Wiliknsons reagent

Presented by Shikha Popali and Harshpal singh Wahi students from Gurunanak college of pharmacy, Nagpur in Department of pharmaceutical Chemistry. The explained topic is seful for every chemistry student and for others too

1. Wilkinson’s Reagent SHIKHA D. POPALI HARSHPAL SINGH WAHI Department of Pharmaeutical chemistry M.pharm 1 St yr GURUNANAK COLLEGE OF PHARMACY, NAGPUR

2. Wilkinson's catalyst, is the common name for  chloridotris ( triphenylphosphane )rhodium(I), a  cordination complex  of rhodium with the formula RhCl (PPh 3 ) 3 (Ph =  phenyl ). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in  tetrahydrofuran  or chlorinated solvents such as dichloromethane . The compound is widely used as a catalyst for hydrogenation of  alkenes . Introduction : tris(triphenylphosphine) chlorhodium 2

3. Structure and basic properties According to single crystal X-ray diffraction the compound adopts a slightly distorted square planar structure. In analyzing the bonding, it is a complex of Rh(I), a  d 8  transition metal ion. From the perspective of the 18-electron rule, the four ligands each provides two electrons, for a total of 16-electrons. As such the compound is coordinatively unsaturated. Furthermore, in solution the complex undergoes fast dynamic exchange processes both intermolecular (one of the PPh 3  is labile and is constantly exchanged between the inner coordination Rh sphere and the solution, although this fast equilibrium is really balanced towards the starting triphosphine complex, K = 10 −5  M) and intramolecular (fluxionality). 3

4. Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol. Triphenylphosphine serves as a two-electron reducing agent that oxidizes itself from oxidation state (III) to (V). In the synthesis, three equivalents of triphenylphosphine become ligands in the product, while the fourth reduces rhodium(III) to rhodium(I). RhCl 3 (H 2 O) 3  + 4 PPh 3  → RhCl (PPh 3 ) 3  + OPPh 3  + 2 HCl + 2 H 2 O Catalytic applications Decarbonylation of Aldehyde This reaction is of value when a carbonyl group is needed foe activation in a step in a synthetic sequence and has to be removed subsequently. Decarbonylation wih Wilkinson’s catalyst goes through complexes of the type is an intramolecular reaction and gives a retention of configuration at a chiral R. Synthesis 4

5. Decarbonylation of an aldehyde with Wilkinson’s catalyst 5

6. 2. Formation of Carbon-carbon bonds In the example, the first step involves a ligand exchange by a combination of ligand association-dissociation step and a methyl group is inserted into the coordination sphere of rhodium. An oxidative addition then incorporates the phenyl group into the rhodium coordination sphere and subsequent reductive elimination then joins methyl group and the benzene ring to yield toluene in the last step. 6

7. 3 . Hydrogenation of alkenes using Wilkinson’s catalyst 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 H 2  to the metal. Subsequent π-complexation of alkene, migratory insertion (intramolecular hydride transfer or olefin insertion), and reductive elimination complete the formation of the alkane product. Mechanism of hydrogenation with Wilkinson’s catalyst 7

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