2/1/2016WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY CSIRNET IITJEE Chemistry Chemistry Organic Chemistry Physical Chemistry Inorganic Chemistry General Chemistry Question Bank AdiChemistry Forum A careful balance between Science and Silence brings Peace and Happiness in the world. Learn Rajyoga Meditation to restore peace in your life. AdiChemistry Home ORGANOMETALLIC CHEMISTRY WILKINSON'S CATALYST RhCl(PPh 3)3 Chlorotris(triphenylphosphine)rhodium(I), is known as Wilkinson’s catalyst. It is used as a homogeneous hydrogenation catalyst. It is a square planar 16electron complex. The oxidation state of Rhodium in it is +1. Preparation: * It can be prepared by reacting RhCl3.3H2O with excess PPh 3 in EtOH. Uses: It is used in the selective hydrogenation of alkenes and alkynes without affecting the functional groups like: C=O, CN, NO2, Aryl, CO2R etc. http://www.adichemistry.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst.html 1/10 adichemistry.e. The oxidation state of Rh is increased to +3. Immediately and finally. this path is referred to as dihydride path . Since the activation of dihydrogen occurs before addition of olefin. Initially. Now one of the hydrogen undergoes migratory insertion at the double bond.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst.. http://www.2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY MECHANISM OF HYDROGENATION CATALYTIC CYCLE Indeed. The oxidation state of Rh is decreased to +1 and the catalyst is regenerated. the solvent molecule fills the vacant site. other paths and intermediates are also possible under the given reaction conditions (see below). Wilkinson's catalyst is a precatalyst that is converted to an active form by losing one triphenylphosphine ligand before entering the catalytic cycle.html 2/10 . Thus formed dihydrido complex binds to the olefin in the next step with the concomitant loss of solvent or PPh 3 ligand. the catalyst activates the molecular dihydrogen by oxidative addition mechanism to give a 18 valence electron dihydrido complex. the alkane is released rapidly by an irreversible reductive elimination step that completes the catalytic cycle. Usually. Rate Determining Step (RDS). However. This is a slow step i. com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst.html 3/10 . Though ethylene cannot be hydrogenated in presence of Wilkinson's catalyst under normal conditions.adichemistry. hydrogen transfer can be achieved with preformed http://www.2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY Above mechanism is supported by following observations. * The rate of reaction decreases when excess of PPh 3 is added; indicating the initial dissociation of one of the PPh 3 ligand before dihydrogen activation. * It is observed that strong πacids like ethylene act as poisons by binding strongly with the electron rich Rh metal center and inhibit hydrogenation. The rates of hydrogenations decrease with increase in the alkyl group substitution on double bond mirroring their relative binding affinities to the metal center.html 4/10 .com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst. This indicates the formation of dihydrido complex that transfers both of its hydrido ligands to the olefin and that the final reductive elimination step is very fast and irreversible. Therefore. oxidative addition of dihydrogen prior to the binding of olefin is crucial. when an equimolar mixture of H2 and D2 are used. It is also partly due to steric factors.adichemistry. * There is minimal scrambling of H/D in the product.2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY dihydrido complex. * Less substituted and sterically less hindered double bonds are selectively http://www. SELECTIVE HYDROGENATION BY WILKINSON'S CATALYST Wilkinson's catalyst can be used to achieve selective hydrogenations. for a successful hydrogenation. 2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY hydrogenated. are unaffected. The selectivity can be enhanced by using acidic alcoholic cosolvents. Aryl. * Terminal alkynes are hydrogenated more rapidly than terminal alkenes.adichemistry. * Functional groups like C=O. * Isolated double bonds are rapidly hydrogenated over conjugated dienes. * Exocyclic double bonds are selectively hydrogenated over endocyclic double bonds. CO2R etc. * Cis alkenes are reduced rapidly than trans alkenes.html 5/10 .. The http://www.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst. C=N. NO2. com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst. Note: But sterically unhindered aldehyde groups are susceptible to decarbonylation.html 6/10 . STEREOCHEMISTRY OF WILKINSON'S HYDROGENATION http://www. It may be due to easy coordination of olefin to the catalyst that is assisted by polar functional group.2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY compatibility of Wilkinson's catalyst with polar multiple bonds indicates the metal hydride bond is primarily covalent in character. also rendering the catalyst ineffective.adichemistry. * Unsaturated substrates containing polar functionality are hydrogenated more rapidly. a racemic mixture is formed. It is stereospecific in the sense that one stereo isomer gives only one stereo isomer as exclusive product. It is due to stereospecific nature of the mechanism.g.2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY STEREOCHEMISTRY OF WILKINSON'S HYDROGENATION Hydrogenations catalyzed by Wilkinson's catalyst involve stereospecific syn hydrometallation of the multiple bond followed by stereospecific reductive elimination. Whereas.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst.html 7/10 . This is also partly due to other mechanisms operating under reaction conditions. Hence the hydrogenation of olefins or alkynes result in syn addition products. Hence Wilkinson's hydrogenation is both stereospecific as well as stereoselective. However it is also stereoselective since only one diastereomer is formed selectively as major product. E. http://www. with fumaric acid. Hydrogenation of Maleic acid or Fumaric acid with D2 in presence of Wilkinson's catalyst is diastereoselective.adichemistry. Hydrogenation of Maleic acid with D2 give meso compound exclusively. It is observed that hydrogenation of alkynes give cis alkenes as major products. 2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY The diastereoselectivity of Wilkinson's hydrogenation is evident in the formation of endo product as major one with the following bicyclic system.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst.html 8/10 . DECARBONYLATION WITH WILKINSON'S CATALYST http://www.adichemistry. The catalyst binds to the double bond from the least hindered exo face of the bicyclic system that is followed by syn addition of two hydrogen atoms. since the catalyst cannot be regenerated.html 9/10 . The complex. chlorocarbonylbis(triphenylphosphine) rhodium. the Wilkinson's catalyst can be used to bring about decarbonylation of aldehydes through oxidative addition route. which removes CO ligand from chlorocarbonylbis(triphenylphosphine) rhodium to regenerate the active form of catalyst.adichemistry.2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY DECARBONYLATION WITH WILKINSON'S CATALYST Since Rh(II) is a low valent and coordinately unsaturated metal and can bind to CO strongly.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst. Hence stoichiometric amounts of the complex is required to carry out this conversion. Mechanism: The process is noncatalytic. formed during the reaction is quite stable and it is not possible to dissociate CO ligand at mild temperatures. http://www. However. it is possible to make decarbonylation reaction catalytic with Wilkinson's catalyst by employing DiPhenylPhosphoryl Azide (DPPA). 2/1/2016 WILKINSON'S CATALYST | HYDROGENATION | MECHANISM | ADICHEMISTRY Author: Aditya vardhan Vutturi Google+ profile http://www.com/inorganic/organometallic/catalysis/wilkinson/wilkinsonscatalyst.adichemistry.html 10/10 .