Hydrodehalogenation of Polychlorinated Aromatic Halides by Hypophosphite with Pd/C Catalyst under Multiphase Conditions

Polyhalogenated benzenes in the presence of a Pd/C catalyst with sodium hypophosphite in a multi- phase system consisting of a hydrocarbon solvent, concentrated aqueous alkali and a quaternary onium salt undergo reduction with rapid and progressive displacement of all their chlorine atoms. The onium salt, being insoluble in both the organic and the aqueous phases, coats, as a third liquid phase, the Pd/C catalyst. The strong alkaline medium and the phase-transfer agent are synergic. Operating at 50 “C, 1,2,4,5-tetrachlorobenzene gives after 2.5 h a 99% yield of benzene. Enhancement of the reaction rate, compared with already known methods, has been attributed to the transfer of phosphite anion from the aqueous solution to the catalyst surface, to partition of halogenated compounds between the hydrocarbon solution and the liquid phase of the phase- transfer agent and to the rapid removal of HCI adsorbed on Pd/C by the alkaline medium. The reaction is also effective with aryl bromides and deactivated p-methoxyaryl halides.