Journal of the American Chemical Society, Vol.132, No.1, 390-395, 2010
The Stability of Aryl Carbanions Derived from Pyridine N-Oxide: The Role of Resonance in Stabilizing Aryl Anions
The gas phase stability of carbanions centered at various positions on pyridine N-oxide were investigated by computational and experimental methods. In addition, G3MP2 computations were completed on ring-deprotonated pyridine and A methylpyridinium. With these species, the effect of a nitrogen-centered positive charge on carbanion stability was assessed. Introduction of the nitrogen-oxide group into the benzene ring decreases the Delta H-acid by similar to 20 kcal/mol, but surprisingly, the effect is nearly independent of the position of the group (ortho, meta, or para). The results indicate that the N-oxide offers a balance of field, resonance, and local effects that cancels out any positional preferences. G3MP2 calculations indicate that a similar lack of positional selectivity is seen in nitrobenzene and benzonitrile. Overall, the data suggest that pi-effects are limited in phenyl anions, and as a result, ylide-like, rather than carbene-like, resonance structures are most important in the anions derived from ring deprotonation of arenes and heterocycles of these general types.