Catalytic reaction schemes

In the simplest catalytic reaction scheme (Figure 2.16) a fast and reversible couple, P/Q serves as catalyst (mediator) for the reduction (taken as an example, transposition to oxidation being straightforward) of the substrate A. Instead of taking place at the electrode surface, electron transfer to A occurs via... 

These NMR experiments provided great insight into the catalytic reaction. Whereas the catalytic reaction requires a high reachon temperature of 100 °C, aU three transformations in Scheme 3.5 proceed at 25 °C. In the same paper [16], Hayashi answered the question of why the catalytic reaction does not take place at the lower temperature. An outline of the reason is illustrated in Scheme 3.6. In the catalytic reaction, Rh(acac)(BINAP) is involved as a significant intermediate, because Rh(acac)(C2H4)2 is used as the rhodium precursor. It was confirmed that the hydroxo-rhodium complex is immediately converted into Rh(acac) (BINAP) by the reaction with 1 equiv. acetylacetone at 25 °C, in which the transmetallation from boron to rhodium is very slow at the same temperature. Thus, the acetylacetonato Hgand inhibits the catalytic reaction (Scheme 3.6, path a). 

According to Shaipless, two cycles operate in the catalytic reaction (Scheme 39) (88c, 9CF). The first cycle is highly enantioselective, whereas the second is poorly enantioselective. Hydrolysis of the key intermediate formed from B and oxidant is not very fast. The second osmylation of olefinic substrate occurs as the intermediate enters the undesired catalytic cycle. Therefore, slow addition of olefinic substrates to minimize the second cycle is essential for obtaining high ee. Use of potassium hexacyanoferrate(III) as oxidant in a 1 1 tert-butyl alcohol-water two-layer system can suppress the second cycle and lead to high enantioselectivity (91). This procedure allows the convenient synthesis of 3-lactams from 2-octenoate. 

At temperatures above ca. 200°C, the decarbonylation reaction can be driven catalytically (1,4,14, 20). Scheme I illustrates the proposed catalytic reaction scheme (15,16). This catalytic reaction is slow (activity for benzaldehyde decarbonylation at 178°C is 10 turnovers hr-1) presumably because the oxidative addition of RCOX to RhCl(CO)(PPh3)2 is difficult (7, 21, 22). Consistent with this, the rate is significantly greater when IrCl(CO)(PPh3)2 is used as the catalyst (benzaldehyde, 178°C, activity is 66 turnovers hr-1) (23). Oxidative addition to iridium complexes is well known to be more facile than with their rhodium analogues. 

Organic electrochemical reactions are classified in the same way as other organic reactions [1,2]. The most important prototypes include additions (Scheme 6.1) [ 13,14], eliminations (Scheme 6.2) [15, 16], substitutions (Scheme 6.3) [17, 18], couplings and dimerisations (Scheme 6.4) [19-21], cleavages (Scheme 6.5) [22,23], and catalytic reactions (Scheme 6.6) [24,25]. Hundreds of other examples maybe found in the literature [1,2]. 

FIGURE 3.3. Proposed phase transfer catalytic reaction scheme between CsHjONa and DPPC in the W/NB system. 

This corresponds to the activation energy Ea2 of the elementary step of the product P elimination from intermediate K-, and equals approximately (to an accuracy of RT) the heat of the formation of the transition state of elementary reaction 2 from the standard state of intermediate Ki (Figure 4.2A). Note that here and in other examples of catalytic reaction schemes with the high occupation of the active center with intermediates the value of the apparent activation energy does not follow the statement in Section 1.4.5 on the apparent activation energies of noncatalytic consecutive processes. 

Catalytic reaction schemes for laccase and ceruloplasmin have been formulated on the basis of the mechanistic studies and the state of characterization of the copper redox centers at this time. They are outlined in the reviews on laccase by Reinhammar (10) and on ceruloplasmin by Ryden (26). The degree of correctness of these reaction schemes is rather limited due to the fact that the structure and spatial arrangement of the copper centers were unknown at this time. 

The process chemistry of the methanol carbonylation reaction is summarized in Scheme 1. This catalytic reaction scheme depicts the balanced relationship between the methanol carbonylation, the WGSR and the iodide cycles under both regimes of water concentration. Within the scope of methanol carbonylation in an aqueous/acetic acid medium, the overall reaction rate depends not only on the nature of the rate-determining step(s), but also on reaction conditions influencing the steady-state concentration of the active Rh species, [Rh(CO)2l2]. ... 

In the catalytic reaction scheme, a species Z, usually nonelectroactive, reacts in the following chemical reaction to regenerate starting material. Thus the problem would involve consideration of a second-order reaction and the diffusion of species Z. 

It is obvious from the above discussion that the existing number of observations is insufficient to allow the formulation of a general catalytic reaction scheme for these enz5mies. In spite of the differences in the presteady state behavior of fungal and Rhus laccases, it seems unlikely that they utihze different catalytic mechanisms, and the two enzymes probably behave identically during steady state catalysis. Elucidation of the mechanism will thus require knowledge of their steady state structure and behavior. 

The autocatalytic uric acid mediated catalytic reaction (Scheme 5.4) can be avoided if voltammetry is restricted to potentials below the uric acid/uric acid... 

In palladium series, Ritter has described the formation of dinuclear Pd(III) intermediates 15 [61] in Pd-catalyzed aromatic C-H acetoxylation reaction of phenylpyridine previously reported by Sanford [62]. Ritter demonstrated, thanks to a thorough experimental and theoretical investigation, that bimetallic redox synergy between the two metals is responsible for the facility of the reductive elimination step involved in this kind of catalytic reaction (Scheme 14) [63]. 

More recently, specific reactors are used for nonsteady-state studies of catalytic kinetics which also allow observation of the intermediate steps in a reaction mechanism [22]. Transient studies of catalytic reaction schemes and kinetics are discussed in Chapter 10. 

Laurent E, Delmon B. Study of the hydrodeoxygenation of carbonyl, carboxylic and guaiacyl groups over sulfided CoMo/(-Al203) and NiMo/(-Al203) catalysts. I. Catalytic reaction schemes. Appl. Catal. A 1994 109 77. 

Independently from the synthetic way employed, the imido-halide clusters react with CO under very mild conditions to afford phenylisocyanate. Interestingly, the order of efficiency of halides in this reaction is the same as foimd in our catalytic reactions (Scheme 11) ... 

In recent years the nonsteady state mode has been used to an increasing extent because it permits accessing intermediate steps of the overall reaction. Very complete reviews of this topic are presented by Mills and Lerou [1993] and by Keil [2001]. Specific reactors have been developed for transient studies of catalytic reaction schemes and kinetics. One example is the TAP-reactor ( Transient Analysis of Products ) that is linked to a quadrupole mass spectrometer for on line analysis of the response to an inlet pulse of the reactants. The TAP reactor was introduced by Cleaves et al. in 1968 and commercialized in the early nineties. An example of appUcation to the oxidation of o.xylene into phthalic anhydride was published by Creten et al. [1997], to the oxidation of methanol into formaldehyde by Lafyatis et al. [1994], to the oxidation of propylene into acroleine by Creten et al. [1995] and to the catalytic cracking of methylcyclohexane by Fierro et al. [2001], Stopped flow experimentation is another efficient technique for the study of very fast reactions completed in the microsecond range, encountered in protein chemistry, e.g., in relaxation techniques an equilibrium state is perturbed and its recovery is followed on line. Sophisticated commercial equipment has been developed for these techniques. 

Various catalytic cycles of dihydroxylation have been proposed by Sharp less [34] and Corey [35]. According to Sharpless, two cycles operate in the catalytic reaction (Scheme 14.7). The first one is highly enantioselective, whereas the second cycle, which includes formation of Os complex with two moles of alkene IC-ID, is poorly enantioselective [36]. 

The procedure involves copper(I)-catalysed conjugate addition of MeMgBr to an a,p-unsaturated thioester 4, followed by reduction to the corresponding aldehyde and subsequent olefination to elongate the chain. This yields anew a,p-unsaturated thioester 6, which, in turn, can be subjected to the next catalytic reaction (Scheme 5). 

As follows from Fig. 10.5 enzymes entrapped in soluble IPEC can easily be separated from the reaction system, i.e. from the reaction products by appropriate change of pH or ionic strength. Compacting and transition of enzyme/IPEC species into an insoluble state immediately terminates or strongly inhibits a catalytic reaction. Scheme VII illustrates the molecular mechanism of these transitions. 

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