Formation of a σ-complex from dicyanopyridinyl and hydroxyalkyl radicals. An ESR study of the photoinitiated complex forming reactions of aryl ketones with 2,4-pyridinedicarbonitrile and alcohols
✍ Scribed by Bruno M. Vittimberga; Anne L. Rieger
- Publisher
- Journal of Heterocyclic Chemistry
- Year
- 2000
- Tongue
- English
- Weight
- 451 KB
- Volume
- 37
- Category
- Article
- ISSN
- 0022-152X
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✦ Synopsis
Abstract
When a 1° or 2° alcohol solution of benzophenone and 2,4‐pyridinedicarbonitrile (1) is irradiated, the 2,4‐dicyanopyridinyl radical (2) is formed immediately. When the irradiation is performed in the cavity of an ESR spectrometer, the spectrum amplitude increases to a near steady‐state level, α. When the illumination is discontinued, a sharp rise in amplitude is observed to the β level. These results can be interpreted in terms of an equilibrium between free 2 and an ESR‐silent complex of 2 (concentrations proportional to α and β ‐ α, respectively) with K~eq~ = (β ‐ α)/α. The unprecedented increase in signal amplitude when irradiation is stopped requires an alcohol; the effect is not observed when other solvents are used. K~eq~ was found to correlate with the steric bulk of the alcohol solvent. These and other results lead to the conclusion that hydroxyalkyl radicals react reversibly with 2 to form the ESR‐silent complex.