Hydropalladation–dehydropalladation

can lead to alkene isomerization Reversible β-hydride eliminations provide a mechanism for interconverting regioisomers of an alkene, and the following reaction sequence also illustrates another point about the reductive elimination step: it is a syn elimination, and the C–Pd and C–H bonds have to eclipse one another for the Pd–H bond to form. Oxidative addition of the aryl iodide to a palladium(0) complex, formed from Pd(OAc)₂ by reduction, gives the active palladium(II) complex ArPdOAcL₂. Carbopalladation occurs as expected on an electron-rich alkene to give the prod uct of aryl addition to the oxygen end of the alkene in a syn fashion. β-Hydride elimination must occur away from the aryl group because there is only one C–H bond syn to the C–Pd bond. The alkene has moved one position round the ring.

Hydropalladation in the reverse sense gives a new σ complex, which could eliminate either the black or the green hydrogens. Elimination of the green H gives the enol ether, which is the most stable alkene possible due to conjugation.

This product now undergoes a second Heck reaction involving naphthyl iodide:

The initial mechanism is much the same. However, the enol ether has two diastereotopic faces: syn or anti to the aromatic substituent (Ar1) introduced in the first step. Palladium is very sensitive to steric effects and generally forms less hindered complexes where possible, so the palladium (II) complexes the face of the enol ether anti to Ar1. This in turn controls all the subsequent steps, which must be syn, leading to a final product with anti-stereochemistry. The requirement for syn β-hydride elimination also explains the regiochemical preference of the elimination. In the σ-bonded cyclic structure there is only one hydrogen (green) that is syn to the palladium; the one on the carbon bearing the naphthyl substituent is anti and cannot be eliminated. Further migrations of the alkene by hydropalladation are prevented by the silver carbonate, which rapidly removes iodide from the intermediate, preventing read dition of Pd–H to the alkene.

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مواضيع ذات صلة

The Suzuki coupling couples boronic acids to halides

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

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