Hydropalladation-dehydropalladation

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 order to avoid or minimize redundancy, the 19 General Patterns are classified into six categories, as summarized in Table 1, and all patterns in each category are discussed as a unit. Many processes of Pd complexes involve a pair of patterns that are miCToscopic reversals of each other, such as complexation and decomplexation (or dissociation) in ligand substitution. Migration of Pd via a series of hydropalladation-dehydropalladation and reversible carbonylation via migratory insertion-deinsertion are additional representative examples. 

The ability of Pd to participate in both hydrometallation and dehydrometallation snggests that Pd and its complexes can serve as catalysts for isomerization of alkenes and alkynes via a series of hydropalladation-dehydropalladation, that is 1,2-H shift. On the other hand, Pd-induced aUylic rearrangement provides a mechanism whereby alkenes can iso-merize via 1,3-shift. In either of these processes, the crucial reqnirement is the presence or ready availability of an empty coordination site. As amply demonstrated throughout this Handbook, Pd is imminently capable of serving as the catalyst center for isomerization of alkenes and alkynes. 

Pd(OAc)2 can catalyze the reaction of 2,3-allenoic acids with non-allylic alkenyl bromides with a terminal C=C bond leading to P-alkenyl butenolides 61 and 62 (Scheme 29). The reaction proceeded via oxypalladation-carbopal-ladation, repeated P-dehydropalladation/hydropalladation-dehalopalladation, in which Pd(II) is the catalytically active species [21]. 

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