Cationic palladium complexes

Hydrosilylation of dienes accompanied by cyclization is emerging as a potential route to the synthesis of functionalized carbocycles. However, the utility of cycliza-tion/hydrosilylation has been Umited because of the absence of an asymmetric protocol. One example of asymmetric cycUzation/hydrosilylation has been reported very recently using a chiral pyridine-oxazoUne ligand instead of 1,10-phenanthroline of the cationic palladium complex (53) [60]. As shown in Scheme 3-21, the pyridine-oxazoUne Ugand is more effective than the bisoxazoUne ligand in this asymmetric cyclization/hydrosilylation of a 1,6-diene. 

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]. 

Cationic palladium complex 121 reductively coupled enynes (Eq. 20) using trichlorosilane as the stoichiometric reductant [71]. This combination of catalyst and silane afforded silylated methylenecyclopentanes such as 122 in good yield from enynes such as 123. Attempts to develop an enantioselective version of this reaction were not successful [71]. When enediyne 124 was cyclized in the presence of trichlorosilane, the reaction favored enyne cycli-zation 126 by a 3 1 ratio over diyne cyclization to 125 (Eq. 21). In contrast, when the more electron-rich dichloromethylsilane was used as the reductant, diyne cyclization product 125 was preferred in a ratio of 4 1 [71]. Selectivities of up to 10 1 for enyne cyclization were observed, depending on the substrate employed [72],... 

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+. 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

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]. 

Less attention has been paid to the use of amines as nucleophiles in the telomerization reaction. A single report from Nolan and co-workers [233] has shown that well-defined cationic palladium complexes are efficient catalysts in the telomerization of butadiene with amines under mild conditions (Table 10). In the case of primary amines, the concentration of the reactants and their steric hinderance dictates the formation of a mono- or double-alkylated product. 

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... 

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

Figure 3.42 Cationic palladium complexes with a chelating imino functionalised NHC ligand.

Dendrimeric macromolecules containing germanium have also been made via hydrogermylation. The cycliza-tion/hydrogermylation of functionalized 1,6-dienes catalyzed by cationic palladium complexes has also been carried out. Hydrogermylation is also an effective synthetic method for the synthesis of germafranes. ... 

Dipolar cycloaddition of nitrones to olefins is also catalyzed by chiral cationic palladium complexes [66]. When nitrone 119 is treated with 3-(( )-2-butenoyl)-l,3-oxazoli-din-2-one (120) under the influence of (5)-BINAP PdCla (10 mol %) and AgBF4 (20 mol %), isoxazolidine 121 is obtained in 61 % yield as a 45 55 mixture of endo and exo forms. The endo isomer is obtained in 91 % ee (Sch. 32). The chemical yield and endo selectivity are improved by employing (5 )-p-Tol-BINAP as chiral ligand. 

The cationic palladium complexes [(R3P)3PdH] (R = CH3 and C2H5) insert CS2 to give [(R3P)2Pd(HCS2PR3)] products when the attack of R3P on the carbon atom of... 

A similar type of cationic palladium complex 21 was also obtained by the action of antimony(V) chloride upon the cyclopropenylidene-palladium complex 20, ° which was prepared from the 3,3-dichlorocyclopropene 19 and palladium black. 

Nakai and a coworker achieved a conceptually different protonation of silyl enol ethers using a chiral cationic palladium complex 40 developed by Shibasaki and his colleagues [61] as a chiral catalyst and water as an achiral proton source [62]. This reaction was hypothesized to progress via a chiral palladium enolate which was diastereoselectively protonated by water to provide the optically active ketone and the chiral Pd catalyst regenerated. A small amount of diisopropylamine was indispensable to accomplish a high level of asymmetric induction and the best enantioselectivity (79% ee) was observed for trimethylsilyl enol ether of 2-methyl-l-tetralone 52 (Scheme 11). 

Currently, a significant body of work deals with the use of chiral cationic palladium complexes bearing ligands of the BINAP type or related bisphosphine ligands such as SEGPHOS (Fig. 3). These are based on the pioneering work from Sodeoka on the direct formation of chiral palladium enolate complexes from the palladium precursors and 1,3-dicarbonyl compounds [10, 23]. Within this context, the combination of cationic BINAP-Pd complexes and N-fluoro-bis(phenylsulfonyl)imine (NFSI) was introduced by Sodeoka for the realization of an extremely efficient a-fluorination of 3-keto esters (Scheme 5). 

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