Kinetic Studies of Carbocation-Carbanion Combinations: Key to a General Concept of Polar Organic Reactivity

Abstract

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.

A further exploration of a nucleophilicity index based on the gas-phase ionization potentials

“…They have first observed that the reactions rates of many carbocations with uncharged nucleophiles follow a linear free energy relationship, namely log k ¼ sðN þ EÞ, with E and N parameters defining the electrophilicity and nucleophilicity powers of reacting species, respectively, and where s describes the sensitivity of the nucleophile involved in the reaction. With this equation, these authors have established a basis for building experimental scales of relative electrophilicity and nucleophilicity for a large variety of electrophile-nucleophile combinations [16][17][18][19].…”

Reactivity scales for quantifying polar organic reactivity: the benzhydrylium methodology

“…For that reason it was possible to measure the rates of the reactions of stabilized carbanions with weakly electrophilic benzhydrylium ions (E<À7) in DMSO. 17 By defining the electrophilicity parameters E as solvent-independent constants, it was possible to link the two data sets as shown in Fig. 6.…”

“…Plot of (lg k)/s N versus the nucleophilicity parameter N for the reactions of benzhydryl cations and quinone methides with p-nucleophiles (in CH 2 Cl 2 ) and carbanions (in DMSO). 17 that purpose, 15 as they can be considered as oxy-substituted benzhydrylium ions, which again have the same steric environment around the reaction center. In cases where the attack of the carbanions XYCH À at the quinone methides was reversible, Roland Lucius studied the reactions in the presence of excess CH acids XYCH 2 , which protonate the intermediate phenolate ions and thus give rise to second-order kinetics with rate-determining CC-bond formation (Scheme 3).…”

Nucleophilicity of the bis-(4-nitrophenyl)-methyl anion

“…4 with 1) causes an increase of N by 3 units. With an N parameter of 19.9 the nucleophilicity of anion 1 in DMSO is comparable to those of the nitromethyl anion 14 and the malononitrile anion in the same solvent 6 and thus ranks among the most reactive carbanions so far characterized in our nucleophilicity scales. …”

“…The anionic adducts 3 -obtained by mixing equimolar amounts of the electrophiles 2 and the potassium salt of the carbanion 1 in d 6 -DMSO solutions were investigated by 1 H NMR spectroscopy (Scheme 1). Their neutral analogues which were supposed to be formed by protonation of 3 -with aqueous HCl decomposed to the starting materials.…”