Acid cocatalyst

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

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]. 

In addition, acid cocatalysts can assist the formation of the enamine. With very basic, nucleophilic amines, such as pyrrolidine and its derivatives, acid catalysis is not necessarily required for enamine formation. However, with less basic amines, Brpnsted or Lewis acids are often used to assist in enamine formation (Scheme 7). 

Momiyama and Yamamoto have recently demonstrated that acid cocatalysts can even influence the outcome of enamine-mediated reactions [63]. In their studies of the acid-catalyzed O- and A-nitroso aldol reaction, they found that the nature of the acid catalyst dictates the regioselectivity of the reaction between preformed enamine species A carboxylic acid catalyst promoted the 0-nitroso aldol reaction whereas a hydrogen bonding catalyst catalyzed the formation of an A-adduct, both in high enantioselectivities(Scheme 10). 

Carbonylation of Alcohols - Pd(tppts)3 catalyses the carbonylation of benzylic alcohols to the corresponding phenylacetic acids, in the presence of a Bronsted acid cocatalyst such as H2S04 or p-CH3C6H4S03H in biphasic aqueous/organic media (no organic solvent).305,451 For example, benzyl alcohol was converted to phenylacetic acid (Equation 6) and l-(4-isobutylphenyl)ethanol (IBPE) to ibuprofen (Figure 9). 

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... 

The majority of catalytic carbonylations employ palladium catalysts and the water soluble complex, Pd(tppts)3, is easily prepared by reduction of PdCl2/tppts with CO in water at room temperature [39]. Hence, this complex is readily generated in situ, under carbonylation conditions. It was shown to catalyze the car-bonylation of alcohols [40, 41] and olefins [42-44], in the presence of a Bronsted acid cocatalyst (Fig. 7.10). 

The reaction is proposed to involve the formation of an intermediate carbe-nium ion (hence the need for an acid cocatalyst) which reacts with the Pd(0) complex to afford an alkylpalladium(II) species (see Fig. 7.11) [44]. 

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,... 

Indeed, a combination of tributyltln hydride, Pd° catalyst and a weak acid, such as ammonium chloride, forms an effective, yet mild tool for conjugate reduction of a, -unsaturated aldehydes and ketones. Similar results are obtained with other acidic cocatalysts, such as zinc chloride, acetic acid and tributyltln triflate. With this system, reductions occur with high regloselectivity, providing a useful approach for deuterium incorporation into either the P- or a-position by using either tributyltln deu-teride or D2O, respectively (Scheme 57). ... 

Later, the scope of this methodology was successfully extended to the intramolecular version by List and coworkers [14]. By employing 9-amino-9-deoxyepiquinine 24 as a catalyst (20 mol%) and an acid cocatalyst (AcOH, 60 mol%), 5-substituted-3-methyl-2-cydohexene-l-ones (26) were obtained with high enantioselectivity (up to 94% ee) from the diketones 25 via the intramolecular aldol reaction (Scheme 8.8). The chiral enones 26 are valuable synthetic building blocks for the synthesis of many biologically important compounds (e.g., HIV-1 protease-inhibitive didemnaketals). The pseudoenantiomeric quinidine analogue 23 of 24 also provided the opposite... 

In 2008, Ye and coworkers also developed a new type of multifunctional cinch-onidine-based catalyst, such as 119 having an additional primary amine moiety, for the Michael addition of nitroalkane to cydic enones [32], In the presence of an acid cocatalyst, the primary amine moiety of 119 can act as a Lewis base to activate the Michael acceptor via iminium formation. The catalysts 119a and 119b (5 mol%) provided quite excellent enantioselectivity (up to 98% ee) for the Michael addition of nitroalkanes to cyclohexenone (Scheme 9.40). The observed retardation of the reaction rate and the opposite sense of enantioselectivity obtained with the catalyst 119b indicated the importance of the configuration of the cydohexane... 

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... 

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