Rhodium complexes hydrogen pressure

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

The combined information gathered from kinetic studies,184 in situ high-pressure NMR experiments,184,185,195 and the isolation of intermediates related to catalysis, leads to a common mechanism for all the hydrogenolysis reactions of (102)-(104) and other thiophenes catalyzed by triphos- or SULPHOS-rhodium complexes in conjuction with strong Bronsted bases. This mechanism (Scheme 41) involves the usual steps of C—S insertion, hydrogenation of the C—S inserted thiophene to the corresponding thiolate, and base-assisted reductive elimination of the thiol to complete the cycle.184 185 195-198... 

PEGs with average molecular weights above 1000 are waxy solids under ambient conditions, but they melt under C02 pressure to become liquids under typical conditions of scC02 catalysis [63], The approach was demonstrated for the rhodium catalysed hydrogenation of styrene as a test reaction using Wilkinson s complex [(PPhs RhCl] as the catalyst (Scheme 8.6) [61],... 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

The rhodium hydride complex, 18b, was the only rhodium complex observed during the hydroformylation reaction at increased partial hydrogen pressure of 32 bar, as concluded from the absence of carbonyl signals in the IR difference spectra. The spectrum of the rhodium hydride resting state is taken as background at... 

The rhodium complexes were prepared in situ from the rhodium precursor [Rh(nbd)2](C104) (nbd = 2,5-norbornadiene) and applied in the hydrogenation experiments under an initial hydrogen pressure of 5 bar at 35°C. The dendrimer structure had almost no effect on the activity of the catalyst in the batch-wise rhodium-catalyzed hydrogenation of dimethyl itaconate (Scheme 4). 

Kollner et al. (29) prepared a Josiphos derivative containing an amine functionality that was reacted with benzene-1,3,5-tricarboxylic acid trichloride (11) and adamantane-l,3,5,7-tetracarboxylic acid tetrachloride (12). The second generation of these two types of dendrimers (13 and 14) were synthesized convergently through esterification of benzene-1,3,5-tricarboxylic acid trichloride and adamantane-1,3,5,7-tetracarboxylic acid with a phenol bearing the Josiphos derivative in the 1,3 positions. The rhodium complexes of the dendrimers were used as chiral dendritic catalysts in the asymmetric hydrogenation of dimethyl itaconate in methanol (1 mol% catalyst, 1 bar H2 partial pressure). The enantioselectivities were only... 

When the catalyst was used for simple olefin systems, it was not as active as with the amino acid precursors. Table III shows the relative rates for a variety of substrates, special care being taken in each case to purge oxygen. The slow rate of a-phenylacrylic acid was unexpected, but, it may be the result of a stable olefin-rhodium complex similar to the one Wilkinson (15) experienced with ethylene. Such a contention is consistent with the increased speed of hydrogenation with increased pressure. 

Effect of Hydrogen Pressure. Reactions carried out at different pressures of hydrogen resulted in essentially the same second-order rate constant, k = 6.2 x 104M 1 min-1 (first order in rhodium complex and first order in hydrogen), as shown in Table III. The solubility of hydrogen in toluene was calculated from data available in the International Critical Tables. All three reactions resulted in linear... 

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