Platinum Metals Rev., 1968, 12, (4), 135
Phosphine Complexes of Rhodium and Ruthenium as Homogeneous Catalysts
Hydroformylation of Olefins
One of the most rapidly advancing areas of physical-inorganic chemistry is undoubtedly the application of phosphine complexes of the platinum metals as homogeneous catalysts for hydrogenation and hydroformylation of olefins. In a paper presented at the recent symposium on “New Olefin Reactions” at the University of Manchester Institute of Science and Technology, Professor Geoffrey Wilkinson of Imperial College described recent new work in this field: the reactions he discussed are characterised by a degree of specificity not commonly encountered in other catalytic processes.
Perhaps his most startling disclosure was that concerning the rhodium complex trans -RhH(CO)(PPh3)2 which catalyses the hydroformylation of olefins under ambient conditions. With the widely used cobalt catalyst CoH(CO)4 temperatures of about 100°C and pressures in excess of 100 atmospheres have to be used. When α-olefins are hydro-formylated with the rhodium catalyst, the ratio of straight- to branch-chain aldehyde is about 20, which is much higher than found with the cobalt catalyst. The reaction is believed to involve the five-coordinate RhH(CO)2(PPh3)2 as an intermediate.
Professor Wilkinson also reported a new, extremely active homogeneous catalyst for olefin hydrogenation: this is the ruthenium complex RuClH(PPh3)3, also a five-coordinate species. This complex is even more active than the rhodium complex RhCl(PPh3)3 but it is selective for α-olefins, as internal olefins are reduced more slowly by a factor of about a thousand. This specificity is explained by the steric effects of the bulky triphenylphosphine groups. Unfortunately, solutions of the ruthenium complex are very sensitive to air and tend to lose activity during use.
These disclosures give further proof of the power of homogeneous catalysis as a technique for organic reactions, although many difficult technical problems will need to be overcome before reactions such as those described find commercial application.