Simultaneous catalysis

A possibility that was proposed quite early for the glucose mutarotation, and that could conceivably be of importance for other reactions, is simultaneous catalysis by an acid and a base. It will be recalled from Section 8.1 that hydration requires addition of a proton at one site and removal of a proton from another. If both these processes were to occur in one step, either by means of separate acid and base molecules acting together or by action of a single molecule containing both an acidic and a basic center, we would designate the process as a concerted acid and base catalysis (Equation 8.39).60 Swain found that the rate of... 

We developed an assay for the cleavage of uridyluridine (39, UpU) by various catalysts, and used it to study the cleavage of this dimeric piece of RNA [126]. We saw that high concentrations of imidazole buffer could catalyze this cleavage, mimicking the high effective local concentrations of imidazole in the enzyme, and concluded that with this buffer there was sequential base, then acid, catalysis [127]. Of course, simultaneous catalysis by two different buffer species by a three-body collision is unlikely unless they are linked in the same catalyst - the enzyme or the artificial enzyme. 

Sodium TV-chloro-/ -toluenesulfonamide (chloramine T, CAT) is a stable N—Cl derivative, which has been widely used in organic synthesis, particularly as an oxidant768. Kinetic study on the chlorination of / -aminobenzoic acid by CAT reveals simultaneous catalysis by H+ and Cl"769. 

The coupled reaction concept adequately illustrates the principle of energy conservation and utilization in living cells. However, the actual mechanisms of energy coupling in living cells seldom involve the simultaneous catalysis of two reactions. Instead, the net effect is generally obtained by catalyzing two... 

Equations (7) and (8) are special cases for aqueous solutions of the equation for generalized acid-base catalysis. As shown by Lowry 54a), the muta-rotations of sugars are reactions involving simultaneous catalysis by both acids and bases, in the generalized concept of acids and bases proposed by Lowry and by Bronsted. Water functions as a complete catalyst because of its amphoteric dissociation into ions H20<- H+ + OH. Acids or bases alone are not effective catalysts but in mixture are complete catalysts. 

The oxidation of aspirin by A-sodio-A-bromobenzenesulfonamide (bromamine-B, BAB) in aqueous HCIO4 is first order in BAB, fractional order in aspirin and inverse fractional order in H+. The reaction proceeds via decarboxylation, bromination, and AcOH loss, affording the product 2,4,6-Br3CeH20H. The oxidation of D-cycloserine (CS) by BAB in HCl is first order in BAB and fractional order each in CS and H+. The proposed mechanism assumes simultaneous catalysis by H+ and Cl ions and is consistent with the observed kinetic results. The oxidation of diethylamine by BAB catalysed by ruthenium(III) is first order in oxidant, catalyst and substrate, but inverse fractional order in H+. A mechanism in which the metal coordinates the nitrogen atom of the amine before a slow electrophilic attack on the nitrogen by BAB, elimination of HBr, attack by water, and disproportionation affords acetaldehyde and ethylamine, which undergoes a similar oxidation. " ... 

In all surfactant solutions 5.2 can be expected to prefer the nonpolar micellar environment over the aqueous phase. Consequently, those surfactant/dienophile combinations where the dienophile resides primarily in the aqueous phase show inhibition. This is the case for 5.If and S.lg in C12E7 solution and for S.lg in CTAB solution. On the other hand, when diene, dienophile and copper ion simultaneously bind to the micelle, as is the case for Cu(DS)2 solutions with all three dienophiles, efficient micellar catalysis is observed. An intermediate situation exists for 5.1c in CTAB or C12E7 solutions and particularly for 5.If in CTAB solution. Now the dienophile binds to the micelle and is slid elded from the copper ions that apparently prefer the aqueous phase. Tliis results in an overall retardation, despite the possible locally increased concentration of 5.2 in the micelle. 

Acid catalysis is an important kinetic phenomenon, and its study often requires the use of concentrated acid solutions, in which the conventional pH scale is not applicable. In sueh solutions (e.g., sulfuric acid-water mixtures covering the full range of compositions) the acid component simultaneously functions both as an acid and as a solvent thus, a medium effect is superimposed on the acidity effect. In this section we briefly describe the acidity function approach to coping with this problem. (A comparable approach can be taken to the study of highly... 

The applications of quantitative structure-reactivity analysis to cyclodextrin com-plexation and cyclodextrin catalysis, mostly from our laboratories, as well as the experimental and theoretical backgrounds of these approaches, are reviewed. These approaches enable us to separate several intermolecular interactions, acting simultaneously, from one another in terms of physicochemical parameters, to evaluate the extent to which each interaction contributes, and to predict thermodynamic stabilities and/or kinetic rate constants experimentally undetermined. Conclusions obtained are mostly consistent with those deduced from experimental measurements. 

As shown above, quantitative structure-reactivity analysis is very useful in elucidating the mechanisms of cyclodextrin complexation and cyclodextrin catalysis. This method enables us to separate several intermolecular interactions, acting simultaneously,... 

The inequality indicates that if a concerted mechanism (where b4 and b2 change simultaneously) gives a Ag which is much lower than our stepwise estimate, we will have smaller Ag< age. This possibility, however, is not supported by detailed calculations (Ref. 6). Direct information about Ag age can be obtained from studies of model compounds where the general acid is covalently linked to the R-O-R molecules. However, the analysis of such experiments is complicated due to the competing catalysis by HaO+ and steric constraints in the model compound. Thus, it is recommended to use the rough estimate of Fig. 6.8. If a better estimate is needed, one should simulate the reaction in different model compounds and adjust the a parameters until the observed rates are reproduced. 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

Both of the components of MOFs, metal ions and ligands, can be involved in heterogeneous catalysis. The framework as a whole can also serve as a heterogeneous carrier that hosts catalytic functionahties. The central question of this section is whether it is possible to introduce reactive centers that are designed to transform organic molecules and that wiU simultaneously not affect the organic components of the frameworks themselves. 

Recently, recombinant biocatalysts obtained using Escherichia coli cells were designed for this process. The overexpression of all enzymes required for the process, namely, hydantoinase, carbamoylase, and hydantoin racemase from Arthrobacter sp. DSM 9771 was achieved. These cells were used for production of a-amino acids at the concentration of above 50 g 1 dry cell weight [37]. This is an excellent example presenting the power of biocatalysis with respect to classical catalysis, since a simultaneous use of three different biocatalysts originated from one microorganism can be easily achieved. 

EfiBdent hydrogen supply iiom decalin was only accomplished by the si terheated liquid-film-type catalysis under reactive distillation conditions at modaate heating tempaatures of 210-240°C. Caibcm-supported nano-size platinum-based catalysts in the si ietheated liquid-film states accelerated product desorption fixjm file catalyst surface due to its temperature gradient under boiling conditions, so that both hi reaction rates and conversions were obtained simultaneously. 

The HKR reactions follow the cooperative bimetallic catalysis where epoxide and nucleophile activate simultaneously by two different (salen)Co-AlX3 catalyst molecules. The linking of two (salen)Co unit through the A1 induces the cooperative mechanism, albeit through a far less enantio-discriminating transition state than that attained with the catalyst la and la (Scheme2). 

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