Sequential catalytic reactions

Here, the chosen domain for our case study is on-board hydrogen production to supply pure H2 to a fuel cell in an electrical car. Among the sequential catalytic reactions that take place for H2 production, the hydrogen purification units are located downstream, after the primary reforming of hydrocarbons into a CO-H2 mixture or Syngas units. They consist of Reaction (1) the water-gas shift (WGS) reaction and Reaction (2), the selective or preferential oxidation of CO in the presence of hydrogen (Selox). 

Keywords Cascade catalytic reactions Ruthenium catalysis Sequential catalytic reactions... 

In this section, we will first present sequential catalytic reactions, in which the compound formed during the first step is used as the substrate for the second reaction, both catalytic reactions being catalyzed by ruthenium species arising from a unique precursor. The ruthenium-catalyzed Rosenmund-Tishchenko reactions represent one of the first examples of such chemical transformations (Scheme 21) [44]. 

As illustrated above sequential catalytic reactions, performed in the same flask, either by modifying the initial ruthenium complex to create a new catalyst, or by introduction of another metal catalyst have been developed with efficiency for the production of useful molecules or polymers. 

We can easily predict that cascade and sequential catalytic reactions will be the subject of important investigations not only by promoting the cooperation of several metal catalysts but also by organizing the tolerance and the cooperative work of metal and organo catalysts, and of metal and enzyme catalysts. This will be possible through a deep understanding of the mechanisms of each catalytic system, so as to organize their mutual tolerance. 

Fo, it seems, has only one proton channel and thus only one catalytic site can be coupled to a proton channel at atime. A sequential catalytic reaction in ATP synthesis would therefore require arotation of parts ofFi and Fq, or at least parts of each, relative to one another, and imply an asymmetric structural and functional configuration ofthe ATP synthase enzyme. The binding changes thatoccur during the catalytic process apparently result from the rotation of some subunit or assembly of subunits of Fq relative to those of Fi. An account ofthe detailed mechanism of subunit movement will be presented below. 

An interesting, as yet unstudied, potential application of immobilized enzyme or catalytic membranes is their use in the conduct of sequential catalytic reactions, as illustrated in Fig. 9.1. Rapid and slow catalytic reactions with different catalysts can be conducted consequently in different converters fluid leaving one membrane converter can be delivered to a second catalytic membrane containing a different catalyst operative on the product of the first transformation. Since there can be no back diffusion from one membrane to the other, there is no chance for products of the second reaction to be acted upon by the first catalyst or to interact with the initial substrate. Thus, cross-reaction between different intermediates and different catalysts is avoided. This may allow continuous, sequential catalytic reactions, impossible to perform concurrently, to be carried out efficiently and rapidly. 

Scheme 4.17 (a, b) Some sequential catalytic reactions involving an intramolecular C-H methine activation... 

Figure 4.7 Two of the enzymatic activities involved in the biosynthesis of tryptophan in E. coli, phosphoribosyl anthranilate (PRA) isomerase and indoleglycerol phosphate (IGP) synthase, are performed by two separate domains in the polypeptide chain of a bifunctional enzyme. Both these domains are a/p-barrel structures, oriented such that their active sites are on opposite sides of the molecule. The two catalytic reactions are therefore independent of each other. The diagram shows the IGP-synthase domain (residues 48-254) with dark colors and the PRA-isomerase domain with light colors. The a helices are sequentially labeled a-h in both barrel domains. Residue 255 (arrow) is the first residue of the second domain. (Adapted from J.P. Priestle et al., Proc.

The catalytic reaction can be conveniently divided into a number of sequential steps, all of which impact on the overall efficiency of the reaction. First the reactants must diffuse to the catalyst surface the rate of diffusion depends on several factors including fluid density, viscosity and fluid flow rate. Whilst some reaction will take place at the external surface, the majority of reactants will need to diffuse into the internal pores. For a... 

Hartwig and coworkers reported an approach to address this limitation involving tandem catalytic reactions. In this tandem process, sequential palladium-catalyzed isomerization of the branched isomer to the linear isomer, followed by iridium-catalyzed allylic substitution leads to the branched product with high enantiomeric excess [105]. More specifically, treatment of branched allylic esters with catalytic amounts of the combination of Pd(dba)2 and PPhs led to rapid isomerization of the branched allylic ester to the linear isomer, and the linear isomer underwent allylic substitution after addition of the iridium catalyst and nucleophile (Scheme 31). 

The CD fragment 1s synthesized starting with resolved bicyclic acid 129. Sequential catalytic hydrogenation and reduction with sodium borohydride leads to the reduced hydroxy acid 1. The carboxylic acid function is then converted to the methyl ketone by treatment with methyl-lithium and the alcohol is converted to the mesylate. Elimination of the latter group with base leads to the conjugated olefin 133. Catalytic reduction followed by equilibration of the ketone in base leads to the saturated methyl ketone 134. Treatment of that intermediate with peracid leads to scission of the ketone by Bayer Villiger reaction to afford acetate 135. The t-butyl protecting... 

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

Interestingly, research has started on single chamber SOFC (SC-SOFC) concepts. However, the SC-SOFC exhibits inherently low power density and is therefore primarily of academic interest. It has the potential to relax cell component requirements and probably to ease manufacture. The principle of SC-SOFC is that it is fed by an air fuel mixture which flows onto the PEN contained in a single compartment, avoiding the use of gas separator plates and high temperature sealants. The fluid may flow simultaneously or sequentially along the electrodes. Both electrodes are either built onto the same side of the electrolyte some distance apart or on opposite sides. Low temperature operation would apparently suppress direct combustion of the air fuel mixture provided the electrode materials chosen are highly selective towards their respective catalytic reactions. SC-SOFC stacks may hold prospects in specific applications where the reaction products are the prime focus. 

In this context, a new concept for high-throughput screening was developed [109], De Bellefon et al. [109] reported a dynamic sequential method to screen liquid/ liquid- and liquid/gas-phase catalytic reactions by applying the method of the injection of different samples followed by barrier liquid. Although the pulsed input to establish spatially separated samples, this method might also be applicable to the study of the dynamic behavior in gas-phase reactions. 

Scheme 3.16 A series of different catalytic reactions carried out sequentially with the cartridge catalyst system I/[Rh(acac)(C0)2]/scC02 pinBH = pinacolborane (Reproduced from Ref. [76], with the permission of John Wiley and Sons)...

The rotaxane assembly is adopted by many enzymes that operate on nucleic acids and proteins. In the case of processive enzymes, the catalytic reaction drives the sequential motion of the enzyme on its polymeric substrate. Therefore, these enzymes can be viewed as molecular motors powered by chemical reactions and moving one-dimensionally on a track, in which fuel is provided by the track itself. An initial attempt to carry out processive catalysis with a synthetic rotaxane has been described [69]. 

Fig. 3.1 Classification of functional roles of subunit assembly and disassembly.4 a) The assembly of identical subunits is essential for the 9catalytic activity, b) The assembly of nonidentical subunits is essential for the catalytic activity, c) The assembly of active subunits enhances the catalytic activity, d) Sequential metabolic reactions are efficiently catalyzed by the assembly of subunits, e) The assembly of subunits is required for the expression of regulatory properties, f) The assembly of nonidentical subunits diminishes the catalytic activity. See text for details. (Reproduced with permission from S. Tokushige, Kagaku Zokan, 103, 41 (1984), in Japanese)).

A second basic interaction pathway between transition metal complexes and organic substrates is SET (Path B). The overall processes can involve one individual or several sequential SET steps. For the latter, timing and direction of SET steps determine the reaction outcome significantly. The catalyzed reaction can proceed either as redox-neutral processes, in which oxidative and reductive SET steps are involved in the catalytic cycle, or as overall oxidative or reductive catalytic reactions, where two oxidative or reductive SET steps occur consecutively in the catalytic cycle. The third pathway (Path C) consists of a direct atom or group abstraction by the metal complex, which is possible for a weak R-X bond. 

Of the two mechanistic pathways, i.e., via palladacyclization or via hydropalladation-cyclic carbopalladation, the latter seems to be more suitable for the development of sequentially catalyzed processes. Considering cycloisomerizations via the hydropalladation-cyclic carbopalladation route the catalytic reaction can terminate by /1-hydride elimination giving rise to the formation of dienes and derivatives thereof (Scheme 79). Alternatively, the alkyl-Pd species formed in the cyclic carbopalladation can be susceptible to subsequent transmetallation with organometallic substrates. Then, a reductive elimination could conclude this second Pd-mediated step releasing the Pd(0) species for a new catalytic cycle. 

Abstract Ruthenium holds a prominent position among the efficient transition metals involved in catalytic processes. Molecular ruthenium catalysts are able to perform unique transformations based on a variety of reaction mechanisms. They arise from easy to make complexes with versatile catalytic properties, and are ideal precursors for the performance of successive chemical transformations and catalytic reactions. This review provides examples of catalytic cascade reactions and sequential transformations initiated by ruthenium precursors present from the outset of the reaction and involving a common mechanism, such as in alkene metathesis, or in which the compound formed during the first step is used as a substrate for the second ruthenium-catalyzed reaction. Multimetallic sequential catalytic transformations promoted by ruthenium complexes first, and then by another metal precursor will also be illustrated. 

During the last decade, molecular ruthenium catalysts have attracted increasing interest for organic synthesis due to their ability to perform specific new reactions with a large panel of applications. Beside individual catalytic transformations, a variety of multi-step catalytic transformations in one pot have appeared. These transformations present practical and economic advantages as far as they are efficient, selective and proceed with atom economy. Ruthenium catalysis has entered this field with a variety of cascade and sequential catalytic transformations. 

In this review, we will focus on cascade and sequential catalytic transformations in which the first one is a ruthenium-catalyzed reaction. This will include ... 

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