Alkenes migrating carbon stereochemistry

As p-hydride elimination is reversible, hydropalladation with the opposite regiochemistry provides a mechanism for forming regioisomers of the alkene. This allows the most stable alkene that is accessible by the hydropalladation-dehydropalladation sequence to dominate. The only restriction is that all of these processes are syn. The migration can be prevented by the addition of bases like silver carbonate, which effectively removes the hydrogen halide from the palladium complex as soon as it is formed. This synthesis of a complex trans dihydrofuran involves the Heck reaction followed by alkene isomerization and then a Heck reaction without migration to preserve the stereochemistry. 

In the stepwise mechanism, the stereospedficity of the elimination of the silyl-oxide from the -hydroxyalkylsilane can be explained as follows. Both migration of the sUyl group from carbon to oxygen and elimination of the trimethylsilyloxide moiety to form a double bond are so rapid that no rotation about the C-C bond is usually observed during these steps, giving the alkene with the same stereochemistry as that obtained by a concerted mechanism. 

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