Carbopalladation hydride termination

As a typical intermolecular carbopalladation and termination, hydroarylation of alkynes are carried out extensively in the presence of HCO2H as a hydride source. Formation of regioisomers is observed in the reaction of asymmetric alkynes, and ratios depend on the nature of the substituents. High regioselectivity was observed in the reaction of the tertiary propargylic alcohol 14 to give 15 as a major product [5]. The (Z)-2-arylcinnamates 17, rather than 3-arylcinnamate 18, was obtained by the hydroarylation of methyl phenylpropiolate (16) [6]. 3-Substituled quinoline 21 was prepared by the regioselective hydroarylation of 19, followed by treatment of 20 with an acid without isolation [6]. 

Cascade carbopalladation sequences after attack on an alkene are most commonly terminated by dehydropallada-tion, if a / -hydride is available in a yy//-orientation. The intramolecular carbopalladation starting from the monocyclic diene 122 with a geminally disubstituted alkene terminator leads to a neopentylpalladium intermediate 123, which cannot undergo /J-dehydropalladation, but continue the cascade by a carbopalladation to eventually form the... 

Palladium-catalyzed vinylations of aryl halides are generally referred to as the Heck reaction (for reviews on the Heck reaction see [34-40]), a versatile process that can be performed inter- and intramolecularly [41]. In the Heck reaction the carbon-carbon single bond forming step is an insertion of an al-kene into the aryl-Pd bond, i.e., a carbopalladation, giving rise to an alkyl-Pd species. If this insertion is terminated by /1-hydride elimination the expected vinylation product is the outcome of the classical Heck reaction. Likewise, reversible insertion of a highly strained olefin where the /1-hydride elimination is suppressed leads to an entry to multiple Pd-catalyzed bond forming processes. 

Of the two mechanistic pathways, i.e., via palladacyclization or via hydropalladation-cyclic carbopalladation, the latter seems to be more suitable for the development of sequentially catalyzed processes. Considering cycloisomerizations via the hydropalladation-cyclic carbopalladation route the catalytic reaction can terminate by /1-hydride elimination giving rise to the formation of dienes and derivatives thereof (Scheme 79). Alternatively, the alkyl-Pd species formed in the cyclic carbopalladation can be susceptible to subsequent transmetallation with organometallic substrates. Then, a reductive elimination could conclude this second Pd-mediated step releasing the Pd(0) species for a new catalytic cycle. 

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

Norbomene can serve as an excellent external relay, for example, for the alkenylpalla-dium species first formed by intramolecular carbopalladation of an ort/to-alkynyl-substituted arylpalladium halide. The 2-alkenylnorbonylpalladium halide formed by intermolecular carbopalladation then undergoes another intramolecular carbopalladation, this one in a 3-exo-trig mode, and subsequent /3-hydride elimination terminates the cascade process to yield a cyclopropanated norbomene derivative (Scheme... 

Cascade carbopalladation sequences after attack on an alkene are most commonly terminated by dehydropalladation, if a /S-hydride is present in a yn-orientation (see above). 

IV Pd-CATALYZED REACTIONS INVOLVING CARBOPALLADATION B.i. Termination by Hydride... 

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