Rhodium, catalytic

Following these early investigations, the major breakthrough in this field has been the change in both the structure of the ligand frame and that of the anion in the starting rhodium catalytic precursor. The [Rh(TFA) (S)-(Cp,Cp)oxoPro-... 

Fig. 1 Phosphorus ligands 1-5 providing high enantioselectivity in asymmetric hydro-formylation. Ligand 1 for Pt systems. Ligands 2-5 for rhodium catalytic systems...

This chapter mainly focuses on the latest achievements and recent developments in asymmetric hydroformylation. Since several reviews have been made in the last decade [9,14-16], the chapter discusses the contributions reported between 2000-2005 in particular, although the main diphoshites and phosphine-phosphite rhodium catalytic systems discovered since 1995 are also considered because of their significance in the subject. Particular attention is paid to mechanistic aspects and characterization of intermediates in the case of the hydroformylation of vinyl arenes because this is one of the most important breakthroughs in the area. The application of this catalytic reaction to different type of substrates, in particular dihydrofurans and unsaturated nitriles is the other main subject of this chapter because of their interest in organic synthesis and their industrial relevance. 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

This material has a dielectric constant of 43.3 at 30UC it has very low proton basicity (pA, = — 12.9) and is a weak Lewis base (117). Indeed, sulfolane is an excellent solvent for the rhodium catalytic system, giving good rates... 

When compared to the rhodium catalytic system, it can be seen that under identical conditions of temperature and pressure the iodide-promoted ruthenium system produces ethylene glycol at a comparable or somewhat lower rate. However, the rate of methanol formation is substantially higher than for the rhodium system. Thus, the overall activity of this ruthenium system is higher than that of the rhodium-based system, but the selectivity to the two-carbon product is lower. 

In both catalytic cycles none of the complexes exceed 18 electrons. In order to account for the effect of phosphine on the nib ratio in rhodium catalysis (vide infra), an associative mechanism has been proposed in which alkene coordinates directly to the 18-electron HRh(CO)2L2 complex. Since this gives a 20-electron complex, this mechanism is not particularly attractive. A number of the intermediates in the rhodium catalytic cycle have been verified by various spectroscopic techniques.13,14... 

In the case of rhodium catalytic systems the observed regularities in hydroformylation have been interpreted on the basis of the currently accepted mechanism for hydroformylation 15). In the following sections the above explanation is critically reviewed and applied to more recent experiments of asymmetric hydroformylation with different catalytic systems. 

Scheme 9.4 illustrates the steps involved in the phosphine-rhodium catalytic cycle. The immediate precursor to the active species is either HRh(CO)2(PR3)2 or HRh(CO)(PR3)333 depending on the concentration of CO with respect to phosphine. 

A comparably efficient kinetic resolution of 3-alkylitaconates can be achieved with this rhodium catalytic system (entry 3). The corresponding diacid affords an effective catalytic kinetic resolution using the standard protocol for asymmetric transfer hydrogenation43. 

The carbonylation of methanol in acetic acid represents an important industrial process, which has been developed by Monsanto Corporation using a homogeneous rhodium complex. Extensive investigations on the rhodium catalytic system have been carried out and Liu et al. [77] have studied the use of PVP-stabilized Rh nanoparticles for this reaction (Scheme 11.10). The stable PVP-Rh colloid presents a lower activity than Monsanto s homogeneous catalyst under the same drastic conditions (140°C, 54bar). However, the colloidal metal catalyst could be reused several times with an increased activity (TON = 19700 cydes/atom Rh), which... 

A highly efficient and convenient rhodium catalytic system was developed for the direct hydroheteroarylation of un saturated compoiuids with heteroarenes (Scheme 10) [28]. A base cocatalyst was found to be crucial for the heteroarene C-H activation step. Substrate scope was very broad, including both electron-deficient pyridine N-oxides and electron-rich azoles. The identical catalytic system was found to be appropriate for hydroheteroarylation of both alkenes and alkynes with excellent regioselectivity and stereoselectivity. 

The concept of a single atmular duct yielding a low pressure drop can also be used for coating with additional supports such as y- or a-alumina. This was applied, for example, for CPO of light alkanes using platinum and rhodium catalytically active species [24]. 

For the rhodium catalytic systems containing BDPP or CHIRAPHOS the regioselectivities depend in the same way on pressure and temperature. The amount of branched aldehyde increases when the pressure is raised and/or... 

Several diaryl phosphines have been sulphonated to form a rhodium catalytic precursor for the hydroformylation of styrene in an aqueous solution. The enantioselectivities reported for (S,S)-BDPPTS are in the range of 14-17% only [58]. 

On the other hand, the rhodium catalytic system with Lil stabilizer/promoter, allows operation at around 5% w/w, and the process has been developed by Celanese [15, 30]. Here, the signiflcant amount of methyl acetate reduces the concentration of HI, which is well known to lead to the formation of the [Rhl4(CO)2]" species. This complex is inactive during carbonylation but responsible for the WGS reaction (Eq. 20.2). The role of Lil is not only to stabilize the Li[Rhl2(CO)2] catalytic species but also to allow the two reactions in Equations 20.7 and 20.8. 

Ruthenium- and rhodium-catalytic systems for the direct cross-dehydrogenative coupling (CDC) of acrylamides with electron-deficient alkenes forming (Z, )-dienamides using copper(II) acetate as the oxidant has been developed. Both methods exhibit wide functional group compatibility and substrate flexibility. It is proposed that the reaction is initiated by cyclometalation of acrylamide by amide-directing C-H bond activation. Coordination of the alkene to the metal centre, followed by insertion of the carbon-carbon double bond, forms a seven-membered ruthacycle or rhodacycle species. Subsequent -elimination occurs to afford the desired (Z, )-dienamide. A CDC between two heteroarenes is effected with copper(II) acetate in the absence... 

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