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 N-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 DeltaH(acid) by approximately 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.
The gas phase stability of carbanions centered at various positions onpan class="Chemical">pyridine N-oxide were investigated by computational and experimental methods. In addition, G3MP2 computations were completed on ring-hemical">pan class="Chemical">deprotonated pyridine and N-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 DeltaH(acid) by approximately 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.
Authors: Xue-Bin Wang; Jelena E Dacres; Xin Yang; Lev Lis; Victoria M Bedell; Lai-Sheng Wang; Steven R Kass Journal: J Am Chem Soc Date: 2003-06-04 Impact factor: 15.419