Lewis palladium catalysts

For the synthesis of heterocycles, an efficient strategy has been introduced utilizing the dual transition metal sequences (Scheme 6).11,lla The key issue is the compatibility of the two catalyst systems. Jeong et al. studied the one-pot preparation of bicyclopentenone 35 from propargylsulfonamide 33 and allylic acetate.11 This transformation includes two reactions the first palladium-catalyzed allylation of 33 generates an enyne 34 and the following Pauson-Khand type reaction (PKR) of 34 yields a bicyclopentenone 35. The success of this transformation reflects the right combination of catalysts which are compatible with each other because the allylic amination can be facilitated by the electron-rich palladium(O) catalyst and the PKR needs a Lewis-acidic catalyst. Trost et al. reported the one-pot enantioselective... 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Peng et al. report the dimerization of styrene in ILs using palladium-Lewis acid catalysts and compare these results for the same dimerization in organic solvents. The stoichiometry of the reaction was given by the following. Scheme 6 ... 

Lewis acid catalysts activate the aldehyde by coordination to the carbonyl oxygen. Shibasaki et al. [13] were able to demon,strate that the activation of the enol ether is possible too. The reaction of the aldehyde 37 with the silyl enol ether 38 in the presence of the catalyst 39 proceeds with good, but still not excellent enantioselectivity to yield the aldol adduct 40. Only 5 mol % of the chiral palladium(II) complex 39 was used (Scheme 6a). Activation of the Pd(lI)-BINAP complex 39 by AgOTf is necessary. Therefore, addition of a small amount of water is important. 

Oraganosilicon compounds react with a wide variety of organic electrophiles in the presence of a palladium catalyst and a Lewis base activator such as a fluoride or hydroxy ion to give the corresponding coupled products. The reaction is applicable to synthesis of diynes, enynes, arylacetylenes, alkenylarenes, biaryls, allylarenes and alkylarenes in addition to 1,3-, 1,4- and 1,5-dienes with tolerance for various functional groups. 

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

The main product of the carbonylation of either 1- or 3-methoxyoctadiene is the linear ester. While the reaction is retarded by halide ligands, the use of a strong Bronsted or Lewis acid lead to improved yields. Screening of different catalysts for the carbonylation step showed that [(methyl-2-allyl)PdCl]2 with [Bu4N]Bp4 as co-catalyst is superior to conventional palladium catalysts. 

The most active nickel and palladium catalysts are either ionic or contain a Lewis acid as a co-catalyst. In the case of palladium, activation has been reported in the presence of BF3 OEt2 [17, 18] while ionic species have been prepared by reacting [( -2-MeC3H4)Pd(cod)] BF4 with a donor ligand [20] or by treating the appropriate halide with a silver salt (e. g., eq. (2)) [16, 19, 20]. 

Shirakawa and Hiyama found that transition metals such as palladium and nickel are efficient catalysts for a series of carbostannylations, the first one of which is the palladium-catalyzed alkynylstannylation of alkynes described in the following section. Both palladium and nickel catalyze the allylstannylation of alkynes (Schemes 5.7.6 and 5.7.7). In contrast to the methods using a radical initiator or a Lewis acid catalyst, these additions proceed with. yyn-selectivity. Both catalysts complement each other in terms of the range of alkynes that can be employed palladium prefers electron-deficient alkynes. 

Palladium catalysts with diimine ligands based on 2,6-diisopropyl aniline polymerize ethene to a rubbery, highly branched polyethene with low glass transition temperatures [12 a]. The interest in these materials results from their possible appheation as a rubber modifier in engineering plastics [3j,k]. However, the activity of the palladium catalysts is not satisfactory for technical use in a world scale plant We intended to improve the activity by increasing the Lewis acidity of the metal center by using relatively electron-deficient bromo phenyl diimine hgands. 

Catalysts (25) are the Lewis acid-Lewis base bifunctional catalysts in which Lewis acid-Al(III) moiety activates acyl iminium ion and the Lewis base (oxygen of phosphine oxide) does TMSCN, simultaneously (Scheme 5.7). Halogen atoms at the 6-position enhanced both yields and enantioselectivity in Reissert-type cyanation of the imino part of 26. However, the order for the activation is not parallel to the electronegativity of the halogen atoms and, moreover, the strong electron-withdrawing trifluoromethyl group provided unexpectedly the worst result for the activation [13]. It is not simple to explain this phenomenon only in terms of the increased Lewis acidity of the metal center. Trifluoromethylated BINOL-zirconium catalysts (28) for asymmetric hetero Diels-Alder reaction (Scheme 5.8) [14], trifluoromethylated arylphosphine-palladium catalyst (32) for asymmetric hydrosilylation (Scheme 5.9) [15], and fluorinated BINOL-zinc catalyst (35) for asymmetric phenylation (Scheme 5.10) [16] are known. 

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

Huser and Perron have extended this work to the isomerization of 2-methyl-3-butenenitrile (2M3 BN) to 3-PN (isomerization step Eq. (6) 92% yield) [17]. This patent mentions the use of iron and palladium catalysts but does not provide examples beyond nickel. In other work these same inventors discuss the use of other water-soluble ligands such as those containing carboxylate, phosphate, and alkyl-sulfonate substituents [18], while also exploring a wide range of Lewis acid co-catalysts for the addition of HCN to 3-pentenenitrile (Eq. 7) [19]. In general, the addi-... 

Hydrocyanation is the formal addition of hydrogen cyanide to alkenes. alkynes, and dienes to yield nitriles. These reactions can be catalyzed by various copper, cobalt, nickel and palladium catalysts modified with phosphanes and phosphites and/or Lewis acids. Hydrocyanation of carbonyl groups in aldehydes and ketones is covered in Section D.l.3.7. 

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