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 popularized its use.
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(III) chloride 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][3] 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 (olefin insertion), and reductive elimination results in extrusion of the alkane product, e.g.:
Other applications of Wilkinson’s catalyst includes the catalytic hydroboration of alkenes with catecholborane and pinacolborane,[4] and the selective 1,4-reduction of α, β-unsaturated carbonyl compounds in concert with triethylsilane.[5] When the triphenylphosphine ligands are replaced by chiral phosphines (e.g., chiraphos, DIPAMP, DIOP), the catalyst becomes chiral and convert prochiral alkenes into enantiomerically enriched alkanes via the process called asymmetric hydrogenation.[6]
Other reactions of RhCl(PPh3)3
RhCl(PPh3)3 reacts with CO to give RhCl(CO)(PPh3)2, which is structurally analogous to Vaska's complex but much less reactive. The same complex arises from the decarbonylation of aldehydes, although the reaction is stoichiometric:
- RhCl(PPh3)3 + RCHO → RhCl(CO)(PPh3)2 + RH + PPh3
Upon stirring in benzene solution, RhCl(PPh3)3 converts to the poorly soluble red-colored species Rh2Cl2(PPh3)4. This conversion further demonstrates the lability of the triphenylphosphine ligands.
References
- ^ 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ A. J. Birch, D. H. Williamson (1976). Organic Reactions. 24: 1ff.
{{cite journal}}: Missing or empty|title=(help) - ^ B.R. James, Homogeneous Hydrogenation. John Wiley & Sons, New York, 1973.
- ^ 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.
- ^ I. Ojima, T. Kogure, Y. Nagai (1972). "Selective reduction of α,β-unsaturated terpene carbonyl compounds using hydrosilane-rhodium(I) complex combinations". Tetrahedron Lett. 13 (49): 5035–5038. doi:10.1016/S0040-4039(01)85162-5.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ W. S. Knowles (2003). "Asymmetric Hydrogenations (Nobel Lecture 2001)". Advanced Synthesis and Catalysis. 345: 3–13. doi:10.1002/adsc.200390028.