Pyridines, alkyl-, anodic oxidation

Dihydropyridines and their /V-alkyl derivatives undergo anodic oxidation in basic medium to the corresponding pyridines (reaction 27). The process may be complicated by the presence of other moieties for example, a nitro group may reductively condense... 

The substitution path b) (Eq. (94) ), is favored by the following experimental conditions low current density, grapliite as anode material, alkaline medium, water or water-pyridine as solvent, and admixture of foreign ions e.g., bicarbonate, sulfate, perchlorate, dihydrogen phosphate, Pb2+, Mn2+, Cu2+, Fe2+, Co2+. The carbonium ion path b) can furthermore be expected for carboxylates RR CHOO with a-substituents R such as alkyl, phenyl 198 hydroxy, halogen 1 amino, or alkoxy. These substituents facilitate oxidation of the intermediate alkyl radical R to the carbonium ion R+. Product formation occurs via carbonium ions and not, as is also conceivable, via mixed coupling of R with Nu ... 

Alkyl-substituted (or arylakyl-substituted) pyridines may be oxidized to the corresponding acids in high yield in acid solution on a Pb02 anode in a divided cell a continuous removal of the products is an advantage [230]. 

Alkyl derivatives of 7r-electron-deficient heterocyclic compounds may be oxidized anodi-cally to carboxylic acids alkyl and arylalkyl pyridines can be oxidized to the corresponding acids in good yields at a lead dioxide anode in aqueous sulfuric acid [229, 230, 376-378]. 

The formation of alcohols or ethers is favored by low current densities, porous graphite as anode material, a solvent such as water/pyridine instead of methanol, a pH of the electrolyte above 7 and anions, such as bicarbonate, sulfate or perchlorate, as additives. Electron-donating substituents at C-2 of the carboxylic acid favor the oxidation of the intermediate radical. Thus a-alkyl, a-cycloal-kyl207,208 a-chloro, a-bromo, a-amino, a-alkoxy, a-hydroxy, a-acyloxy and a,a-diphenyl substituents more or less promote the formation of nonKolbe products. 

---