Work

The Stability of Aryl Carbanions Derived from PyridineN-Oxide: The Role of Resonance in Stabilizing Aryl Anions

Year: 2009

Type: article

Abstract: 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.

Source: Journal of the American Chemical Society

DOI: 10.1021/ja906473v

Authors: Scott K. Koehn, Ngoc L. Tran, Scott Gronert, Weiming Wu

Institutions: Virginia Commonwealth University, San Francisco State University

Language: English

Topic: Chemical Reaction Mechanisms

Subfield: Organic Chemistry

Field: Chemistry

Domain: Physical Sciences

Open Access status: green

Funders: National Institute of General Medical Sciences, Division of Chemistry

Awards: MBRS SCORE at San Francisco State University, G807401478