Aromatic substitution. Part XXVI. The alkaline ferricyanide oxidation of pyridinium salts. Effects of substituents and mechanism
- 1 January 1971
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society B: Physical Organic
- p. 131-136
- https://doi.org/10.1039/j29710000131
Abstract
The mechanism of alkaline ferricyanide oxidation of pyridinium salts has been studied as has the quantitative effect of a 3-substituent upon orientation and reactivity of the nucleus. A3-methyl group activates slightly (H MeK= 1·13) and directs mainly to C(2); a 3-cyano-group activates appreciably (H CNK= 20·2) and also directs mainly to C(2); a 3-methoxycarbonyl group deactivates (H CO2MeK= 0·047) and directs exclusively to C(6). The C–H bond cleavage step is not rate-determining in the oxidation of 3-picoline methiodide. Rate-determining addition of OH– cannot explain the substituent effects. Base-catalysed H–D exchange has been studied; nuclear proton abstraction cannot account for the observed orientation. It is suggested that the rate-determining step is the formation of a complex which reacts with more ferricyanide, oxidation taking place within a second complex. A 3-methyl group facilitates the approach of the polarisable Fe(CN)6 3–. 3-Substituents exert both electronic and steric effects.Keywords
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