Cesium promoter similarity

Fluoride ion is effective in promoting the reduction of aldehydes by organosil-icon hydrides (Eq. 161). The source of fluoride ion is important to the efficiency of reduction. Triethylsilane reduces benzaldehyde to triethylbenzyloxysilane in 36% yield within 10-12 hours in anhydrous acetonitrile solvent at room temperature when tetraethylammonium fluoride (TEAF) is used as the fluoride ion source and in 96% yield when cesium fluoride is used.83 The carbonyl functions of both p-anisaldehyde and cinnamaldehyde are reduced under similar conditions. Potassium bromide or chloride, or tetramethylammonium bromide or chloride are not effective at promoting similar behavior under these reaction conditions.83 Moderate yields of alcohols are obtained by the KF-catalyzed PMHS, (EtO SiH, or Me(EtO)2SiH reduction of aldehydes.80,83,79... 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

As far as finding improved catalysts or operating conditions is concerned, a variety of experimental studies have been conducted. Promotion by alkali metal salts [11], especially cesium [12,13], has been shown to increase the EO selectivity. Similarly, it is observed that addition of chlorine to the catalyst results in higher EO selectivity [14-18]. Atmospheric pressure experiments in our laboratory have indicated that Cu-Ag bimetallic catalysts are more effective than pure Ag [19]. Furthermore, it is observed that after addition of Cs and Cl to the Cu-Ag bimetallic catalyst, it outperforms the corresponding promoted silver catalyst [20]. Published experimental studies have t)q)ically focused on a limited number of catalyst components, whereas one could potentially explore a wide variety of catalyst combinations using theoretical tools. 

The preparation of Ru supported catalysts by sol-gel method, indeed, was extended to obtain new formulations by changing the type of support. Alkali-promoted Ru/MgO systems were prepared starting from magnesium ethoxide, Ru3(CO)i2 and a cesium compound [9]. The gels were subjected to an activation/reduction procedure to substantially obtain Ru-CsOH/MgO and then tested as catalysts in the ammonia synthesis at atmospheric pressure. It was evidenced that the sol-gel prepared Cs-promoted Ru/MgO catalysts are much more active, under similar reaction comlitions, than the analogous catalysts prepared by the impregnation procedures reported in literature [10]. 

Electrochemical techniques have often been used to study the thermodynamics of solutions. The present study shows that the oxidation of titanium ions in fused alkali chlorides depends dramatically on the composition of the melt. Large cations weakly bonded to chloride ions, such as cesium ions, promote the formation of titanium hexachlorocomplexes, TiCl or TiCl , and results in a large stability range for the high oxidation states. Ti " reduces directly to metallic titanium. This behaviour is similar to that obtained with stronger ligands such as fluoride ions [9, 22]. 

As promoters, the loading amount of alkali metal is more than that of alkaline earth metal. Rare earth metal is similar with alkaline earth metal. The relationship between the distribution of cesium and barium on the Ru/AC surface and content are shown in Fig. 6.24. ... 

Similar olefination reactions were performed in weakly hydrated solid/liquid media (Mouloungui et ai, 1989). Depending on the nature of the solid inorganic base, the presence of small amounts of water was able to promote the olefination reaction. For example, potassium carbonate allows the reaction of furfural with ethyl (diethylphosphono)acetate in the presence of one or two equivalents of water, whereas no effect is observed for the cesium carbonate reaction. 

The cesium hydroxide-promoted addition of benzimidazole to phenylacetylene proceeded with exclusive anti-Markovnikov regioselectivity and complete stereoselectivity for the formation of the Z-isomer (Scheme 3.132) [142]. Similar reactivity was obtained for pyrrole and imidazole however, acyclic secondary amines such as diphenylamine or phenytmethylamine afforded mixtures of the stereoisomers. It should be noted that these additions were still regioselective for the anti-Markovnikov product. In related work, the base-assisted synthesis of enamines through the addition of indole derivatives to TMS-protected alkynes has been achieved using KOH as the promoter (Scheme 3.133) [143]. 

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