Wilkinson's catalyst

Wilkinson's catalyst is the common name for chlorotris(triphenylphosphine)rhodium(I), named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson. The compound is a square planar, 16-electron complex, and is usually isolated in the form of a red-violet crystalline solid with a melting point of 245–250°C. Wilkinson's catalyst may be prepared by reducing rhodium(III) chloride trihydrate with ethanol (which also serves as the solvent) in the presence of excess triphenylphosphine.¹

The most common use of Wilkinson's catalyst is as a homogeneous catalyst in the hydrogenation of alkenes,² the mechanism of which involves the initial dissociation of one or two triphenylphosphine ligands to give 14 or 12-electron complexes respectively, followed by oxidative addition of H₂ to the metal. Subsequent π-complexation of alkene, intramolecular hydride transfer, and reductive elimination results in extrusion of the alkane product, e.g.:

Other applications of Wilkinson’s catalyst include: catalytic hydroboration of alkenes using catecholborane and pinacolborane,³ and the selective 1,4-reduction of α, β-unsaturated carbonyl compounds in concert with triethylsilane.⁴ When the triphenylphosphine ligands are replaced by chiral phosphines (e.g. Chiraphos, DIPAMP, DIOP), the catalyst becomes chiral and is capable of making chiral alkanes from prochiral alkenes through asymmetric hydrogenation.⁵

References

1. J. A. Osborn, F. H. Jardine, J. F. Young, G. Wilkinson, J. Chem. Soc. A. 1966: 1711.
2. (a) A. J. Birch, D. H. Williamson, Org. React. 24:1 (1976); (b) B.R. Jones, Homogeneous Hydrogenation. John Wiley & Sons, New York, 1973.
3. D. A. Evans, G. C. Fu, and A. H. Hoveyda, J. Am. Chem. Soc. 110: 6917 (1988).
4. I. Ojima, T. Kogure, Y. Nagai, Tetrahedron Lett. 1972: 5035.
5. W. S. Knowles, Adv. Synth. Catal. 2003, 345(1&2), 3–13.