Hydrodefluorination of fluorobenzene catalyzed by rhodium metal prepared from [Rh(COD)2]+BF4− and supported on SiO2 and Pd-SiO2

The effects of solvent, pH, temperature, and hydrogen pressure were determined for the reaction of fluorobenzene (PhF) with hydrogen gas catalyzed by supported rhodium metal on silica. Non-polar and aprotic solvents, heptane and 1,2-dichloroethane (DCE), favor hydrogenation to fluorocyclohexane (CyF). Polar and protic solvents, heptane/methanol and heptane/water, favor hydrodefluorination to benzene (PhH) and subsequent hydrogenation to cyclohexane (CyH). The hydrodefluorination–hydrogenation of fluorobenzene to cyclohexane via benzene proceeds fastest in heptane/water under acidic conditions. For the hydrodefluorination of fluorobenzene in heptane/water, the hydrodefluorination product, benzene, inhibited further hydrodefluorination of fluorobenzene. In heptane or DCE, a side reaction between fluorocyclohexane and hydrogen fluoride, not catalyzed by rhodium, was discovered that provides an alternate pathway to cyclohexane. Reaction conditions can be modified to yield primarily fluorocyclohexane, benzene, or cyclohexane.