α-Substituted toluenes as carbon acids: structural reorganization and free energy changes upon carbanion formation

Abstract

¹ H, ¹³C, and ¹⁹F n.m.r. data of benzyl carbanions PhCH^–X (VI) and p-FC₆H₄CH^–X have been found to be appropriate monitors for probing the structural reorganization undergone by the precursor carbon acids PhCH₂X upon deprotonation. Even in the highly dissociating Me₂SO as solvent benzyl carbanions exhibit ion-pairing phenomena: these have been proved by the effect exerted by the [2.2.1] cryptand on ¹³C shifts, and by changing the countercation. ¹³C Parameters of benzyl anions (VI) provide experimental access to negative π charge density maps through equation (1). It is shown that the solvent greatly assists delocalization of the negative charge in these systems. The substituent-induced variations of the para-monitor in the anions are linearly related to pK_a variations of the precursor acids. Carbanions appear to belong to two different classes depending on the mechanism with which the charge stabilizes the system. Thus, in enolate and nitronate anions the stabilization is provided by extensive charge transfer from the carbanion carbon, in contrast to carbanions activated by the third-row elements (P,S) and by the cyano group. Substituent electron demands q_x(the amount of charge transferred to the activating substituent X) govern the charge distribution in the anions in related ways, the acidity of the precursor acids PhCH₂X, and the sensitivities of these latter to remote aryl substitution.