Homogeneous Catalytic Reaction Schemes

FIGURE 2.15. Double potential step chronoamperometry for electrodimerizations. Variation of the normalized anodic-to-cathodic current ratio RDPS — [—ia(2tR)/ic(tR) /(l — 1 / /2), with the dimensionless kinetic parameter X defined as  

FIGURE 2.16. Homogeneous catalysis electrochemical reactions. Reaction scheme and typical cyclic voltammetric responses. The reversible wave pertains to the mediator alone. The dotted curve is the response of the substrate alone. The third voltammogram corresponds to the mediator after addition of the substrate. 

Homogeneous Electron Transfer as the Rate-Determining Step The 

FIGURE 2.1 7. Homogeneous catalysis electrochemical reactions. Kinetic zone diagram in the case where the homogeneous electron transfer step is rate limiting. 

In the no substrate consumption zone, one passes upon increasing Xe (i.e., increasing the rate constant and/or decreasing the scan rate) from a 

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Most recently, Milstein and coworkers reported a new type of homogeneous catalytic reaction that generates silanones from secondary silanols under extremely mild conditions. The platinum complex (dmpe)Pt(Me)OTf) (27) (dmpe = Me2PCH2CH2PMe2, OTf = OSO2CF3 was treated with an equimolar amount of the silanol, (i-PrhSilKOII) (28), in acetone to yield a new dimeric hydrido-bridged complex 29 and the trimer 30 of diisopropylsilanone, 31 (Scheme 12)29. 

Ferrocenyl diphosphine core-functionalized carbosilane dendrimers have been prepared as ligands for homogeneous catalytic reactions applied in a CFMR by Van Leeuwen et al. [20,21,67,68]. The syntheses of these dppf-like ligands (Go-G2)-17 were performed using carbosilane dendritic wedges with an aryl bromide as focal point. These wedges were coupled to the core via quenching of the lithiated species with ferrocenyl phosphonites (Scheme 11). 

Nonetheless, the transposition of homogeneous catalytic reactions from unsupported to dendrimer-supported catalysts is still not straightforward. Various dendritic effects , positive and negative ones, on the activity, selectivity, stability and solubility of metallodendrimer catalysts have been observed in this respect. In our own research we have found that a high concentration of metal centers at periphery-functionalized metallodendrimers may translate into a decrease in the catalytic performance due to undesirable side-reactions between the catalytic sites at the dendrimer surface (Fig. 4 and Scheme 4). In contrast, when the exact same catalyst is located at the focal point of a dendron, this matter is avoided by isolating the active site, thereby providing a more stable albeit less active catalyst (Scheme 13). 

We have dealt above with a pure case of doublet reaction and other reactions. It is only natural that there can be some quite reasonable deviations. The bonds requiring activation must come into contact with the catalyst. If, for instance, one part of the index of the doublet reaction does not require activation, it can react without coming into contact with the catalyst. Such a half-doublet scheme for esterification in solution was given by the author 37). Hence, the transition from heterogeneous to homogeneous catalytic reactions occurs, and in the extreme case in which both the bonds of the doublet group do not require activation for the reaction, to noncatalytic reactions. 

The kinetic scheme of homogeneous catalytic reactions is often discussed in terms of a two-step sequence... 

Using a fluorous palladacycle catalyst 10 originating from the corresponding fluorous Schiff base and palladium acetate, a fluorous Mizoroki-Heck reaction was achieved with an excellent turnover number (Scheme 12). A homogeneous catalytic reaction system was obtained when DMF was used as the solvent. After the reaction, perfluorooctyl bromide was added to facilitate the separation of DMF (containing the products and amine salts) from the catalyst phase. The resulting lower fluorous layer was condensed under vacuum and the catalyst residue was used in a second run. In this reaction, the palladacycle catalyst appears to act as a source of palladium nanoparticles, which are thought to be the dominant active catalyst. 

It wiU be stressed that agreement of the macrokinetic law with the expression obtained from the formal scheme of a homogeneous catalytic reaction is yet no proof that the reaction mechanism satisfying this scheme is correct. 

The catalytic formation of cyclic carbonates via aluminium porphyrins was probably the trigger for the investigation run by Kasuga and coworkers in 1996 with aluminium phthalocyanines as catalysts.In general phthalocyanines display the same planar N4-coordination geometry as the porphyrins as shown in Scheme 18.35 with the advantage of an easier synthesis and have been used in many homogeneous catalytic reactions. 

Fig. 4.4 Measured and predicted distributions of the OH radical for Cases 8-11. (a) OH-LIF, (b) numerical predictions with the Qin et al. (gas-phase) and pressure-corrected (catalytic) reaction schemes. The green arrows define the onset of homogeneous ignition. Predicted OH...

Homogeneous hydrothiolation of alkynes was achieved by using CpNi(IMes)Cl (IMes = A, M-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Under the optimized conditicHis (8), CpNi(IMes)Cl catalyzed the reaction of PhSH with 1-heptyne in the presence of Et3N in toluene-tfg to give 9 (66%) and 11 (8%) without other byproducts. This catalytic reaction (Scheme 10) starts from the formaticm of the thiolato Ni(II)... 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

In the various homogeneous catalytic schemes, the solvent may be coordinated to the metal or may simply be present as bulk solvent. When a ligand leaves the coordination sphere of a metal, it may be replaced by a molecule of solvent in a process that is either associative or dissociative. There is no general way to predict which type of mechanism is operative, so in some cases the substitution reactions will be described as they relate to specific processes. Because substitution reactions have been described in Chapter 20, several other types of reactions that constitute the steps in catalytic processes will be described in greater detail. 

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

Two-Electron Catalytic Reactions In a number of circumstances, the intermediate C formed upon transformation of the transient species B is easily reduced (for a reductive process, and vice versa for an oxidative process) by the active form of the mediator, Q. This mechanism is the exact counterpart of the ECE mechanism (Section 2.2.2) changing electron transfers at the electrode into homogeneous electron transfers from Q, as depicted in Scheme 2.9. In most practical circumstances both intermediates B and C obey the steady-state approximation. It follows that the current is equal to what it would be for the corresponding EC mechanism with a... 

The reaction scheme shown in Scheme 5.4 is the same as in the homogeneous case except that all forms of the enzymes are now immobilized onto the electrode surface. The cosubstrate is still in solution. The current is composed of two terms, one pertaining to the diffusion of the cosubstrate and the other to the catalytic reaction ... 

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