Hydrodehalogenation of aryl halides by hydrogen gas and hydrogen transfer in the presence of palladium catalysts

Catalytic hydrodehalogenation of halobenzenes over palladium supported catalysts was studied. Two different hydrogen sources were used: molecular hydrogen gas and hydrogen transfer from HCOOK and 2-propanol. HCOOK hydrogen transfer competitive reactions confirm the strong adsorption of iodobenzene over the catalyst which inhibits the adsorption of the other aryl halides and the donor. However, when using propan-2-ol as donor, iodobenzene is easily hydrodehalogenated and even fluorobenzene can be reduced. The basicity of the support plays an important role in the final performance of the catalytic system. Thus Pd/MgO is the best catalyst among all solids tested. Finally, hydrogen gas liquid-phase catalytic hydrodehalogenation indicates that reactivity order (BrBz>ClBz>IBz>FBz) depends on both the C-X bond dissociation energy and the adsorption strength of the halobenzene.