Palladation carbo

The oxidative coupling/cyclization process occurs via stoichiometric carbo-palladation using a Pd(II) species, typically Pd(OAc)2. In an early example, submission of diphenylamines 3 to the palladium(II)-promoted oxidative intramolecular cyclization conditions yielded carbazoles 4 [15-... 

The resulting <7 alkyl bond in such complexes is very reactive, especially towards carbon-carbon bonds. Thus an alkene in the reacting system will lead to coordination followed by migratory inser tion into the palladium-carbon a bond. This process is like hydrometallation and is called carbo palladation as carbon and palladium are attached to the ends of the alkene system. There is nc change in oxidation state during this process, although the ligands (often phosphines) must dissociate to allow coordination of the alkene and associate to provide a stable final 16-electron product. 

Pettier LR, Peyrat JF, Alami M, Brion JD (2004) Unexpected tandem sonogashira-carbo-palladation-sonogashira coupling reaction of benzyl halides with terminal alkynes a novel four-component domino sequence to highly substituted enynes. Synlett 1503-1508... 

Abstract Palladium readily catalyzes the cross-coupling of functionally substituted aryl or vinylic halides and alkynes to afford a wide variety of heterocycles and carbocycles in one efficient step. Terminal alkynes presumably initially generate aryl (vinylic) alkynes, which under the reaction conditions are rapidly cyclized by the palladium or copper salts employed in the first step to produce the final product. Internal alkynes apparently react by carbo-palladation of the alkyne and subsequent intramolecular nucleophilic substitution of the palladium moiety to generate the observed products. A variety of other closely related processes have also been reported, including the simultaneous annulation of alkynes and car-... 

In an alternative route to the prostaglandins from cyclopentadiene, Holton has made use of the technique of so-called carbo-palladation , whereby the two side-chain fragments are attached in sequence to the double bonds of cyclopentadiene via appropriate palladium complexes. Thus treatment of the enamine (58), derived from cyclopentadiene, with lithium tetrachloropalladate and sodium diethylmalonate led to the palladium complex (59) which with di-iso-... 

The proposed mechanism involves an acetoxy palladation of the triple bond in 133 to produce a vinyl-Pd(II) intermediate 135, which further undergoes an intramolecular insertion to give 136 (Figure 9.2). This is followed by insertion through carbo palladation with the formation of bicyclic intermediate 138, which is immediately oxidized by PhI(OAc)2 to give the Pd(IV) intermediate 139 (path a). Another pathway (path b) with the formation of intermediate 139 via 137 has also... 

The use of benzyne derivatives in intermolecular carbo-palladation processes has stood out as a powerful tool for the preparation of 1,2-functionalized arenes in the last decade. Since the pioneering work by Yamamoto in 2000 [34], which introduced arynes as reactive partners in palladium-catalyzed reactions, several groups have exploited the carbopallada-tion of benzyne in three-component couplings. In general, 2-(trimethylsilyl)phenyl triflate derivatives, in combination with a fluoride salt such as CsF, are used as benzyne precursors. 

Kibayashi and coworkers [15] have introduced a sequential cyclic carbo-palladation-Stille vinylations of enyne compounds. Upon treating the enyne 1 and vinyl tributylstannane with catalytic amounts of system Pd2(dba)3-CHCl3/ AcOH (dba, dibenzylideneacetone), the allyl-substituted methylene cyclopentane 2 was formed in 53% yield (Scheme 12.1). The expected Alder-ene product 3 was not detected. This sequence was extended to heteroatom-tethered enynes 4 and vinyl tin compounds 5 to furnish heterocycles 6 in moderate to excellent yields. 

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