Rhodium and Iridium Complexes in Catalysis

During the development of RhCl(PPh3)3 as the first efficient alkene hydrogenation catalyst by Wilkinson et it was realized that only two 

Interesting selectivity effects were found for various dienes and alkynes, the reduction of both of which could be stopped at the monoene stage. Almost exclusive c/5-addition of H2 to the alkyne was observed. Ketones can be reduced, although this is not a result of the cationic character of the system, since the catalysts can be deprotonated with NEt3 to give neutral species that also effect this reduction equally well. This illustrates a possible ambiguity in studying cationic catalysts if dissociation of a positive species, usually H , can occur. Fortunately, in this particular case, each system can be separately prepared and studied. 

As might be expected in a system that binds solvents well, rates of reduction of various substrates are very solvent dependent,the substrate being in competition with the solvent for the metal. 

In contrast to the relatively high solvent dependence, the Osborn system shows a relatively small dependence on L. Wilkinson s catalyst requires PPh3 or ligand of very similar cone angle, because the steric 

Even [Rh(diene)L3]A salts were shown to be active, probably because alkene hydrogenation requires no more than three sites at the metal, two for hydrogen and one for the substrate. Induction periods are sometimes observed in these cases, no doubt because L(or cod) does not have to dissociate before H2 can add. Compared to the Wilkinson system, the side-reaction leading to isomerization of the alkene (e.g., 1-butene 2-butene) is a more serious competitor with hydrogenation itself. In applications requiring specific deuteration or where the isomerization product is nonhydrogenable, this can be a problem 

---