Anodic pyridination of 2-hydroxy-3-methoxy-5-methylbenzaldehyde and its Schiff's base derivatives

Abstract

Anodic oxidation of substituted phenols, 2-hydroxy-3-methoxy-5-methylbenzaldehyde (1) and its Schiff's base derivatives (2) in acetonitrile containing an excess of pyridine gave pyridinated phenols. Pyridination occurred at the methyl group of the hydroxybenzaldehyde (1) and the Schiff's base (2g) derived from p-nitroaniline, while for the other Schiff's base derivatives of (1) studied the pyridinium group was on the ring meta to the hydroxy and ortho to the imino group. The pyridination process was investigated by cyclic voltammetry and controlled potential and constant current electrolysis of (1), (2), and related phenols including 2-hydroxy-3-methoxybenzaldehyde Schiff's bases (4). Two different routes are suggested for pyridination, a process involving a phenoxonium ion and one involving a quinone methide. Under the conditions of controlled potential electrolysis, ring pyridination proceeds through the phenoxonium ion route and side-chain pyridination through the quinone methide route. The role of the imino group in the anodic oxidation of the phenolic Schiff's bases (2) and (4) is discussed. The principal mode is suggested to be catalysis of the dimerisation of the corresponding cation radicals by accepting the proton of the hydroxy-group intramolecularly to remove the positive charge from the reaction centre.