Carbon-palladium bonds hydridopalladium complexes

In analogy to the mechanism of the palladium-catalyzed enyne cyclization, it is postulated that exposure of palladium(O) to acetic acid promotes in situ generation of hydridopalladium acetate LnPd"(H)(OAc). Alkyne hydrometallation affords the vinylpalladium complex A-10, which upon r-carbopalladation of the appendant alkyne provides intermediate B-7. Silane-mediated cleavage of carbon-palladium bond liberates the cyclized product along palladium(O), which reacts with acetic acid to regenerate hydridopalladium acetate to close the cycle (Scheme 33). 

The addition of an organopalladium intermediate to the double bond with subsequent cleavage of palladium hydride and restoration of unsaturation is the basis of another important process involving organopalladium compounds, the Heck reaction. In a catalytic version, the organopalladium intermediate is formed by oxidative addition to carbon-halogen or other carbon-heteroatom bonds, and the hydridopalladium complex is believed to be recycled by base-promoted reductive elimination (Scheme 5.7). 

Olefinic compounds will often insert into carbon-transition metal bonds as CO does, and this reaction is an important step in many catalytic syntheses. When this step is combined with an oxidative addition of an organic halide to a palladium(O) complex in the presence of a base, a very useful, catalytic olefinic substitution reaction results (26-29). The oxidative addition produces an organopalladium(II) halide, which then adds 1,2 to the olefinic reactant (insertion reaction). The adduct is unstable if there are hydrogens beta to the palladium group and elimination of a hydridopalladium salt occurs, forming a substituted olefinic product. The hydridopalladium salt then reforms the... 

The transformation that has come to be known as the Heck reaction is broadly defined as the palladium(O)-mediated coupling of an aryl or vinyl halide or triflate with an alkene. The basic mechanism for the Heck reaction of aryl halides or trifiates (as outlined in more detail in the Key Chemistry), involves initial oxidative addition of the chiral palladium(O) catalyst to afford a a-arylpalladium(II) complex. Coordination of an alkene and subsequent carbon-carbon bond formation by syn insertion provide a a-alkylpalladium(II) intermediate, which readily undergoes P-hydride elimination to release the alkene product. Finally, the hydridopalladium(II) complex has to be converted into the active palladium(O) catalyst to complete the catalytic cycle. 

A variety of palladium-catalyzed organic reactions involve the oxidative addition process. A typical example is seen in the catalytic arylation and alkenylation of olefins (eq (60)) [85]. Aryl- and alkenylpalladium(ll) complexes (9) formed by oxidative addition undergo olefin insertion into the palladium-carbon bond to give an alkylpal-ladium species (10), which provides arylated and alkenylated olefins via p-hydrogen elimination. The hydridopalladium species 11 thus generated is reduced to a Pd(0) species upon its interaction with a base and carries the sequence of reactions... 

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