Palladium is the most widely used metal in homogeneous catalysis

These elementary steps form the basis for most of organo-transition metal chemistry, and are the same regardless of the metal and the detailed structure of the ligands. Transition metal catalysis is an enormous and rapidly expanding fi eld that we just do not have the space to discuss in comprehensive detail. Instead, we will concentrate on the chemistry of one important, and representative, transition metal: palladium. Pd-catalysed reactions are widely used in both industrial and academic laboratories, on both a minute and very large scale. The variety of reactions that can be catalysed by Pd together with the range of functional groups tolerated, and usually excellent chemo- and regioselectivity, means that most syntheses of organic molecules of any complexity will now involve palladium chemistry in one or more key steps.

Let’s start with a review of the basic chemistry of palladium, as you will be seeing many more examples of these steps in specialized situations. Palladium chemistry is dominated by two oxidation states. The lower, palladium(0), present in tetrakis(triphenylphosphine)palladium, for example, is nominally electron-rich, and will undergo oxidative addition with suit able substrates such as organic halides, resulting in a palladium(II) complex. Oxidative addition is thought to occur on the coordinatively unsaturated 14-electron species, formed by ligand dissociation in solution.

The resulting Pd–R σ bond in such complexes is very reactive, especially towards car bon–carbon π bonds. Thus an alkene in the reacting system will lead to coordination followed by migratory insertion into the palladium–carbon σ bond. Like hydrometallation this process is a migratory insertion and is called carbopalladation because carbon and palladium become attached to the ends of the alkene system. There is no change in oxidation state during this process, although the ligands (often phosphines) must dissociate to allow coordination of the alkene and associate to provide a stable final 16-electron product.

With some metals the process of olefi n coordination and insertion may continue, leading to polymerization, but with palladium the metal is expelled from the molecule by a β-hydride elimination reaction and the product is an alkene, plus a Pd(II) complex. For the whole process to be catalytic, this Pd(II) product of β-hydride elimination must be converted to a Pd(0). This occurs in the presence of base, which removes HX from the palladium(II) species. This is another example of reductive elimination: one that forms a hydrogen halide rather than a carbon–carbon or carbon–hydrogen bond, as you saw earlier.

The speed of the β-hydride elimination (which is intramolecular and very fast) means that the original substrate for the oxidative addition reaction must be chosen with care—the presence of hydrogen at an sp³ carbon in the β position must be avoided. Thus, substrates for oxidative addition reactions in palladium chemistry are frequently vinylic, allylic, or aro matic and never ethyl or n-propyl.

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مواضيع ذات صلة

The Suzuki coupling couples boronic acids to halides

Hydropalladation–dehydropalladation

Insertion reactions are reversible

Carbon monoxide incorporation extends the carbon chain

Migratory insertion builds ligand structure

Reductive elimination removes metal atoms and forms new single bonds

Oxidative addition inserts metal atoms into single bonds

Bonding and reactions in transition metal complexes

Ligands can be attached in many different ways

The 18-electron rule

Transition metals extend the range of organic reactions

The Ritter reaction and the Beckmann fragmentation