Wilkinson's catalyst

Chlorotris (triphenylphosphine) rhodium (I) (IUPAC)

Dark red, odorless solid

Fixed

157 ° C.

Poorly in water

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Wilkinson's catalyst is used in the organic chemistry homogeneous catalyst having the empirical formula C54H45ClP3Rh. This is a rhodium complex to propenyl applies for the hydrogenation, hydroformylation, hydrosilation and for the isomerization of allyl groups. The Wilkinson's catalyst is named after its developer, the Nobel Prize winner Geoffrey Wilkinson, named.

Structure and synthesis

When Wilkinson's catalyst is a square-planar rhodium (I ) complex bearing a chloro - and three triphenylphosphine ligands ( PPh3). It is a 16- Valenzelektronenkomplex. It can be synthesized by substitution of triphenylphosphine to rhodium ( III ) chloride in boiling ethanol. Since the reaction takes place in a mixture of acetone and water instead of ethanol, but only in the presence of an excess of triphenylphosphine, it is assumed that the latter not only as a substituent, but also as a reducing agent functions (reduction of Rh (III) to (Rh ( I) ).

Catalytic cycle

The Wilkinson hydrogenation is used for the hydrogenation of alkenes with hydrogen. The decisive factor here is the lability of the phosphine ligand bound, by their cleavage free coordination sites are created. In the first step, a phosphine ligand from the catalyst splits off. Then hydrogen oxidative addition to the previously formed trigonal- planar 14 - Valenzelektronenspezies. Here, a trigonal bipyramidal complex is formed. The oxidation state changes from I to III. The alkene used then coordinates first side-on to the metal. Subsequently, the insertion of the alkene takes place under hydrogenation. It is again a trigonal bipyramidal complex, which now carries a end-on -bonded alkyl radical. The hydrogenation by the second hydrogen bonded ultimately leads to elimination (reductive elimination) of the alkane to regenerate the catalyst species.

By Wilkinson's catalyst selectively terminal double bonds can be hydrogenated. The reaction proceeds much more quickly at these from that present in the molecule, a further non- terminal double bond is not attacked. With bulky substituents on the double bond, as well as tetrasubstituted double bonds, usually no hydrogenation takes place.

Asymmetric hydrogenation

Wilkinson's catalyst can also be used for the asymmetric synthesis of chiral products. To this end, chiral phosphines such as DIOP DIPAMP or be used instead of the achiral triphenylphosphine. Thus, for example be established via an asymmetric hydrogenation with Wilkinson DIPAMP chiral ligand as a chiral center of the medically important amino acid L- DOPA.