M. Schlosser, L. Guio, F. Leroux
Mar 28, 2001
Citations
0
Influential Citations
30
Citations
Journal
Journal of the American Chemical Society
Abstract
Every first-year student is advised not to draw formulas carrying two like charges in close vicinity. Electrostatic repulsion, so goes the argument, would make such structures energetically exorbitant and hence unrealistic. This plausible rule of thumb is confirmed by recent computational work at the MP2, MP4, and B3LYP levels of theory. Proton abstraction from a “naked” phenyl anion by another phenyl anion is a highly endothermal process, requiring reaction enthalpies of 112, 96, and 91 kcal/mol at rtho, meta, and para positions. 1 However, the situation changes profoundly when the phenyl anion-promoted deprotonation of phenyllithiumis examined. This time the process is exothermal at ortho andpara positions by approximately 10 and 5 kcal/mol, respectively. 1 In other words, the chelating and π-coordinating properties of the lithium atom render phenyllithium more acidic than benzene is! With this as a background it should be intriguing to assess computationally (i.e., under gas phase conditions) and experimentally (in solution) the relative basicities of the three phenylenedilithiums which at least formally result when one phenyllithium cannibalizes another one. In this context recent work by Bickelhaupt et al .2 deserves attention. 1,3,5-Trilithiobenzene proved to be too basic to be generated from 1,3,5-tribromobenzene by permutational halogen/metal exchange even when tert-butyllithium was employed as the reagent. 5-Bromo-1,3-phenylenedilithium was obtained almost exclusively under optimized conditions. 2 1,3,5-Trilithiobenzene (79%) did form along with 1,3phenylenedilithium (13%) and phenyllithium (7%) as byproducts when the halogen in 1,3,5-tribromobenzene was replaced reductively using the 4,4′-di-tert-butylbiphenyl/lithium “radical-anion”. 2