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 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.
Koehn
Scott K
SK
Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284, USA.
Tran
Ngoc L
NL
Gronert
Scott
S
Wu
Weiming
W
D001554:Benzene / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization
D011725:Pyridines; C013229:pyridine N-oxide; D001554:Benzene