Reductive elimination removes metal atoms and forms new single bonds

If we want to use organometallic chemistry to make organic compounds other than those containing metals, we must be able to remove the ligands from the coordination sphere of the metal at the end of the reaction. Neutral organic species such as alkenes, phosphines, and carbon monoxide can simply dissociate in the presence of other suitable ligands but those that are bound to the metal with shared electrons require a more active process. Fortunately, most reactions that occur around a transition metal are reversible and so the reverse of oxidative addition, known as reductive elimination, provides a simple route for the release of neutral organic products from a complex. Our general reaction shows M(II) going to M(0), releasing X–Y. These two ligands were separate in the complex but are bound together in the product. A new X–Y σ bond has been formed.

The ligands to be eliminated must be cis to one another for reductive elimination to occur. This is because the process is concerted. Two examples from palladium chemistry make this point clear. Warming in DMSO causes ethane production from the fi rst palladium complex because the two methyl groups are cis in the square planar complex. The more elaborate second bisphosphine forces the two methyl groups to be trans and reductive elimination does not occur under the same conditions.

Of course, no one wants to make ethane that way (if at all) but many other pairs of ligands can be coupled by reductive elimination. Reductive elimination is one of the most important methods for the removal of a transition metal from a reaction sequence, leaving a neutral organic product. We will see many examples as the chapter develops but here is an indole synthesis that depends on a reductive elimination at palladium as a last step. In the starting material, palladium has two σ bonds sharing electrons with C, and is Pd(II). In the reaction the two C substituents bond together to form the indole ring and a Pd(0) species is eliminated.

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مواضيع ذات صلة

The Suzuki coupling couples boronic acids to halides

Hydropalladation–dehydropalladation

Palladium is the most widely used metal in homogeneous catalysis

Insertion reactions are reversible

Carbon monoxide incorporation extends the carbon chain

Migratory insertion builds ligand structure

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