Lewis acids as co-catalysts

On the technical scale, olefin polymerization is based on the activation of a suitable zirconocene precursor, typically Cp2ZrMe2 or Cp2ZrCl2, by a strong Lewis acid as co-catalyst. Mechanistic studies on Ziegler-Natta-like polymerization have long focused on the role of cationic alkylzirconocene species in the catalysis. There have been previous mass spectrometric studies of isolated metallocene ions in the gas phase, most notably by Richardson and co-workers. The failure to observe olefin oligomerization in this study was attributed to incomplete thermaliza-... 

Since the rate enhancement of MBH reaction in protic solvents or by use of LBBA bifunctional catalysts and Lewis acids as co-catalysts is attributed to the involvement of hydrogen bonding, it seemed that imidazolium-based ionic liquids would be a good choice of solvent for the MBH reaction because the acidic proton at C(2) is known to act as a donor to hydrogen bond acceptors. ... 

Type real activation o-f the nitro compound. This type o-f interaction has been proposed -for the intermediate -formed during the catalytic carbonyl at i on reactions o-f n i troaromat i cs, carried out in the presence o-f a complex in a low oxidation state with a Lewis acid as co-catalyst l35j ... 

The conversion o f the nitro compound with R=R =Me to the isocyanate using a variety o-f palladium halide complexes has also been studied. In a comparative study o-f the rhodiumwith Lewis acids as co-catalysts, the activity and selectivity have been -found to -follow the order (Bu N) fPdCl33-U or VO chlorides > [Rh Rh... 

A recent example with a benzoyl derivative is provided by the asymmetric chlorination of 3-aryl-N-Boc-oxindoles with N-chlorosucdnimide as the chlorine source and C9 0-benzyl quinidine 20 as catalyst (Scheme 6.24) [53]. Comparably, benzoyl esters of quinine were applied with metal-containing Lewis acids as co-catalysts for enantioselective [44-2] cycloadditions-for example, the addition of ketene eno-lates to ortho-quinone [54]. 

Friedel-Crafts reactions are electrophilic substitution reactions in which the electrophile is a Ccirbocation or an acylium ion. The removal of a halide ion from an alkyl halide is the means of generating the ceirbocation. An acylium ion is created by removing a chloride ion from an acid chloride (R-CO-Cl). Both of these processes require a Lewis acid as a catalyst. The most commonly used Lewis acid is aluminum chloride. 

In 2009, Wang and co-workers developed a new Lewis base-Lewis base bifunctional catalysis for the AFC alkylation of indoles with o,p-unsaturated aldehydes with high efficiency and enantioselectivity. In contrast to the reported system utilizing protic acids as co-catalysts, this protocol demonstrated that it was possible to carry out AFC alkylation reaction in the presence of (S)-104 and triethylamine. The addition products 117 were obtained in good to excellent yields (66-95%) with up to 98% ee (Scheme 6.47). 

Keim and co-workers have carried out various alkylation reactions of aromatic compounds in ionic liquids substantially free of Lewis acidity [84]. An example is the reaction between benzene and decene in [BMIM][HS04], which was used together with sulfuric acid as the catalyst (Scheme 5.1-54). These authors have also claimed that these acid-ionic liquids systems can be used for esterification reactions. 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

To understand how to control process conditions to give methyl, 4-pentadienoate, the reaction mechanism must be examined. (See Equation 2.). -palladium hydride elimination from 4 gives rise to trans and cis-methyl penta-, 4-dienoate which is the desired monocarbonylation intermediate for sebacic acid. The desired mono-carbonylation reaction is promoted by low carbon monoxide pressure ( 1000 psig) while high pressure (1800 psig) gives excellent 1,4-dicarbonylation product yield. The mono-carbonylation reaction is also facilitated by using a Lewis Acid as a co-catalyst and iodide as the preferred palladium counter-ion (Table III.). Chloride is the preferred palladium counter-ion for 1,4-dicarbonylation. 

In addition to the familiar and extensive role perfluoroaryl boranes and borates fill as co-catalysts for the coordination polymerization of ethylene and a-olefins, these compounds can act as initiators for other polymerization reactions due to their high Lewis acidity. 

The results mentioned above prompted us to synthesize a more simplified catalyst, scandium tris(dodecyl sulfate) (Sc(DS)3) [23,24]. This new type of catalyst, Lewis acid-surfactant-combined catalyst (LASC) , was expected to act both as a Lewis acid to activate the substrate molecules and as a surfactant to form emulsions in water. Eng-berts and co-workers also reported a surfactant-type Lewis acid, copper bis(dodecyl sulfate) (Cu(DS)2) [25]. Although they studied detailed mechanistic aspects of Diels-Alder... 

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]. 

Sinou and co-workers [73] studied the influence of different surfactants on the palladium-catalyzed asymmetric alkylation of l,3-diphenyl-2-propenyl acetate with dimethyl malonate in presence of potassium carbonate as base and non-water-soluble chiral ligands. Best results in activity and enatioselectivity (> 90% ee) were observed with 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as ligand and cetyltrimethylammonium hydrogen sulfate as surfactant in aqueous medium. Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [74]. An acceleration of the reaction was indicated in presence of SDS as anionic surfactants. An additional promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant-combined catalysts) as shown in Eq. (3). Surfactant the anion of dodecanesulfonic acid. 

Water is likely to be present in aU practically relevant catalytic applications unless extreme precautions are taken or the system is self-drying (e.g., due to the fact that strong Lewis-acids or metal alkyls are used as co-catalysts). Water will influence the ionic liquid s thermal stability significantly if any part of the ionic liquid is prone to hydrolysis. Apart from the weU-known hydrolysis lability of tetrafluoroborates and hexafluorophosphates, water will thus also affect the stability of ester functionalities in the ionic liquid, e.g. the stability of alkyl sulfate anions. The presence of Bronsted acidity in the reaction system will further promote this kind of thermally induced hydrolysis reaction. Additionally, in strong Lewis-acidic ionic liquids care has to be taken to avoid incompatibilities between oxygen and nitrogen functionalities in the reactants or impurities and the ionic liquid s Lewis acidic group (usually a complex anion). It is for example obvious that the Pd-catalyzed dimerization of methylacrylate caimot be carried out in acidic chloroaluminate ionic hquids since the ionic liquid s anion would decompose in an irreversible reaction with the substrate methylacrylate. 

Water-stable Lewis acids as catalysts for aldol reactions were developed by Kobayashi and co-workers [22]. A high promotion could be observed by combination of Lewis acid and surfactant (LASCs = Lewis acid-surfactant combined catalysts as shown in Eq. (2)). The surfactant is here the anion of dodecanesulfonic add. 

Electrophiles do not have to be cations tris(pentafluorophenyl)borane B(C6Fs)3 and related Lewis acids are very efficient hydride and alkyl anion abstractors and have found important applications as co-catalysts in the homogeneous Ziegler-Natta polymerization of olefins.The quest for better activators with potential... 

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