Metallacarboranes in Catalysis. 6. Kinetics and Mechanism of Alkene Hydrogenation and Isomerization Catalyzed by Rhodacarborane Clusters. A Search for Cluster Catalysis

Article abstract of DOI:10.1021/ja00322a042

In a search for cluster-catalyzed reactions the rhodacarboranes (1), (II), (III), and (IV) were employed as catalyst precursors in a study of 1-hexene (B) isomerization.Precursors I, III, and IV were examined in the hydrogenation of 3-methyl-3-phenyl-1-butene (A).Complete rate laws were developed for isomerization and hydrogenation.Studies with D2 demonstrated the presence of reversible alkylrhodium formation during hydrogenation.The use of D-labeled catalyst precursors, etc., proved that the Rh-H ligand of the closo precursors was not directly involved in either alkene isomerization or in hydrogenation.Competitive isomerization and hydrogenation of 1-hexene catalyzed by precursors I and IV suggested the presence of a common intermediate for these two reactions.Extensive intermolecular D scrambling was observed in equilibration experiments which employed propene-1,1,1-d3 and propene-2-d with precursor I and isotopically normal propene.The slow regiospecific transfer of deuterium from carbon in A to the 9, 10, and 12 boron vertices in I was observed and is believed to proceed via H-Rh-B bonded intermediates.The mechanistic implications of these and other observations are integrated into a mechanistic scheme which is based upon the prior equilibrium of Rh(3+) closo- and Rh(1+) exo-nido-rhodacarboranes which, in the presence of alkene, produce an equilibrium concentration of a key (phosphine)(alkene)Rh(1+) exo-nido intermediate regardless of the closo or exo-nido nature of the catalyst precursor used.Alkene isomerization is thought to involve η³-allylic intermediates produced from the exo-nido alkene complex.Hydrogenation appears to proceed via oxidative addition of H2 to this same complex followed by rate-determining decomposition of the hydridorhodium alkyl produced by this means.These kinetic characteristics may have their origin in the weak electron-donor properties of the chelated exo-nido-C2B9H12(1-) ligands which are attached to Rh(1+) or Rh(3+) in the exo-nido intermediates by a pair of B-H-Rh three-center, two-electron bonds.