Cationic palladium complexes mechanisms

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The reactions catalyzed by cationic palladium complexes are believed to proceed via a different mechanism (Scheme 67).273 Initially, a cationic silylpalladium(n) species is generated by cr-bond metathesis of the Br-Pd+ with a silylstannane. Subsequently, the alkyne and alkene moieties of the 1,6-diyne successively insert into the Pd-Si bond to form a cationic alkylpalladium(n), which then undergoes bond metathesis with silylstannane to liberate the product and regenerate the active catalyst species, S/-Pd+. 

Figure 22-4 Mechanism for the hydrosilylation and dehydrogenative silylation of 1-alkenes catalyzed by cationic palladium complexes Pd represents [(phen)Pd]+. The palladium alkene complex A is the resting state of the cycle. Cycle I denotes the hydrosilylation cycle, Cycle II describes the dehydrogenative silylation reaction.

A related mechanism has been established for hydrosilylations catalyzed by cationic palladium complexes and is shown in Fig. 22-4. The mechanisms for both... 

An interesting variant of oxidative carbonylation was recently achieved by ConsigUo and co-workers. In the presence of a cationic palladium complex it was possible to introduce three carbonyl groups in the oxidative carbonylation of ethylene or propylene, one of them under the form of ketone, using (Pd(H20)2[(5)-2,2 -dimethoxy-6,6 -bis(diphenylphosphino)biphenyl] (CF3S03)2 as catalyst precursor Scheme 15 also shows the proposed mechanism. 

The reductive carbonylation of acetylenes proceeds via a different mechanism compared to the carbonylation of olefins, but through the addition of palladium hydride species to the triple bond. The most probable source of PdH is the WGS reaction, so water is required at two key steps of this catalytic cycle. Depending on conditions, the nature of the catalyst, and promoter additives, the carbonylation of acetylenes can lead to different products. An important role of cationic palladium complexes that readily form in the presence of water has been disclosed. "... 

For the open mechanism, the most characteristic features are the absence of a cyclic species, and the formation of a cationic palladium complex. Experimentally... 

Seayad A, Jayasree S, Damodaran K, Toniolo L, Chaudhari RV. On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex. J. Org. Chem. 2000 601 100-107. 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

The discovery in the early 1980s that cationic palladium-phosphine complexes catalyse the copolymerisation of carbon monoxide with ethene or a higher a-olcfin to yield perfectly alternating polyketones has since attracted continuous increasing interest [1,2]. This is because the monomers are produced in large amounts at a low cost and because polyketones represent a new class of thermoplastics of physical-mechanical and chemical properties that have wide applications [3-6]. In addition, easy functionalisation can open the way to a large number of new materials [7]. The copolymerisation has... 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

This chemistry has been extended to produce palladium(O) intermediates (48). Much of the chemistry is similar to that of the Pt analogues except that the palladium(O) complexes are more unstable and difficult to isolate. A reaction characteristic of palladium is the addition of allyl compounds to form cationic allyl complexes, Equation 16. This has been postulated (49) as a key step in the mechanism for Pd(diphos)2 catalyzed reactions of allyl compounds. 

Following the usual mechanism (pp. 1330-4), the palladium complexes to the face of the alker.e opposite the bridge and the ester group leaves to give an allyl cation complex. This is attacked by the malonate anion from the opposite face to the palladium. So the overall resrdt is retention, the starting material giving the cis product. 

Carbonylation of the isomeric cationic solvated complexes of platinum(II) C and D [the corresponding palladium(II) species react at a much faster rate] proceeds via an alkyl migration mechanism in a study based on NMR detection of the intermediates. This conclusion is in agreement with the available data in the literature kinetic data should, however, complement the information concerning the palladium(II) and plati-num(II) systems. 

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