Product rates rhodium catalysis

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

Hydration of nitriles providing carboxamides is usually carried out m strongly basic or acidic aqueous media - these reactions require rather bars conditions and suffer from incomplete selectivity to the desired amide product. A few papers in the literature deal with the possibihty of transition metal catalysis of this reaction [28-30]. According to a recent report [30], acetonitrile can be hydrated into acetamide with water-soluble rhodium(I) complexes (such as the one obtained from [ RhCl(COD) 2] and TPPTS) under reasonably mild conditions with unprecedently high rate... 

It is interesting to note that, when operating at approximately 5% water content, and with good reaction rates, it is possible to save about 30% of the energy costs of the process when compared to the Monsanto process. In addition, the current price of rhodium is 20-fold that of iridium. Even with annual unit capacity productions as large as 500 000 tons, it is still possible to improve catalysis and to achieve significantly better performances. 

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. 

In asymmetric catalysis a prochiral substrate binds to an enantiomerically pure catalyst to generate a pair of diastereomeric intermediates. The energy difference and the rate of exchange between them controls the optical yield (e.e.) of the final product. In the case of a-aminocinnamic acid derivatives, the acyl auxiliary on the nitrogen is required to enable the substrate to form a chelate complex with rhodium.12 The mechanism of this reaction is shown in Fig. 22-3 the ligand in this case is DIPAMP (22-XV). 

The regioselectivity in the hydroformylation of monohydroxyben-zenes was the same for the host catalyst and a triphenylphosphine complex of rhodium (with a molar product ratio linear branched = 2 1) (35). No isomerization was observed. Rates of the conversion of monohydrox-ybenzenes when host-guest catalysis is employed are lower than when a triphenylphosphine complex of rhodium is used. In contrast, dihydroxy substrates, which are more strongly boimd to the host, react at higher rates, with an initial increase by a factor of 4 relative to catalysis by the bare rhodium complex. Dihydroxybenzenes also gave the selectivities to linear products (linear branched molar product ratio > 20 1, see below). At 30% conversion, product inhibition took place. 

It must be also noted that supported ionic liquid phase (SILP) catalysis can also be successfully combined with supercritical fluids. Cole-Hamilton et al. [127] have reported recently high activity (rates up to 800 h ), stable performances (>40 h) and minimum rhodium leaching (0.5 ppm) in the hydroformylation of 1-octene using a system that involves flowing the substrate, reacting gases and products dissolved in... 

Supramolecular chemistry has been a very popular research topic for three decades now. Most applications are foreseen in sensors and opto-electronical devices. Supramolecular catalysis often refers to the combination of a catalyst with a synthetic receptor molecule that preorganizes the substrate-catalyst complex and has also been proposed as an important possible application. The concept, which has proven to be powerful in enzymes, has mainly been demonstrated by chemists that investigated hydrolysis reactions. Zinc and copper in combination with cyclodextrins as the receptor dramatically enhance the rate ofhydrolysis. So far, the ample research devoted to transition metal catalysis has not been extended to supramolecular transition metal catalysis. A rare example of such a supramolecular transition metal catalyst was the results of the joined efforts of the groups of Nolte and Van Leeuwen [SO], They reported a basket-shaped molecule functionalized with a catalytically active rhodium complex that catalyzed hydrogenation reactions according to the principles of enzymes. The system showed substrate selectivity, Michaelis Menten kinetics and rate enhancement by cooperative binding of substrate molecules. The hydroformylation of allyl catachol substrates resulted in a complex mixture of products. 

Catalysed Aquation.— In 1971 Rudakov and Kozhevnikov reported a correlation between the rate constants for metal-ion (M +) catalysed solvolysis of t-butyl halides and the stability constants of the respective complexes Now these authors have shown that a similar correlation applies, albeit rather approximately, to metal-ion catalysis of aquation of halogeno-cobalt(ra), -chromium(ni), and -rhodium(in) complexes. In fact the catalysts mentioned include not only metal ions such as Hg , Tl +, and Ag+, but also complexes such as HgCl+ and T1C1 +. This correlation can be improved by making an allowance for the different coulombic repulsions in systems of different charge products. If the rate constant for the catalysed aquation is ki and that for uncatalysed aquation Ato, the stability constant of the metal-ion complex produced K, the product of the charges on the reactants zazb, and C is a coulombic interaction constant, then the correlation conforms to the equation... 

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