Metal cationic palladium complex

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]. 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

Unlike early transition metal polymerization catalysts which do not tolerate functional groups, cationic palladium complexes are able to copolymerize ethylene with methyl acrylate.128... 

Indeed, direct measurements of the rates of insertion of CO and ethylene into alkyl-metal olefin and acylmetal olefin complexes show that the insertion of ethylene into the metal-acyl linkage is faster than the insertion of ethylene into the metal-alkyl linkage. Comparisons of these rates for insertions into cationic palladium complexes containing phenanthroline and bis-diphenylphosphinopropane as ancillary ligand have been made by Brookhart and co-workers. These reactions are shown in Equations 9.69 and 9.70. A summary of the barriers for insertion is provided in Table 9.2. The rate of insertion of ethylene into the metal-acyl bond is orders of magnitude faster than the rate of insertion of ethylene into the metal-alkyl bond. - - ... 

Although Pd-catalyzed intramolecular hydroamination reactions of alkynes have been known for ten years, analogous transformations of unactivated alkenes have only recently been developed [33]. Key to the success of these studies was the use of a cationic palladium complex bearing a pyridine-derived P-N-P pincer ligand (29). For example, treatment of 26 with catalytic amounts of 29, AgB F4, and Cu(OTf)2 led to the formation of pyrrolidine 27 in 88% yield with 4 1 dr (Eq. (1.13)). Detailed mechamstic studies have indicated these transformations proceed via alkene coordination to the metal complex followed by outer-sphere aminopaUadation to provide 28. Protonolysis ofthe metal-carbon bond with acid generated in situ leads to formation of the product with regeneration of the active catalyst. 

Copolymerization of isoprene with acrylic monomers is also possible [516,522,523]. The copolymers can be synthesized with statistical as well as strictly alternating structure [524]. Ethylaluminum dichloride is used as a catalyst. The dine units are incorporated predominantly with trans-l,A enchainment. Addition of transition metal compounds or radical-forming compounds increases the polymerization rate [525,526]. Cationic palladium complexes are used as highly active catalysts in the polymerization of isoprene with diethylamine [527]. 

More remarkably, structurally well-characterized Phen-containing transition metal complexes have been prepared and further applied in challenging organic transformations. Thus, cationic palladium complexes of t)fpe Pd(phen)2X2 (where X = CF3SO3, PFs, BF4) are very stable and found fairly active in combination with either benzoic acids or phosphorus acids as cocatalysts in the conversion of nitroarenes to the corresponding carbamate derivatives. Extensive studies have evidenced the key role of the nature of counteranion X of such Phen-based catalysts in the reaction outcome and even metaUacycles of type A have been isolated and identified as active intermediates in the reductive carbonyla-tion of nitrobenzene (eq l). ... 

The cationic palladium complex [(dppp)Pd(H20)2] catalyzes the carbonylation of styrene under CO/H2 to give l,5-diphenylpentan-3-one and l,5-diphenylpent-l-en-3-one (40) (5 95), indicating that insertion of a second styrene molecule into the Pd—COR bond is faster than metal-acyl bond hydrogenolysis and hydroformylation. A crystallographically characterized dimer, [(dppp)Pd(/i-OH)] has only low activity. The carbonylation of styrene with Pd(OAc)2/p-tolS03H/benzoquinone/L is ligand-dependent if L = phenanthroline, a syndiotactic copolymer is produced whose regio-selectivity indicates chain-end control of the insertion process (Scheme 25). 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

In the ligand l,2-bis-(2-pyridylethynyl)benzene the pyridyl N atoms easily attain the appropriate separation for trans-chelation of metal cations. The 1 1 complex of the ligand with palladium(II) chloride has been structurally characterized.171,182... 

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]. 

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

In accord with calculations performed by Cavell et al. [110], the oxidative addition of C2-X functionalized azolium cations (X = halogen) to metal centers proceeds faster and with a more favorable reaction enthalpy than the oxidative addition of the C2-H substimted imidazolium cations [118, 119]. The former reaction was applied successfully for the preparation of nickel and palladium complexes bearing a variety of different ylidene ligands [119]. 

The metal-bound RCN group is also activated on coordination towards nucleophilic attack by alcohols, thiols or amines to give stable N-bonded iminoether, iminothioether and amidine complexes respectively.332 Several cationic cyanobenzylpalladium(II) complexes have been prepared, and the reactivity of the CN group towards nucleophiles has been studied.333,334 The palladium complex (97) reacts with aromatic amines to give chelated amidino complexes (98) and the reaction has been studied kinetically.333 In this case intermediates with the nitrile group bonded side-on are considered to be involved. 

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