Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Catalytic Carbon—Carbon Coupling Reactions
19.2 Reactions of Transition Metal Complexes

In this section, we will examine the structure and type of reactions that selected coordination transition—metal complexes undergo. Some of these reactions are already mentioned in previous chapters, but in this section, more details will be provided about the mechanism for these reactions.

19.2.1 Oxidative Addition Reactions

In oxidative addition, a metal is inserted into a sigma bond. We have already seen this type reaction in the synthesis of the Grignard reagent, in which magnesium is inserted into the C─X bond to form R-Mg-X, where X is a halogen. For Reaction (19-1), the metal Mg in the reactant is oxidized to Mg2+. As a result, the term oxidative addition is used to describe this type of reaction.

(19-1)Image

As we have mentioned in earlier chapters, Victor Grignard discovered this type of reaction, and the reaction is now known as the Grignard reaction and the organomagnesium halides generated are known as Grignard reagents.

For transition metals, the same type of reaction is possible as shown below in Reaction (19-2), in which Pd is inserted into a Ph─X bond, where Ph is the phenyl group (—C6H5) and X is a halogen.

(19-2)Image

Note the change in oxidation state of palladium, it is Pd(0) in the reactant and is oxidized to Pd(II) in the product. The formation of this type of complex is key to the reactions that will be discussed throughout this chapter.

19.2.2 Transmetallation Reactions

Another important reaction that coordinated transition—metal complexes can undergo is transmetallation as shown in Reactions (19-3) through (19-5a).

(19-3)Image

(19-4)Image

(19-5a)Image

These reactions are very straightforward in that one group is transferred from a coordinated metal ion complex to another. We have seen a similar type of reaction before in the synthesis of organocuprates, also known as the Gillman reagent and given in Reaction (19-5b).

(19-5b)Image

19.2.3 Ligand Migratory Insertion Reactions

Another type of a reaction of metal ion complex is the possibility of migratory insertion of a ligand in a bond of the metal ion, as shown below in Reaction (19-6).

(19-6)Image

These reactions are called 1,2 insertion since the insertion takes place across two carbons of the alkene, and this type of reaction is also key in coupling reactions, as we will see later in the chapter.

19.2.4 β-Elimination Reactions

As the name implies, these reactions involve the migration of a atom or group in the β-position of the metal along with its bonding electrons, to the metal. This reaction is essentially the reverse of ligand insertion as discussed in the previous section. An example is shown below in Reaction (19-7).

(19-7)βImage

This type of elimination is also known as an intramolecular syn-elimination.

19.2.5 Reductive Elimination Reactions

As the name implies, this type of reaction is essentially the reverse of oxidative addition. For these reactions, two ligands bonded to the metal leave. An example is given in Reaction (19-8).

(19-8)Image

For Reaction (19-8), a base is introduced to abstract a proton and the bonding electrons are used to reduce the Pd2+ to Pd (0) metal while the other ligand leaves with its bonding electrons. Another example of this type of reductive elimination reaction is given in Reaction (19-9).

(19-9)Image

For this reaction, which is an intramolecular elimination, the phenyl group migrates to form a new carbon—carbon bond while the Ni is reduced from Ni(II) to Ni(0).

Problem 19.1

For each of the following reaction described above the arrow, give the products.

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