Wilkinson's catalyst

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Wilkinson's catalyst
Wilkinson's catalyst
Wilkinson's catalyst
Wilkinson's catalyst
Wilkinson's Catalyst
Names
IUPAC names
(SP-4)chloridotris(triphenylphosphane)
rhodium
Other names
Rhodium(I) tris(triphenylphosphine) chloride,
Wilkinson's catalyst,
Tris(triphenylphosphine)rhodium(I) chloride
Identifiers
14694-95-2
EC Number 238-744-5
Jmol 3D model Interactive image
PubChem 84599
RTECS number none
Properties
C54H45ClP3Rh
Molar mass 925.22 g/mol
Appearance red solid
Melting point 245 to 250 °C (473 to 482 °F; 518 to 523 K)
insoluble in water
Solubility in other solvents benzene
Structure
square planar
Hazards
Main hazards none
R-phrases none
S-phrases S22 S24/25
Related compounds
Related compounds
triphenylphosphine
Pd(PPh3)4
IrCl(CO)[P(C6H5)3]2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Wilkinson's catalyst is the common name for chloridotris(triphenylphosphane)rhodium(I), a coordination compound with the formula RhCl(PPh3)3 (Ph = phenyl). It is named after the chemist and Nobel Laureate, Sir Geoffrey Wilkinson, who popularized its use.

Structure and basic properties[edit]

The compound is a square planar, 16-electron complex. It is usually obtained in the form of a red-violet crystalline solid from the reaction of rhodium(III) chloride hydrate with excess triphenylphosphine.[1] The synthesis is conducted in refluxing ethanol which helps with the reduction.[2][3] Triphenylphosphine serves as the reducing agent yielding triphenylphosphine oxide.

RhCl3(H2O)3 + 4 PPh3 → RhCl(PPh3)3 + OPPh3 + 2 HCl + 2 H2O

Catalytic applications[edit]

Wilkinson's catalyst catalyzes the hydrogenation of alkenes.[4][5] The mechanism of this reaction 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.:

RevisedCatCycle.png

Other applications of Wilkinson's catalyst includes the catalytic hydroboration of alkenes with catecholborane and pinacolborane,[6] and the selective 1,4-reduction of α,β-unsaturated carbonyl compounds in concert with triethylsilane.[7] When the triphenylphosphine ligands are replaced by chiral phosphines (e.g., Chiraphos, DIPAMP, DIOP), the catalyst becomes chiral and converts prochiral alkenes into enantiomerically enriched alkanes via the process called asymmetric hydrogenation.[8]

Other reactions of RhCl(PPh3)3[edit]

RhCl(PPh3)3 reacts with CO to give trans-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:

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 [RhCl(PPh3)2]2. This conversion further demonstrates the lability of the triphenylphosphine ligands.

Catalytic improvements[edit]

The addition of a Barton's base such as 2-tert-butyl-1,1,3,3-tetramethylguanidine poses dramatic implications on the classic Rh(III)-mediated pathway of hydrogenations with Wilkinson's catalyst and H2. Following the customary initial oxidative addition, a barrierless reductive elimination of HCl from the traditional Rh(III)–H2 intermediates instantly produces remarkably reactive Rh(I)–H species. This mechanistic event translates into a notable boost on the performance of Wilkinson's catalyst on the hydrogenation of several alkenes and internal alkynes.[9]

See also[edit]

References[edit]

  1. ^ Bennett, M. A.; Longstaff, P. A. (1965). "Complexes of Rhodium(I) with Triphenylphosphine.". Chem. Ind. (London) 1965: 846. 
  2. ^ 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". J. Chem. Soc. A 1966: 1711–1732. doi:10.1039/J19660001711. 
  3. ^ Osborn, J. A.; Wilkinson, G. (1967). "Tris(triphenylphosphine)halorhodium(I)". Inorg. Synth. 10: 67. doi:10.1002/9780470132418.ch12. 
  4. ^ Birch, A. J.; Williamson, D. H. (1976). "Homogeneous Hydrogenation Catalysts in Organic Synthesis". Org. React. 24: 1. 
  5. ^ James, B. R. (1973). Homogeneous Hydrogenation. New York, NY: John Wiley & Sons. 
  6. ^ Evans, D. A.; Fu, G. C.; Hoveyda, A. H. (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. 
  7. ^ Ojima, I.; Kogure, T. (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. 
  8. ^ Knowles, W. S. (2003). "Asymmetric Hydrogenations (Nobel Lecture 2001)". Adv. Synth. Catal. 345 (12): 3–13. doi:10.1002/adsc.200390028. 
  9. ^ Perea Buceta, Jesus E.; Fernández, Israel; Heikkinen, Sami; Axenov, Kirill; King, Alistair W. T.; Niemi, Teemu; Nieger, Martin; Leskelä, Markku; Repo, Timo (2015-11-23) [2015]. "Diverting Hydrogenations with Wilkinson's Catalyst towards Highly Reactive Rhodium(I) Species". Angew. Chem. Int. Ed. 54 (48): 14321–14325. doi:10.1002/anie.201506216. ISSN 1521-3773.