Wilkinson's catalyst
Encyclopedia
Wilkinson's catalyst is the common name for chlorotris(triphenylphosphine)rhodium(I), a coordination 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.
with excess triphenylphosphine
. The synthesis is conducted in refluxing ethanol. Triphenylphosphine
serves as the reducing agent yielding triphenylphosphine oxide
.
of alkenes. 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.:
Other applications of Wilkinson’s catalyst includes the catalytic hydroboration of alkenes with catecholborane and pinacolborane, and the selective 1,4-reduction of α, β-unsaturated carbonyl
compounds in concert with triethylsilane
. 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.
(but much less reactive). The same complex arises from the decarbonylation of aldehyde
s:
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.
Geoffrey Wilkinson
Sir Geoffrey Wilkinson FRS was a Nobel laureate English chemist who pioneered inorganic chemistry and homogeneous transition metal catalysis.-Biography:...
who popularized its use.
Structure and basic properties
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) chlorideRhodium(III) chloride
Rhodium chloride refers to inorganic compounds with the formula RhCl3n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt...
with excess triphenylphosphine
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
. The synthesis is conducted in refluxing ethanol. Triphenylphosphine
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
serves as the reducing agent yielding triphenylphosphine oxide
Triphenylphosphine oxide
Triphenylphosphine oxide is the chemical compound with the formula OP3. Often chemists abbreviate the formula by writing Ph3PO or PPh3O . This white crystalline compound is a common side product in reactions involving triphenylphosphine...
.
- RhCl3(H2O)3 + 4 PPh3 → RhCl(PPh3)3 + O=PPh3 + 2 HCl + 2 H2O
Catalytic applications
Wilkinson's catalyst catalyzes the hydrogenationHydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
of alkenes. 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
Oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre...
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, and the selective 1,4-reduction of α, β-unsaturated carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
compounds in concert with triethylsilane
Triethylsilane
Triethylsilane is a silane with the molecular formula C6H16Si. It is a trialkylsilicon hydride compound with a reactive Si-H bond. This colorless liquid is often used in organic synthesis, specifically for the hydrosilation of olefins to give alkyl silanes...
. When the triphenylphosphine ligands are replaced by chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
phosphines (e.g., chiraphos
Chiraphos
Chiraphos is a chiral diphosphine employed as a ligand in organometallic chemistry. This bidentate ligand chelates metals via the two phosphine groups. Its name is derived from its description — being both chiral and a phosphine...
, DIPAMP
DIPAMP
DIPAMP is an organophosphorus compound that is used as a ligand in homogeneous catalysis. It is a white solid that dissolves in organic solvents. Work on this compound by W. S. Knowles was recognized with the Nobel Prize in Chemistry. DIPAMP was the basis for of the first practical asymmetric...
, DIOP), the catalyst becomes chiral and converts prochiral
Prochiral
In stereochemistry, prochiral molecules are those that can be converted from achiral to chiral in a single step.If two identical substituents are attached to a sp3-hybridized atom, the descriptors pro-R and pro-S are used to distinguish between the two...
alkenes into enantiomerically enriched alkanes via the process called asymmetric hydrogenation.
Other reactions of RhCl(PPh3)3
RhCl(PPh3)3 reacts with CO to give trans-RhCl(CO)(PPh3)2, which is structurally analogous to Vaska's complexVaska's complex
Vaska's complex is the trivial name for the chemical compound trans-chlorocarbonylbisiridium, which has the formula IrCl[P3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands, carbon monoxide, and a...
(but much less reactive). The same complex arises from the decarbonylation of aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s:
- 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.