Ethanol catalysts, rhodium complexes

The sol-gel entrapment of the metal complexes [Ru(p-cymene)(BINAP)Cl]Cl and the rhodium complexes formed in situ from the reaction of [Rh(COD)Cl]2 with DlOP and BPPM has been reported by Avnir and coworkers [198]. The metal complexes were entrapped by two different methods the first involved addition of tetramethoxysilane to a THF solution of the metal complex and triethylamine, while the second method was a two-step process in which aqueous NH4OH was added to a solution of HCl, tetramethoxysilane and methanol at pH 1.96 followed by a THF solution of the appropriate metal complex. The gel obtained by each method was then dried, crushed, washed with boiling CH2CI2, sonicated in the same solvent and dried in vacuo at room temperature until constant weight was achieved. Hydrogenation of itaconic acid by these entrapped catalysts afforded near-quantitative yields of methylsuccinic acid with up to 78% e.e. In addition, the catalysts were found to be leach-proof in ethanol and other polar solvents, and could be recycled. 

Wan and Davis135,138 modified rhodium complexes with the water soluble chiral tetrasulfonated binap ligand 26 (Table 2) and used them as catalysts in the asymmetric hydrogenation of 2-acetamidoacrylic acid in aqueous media. The e.e. observed in neat water using Rh/26 was approximately the same as that obtained with the unsulfonated Rh/binap in ethanol (68-70% versus 67%).135... 

Acetic acid (CH3COOH) is a bulk commodity chemical with a world production of about 3.1 x 106 Mg/year, a demand increasing at a rate of +2.6% per year and a market price of US 0.44-0.47 per kg (Anon., 2001a). It is obtained primarily by the Monsanto or methanol carbonylation process, in which carbon monoxide reacts with methanol under the influence of a rhodium complex catalyst at 180°C and pressures of 30-40 bar, and secondarily by the oxidation of ethanol (Backus et al., 2003). The acetic fermentation route is limited to the food market and leads to vinegar production from several raw materials (e.g., apples, malt, grapes, grain, wines, and so on). 

A rhodium complex with PNIPAM modified with propyldiphosphine 49 groups was more active in hydrogenation of 1-octadecene and 1-dodecene in a system containing heptane and 90% aqueous ethanol (Experiment 11-3, Section 11.7). At 22 °C the catalyst is virtually insoluble in heptane and there is no reaction, but at 70 °C this system forms a homogeneous solution of the polymer and the reaction takes place. The catalyst can be reused without loss of activity [94],... 

Limiting the reaction to the addition of one mole of H2 showed that the heterogenized catalyst was 2-4 times more selective toward side-chain hydrogenation [polystyrene 2% DVB, anchored RhCl(PPh3)3] in benzene than the homogeneous rhodium complex. In the solvent benzene ethanol (1 1), a poorer swelling solvent, the selectivity was enhanced. 

The hydrogenation of (Z)-a-Af-acetamidocinnamic acid in ethanol-toluene with complexes 30 and 31 led to complete conversion. It was observed that the ratio of the two solvents plays an important role in the obtained enantiomeric excess of the reaction (Table 22.9). For 30, an increase in the ee was observed with decreasing ethanol content, whereas for 31 the reverse behavior was noted. For (R,R,R)-31 complex an ee of ca. 80% was obtained at an ethanol-toluene ratio of 2/1. This value is comparable with literature values for the rhodium-DIOP catalyst, and together with the solvent-dependent behavior of 31, it is in agreement with the assumed dissociation mentioned above [44]. 

Rhodium(III) complexes [e.g. (i-Pr,P)2Rh(H)Cl2] in the presence of quaternary ammonium salts are excellent catalysts for the hydrogenolysis of chloroarenes under mild conditions [5] other labile substituents are unaffected. Hydrodehalogenation of haloaryl ketones over a palladium catalyst to give acylbenzenes is also aided by the addition of Aliquat [6]. In the absence of the phase-transfer catalyst, or when the hydrogenation is conducted in ethanol, the major product is the corresponding alkyl-benzene, which is also produced by hydrodehalogenation of the halobenzyl alcohols. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Hydroamination of allenes and 1,3-dienes in the presence of Ni(II), Pd(II), and Rh(III) complexes yields product mixtures composed of simple addition products and products formed by addition and telomerization.288 Nickel halides308 and rhodium chloride309 in ethanol [Eq. (6.51)] and Pd(n) diphosphine complexes310 are the most selective catalysts in simple hydroamination, while phosphine complexes favor telomerization 288... 

Butadiene and ethylene are codimerized with a soluble rhodium-phosphine complex as the catalyst. Very little has been reported on the mechanistic evidence for this reaction. However, a catalytic cycle as shown in Fig. 7.9 involving a rhodium hydride seems likely. Reducing rhodium trichloride with ethanol in the presence of a tertiary phosphine generates the hydride complex 7.32. The 1,4-hydride attack on the coordinated butadiene gives an rf-allyl complex. This is shown by the conversion of 7.33 to 7.34. Ethylene coordination to 7.34 produces 7.35. 

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