Oxidative activation pyridine exchange reactions

The redox reactions of metallo phthalocyanine electrodes cause a selective and efficient catalysis for many photochemical and electrochemical reactions at moderate potentials [3,57,70,161,171,172]. The electrons are transported by electronically conductive polymers and also by electron exchange reaction in the oxidizable or reducible groups present in the incorporated polymer. Phthalocyanine and their derivatives have been studied for the catalytic behaviour in the oxidation of acetaldehyde ethylene acetate, iodide and other aldehydes such as benzaldehyde and acrolein [3,57,70,120,231-235]. In the latter cases pyridine was used as an activator. The oxidation reactions are found to follow through a peroxy intermediate. The effectiveness of the catalyst for the reaction depends on the purity, uniformity, stability and turnover number of the phthalocyanine polymer. Oxidative polymerization of phenols to polyoxyphenylenes and thiols to sulphides has also been reported [57,70,120,236-238]. 

The formation of a bridged, activated intermediate such as V is a well known hypothesis for explanation of many electron exchange and oxidation-reduction reactions (14). The two-electron reduction of niobium by pyridine makes this step necessary, since niobium (III) must be formed only as an intermediate. No evidence was found for the presence of niobium (III) in the final products. If... 

As in pyridine, it is the nitrogen in quinoline which undergoes protonation, alkylation, acylation and, with peroxyacids, oxidation to the A-oxide. S Ar reactions occur on the ring C-atoms, preferentially on those of the more activated benzene moiety. The relative reactivities of the individual heteroaromatic positions were determined by an acid-catalysed H-D exchange with D2SO4. This demonstrated that the SgAr process occurs via the conjugated acid, i.e. the quinolinium ion, and the positional selectivity C-8 > C-5/C-6 > C-7 > C-3 was observed. 

The electron-withdrawing effect of typical azines makes chlorine substituents sufficiently reactive that they can participate in palladium-catalysed reactions, even at a pyridine / -position. " a-Activation can serve to allow regioselective reaction in the presence of a /3-halogen (cf. section 2.7.1.1, oxidative addition) and this should be contrasted with lithiation by exchange which shows the opposite regioselectivity. 

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