Coordinately Unsaturated Clusters. A Novel Catalytic Reaction

Article abstract of DOI:10.1021/ja00349a030

The dinuclear rhodium hydride <(μ-H)Rh2>2 is a catalyst precursor for stereoselective hydrogenation of dialkylalkynes and diarylalkynes to the corresponding trans-alkenes.Both H2 and the alkyne complete for reaction with the dimer with the respective products being (H)(μ-H)3Rh24 and (μ-H)2(μ-η²-RC2R)Rh24.The further reaction of the tetrahydride complex with alkyne and also the rearrangement of the η²-alkyne complex yield a bridged vinyl derivative, (μ-H)<μ-η²-RC=C(H)R>Rh24, 3, which has a trans arrangement of alkyl or aryl groups as established by X-ray crystallographic studies.The rate-determining step in the catalytic cycle appears to be hydrogen addition to the vinyl derivative.Competitive with hydrogen addition to the vinyl derivative is diarylalkyne addition and insertion to give the mononuclear complex , which was also defined by X-ray crystallographic study.The latter is a catalyst precursor for alkyne hydrogenation to form cis-alkenes.Dynamic stereochemical features of the vinyl complexes in the solution state are described as well as intermolecular alkyne exchange between free alkyne and the vinyl complexes.