Catalytic cycles bases

The discovery of a new heterodinuclear active site in [NiFe] hydro-genases opens the way for the proposal of catalytic cycles based on the available spectroscopic data on the different active site redox states, namely EXAFS studies that reveal that the Ni-edge energy upon reduction of the enzyme supports an increase in the charge density of the nickel (191). 

A tetracoordinated complex (20)4 was actually isolated. Complex 20 in the presence of ethylene forms the coordinated complex 21, as can be seen from H NMR. Complex 21 is a model of the intermediate for the additional reaction to form C6 dienes. The model catalyst had been shown to be a codimerization catalyst under more severe conditions (high temperature), although the rate of reaction was very slow compared to the practical systems. These studies are extremely useful in demonstrating the basic steps of the codimerization reactions taking place on the Ni atom. The catalytic cycle based on these model complexes as visualized by Tolman is summarized in Scheme 7. A more complete scheme taking into consideration by-product formation can be found in Tolman (40). 

The first example of such a process was reported in 1994 by Asami, who noticed that LDA was less reactive than HCLA 53 toward oxirane and thus proposed its use as a co-base in a catalytic cycle . Based upon this seminal result, the system has been extended to other HCLAs and various co-bases have been tested Selected results for the asymmetric rearrangement of cyclohexene oxide mediated by sub-stoichiometric quantities of HCLA are collected in Table 4. 

Catalytic Cycle Based on Carbene Generation of Ylide... 

Catalytic Cycle Based on Sulfide Alkylation and Sulfonium Salt Deprotonation... 

The hierarchy of Figure 1 is also in many ways a road map. The hierarchy helps to illustrate the manner in which the different models communicate. Traditionally, the mechanistic model could only communicate with global models through molecular models. That is, thermal and catalytic cycles based on mechanistic chemistry, such as Rice-... 

In the Shell process for MMA manufacture an alternative mechanism involving a hydride intermediate may be invoked. Draw a catalytic cycle based on such a mechanism. 

Metal cluster compounds of the carbonyl type have often served as catalyst precursors, that is, as the source of fragments that are the actual catalysts,18 although there have been relatively few fully detailed descriptions of catalytic cycles based on clusters.19 Triruthenium cluster compounds have been found to catalyze hydrogen... 

It has been shown that ligands such as phosphines can control the course of homologation reaction to a large extent. Tims, it seems likely that the Ugands are coordinated to the metal center during the catalytic cycle. Based on this assumption and on the observation that the best results were obtained with a cobalt-phosphine-iodide ratio of 1 2 2, Roper and Loevenich proposed the mechanism shown in Scheme 2. 

The mechanism of the NiFe hydrogenase has been treated by caleula-tional methods, with some interesting conclusions (Pavlov et al., 1998). Scheme 1 of this reference proposes a catalytic cycle based on these results. It was proposed that Fe binds H2 and that a low spin Fe is essential for het-erolytic cleavage of the H6H bond. The next step is proposed to be hydride transfer to Fe and proton transfer to a ligated cysteine thiolate, whieh leads to decoordination of the cysteine and concurrent bridging of the N of CN... 

Another impetus to mechanistic studies arose from the recognition that compounds of these d ions were those on the energy borderline between stable 18-electron and 16-electron molecules (1) and that the reactions involving transitions between these states are those encountered in catalytic cycles based on these compounds. Nucleophilic ligand substitution, involving association of an entering nucleophile with a square-planar compound, is just one example of the easy 16- 18- 16... 

The final step of the catalytic cycle, base-assisted reductive elimination, has been addressed by Deeth et al. [14]. In their calculations, the authors investigated palladium complexes with the chelating diaminomethane H2N(CH2)NH2 and di-phosphinomethane H2P(CH2)PH2 ligands. Within this system, they found that the postulated hydrido-olefin complex, which is usually formed by p-H elimination of the y9-agostic insertion product, is in fact not a stable minimum structure in this particular case (eq. (11)) [14]. 

The catalytic cycle, based on gold, is thus verified to be as follows [326] ... 

A catalytic cycle based on these observations is shown in Scheme I. The decomposition of the starting material HW2(CO)10 to HW(C0)5 and its subsequent reaction with C02 to yield HC02W(C0)5 have been reported elsewhere (5,6). The presence of formic acid, which subsequently undergoes esterification to afford methyl formate, was detected by GC when the reaction was carried out in benzene. 

An example, developed by our group, has been presented 3-chloro-propenyl pivalate, the new synthon we propose as a formal a-hydroxy allyl anion, and chromium(II) chemistry allow to achieve the formal diastereo- and enantio-selective a-hydroxyallylation of carbonyl compounds by applying a catalytic cycle based on an in situ produced Salen-chromium complex (Table 8.3). 

Whether or not catalytic cycles based on such ionic hydrogenation mechanisms can be devised remains to be seen. 

In the initial publication [52], a catalytic cycle based on the iodine(V) species was proposed for this reaction (Scheme 4.26) however, more recent studies (Section 4.2) have demonstrated that the oxidation of... 

The accelerating effect of adding small amounts of L to the dimer (Figure 2.4) suggests a step such as reaction (67) in a more complete mechanism, which might also include an additional catalytic cycle based... 

Fig. 3 Catalytic cycle based on the dominant A pathway as proposed by Landis and coworkers...

Strained square planar structure of the PSiP-linkage would facilitate structural change to a trigonal bipyramidal geometry, which might allow coordination of CO2 for facile carboxylation. (iii) The pincer structure would enable a catalytic cycle based on Pd(II) without liberation of Pd(0), which causes side reactions through oxidative cyclization of unsaturated hydrocarbons. 

Stoltz and co-workers proposed a catalytic cycle based on observations including the formation of diallylated Meldmm s acid and an understanding of the DAAA process (Scheme 4.32). Kinetic studies suggested the P-keto ester reacts very quickly to generate a Pd-enolate or Pd-carboxylate and this then undergoes asymmetric protonation in a slower second step. The catalytic cycle begins with... 

Chemistry of the Individual Steps of the Classic Catalytic Cycle Based on Results of Model Reactions and Spectroscopic Investigations... 

Metallabenzenes have been invoked as possible intermediates in several other reaction types. Schrock, " for example, proposed tungstenabenzenes as possible intermediates in certain alkyne metathesis reactions that proceed by associative mechanisms. Shown in Scheme 32 is a proposed sequence for the metathesis of 3-heptyne to 3-hejQTie and 4-octyne using a tungstenacyclobutadiene complex as catalyst. The postulated metallabenzenes are formed by alkyne insertion into the metal carbon bonds of the metallacyclobutadienes. Of course, it is also possible to envisage a catalytic cycle based on Dewar metallabenzene intermediates. 

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