Wilkinson catalyst, structure

All of these reactions involve transition metals such as palladium, copper, and ruthenium, usually in complex with certain types of ligands. After we see the practical applications of these reactions for carbon—carbon bond formation, we shall consider some general aspects of transition metal complex structure and representative steps in the mechanisms of transition metal—catalyzed reactions. We shall consider as specific examples the mechanism for a transition metal—catalyzed hydrogenation using a rhodium complex called Wilkinsons catalyst, and the mechanism for the Heck—Mizoroki reaction. 

Catalyst or Catalyst Precursor In Situ Catalyst or Preformed Catalyst. A major consideration in homogeneous processes is whether to use a catalyst precursor or to use a compound actually involved in the catalytic cycle. A catalyst precursor is a compound that is not itself the catalyst but can react to form a catalyst. For example, rhodium trichloride does not itself catalyze hydrogenation but can be reacted with triphenylphosphine to form Wilkinson s catalyst (1). Catalyst precursors are generally cheaper and are easier to handle than the catalysts themselves. However, a catalyst formed in the complex chemical mixture of a reaction vessel will normally be less pure than if it were synthesized separately. In some cases, catalyst purity is critical to the reaction. For example, in enantioselective reactions an optically pure catalyst may be needed. Small differences in catalyst structure can have a large effect on enantioselectivity (2). 

Figure 4 Potential energy profile of the entire catalytic cycle of the Halpem mechanism of olefin hydrogenation by the Wilkinson catalyst, in kcal mol at the RHF level relative to 13 + C2H4 + H2. Numbers in parenthesis are MP2 energies for the RHF structures, relative to 15...

Write the structure obtained by hydrogenation of ipsdienol, a pheromone of the Norwegian spruce beede, using one equivalent of hydrogen gas and the Wilkinson catalyst. 

Write the structure of the product obtained by catalytic reduction of each of the following compounds using the Wilkinson catalyst and one molar equivalent of hydrogen gas. 

Werner s coordination theory, 1, 6 Whewellite structure, 6, 849 Wickmanite structure, 6, 849 Wilkinson s catalyst, 6, 239 Wilson s disease, 5, 721 copper, 6,648 removal, 6,769 copper complexes, 2,959 copper metabolism, 6,766 radiopharmaceutical agents, 6,968 Wolfram s red salt, 5,427 Wurzite... 

In some cases an alternative sequence involving addition of hydrogen at rhodium prior to complexation of the alkene may operate.11 The phosphine ligands serve both to provide a stable soluble complex and to adjust the reactivity at the metal center. The a-bonded intermediates have been observed for Wilkinson s catalyst12 and for several other related catalysts.13 For example, a partially hydrogenated structure has been isolated from methyl a-acetamidocinnamate.14... 

Some disagreement exists regarding the structure of the transition state (whether H or P3 is trans to the alkene) and whether or not solvent molecules occupy sites that are apparently vacant. In spite of some uncertainty regarding these details, the major issues regarding the catalyzed hydrogenation of alkenes using Wilkinson s catalyst are fairly well understood. 

Another important use for Wilkinson s catalyst is in the production of materials that are optically active (by what is known as enantioselective hydrogenation). When the phosphine ligand is a chiral molecule and the alkene is one that can complex to the metal to form a structure that has R or S chirality, the two possible complexes will represent two different energy states. One will be more reactive than the other, so hydrogenation will lead to a product that contains predominantly only one of the diastereomers. 

Methyl deoxypodocarpate 127 (Scheme 1) 129) represents a simple problem since the ketone 132 is well-known and readily available from Hagemann s ester in three steps. The problem of geminal alkylation of this ketone stems from its existence as an EjZ mixture of ring fusion isomers. Recognizing that decarbonylation of aldehydes occurs readily with Wilkinson s catalyst creates a structural equivalence of an acetaldehyde chain and a methyl group as in 128. This simple relationship immediately establishes several options, a simple one uses a thioacetal such as 129 as a synthon for the aldehyde. The presence of a carbonyl group three carbons away... 

C. Hofstetter, P. S. Wilkinson, T. C. Pochapsky, NMR Structure Determination of Ion Pairs Derived from Quinine A Model for Templating in Asymmetric Phase-Transfer Reductions by BH4" with Implications for Rational Design of Phase-Transfer Catalysts , J. Org. Chem 1999, 64, 8794-8800. 

Work with supported ionic liquids was extended to a cationic polymer, poly (diallyldimethylammonium chloride), which has quaternary ammonium functional groups (Fig. 16) 268). The extra-structural counter anion is Cl . The polymer was applied to simultaneously incorporate an ionic liquid and a transition-metal catalyst via a simple mixing of the components. Wilkinson s catalyst and [BMIM]PF6 were... 

Fig. 15.47 Structure of Wilkinson s catalyst. (PhiP)iPhCI. I From Jardine, I-. H. 1 n/ji. Inut-ft. Client. 1981. 28. 63 Reproduced with permission.]...

The cyclotrimerisation of dimethylcyclopropene is effected by Wilkinson s catalyst RhCl(PPh3)3 to provide a rhodacycloheptane derivative 111 which has been structurally characterised (Fig. 35). The metallacycle is cleaved upon treatment with carbon monoxide to provide both cyclohexane (reductive eliminations) and cycloheptanone (CO insertion) products (Scheme 34).120... 

Like most rules in bonding theory and structural chemistry, the 18-electron rule is often violated, even among the nonclassical complexes containing n acceptor ligands where, we have suggested, it should apply. Square planar complexes follow a 16-electron rule examples include Wilkinson s catalyst Rh(PPh3)3Cl (see also Section 9.8) and the vast number of square Pt(II) complexes. 

Although the structure of the active catalyst obtained in solution was uncertain, the Monsanto group suggested at the time that Wilkinson-type Rh(I) complexes might be involved. They did not speculate on how the reduction of Rh(HI) to Rh(I) might be accomplished, but one possibility is... 

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