Alkenes coordinated to palladium (II) are attacked by nucleophiles

Now for another case where a transition metal catalysis facilitates a reaction that would not occur under normal conditions: nucleophilic attack on an isolated double bond. Usually alkenes react with nucleophiles only when conjugated with an electron-withdrawing group. But coordination of an electron-rich alkene to a transition metal ion such as palladium (II) changes its reactivity dramatically: electron density is drawn towards the metal and away from the π orbitals of the alkene. This leads to activation towards attack by nucleophiles, just as in con jugate addition, and unusual chemistry follows. Unusual, that is, for the alkene; the palladium centre behaves exactly as expected.

Many nucleophiles, such as water, alcohols, and carboxylates, are compatible with an alk ene–Pd (II) complex and can attack the complexed alkene from the side opposite the palla-dium. The attack of the nucleophile is regioselective for the more substituted position. These parallels attack on bromonium ions but is probably governed by the need for the bulky palladium to be in the less hindered position.

The resulting Pd (II) σ-alkyl species decomposes by β-hydride elimination to reveal the substituted alkene. Reductive elimination of a proton and the leaving group, usually chloride, leads to palladium (0). The weakness of this reaction is that the catalytic cycle is not complete: Pd (II), not Pd (0), is needed to complex the next alkene.

There are two solutions to this problem. We could use stoichiometric Pd(II) but this is acceptable only if the product is very valuable or the reaction is performed on a small scale. It is better to use an external oxidant to return the palladium to the Pd(II) oxidation state so that the cycle can continue. Air alone does not react fast enough (even though Pd(0) must be protected from air to avoid oxidation) but, in combination with copper (II) chloride, oxygen completes the catalytic cycle. CuCl₂ oxidizes Pd (0) to Pd (II) and is itself oxidized back to Cu (II) by oxygen, ready to oxidize more palladium.

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مواضيع ذات صلة

Oxypalladation and the Wacker oxidation

Palladium-catalysed amination of aromatic rings

Intramolecular alkylations make rings

Vinyl epoxides provide their own alkoxide base

Allylic electrophiles are activated by palladium (0)

Coupling to alkynes: the Sonogashira reaction

The Suzuki coupling couples boronic acids to halides

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