Selective hydrogenolysis of chlorofluorocarbons by RhCl(PPh₃)₃

Factors affecting hydrogenolysis of CFC-113a (CF₃CCl₃) catalyzed by RhCl(PPh₃)₃ were studied to have an insight on the mechanism of the reaction. A slightly polar solvent such as THF is found to be appropriate for selective hydrogenolysis of fluorochlorocarbons. Kinetic analysis showed the reaction was first-order with respect to the concentrations of catalyst and substrate, respectively. Reaction rates were dependent on the concentration of Cl^- ions added; the reaction rate increased initially but remained constant at the higher Cl^- concentration. Striking effect of halide ions were rationalized by increased nucleophilicity of [RhX₂(PPh₃)₂]^- (X = Cl, Br, I), a product from the initial reaction between catalysts and added halide ions, toward CFC-113a. Hydrogenolysis of CFC-113 (CF₂ClCFCl₂) produced HCFC-123a (CF₂ClCHClF) and chlorotrifluoroethene (CF₂=CFCl), a product from β-elimination of HCFC-123a, indicating nucleophilic attack of metal complexes to more electropositive carbons actually occurred. These results appears to support the S(N)2 mechanism for the oxidative addition of CFC-113a. Added amines decreased reaction rates most probably by inhibiting the formation of more nucleophilic complexes mentioned above. Added proton induced decrease in reaction rates. Effect of added phosphines is rather complicated; reaction rates initially decline to the range where the ratio of [PPh₃] to [catalyst] is 6 but increase thereafter. This rate enhancement strongly indicates the existence of an independent phosphonium-containing reaction cycle. Light brownish compound was actually isolated and was spectroscopically characterized as [Ph₃P(CCl₂CF₃)]Cl. Plausible reaction cycles are proposed and the rate equations for this proposed mechanism are derived.