Addition of H2-D2 Mixtures to 2-Butyne and Norbornene Catalyzed by Solutions of Chlorotris(triphenylphosphine)rhodium(I), Hydridocarbonyltris(triphenylphosphine)rhodium(I), and Hydridotetrakis(5-phenyl-5-H-dibenzophosphole)rhodium(I)

The use of H2/D2 mixtures in place of H2 as a probe of the mechanisms of hydrogenation is explored using the catalyst precursors chlorotris(triphenylphosphine)rhodium(I) (I), hydridocarbonyltris(triphenylphosphine) rhodium(I) (II), and hydridotetrakis(5-phenyl-5-H-dibenzophosphole)rhodium (I) (III). Benzene solutions of these complexes catalyze the H2-D2 equilibration, the relative rates decreasing in the order III > II > I. The accepted mechanism of hydrogenation catalyzed by I does not accommodate the observed isotopic exchange, which is inhibited by added acids (trifluoroethanol, 1,3,4-trichlorophenol) and accelerated by small amounts of triethylamine; the formation of hydridotris(triphenylphosphine)rhodium(I) is implicated. The H2 and D2 added to an unsaturated hydrocarbon is randomized with complexes II and III as the catalyst, whereas the molecular identity of H2 and D2 is retained if the dihydrido is the catalyst.