Hydroisomerization of olefins catalyzed by tris(triphenylphosphine)rhodium (I) halides

Abstract

During the hydrogenation of cis-2-pentene (at 30°C and atmospheric pressure) catalyzed by tris(triphenylphosphine)rhodium chloride in benzene + ethanol, isomerization to trans-2-pentene and to 1-pentene, the latter predominating, occurs at about one-third as fast as hydrogenation. The relative isomerization rate is less with the bromo- and iodo-complexes. From trans-2-pentene, which is much less reactive, the cis-isomer is formed in the presence of each complex when at 0.005 M concentration, but as 0.01 M only 1-pentene is formed with the chloro-complex and no isomeric products at all with the iodo-complex. 1-Pentene yields small amounts of both 2-pentenes with the chloro-complex, but not with the others. The results are interpreted in terms of the formation of pentyl radicals from co-ordinated olefins in configurations of different energy : the effect of the size of the halogen atom in restricting rotation of the pentyl radicals is clearly demonstrated.