Rhodium complexes Subject

Formation of the methyl-rhodium complex is analogous to the formation of CH3-C(C0)4 from CH30H2 arid Co(C0K as proposed by Wender. The difference here is that the nature of the active rhodium species is not known. Under the present conditions,homologation does not occur because CO is not present however, addition of the methyl-rhodium species to benzaldehyde must occur as shown in (19), metal adds to the oxygen. The product in (19) is then subject to acid catalyzed etherification to obtain the methyl ether. 

Steric effects were also found to be important for determining the reactivity of rhodium complexes containing N-heterocyclic carbene (NF1C) ligands [47] (Scheme 10), which have been the subject of intense in-... 

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

One of the success stories of transition metal catalysis is the rhodium-complex-catalyzed hydrogenation reaction. Asymmetric hydrogenation with a rhodium catalyst has been commercialized for the production of L-Dopa, and in 2001 the inventor, Knowles, together with Noyori and Sharpless, was awarded the Nobel Prize in chemistry. After the initial invention, (enantioselective) hydrogenation has been subject to intensive investigations (27). In general, hydrogenation reactions proceed... 

Applying P-31 NMR to the field of hydroformylation catalysis by triphenylphosphine rhodium complex-based systems is the subject of this chapter. These hydroformylation catalyst systems are of high academic and technological interest. They are effective for hydroformylat-ing 1-olefins at low pressure and temperature and exhibit a high selectivity to n-aldehydes ... 

In 1986, Reetz et al. reported that chiral Lewis acids (B, Al, and ll) promoted the aldol reaction of KSA with low to good enantioselectivity [115]. The following year they also introduced asymmetric aldol reaction under catalysis by a chiral rhodium complex [116]. Since these pioneering works asymmetric aldol reactions of silyl enolates using chiral Lewis acids and transition metal complexes have been recognized as one of the most important subjects in modern organic synthesis and intensively studied by many synthetic organic chemists. 

A different extractive work-up is based on fluorous biphasic systems. This concept was first introduced for fhe recovery of rhodium complexes from hydroformylation processes [13] and was soon extended to separation procedures in combinatorial chemistry [14]. It has been fhe subject of several reviews [15-21]. 

Styrylboronic ester 24 was subjected to the catalytic hydroboration with cat-echolborane in the presence of rhodium complexes coordinated with chiral bis-phosphine ligands. Oxidation of the resulting 1,2-diboryl product 25 gave optically active 1-phenyl-1,2-ethanediol (26) (Scheme 6) [26]. The reaction with BINAP (7) at -60 °C gave (S)-diol 26 of over 70% ee. 

The reaction system (6-37) includes the thermal azo-extrusion of a cyclic azo compound to a cyclopropane derivative and the direct formation of cyclopropanes, catalyzed by metal complexes. Synthetic routes to cyclopropane derivatives became an important subject in the last two decades, and one frequently used method is the 1,3-dipolar cycloaddition of a diazoalkane to an alkene followed by thermal or photolytic azo-extrusion of the 4,5-dihydro-3//-pyrazole formed to the cyclopropane derivative (6-37 A). This route can be followed in many cases without isolation, or even without direct observation, of the 4,5-dihydro-3//-pyrazole. Therefore, it is formally very similar to cyclopropane formation from alkenes with diazoalkanes, in which a carbene is first formed by azo-extrusion of the diazoalkane (see Sect. 8.3). As shown in pathway (6-37 B), this step can be catalyzed by copper, palladium, or rhodium complexes (see Sects. 8.2, 8.7, and 8.8). There are cases where it is not clearly known whether route A or B is followed. Scheme 6-37 also includes... 

Synthetic binuclear complexes as catalysts, aiming at high activity, have been subjects of intensive research [3], However, the number of novel bimetallic catalysts that are active and actually operating via a bimetallic mechanism is extremely scarce. Stanley and co-workers reported a bimetallic rhodium complex that is an active and selective hydroformylation catalyst that was proposed to operate via a cooperative mechanism. 

The Monsanto rhodium catalyst system has been the subject of numerous reviews [6, 10-17], including a very recent one by Haynes [18]. At high water content, typically more than 8% w/w, the overall rate is first order in both the rhodium complex and the methyl iodide reactant and zero order in both methanol and CO reactants. The catalytic cycle, which is usually adopted is shown in Fig. 20.1. The first step is the CH3I oxidative addition reaction to the [Rhl2(CO)2] active... 

The rhodium hydride, [(Iriphos)RhH], will react with thiophene and benzo[(>] thiophene to yield complexes 129 and 130, respectively, which yield other Rh-containing complexes when they were subjected to electrophilic reagents [7]. The catalytic transformation of BT into 2-ethylthiophenol can be achieved with this soluble rhodium complex (Scheme 141) [157]. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

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