Wilkinson's catalyst

Wilkinson's catalyst is the common name for chlorotris(triphenylphosphine)rhodium(I), a chemical compound with the formula RhCl(PPh₃)₃ (Ph = phenyl). It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularlized its use.

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Structure and basic properties

The compound is a square planar, 16-electron complex and is usually isolated in the form of a red-violet crystalline solid from the reaction of rhodium trichloride with triphenylphosphine. The synthesis is conducted in refluxing ethanol.^[1] Ethanol serves as the reducing agent.

RhCl₃(H₂O)₃ + CH₃CH₂OH + 3 PPh₃ → RhCl(PPh₃)₃ + CH₃CHO + 2 HCl + 3 H₂O

Catalytic applications

Wilkinson's catalyst catalyzes 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

Other reactions of RhCl(PPh₃)₃

RhCl(Ph₃P)₃ reacts with CO to give RhCl(CO)(PPh₃)₂, which is structurally analogous to Vaska's complex. The complex will also decarbonylate aldehydes, although the reaction is stoiochiometric:

RhCl(PPh₃)₃ + RCHO → RhCl(CO)(PPh₃)₂ + RH + PPh₃

Upon stirring in benzene solution, RhCl(PPh₃)₃ loses PPh₃ to give the poorly soluble red-colored species Rh₂Cl₂(PPh₃)₄. This conversion demonstrates the lability of the triphenylphosphine ligands.

References

1. ^ Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. (1966). "The Preparation and Properties of Tris(triphenylphosphine)halogenorhodium(I) and Some Reactions Thereof Including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and Their Derivatives". Journal of the Chemical Society A: 1711 - 1732. doi:10.1039/J19660001711.

1. (a) A. J. Birch, D. H. Williamson, Organic Reactions 1976, volume 24, page 1ff; (b) B.R. James, Homogeneous Hydrogenation. John Wiley & Sons, New York, 1973.
2. D. A. Evans, G. C. Fu and A. H. Hoveyda (1988). "Rhodium(I)-catalyzed hydroboration of olefins. The documentation of regio- and stereochemical control in cyclic and acyclic systems". J. Am. Chem. Soc. 110 (20): 6917-6918. doi:10.1021/ja00228a068.
3. I. Ojima, T. Kogure, Y. Nagai, Tetrahedron Lett. 1972, page 5035.