Overall Catalytic Cycles

Pathway for isomerization of branched alkyi complexes formed from internal olefins  

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The overall catalytic cycle is summarized in Scheme 2.24. The catalytic cycle from 71 to 72 is promoted by addition of 2 mole of carbon monoxide and the catalytic cycle from 72 to 71 releases 2 mole of carbon monoxide. Thus, carbon monoxide acts as the driver of this catalytic cycle. 

Thus, in the overall catalytic cycle it is observed that the various steps necessary can all occur under mild conditions therefore, the use of elevated temperatures (>150°C) in the commercial operation is related to increasing the efficiency of the use of the rhodium catalyst by increasing the reaction rate by use of temperature rather than catalyst level. 

Overall Catalytic Cycle with Specific Intermediates... 

The transition metal catalyzed synthesis of arylamines by the reaction of aryl halides or tri-flates with primary or secondary amines has become a valuable synthetic tool for many applications. This process forms monoalkyl or dialkyl anilines, mixed diarylamines or mixed triarylamines, as well as N-arylimines, carbamates, hydrazones, amides, and tosylamides. The mechanism of the process involves several new organometallic reactions. For example, the C-N bond is formed by reductive elimination of amine, and the metal amido complexes that undergo reductive elimination are formed in the catalytic cycle in some cases by N-H activation. Side products are formed by / -hydrogen elimination from amides, examples of which have recently been observed directly. An overview that covers the development of synthetic methods to form arylamines by this palladium-catalyzed chemistry is presented. In addition to the synthetic information, a description of the pertinent mechanistic data on the overall catalytic cycle, on each elementary reaction that comprises the catalytic cycle, and on competing side reactions is presented. The review covers manuscripts that appeared in press before June 1, 2001. This chapter is based on a review covering the literature up to September 1, 1999. However, roughly one-hundred papers on this topic have appeared since that time, requiring an updated review. 

Reoxidation of the palladium back to the divalent state is most commonly accomplished with CuCl2/02. The overall catalytic cycle then becomes... 

Whiehever mechanism operates, it is clear that the rate of reduetion of the flavin group is totally limited by the cleavage of the aC-H bond sinee the deuterium kinetic isotope effect for this step is around 8 (Miles et al., 1992 Pompon et al., 1980). However, in flavocytochrome 2 the rate of flavin reduetion is some 6-fold faster than the overall steady-state turnover rate (Daff et al., 1996a). As a consequence the flavin reduction step eontributes little to the rate limitation of the overall catalytic cycle (Figure 3). In faet it is eleetron transfer from flavin-semiquinone to b2 -heme that is the major rate-determining step and this is discussed in the following seetion. 

Fig. 1. The overall catalytic cycle of polyketide synthases. Within this biosynthetic scheme, different polyketide synthases can show variability with regard to the length of the polyketide chain, the choice of monomer incorporated at each step, the degree of reduction of each P-keto group, and the stereochemistry at each chiral center. For example, the dashed arrows illustrate how the degree of -ketoreduction can vary at any given carbonyl...

Several studies have demonstrated the ability to observe a complete catalytic cycle in the gas-phase. Wallace and Whetten, and Woste and coworkers combined gas-phase experiments and theoretical calculations to elucidate the fuU catalytic cycle of CO oxidation including intermediate reaction steps [27-29]. Schwarz et al. have also demonstrated a full gas-phase catalytic cycle for the oxidation of CO in the presence of cationic platinum oxide clusters [30]. Furthermore, Armentrout and co-workers have studied the energetics of the individual steps in the overall catalytic cycles and produced a wealth of information on the thermochemistry, structure, and bond energies of transition metal clusters [31]. Clearly, the ability to probe the active sites and intermediates of complex catalytic reactions through gas-phase ion-molecule studies has yielded significant insight into the mechanisms of condensed-phase catalytic processes. 

A fair number of possibilities for explaining HDS reaction schemes have been available for some time in heterogeneous catalysis publications, most of them based on sound experimental evidence and sometimes on extensive calculations. However, because of the intrinsic complexity of the problem, some of the key points of the mechanisms have remained essentially at a controversial or a speculative level over many years. These are mainly related to the nature and the structure of the surface active sites required for each type of reaction involved, and to the details of tine various elementary steps of the overall catalytic cycles. By studying the analogous reactions on... 

The overall catalytic cycle is believed to involve various titanium complexes which all have at least one isopropoxy ligand attached to the metal (Scheme 1). Given this fact, it is evident that kind and structure of the alkoxide can influence the catalysis, in particular the chirality transfer step (4—>5, via 2) and the displacement of the product sulfoxide from 5 to regenerate 3. Evidence for this assumption was obtained in studies with both other titanium alkoxides and alcohols such as methanol. In all cases less efficient catalyst systems resulted. 

The Shilov Pt(II) system for methane functionalization has been studied using computations. For example, using MCl2(H20)2 (M = Pt or Pd) systems as models for Shilov-type reactions, the overall catalytic cycle was studied using a combination of... 

Both the cluster, as well as periodic approaches, will likely play invaluable roles in the future toward the quantitative prediction of transition metal surface chemistry. Herein, we discuss some of the recent developments on the application of DFT-cluster calculations to chemisoiption and reactivity of adsorbates on metal surfaces. We demonstrate how these results can subsequently be used to begin to model overall catalytic cycles and interpret different selective oxidation chemistries. 

These results are subsequently used to elucidate the most favored adsorption sites, the most favored adsorption modes, and overall reaction energies for specific surface reaction steps. From this data, we can begin to construct overall catalytic cycles and examine their likelihood in carrying out proposed process chemistry. More detailed reaction coordinate searches are required to analyze the actual mechanism, where transition states and corresponding activation barriers are rigorously computed. While the transition states found herein appear to be quite reasonable, the predicted activation barriers are slightly high. This may be the result of cluster size effects rather than the DFT accuracy. 

The overall catalytic cycle is depicted in Fig. 5. It details the cooperativity of the chiral nickel catalyst, the lutidine base and the silyltriflate in this uifique catalytic functionalization. 

The mechanistic understanding is that a coordinated aldehyde undergoes enolization within the coordination sphere of the ruthenium catalyst. A subsequent oxidation of the metal upon interaction with the silver(l) complex provides a Ru(IV) catalyst state that is nucleophilicaUy attacked by fluoride. This step installs the carbon-fluorine bond and reduces the ruthenium catalyst back to oxidation state n. Displacement of the product by another substrate molecule closes the overall catalytic cycle. 

An overall catalytic cycle for olehn metathesis reactions catalyzed by (PCy3)2(Cl)2Ru=CHPh (1) and its derivahves is summarized in Fig. 4.32. The hrst... 

ORII. The pH dependence of the observed first-order rate constant showed that a maximum degradation rate was observed at pH 3.5-4.0. This partly results from the fact that the most labile form of the Ru (edta) complex, viz. [Ru (edta)(H20)] , is present in the pH range 4.0-6.0 as a result of the pifa values given in Section V of this chapter. Further, the role of the dangling acetate arm can also contribute toward the stabilization of the [Ru (edta)(OOH)] complex as shown schematically in the overall catalytic cycle summarized in Scheme 10. The studies also showed that the degradation reaction catalyzed by [Ru (edta)(0)] and [Ru (edta)(OH)] are orders of magnitude slower than found for [Ru (edta)(OOH)] (75). 

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