Rhodium structural transformations

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

In addition to the [3-1-2] and [2-I-2-I-1] carbocycHzations that facilitate the formation of five-membered rings, the [4-t-l] carbocycHzation also has merit. Several transition metals have been engaged in this transformation [37], wherein it was found that vinylallene 63 a reacts with 1 equiv of WiUdnson catalyst to afford the planar (T2-bonded (vinylallene)rhodium complex 64a upon simple ligand displacement (Scheme 11.16). The stmcture of 64a was confirmed unambiguously by X-ray crystallography, and represents the first structural characterization of a metallacycle intermediate [38 a]. 

Fig. 7.4 High-resolution transmission electron microscopy (TEM) image of a rhodium on a modified cerium oxide (Ceo.gTbo.202 x) after reduction, showing how the metal is almost epitaxially attached to the support, (a) TEM image (b,c) digital diffraction patterns obtained by Fourier transformation from the images (d) structure model ...

Several chemical transformations of this acyl complex were carried out in order to prove its structure. The reaction of carbon monoxide with the complex gave acyl halide and chlorocarbonylbis (triphenylphosphine)-rhodium (XII). The thermal decomposition of the acyl complex gave rise to a mixture of isomeric olefins. The formation of olefin from the complex can be carried out more smoothly by adding iodine. When iodine was added to the solution of the acyl complex at room temperature, terminal olefin was obtained in high yield. These reactions are summarized below... 

Table 2 also shows that the resulting Rh dispersion is not a fimction of the metal loading but of the structure of the support [12,13], The natme of the support influences dispersion and therefore the size of the metallic particles. There was no correlation between metallic load and dispersion. Lamellar structures (BENa and BENPIL) incorporated larger amounts of rhodium complex but had lower dispersion than the catalysts synthesized on zeolitic products. The influence of the support structure is also reflected in the results for the zeolitic product synthesized with different treatment conditions or in different media, which determine the channel dimensions and the number of anchoring centres in the resulting samples. Higher dispersion was achieved in the more transformed samples, ZE— P, than in ZE—X, and in those synthesized in distilled water (ZEDI-) than in sea water medium (ZESE-). 

In 1990, Brunner [5], McKervey [6], and Ikegami [7] and their respective coworkers independently introduced chiral rhodium(II) carboxylates for asymmetric diazocarbonyl transformations. At that time the only chiral rhodium(II) carboxylates known were those derived from (R) and (S)-mandelic acid which had been prepared by Cotton and co-workers [8] for structural and chiroptical studies. Enantiopure carboxylates (1) on a dirhodium core (substituents varied from H, Me, and Ph to OH, NHAc, and CFj) were assessed by Brunner [5] for enantioselective cyclopropanation of alkenes with ethyl diazoacetate. McKervey... 

The first natural product in this family to be synthesized with this methodology was (+)-erogorgiaene (201) [142], and provided an excellent proof of concept for the more complex structures to follow (Scheme 50). Importantly, this synthesis began with racemic dihydronaphthalene precursor 205, and included a powerful enantiodivergent carbenoid transformation. In this key step, one enantiomer of the substrate was poised to react with the rhodium carbenoid to form the desired C-H... 

In all enantioselective hydrogenations the ability of the substrate to form a chelate ring with the catalyst is extremely important. For this reason the enantioselective reductive ami-nation of ketones is always particularly difficult, because these compounds usually do not have a structure suitable for the required chelation. Burk et al. circumvent this problem by reversible derivatization. The ketones are converted into A -acetylhydrazones, whose structures resemble those of enamides. [11] The C-N double bond can then be hydrogenated by nPr-DuPHOS-rhodium with ee values almost as high as those for C-C double bonds of enamides. The A-acetylhydrazines obtained thus can either be transformed into... 

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