the observed activation free energies of the electrophile-nucleophile combinations are 61-195 kJ mol(-)(1) smaller than the free energy change of electron transfer from nucleophile to electrophile, which definitely excludes outer-sphere electron transfer occurring during these reactions.
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