Lewis acid cocatalyst

Catalytic activity in olefin polymerization is related to the presence of cationic metal-hydrocarbyl species [90], which can be obtained by (i) using oxide supports that have high Br0nsted and Lewis acidity, (ii) the addition of a co-catalyst to a neutral supported species or (iii) modification of the surface with Lewis acid cocatalysts prior to grafting of the metal-hydrocarbyl species (Scheme 11.8a-c) [91-97]. 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

In the absence of Lewis acids, further hydrocyanation of the monoolefin products does not readily occur. However, the addition of a Lewis acid cocatalyst allows pentenenitriles (PN s) to be hydrocyanated to dinitriles. When BD and 4PN are hydrocyanated together with Ni[P(0-p-tolyl)3]4 and ZnCl2 at 80°C, BD hydrocyanates 20 times faster than 4PN. 

Balan and Adolfsson [28] reported a direct catalytic enantioselective three-component aza Baylis-Hillman reaction between arylaldehydes, tosylamides, and Michael acceptors using the quinidine-based Hatekayama catalyst 96 [29] together with titanium isopropoxide as a Lewis acid cocatalyst (Scheme 9.18). High chemical yields and stereoselectivity ranging between 49 and 74% ee were obtained using various substituted arylaldehydes. 

Very recently, the Nelson group expanded scope of this reaction by applying cinchona alkaloid-Lewis acid catalyst systems [142b]. In the presence of O-trimethyl-silylated quinine or quinidine, and LiCICN as Lewis acid cocatalyst, a broad range of aliphatic and aromatic aldehydes was converted into the corresponding... 

The titanium complexes 29 and 30 were found to polymerize ethylene after the addition of the Lewis add AlEtCl2- The reaction of 10 with methyl alumoxane resulted in the m-methyl bridged bimetallic compound 31, Eq. (31), which was able to polymerize ethylene in the absence of a Lewis acid cocatalyst, when dichloromethane was used as solvent... 

These 1987 resnlts concluded that classical metathesis catalyst systems were not sufficient and that Lewis acid cocatalyst-free systems were necessary if successM ADMET condensation polymerization were to become a reality. The key to snccessM ADMET polymerization was demonstrated " nsing the Lewis acid-free tungsten alkylidene metathesis catalyst (5a), the structure of which had been reported by Schrock et just one year earlier. When this... 

Initiation is facilitated by interaction of the Lewis acid with a second compound (called a cocatalyst) that can donate a proton or carbenium ion to the monomer. Typical eocatalysts are water, protonic acids, and alkyl halides. Examples of Lewis acid-cocatalyst interactions are given in reactions (9-29) and (9-32). The general reaction for... 

The use of cocatalysts is desirable and possibly absolutely neces.sary in many Lewis acid systems. The concentration of cocatalyst must be carefully controlled, however, and optimum Lewis acid/cocatalyst concentration ratios can be established for particular polymerizations. This is because the cocatalyst must be more basic than the monomer otherwise the Lewis acid would react preferentially with the monomer. If excess co-catalyst BA is present, however, it can compete with the monomer for reaction with the primary Lewis acid/cocatalyst complex. For example,... 

The expected cycloaddition of ,[i-unsaturated add chloride and chloral indeed occurred in the presence oftertiary amine. The yields could be significantly improved by adding a Lewis acid cocatalyst, Sn(OTf)2, which would facilitate the deprotonation of acid chloride and activate the aldehyde substrate. More satisfactory results could be obtained when acid chloride was added slowly by syringe pump to avoid massive... 

Stable propagating metal carbene complexes may also be observed when certain initiators of the type listed in Table 2.2 are used without a Lewis acid cocatalyst. The propagating species are living and addition of successive batches of different monomers can be used to make block copolymers see Ch. 14. The conversion of the living polymer derived from the first monomer, into the propagating species of the second monomer can be readily followed by H NMR. 

In 2012, Chi et al. disclosed an oxidative y-addition of enals to tri-fluoroacetophenone for the synthesis of unsaturated 5-lactones 114 under NHC catalysis. Scandium triflate/magnesium triflate as relatively strong Lewis acid cocatalysts were found to be effective for enantiocontrol involving the relatively remote enal y-carbon (Scheme 20.49). 

The direct proline-catalysed aldoi in the presence of water has been screened with water-compatible Lewis acid cocatalysts. Chlorides of zinc s group proved best, and the optimized formation of anti-products in >99% ee was obtained with L-proline/ZnCl2 in 4 1 DMSO/water. Adding cobalt(II) chloride as a co-catalyst to L-proline-promoted direct aldols substantially improves selectivity, giving yield/de/ee up to 93/96/99%. Cobalt(II) is proposed to preorganize two proUnes. (g)... 

The use of a co-catalyst was crucial to the development of practical hydrocyana-tion. The rate and catalyst lifetime for hydrocyanation of simple alkenes increases dramatically by conducting the reactions in the presence of a Lewis acid. As shown in Table 16.1, the reaction of propene occurs much faster in the presence of aluminum and zinc halides. Lewis acid cocatalysts also promote isomerization and selective additions during some steps of the hydrocyanation of butadiene. This effect is presented later in this section. 

Copper based systems are generally distinguished by their superior reactivity and their excellent compatibility with functional groups, whereas ruthenium complexes turned out to be less active despite the beneficial effect of Lewis acid cocatalysts. In view of the preliminary observations indicating that styrene polymerised under conditions used for olefin metathesis and/or cyclopropanation, we were prompted to probe the performance of some ruthenium complexes under ATRP conditions. 

Miscellaneous Reactions. Hydwcyanation of olefins was among the very first reactions investigated in aqueous organic biphasic systems. [Ni(TPPTS)4] prepared separately or in situ from Ni(II) salts and TPPTS catalyze efficiently the anti-Markovnikov addition of HCN to butadiene and 3-pentenenitrile and also the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile (Scheme 40). Lewis acid cocatalysts such as ZnCl2 facilitate the reaction. 

Nakao and Hiyama [185] have shown that allyhc cyanides 179 add across alkynes 178 in the presence of a Ni/P(4-CF3-C, H4)3 catalyst to give oHgosubstituted 2,5-hexadienenitriles 180 with defined stereo- and regiochemistry (Scheme 12.84). Use of AlMe2Cl or AlMej as a Lewis acid cocatalyst accelerates the reaction and expands the substrate scope significantly. 

After activation using a Lewis acid cocatalyst such as MAO, catalysts A, B, and C will each have two coordination sites. In C, the two coordination sites are diastereotopic owing to the plane... 

The role of the Lewis acid cocatalyst, in addition to its obvious role in catalyzing hydrolysis of the silyl enol ether, is probably that of polarizing the substrate, thereby facilitating migratory insertion of hydride into Ihe olefin (II to in in Scheme 17). 

The absence of functionality in hydrocarbon polymers limits applications where good adhesive properties, affinities for dyes, permeability, and compatibility with polar polymers are necessary. One of the limitations of conventional Ziegler-Natta catalysts is their intolerance of functional groups due to the high Lewis acidity of the transition metal component and the presence of Lewis acidic cocatalysts based on alkyl aluminums or aluminoxanes, as was anticipated in Section 2. 

Reaction conditions were 200 °C or less and at least 200 atm of carbon monoxide. Benzene, chlorobenzene, cyclohexane, or 1,1,2-trichloro-1,2,2-trifluoroethane were used as solvents. The noble metal could be supported, but even unsupported or in the form of salt, oxide, or complex. Among the Lewis acid cocatalysts used successfully were FeCU, FeCU, FeBrs, AICI3, AlBrs, SnCl4, CuCL, and anhydrous HCl, but the highest yields were obtained with supported metals and with FeCL as cocatalyst. The presence of FeCL allowed for the use of milder experimental conditions with respect to the monometallic system, also affording higher conversions and better selectivities [24-26]. For example, nitrobenzene (24.6 g), Rh/C (5 % on Rh, 5.0 g), anhydrous ferric chloride (0.4 g), under 500 atm of carbon monoxide and at 190 °C for 5.5 h in benzene (100 ml), gave 100 % conversion with formation of PhNCO (35 % after vacuum fractional distillation). Diphenylurea, PhNHC(0)NHPh, and 1,3,5-... 

The role traditionally assigned to the Lewis acid cocatalysts is to promote the insertion of CO into the metal-nitrogen bond of an imido complex, in a way analogous to what long known for the CO insertion into metal-carbon bonds (Scheme 6) ... 

The first well-defined tungsten(Vl) alkylidenes which serve as highly active metathesis inititors were reported by Osborn and co-workers in 1982 [149]. W(=CH-t-Bu)(OCH2-t-Bu)2X2 (13) in combination with AlBrs or GaBrs polymerizes a variety of cycloolefins [61,62,149-151]. Later it has been reported that the related cyclopentylidene complex W(=C(CH2)4)(OCH2-t-Bu)2Br2 polymerizes numerous cycloolefins, e.g., various methoxycarbonyl derivatives of norbomene, even without addition of a Lewis acidic cocatalyst [152-154]. 

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