Cocatalytic effect

The mechanism of the cocatalytic effect is still a matter of investigation. For most of the systems of interest in electrocatalysis, data for characterization of the surface by means of spectroscopic UHV methods are still missing. Also measurements of changes in the electronic properties of the metal in the presence of adatoms in addition to more intensive application of in situ and on-line methods are desirable for a systematic search of new catalytic materials. 

The cocatalytic effects of pinacol in the phase transfer catalysis (PTC) of dihalocarbene additions to alkenes were noted by Dehmlow and co-workers who showed that pinacol accelerates the PTC deprotonation of substrates up to pKa 27.7 Dehmlow also studied the effects of various crown ethers as phase transfer catalysts in the addition of dibromocarbene to allylic bromides.8 In Dehmlow s study, elevated temperature (40°C) and dibenzo-18-crown-6 did not give the highest ratio of addition/substitution to allyl bromide. However, the submitters use of pinacol,... 

Water participates in carbocation formation59,60 by generating the strong acid H[Al(OH)Hlg3],6,59 which forms a carbocation through an alkyl halide intermediate60 or via direct hydride ion abstraction.6 Experiments with heavy water by Pines further proved its cocatalytic effect indicating that HC1 (HBr) formed is not the real activator.60,61... 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

The Feltzin mechanism 73) takes account of the presence of proton donors at the beginning of copolymerization. However, initiation probably proceeds in two ways 74) and depends on the type of the proton donor and its concentration in the copolymerization mixture. If HA in Eq. (45) is alcohol, phenol or moisture, initiation occurs according to Eq. (46), i.e. through interaction with the anhydride yielding an ammonium salt of the monoester. The formation of monoesters as primary active centres accounts here for the lower cocatalytic effect of phenols as compared with alcohols. If the proton donor is a carboxylic acid, activation of the tertiary amine (Eq. (63)) is followed by reaction with the epoxide according to Eq. (76)74. ... 

Figure 2. The cocatalytic effects of various charge carriers on acetylene reduction by the Mo(V)-cysteine complex (0.42 mmol) and NaBH4 (1.0 mmol), plotted against the reduction potentials (V vs. SHE) of the corresponding additives.

Cocatalysis with copper salts has proven its efficiency in the enhancement of the reaction rate, particularly in polar solvents. Discovered by Liebeskind and Fengl , and largely exploited, the cocatalytic effect of Cu(I) depends on the reaction conditions in two... 

Lecithin has some catalytic or cocatalytic effects in multiphase systems because of its surface-active properties. Lecithin is reported to be useful as an emulsifier in the curing of aqueous dispersions of unsaturated polyesters (337). The products are more easily removed from their molds and have improved mechanical properties when lecithin is used. In a fermentation application, 1.5% soybean lecithin acts as an inducer in the preparation of cholesterol esterase using a strain of Pseudomonas bacteria (338). Aside from its role as a catalyst, initiator, or modifier, lecithin may have ancillary uses in catalyst systems as part of a protective coating (339). 

Aromatic hydrocarbons (benzene, toluene and p-xylene) also exert a cocatalytic effect on the conversion of aqueous methanol, which is limited to a narrow temperature range below that at which conversion is complete without the added aromatic. The effect 1s of a mechanistic rather than physico-chemical nature since ethylene with some C-label is obtained from unlabelled methanol in the presence of C-labelled benzene (ref. 17,18). 

Dehmlow, E. V.,H. C. Raths, and H. Soufi, Application of PTC Part 41 Cocatalytic Effects of Pinacol in Phase Transfer Catalysis, ... 

Besides the use as a model system, copper exchanged zeolites themselves are of technical interest, e.g. as hydrocracking catalyst components [1]. The copper ion redox behaviour is important for cocatalytic effects during catalyst activation (reduction of Fe-zeolites ) and catalyst regeneration. 

In principle, hydroxide anion is very difficult to transfer from aqueous to organic phases, yet it is one of the most valuable and most commonly used anions in the PTC systems. Addition of small amounts of alcohols to PTC systems requiring hydroxide transfer causes a dramatic increase in rates. Therefore, addition of alcohol enhances the PTC reaction as the cocatalytic effect. For example formation of alkoxide anions, RO, which are more readily transferred than the highly hydrated hydroxide anion, and which can serve as a strong base just as well as OH", and solvation of the hydroxide with alcohol rather than with water, making the hydroxide anion more organophilic and more easily transferred. ... 

In all four investigated systems, the measured increase of the yield of the intermediate is stronger compared to what is expected purely by the physical solvent effect (Table 14.4 calculation vs experiment ), indicating an additional cocatalytic effect of the IL. 

The change in the reaction rate due to the IL coating (given in brackets in column 4 of Table 14.3) can be estimated by Eq. (14.9a) for a > 0.1. Comparison with the measurements shows that the estimated rates are mostly slightly higher, indicating a cocatalytic effect of the IL. 

Sinn et al. (1961) pointed out that the effect of water in the stereoregular polymerization was like the cocatalytic effect of water in Friedel-Crafts polsmerizations. They concluded, therefore, that the ds polymerization of butadiene was a cationic coordinated mechanism. In this mechanism the monomer was first oriented by the cobalt, then released as the cationic end of the chain added. They suggested that the monomer did not isomerize because it was held in a caged structure. The orientation of the chain end was not considered. It should be noted that, if a relatively free carbonium ion existed, isomerization of the double bond would be expected according to Eq. (39). The cobalt must preserve, therefore, the stereochemistry of the chain end until addition of another monomer unit (Lehr, 1963). 

Kanki, K. Misumi, Y Masuda, T. Remarkable cocatalytic effect of organometaUics and rate control by triphenylphosphine in the Rh-catalyzed polymerization of phenylacetylene. Macromolecules 1999, 32, 2384-2386. 

The cocatalytic effects of the acetate buffer are only possible after ligand exchange with one of the aqua ligands of the catalyst. This process was studied computationally by Zheng and coworkers at the DFT level,by using... 

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