A Green Lindqvist-Type POM/TEMPO/H2O2 Ternary Catalytic System for the Selective Oxidation of Hantzsch 1,4-Dihydropyridines (1,4-DHPs) to Pyridines.

Pyridine derivatives are prepared via the dehydrogenative aromatization of 1,4-dihydropyridines (1,4-DHPs). Among these methods, stoichiometric catalysis with high waste and electrochemical oxidation with less productivity are limited in their use. Notably, methyl groups and nitrogen atoms may impose limitations on oxidation. We revealed the efficient strategy via recyclable Lindqvist-type polyoxometalate (POM) (n-Bu4N)2[Mo6O19] and 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as co-catalysts using the green and economical oxidant hydrogen peroxide. The method applied to the aromatization of a large amount of 1,4-dihydropyridines (1,4-DHPs) incorporating multiple oxidation-prone groups to pyridines with a high conversation rate and selectivity. The Lindqvist-type POM/TEMPO/H2O2 ternary catalytic system was verified on a larger scale; meanwhile, after being recycled for six times, the Lindqvist-type POM catalyst could be used for the oxidation without obvious loss in reaction time and isolated yield (up to 90%). The mechanism of the ternary catalytic system was proposed by the control experiments.