Catalytic route cycle

Fig. 4.15 Catalytic route cycle for Langmuir-Hinshelwood CO oxidation mechanism with dissociative oxygen adsorption.

Visualize the reaction mechanism using a catalytic route cycle approach. 

The head-to-head dimerization with formation of a butatriene derivative was very scarcely observed as the main catalytic route (Scheme 10.19, cycle B). Nevertheless, this was the case with benzylacetylene in the presence of RUH3CP (PCy3) as catalyst precursor in tetrahydrofuran at 80°C which gave more than 95% of (Z)-l,4-diben-zylbutatriene [66], and with terf-butylacetylene with two efficient catalytic systems capable ofgenerating zero-valent ruthenium species, RuH2(PPh3)3(CO) and Ru(cod) (cot) in the presence of an excess of triisopropylphosphine, which led to (Z)-l,4-di-tert-butylbutatriene as the major compound [71-73]. 

The Meerwein arylation is at least formally related to the atom transfer method because a net introduction of an aromatic ring and a chlorine across a double bond is accomplished (Scheme 62). Facile elimination of HC1 provides an efficient route to the kinds of substituted styrenes that are frequently prepared by Heck arylations. Standard protocol calls for the generation of an arene diazonium chloride in situ, followed by addition of an alkene (often electron deficient because aryl radicals are nucleophilic) and a catalytic quantity of copper(II) chloride. It is usually suggested that the copper salt operates in a catalytic redox cycle, reducing the diazonium salt to the aryl radical as Cu1 and trapping the adduct radical as Cu11. 

A better approach to substitution at (1) is the use of catalytic routes. Induction with Me3NO has proved successful for (1) in some cases, but of particular value for a clean reaction with (1) was electron-transfer catalyzed substitution using Ph2CONa in low concentration as a reductive activator. The likely intermediate is the 49-electron species [Ru3(CO)i2], which quickly reacts to form [Ru3(CO)iiL]". Electron-transfer occurs again to form Ru3(CO)iiL and to regenerate [Ru3(CO)i2] to repeat the cycle. The same type of reaction cycle can be induced with [PPN]X, where X = acetate, cyanide, or halides. The [PPN][Ru3(CO)i2-nX] (n = 1, 2, 3) species is the intermediate, and X is displaced by the appropriate ligand to obtain the substitution product. 

Routes for completing the cycle back to the U(III) compounds, and other catalytic routes, are currently under investigation in our laboratory. 

Scheme 20.11 Catalytic epoxidation cycle via the metal-mediated carbene transfer route.

A convenient way to overcome the difficulties in visuafization of reaction networks is to use the catalytic cycle approach similar to finear sequences with the number of nodes (and respective edges) corresponding to the number of steps. For CO oxidation, instead of Fig. 4.14, a reaction route cycle as in Fig. 4.15 can be proposed. In this figure, the nodes contain aU the surface species taking part in the cycle. In such representation graph branches are used to visualize elementary reactions. 

Succinyl-CoA derived from propionyl-CoA can enter the TCA cycle. Oxidation of succinate to oxaloacetate provides a substrate for glucose synthesis. Thus, although the acetate units produced in /3-oxidation cannot be utilized in glu-coneogenesis by animals, the occasional propionate produced from oxidation of odd-carbon fatty acids can be used for sugar synthesis. Alternatively, succinate introduced to the TCA cycle from odd-carbon fatty acid oxidation may be oxidized to COg. However, all of the 4-carbon intermediates in the TCA cycle are regenerated in the cycle and thus should be viewed as catalytic species. Net consumption of succinyl-CoA thus does not occur directly in the TCA cycle. Rather, the succinyl-CoA generated from /3-oxidation of odd-carbon fatty acids must be converted to pyruvate and then to acetyl-CoA (which is completely oxidized in the TCA cycle). To follow this latter route, succinyl-CoA entering the TCA cycle must be first converted to malate in the usual way, and then transported from the mitochondrial matrix to the cytosol, where it is oxida-... 

RhCl(PPhi)i as a homogenous hydrogenation catalyst [44, 45, 52]. The mechanism of this reaction has been the source of controversy for many years. One interpretation of the catalytic cycle is shown in Figure 2.15 this concentrates on a route where hydride coordination occurs first, rather than alkene coordination, and in which dimeric species are unimportant. (Recent NMR study indicates the presence of binuclear dihydrides in low amount in the catalyst system [47].)... 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

One of the most efficient approaches allowing us to investigate in a reasonable time a catalytic cycle on non-periodic materials in combination with reliable DFT functional is a cluster approach. The present study is devoted to the investigation of the effect of the cluster size on the energetic properties of the (p-oxo)(p-hydroxo)di-iron metal active site. As a first step, we have studied the stability of the [Fen(p-0)(p-0H)Fen]+ depending on the A1 position and cluster size. Then, we compared the energetics for the routes involving the first two elementary steps of the N20 decomposition catalytic process i.e. the adsorption and dissociation of one N20 molecule. 

Scheme 5. Condensed free-energy profile (kcalmol-1) of the complete catalytic cycle of the Ci2-reaction channel of the nickel-catalyzed cyclo-oligomerization of 1,3-butadiene, focused on viable routes for individual elementary steps. The favorable [Ni°(r 2-/r<2fts-butadiene)3] isomer of the active catalyst 1/b was chosen as reference and the activation barriers for individual steps are given relative to the favorable stereoisomer of the respective precursor...

The c,c,t-CDT and c,t,t-CDT production paths are shown to be not assisted by incoming butadiene, while the square-planar transition state involved along the all-t-CDT path is significantly stabilized by an axial coordination of butadiene. Hence, the all-t-CDT route becomes the most facile of the three CDT production paths with a free-energy barrier for reductive elimination of 23 kcal mol-1, that perfectly corresponds with experimental estimates.44 Accordingly, the production of C12-cyclo-oligomers requires moderate reaction conditions,9 although 7b represents a thermodynamic sink within the catalytic cycle. 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

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