Hydropalladation–dehydropalladation
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص1081-1082
2025-08-07
539
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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