Liquid phase hydrodechlorination of some chlorinated aromatic nitrogen-containing heterocyclics

Several chlorinated pyridine, indole, quinoline and isoquinoline derivatives are hydrogenated at 80–140 °C in the presence of a silica sol–gel entrapped combined palladium–[Rh(cod)Cl]2 catalyst to give ultimately chorine-free aromatic and/or hydroaromatic heterocyclic compounds. The process takes place stepwise. Except for the hydrogenation of 6-chloroquinoline, in which the hydrogenation of some aromatic double bonds precedes the elimination of the chlorine atom, the first step is always the hydrogenolysis of the halogen atom by which the parent aromatic heterocyclic compound is formed. Quinoline and isoquinoline derivatives form two isomeric tetrahydro compounds which are further hydrogenated to the decahydroquinolines and isoquinolines, respectively. The combined immobilized catalyst is leach-proof and recyclable. Its activity relies on a synergistic effect between the two different metallic nuclei.