Carbopalladation inter-intramolecular reactions

As shown in Eqs. 2 and 3, the carbopalladation of bicyclopropylidene [14,15] and vinylcyclopropane [16] also gives the corresponding CPC-Pd intermediates, which readily undergo P-carbon elimination, hydrogen migration, and the subsequent inter- or intramolecular reaction with nucleophiles. 

Cascade reactions arising from sequential carbopalladations are especially valuable for the construction of various carbo- and heterooligocyclic systans with three, four, or even more annelated rings. The Heck reaction has successfully been employed in various inter-inter-, intra-inter-, as well as all-intramolecular reaction cascades (Sect. IV.3). In the carbopalladation step of the Heck reaction (Scheme 6) a new metal-carbon bond is formed, which, in principle, can undergo any of the typical reactions of a hydride elimination is not too fast. When the /S-hydride elimination is totally suppressed, the palladium species can undergo a number of reactions with the formation... 

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. 

Different regioselectivities were observed in the reaction of 3,4-pentadien-l-ols 387 with aryl iodides. The expected products 390 and 391 from the tt-allylpalladium intermediates 389 were not formed [151]. Exclusive formation of the dihydrofuran 388 is explained by concerted inter- and intramolecular exo-oxypalladation as shown by 392 to give 7r-allylpalladium 393, and reductive elimination gives rise to the dihydrofuran 394, showing that the intramolecular oxypalladation is faster than the intermolecular carbopalladation with Ar-Pd-I. 

Some heterocycles undergo facile inter- and intramolecular arylations. Two mechanistic explanations of the arylation are possible. The Heck-type reaction is one of them. Carbopalladation of one of two double bonds with Ar-Pd-X gives la, which is stabilized by jr-allylpalladium formation. Pd and jS-H is anti in la and hence syn f-H elimination is not possible. Therefore, in stereoisomerization from anti la to syn lb a relationship between Pd and jS-hydrogen occurs, and syn y3-H elimination from lb affords the arylation product (path a). Another explanation is electrophilic substitution with Ar-Pd-X. Since arylations occur mainly at electron-rich carbons of heterocycles, they can be understood as electrophilic substitutions (path b). In some cases, a Heck-type mechanism seems to be more reasonable. Certainly arylations proceed by one of these mechanisms depending on the electronic nature of the substrates and reaction conditions, and hence further mechanistic studies are necessary. 

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