Rhodium catalysts complexes, triarylphosphine

In 1976, water-soluble triarylphosphine, TPPTS (43, Figure 15), was synthesized and used for two-phase hydroformylation system. Here the rhodium catalyst is soluble in water, whereas the substrate and the product remain in organic solvents. The catalytic performance of the rhodium-TPPTS complexes is similar to the ordinary... 

In 2007, Trost and McClory reported the rhodium-catalyzed cycloaromatization of terminal alkynes bearing an amino or a hydroxy group (61 and 63) into the corresponding indoles and benzofurans (62 and 64) (Scheme 21.27) [36]. As described in the preceding section, McDonald et al. reported a similar transformation catalyzed by molybdenum carbonyl complex as a catalyst [10] (see Schemes 21.23 and 21.25). Use of rhodium catalyst provided some advantages in terms of the catalyst turnover and the selectivity. The combination of [RhCl(cod)]2 and a fluorinated triarylphosphine promoted the cyclization efficiently. Later, a ruthenium version of this cyclization was reported, as shown in Scheme 21.22 [31]. 

Most hydroformylation investigations reported since 1960 have involved trialkyl or triarylphosphine complexes of cobalt and, more recently, of rhodium. Infrared studies of phosphine complex catalysts under reaction conditions as well as simple metal carbonyl systems have provided substantial information about the postulated mechanisms. Spectra of a cobalt 1-octene system at 250 atm pressure and 150°C (21) contained absorptions characteristic for the acyl intermediate C8H17COCo(CO)4 (2103 and 2002 cm-1) and Co2(CO)8. The amount of acyl species present under these steady-state conditions increased with a change in the CO/ H2 ratio in the order 3/1 > 1/1 > 1/3. This suggests that for this system under these conditions, hydrogenolysis of the acyl cobalt species is a rate-determining step. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

In 1986 a new process came on stream employing a two-phase system with rhodium in a water phase and the substrate and the product in an organic phase. For propene this process is the most attractive one at present. The catalyst used is a rhodium complex with a sulphonated triarylphosphine, which is highly water-soluble (in the order of 1 kg of the ligand "dissolves" in 1 kg of water). The ligand, tppts (Figure 8.6), forms complexes with rhodium that are most likely very similar to the ordinary triphenylphosphine complexes (i.e. RhH(CO)(PPh3)3). 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

Biologically important substrates can be hydrogenated in aqueous solution by using sulfonated triarylphosphine ligands that confer water solubility upon the catalyst. However, it was later reported that rhodium metal was formed from these complexes in the presence of hydrogen. It was claimed that the role of the ligands is merely to prevent aggregation of the rhodium colloid produced. 

A second alternative for the separation of hydroformylation products from a rhodium [8] or cobalt [9] catalyst is to perform the catalytic reaction in a polar solvent using complexes of monosulfonated trialkyl- or triarylphosphines (e.g., TPPMS). Addition of both water and an apolar solvent such as cyclohexane gives a biphasic system. After separation of the apolar layer, the added apolar solvent must be stripped from the products. In order to form a homogeneous system with new substrate alkene, the polar catalytic phase must be freed from water, e.g., by azeotropic or extractive distillation. Clearly, these extra co-distillation steps are energy-consuming. 

Biphasic catalysis in the presence of water-soluble catalysts has been the most significant development in recent years. After the report of Kuntz on the synthesis of sulfonated triarylphosphine TPPTS (Figure 14.1) and its successful industrial application in Rh-catalyzed hydroformylation of propene, great attention has been focused on the scientific study and industrial application of water-soluble catalysts, especially on water-soluble phosphines [6, 7], phosphites, and other phosphide compounds as well as their rhodium complexes [8]. Among them, TPPTS is the most widely studied and applied. Other important phosphine hgands will he introduced later. 

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