Lewis cocatalyst

The Lewis acids must be used with a protonic cocatalyst such as water or methanol which generates protons through the following kinds of equilibria ... 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Strong protonic acids can affect the polymerization of olefins (Chapter 3). Lewis acids, such as AICI3 or BF3, can also initiate polymerization. In this case, a trace amount of a proton donor (cocatalyst), such as water or methanol, is normally required. For example, water combined with BF3 forms a complex that provides the protons for the polymerization reaction. 

Figure 36 Organoboron polymers of PS with well-defined boron-containing Lewis acids for use as a cocatalyst in metallocene-catalyzed olefin polymerizations. (Adapted from ref. 81.)... 

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

Marks TJ, Yang S, Stem CL, Chen YXE (1996) Organo-Lewis acids as cocatalysts in cationic metallocene polymerization catalysis. Unusual characteristics of sterically encumbered Tris(perfluorobiphenyl)borane. J Am Chem Soc 118 12451-12452... 

The Lewis acidity and reactivity of these alkyl aluminum cocatalysts and activators with Lewis basic polar monomers such as acrylates make them impractical components in the copolymerization of ethylene with acrylates. To address this shortcoming, Brookhart et al. developed well-defined cationic species such as that shown in Fig. 2, in which the counterion (not illustrated) was the now-ubiquitous fluorinated arylborate family [34] such as tetrakis(pentaflurophenyl)borate. At very low methyl acrylate levels the nickel catalysts gave linear copolymers but with near-zero levels of acrylate incorporation. 

R AlB3- (B = OR or NR2). As can be seen from Table XI, the above combination also caused a substantial drop in the catalyst activity, which is expected with a lower overall Lewis acidity of the mixed cocatalyst. The effect of these cocatalysts on the trans/cis ratio of the product is very dependent on the ratios of the two aluminum components. The combination which enables the catalyst system to produce the highest proportion of franj-hexadiene had R2A1B/RA1C12 = 1.5-2 (Table XIII). 

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

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). 

BF3 + H2O Lewis acid Lewis base catalyst cocatalyst... 

The active site is a cationic metallocene alkyl generated by reaction of a neutral metallocene formed from reaction with excess MAO or other suitable cocatalysts such as a borane Lewis acid. This sequence is shown in Figure 5.1 employing MAO with ethylene to form PE. Initiation and propagation occur through pre-coordination and insertion of the ethylene into the alkyl group polymer chain. Here, termination occurs through beta-hydride elimination... 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

Water, alcohols, ethers, or amines can cause inhibition of ionic polymerization. However, these substances can act in different ways according to their concentration. For example, in polymerizations initiated by Lewis acids (BF3 with isobutylene) or organometallic compounds (aluminum alkyls), water in small concentrations behaves as a cocatalyst, but in larger concentrations as an inhibitor (reaction with the initiator or with the ionic propagating species). 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Reactions of the same carbonyl ylide intermediate with aldehydes are even more fruitful. The Rh2(OAc)2 catalyzed reaction proceeds at room temperature in the presence of 2 mol% of the catalyst, but the diastereoselectivity is disappointingly low (endo/exo = 49 51, Scheme 11.56). However, when 10 mol% of the cocatalyst Yb(OTf)3 is added, the reaction becomes highly exo-selective (endo/ exo = 3 97) (198). Suga has extended this Lewis acid catalyzed carbonyl ylide cycloaddition reaction to catalyzed asymmetric versions. The chiral cocatalyst employed is ytterbium(III) tris(5)-1,1 -binaphthyl-2,2 -diyl phosphonate, Yb[(S) BNP]3 (10 mol%). In the reaction of methyl o-(diazoacetyl)benzoate with benzyloxyacetaldehyde in the presence of Rh2(OAc)2 (2 mol%) at room temperature with the chiral Yb catalyst, the diastereoselectivity is low (endo/exo = 57 43) and the enantiopurity of the endo-cycloadduct is 52% ee. 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

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