Rhodium-based catalyst systems

Scheme 39.2 Examples of typical rhodium-based catalyst systems and modified derivatives of triphenylphosphine as used to control their solubility with scC02 in homogeneous or multiphase systems (BARF=tetrakis[3,5-bis (trifluoromethyl)phenyl]borate).

The levels of liquid by-products formed are a significant improvement over those achieved with the conventional high water rhodium based catalyst system and the quality of the product obtained under low water concentrations is exceptional [4],... 

Rhodium has been successfully used in a range of Suzuki-Miyaura type reactions. Satoh and Miura have reported the Suzuki-Miyaura-type cross-coupling of arylboron compounds with aryl halides in the presence of a rhodium-based catalyst system to produce the corresponding biaryls (Scheme 13.21). They also found, unexpectedly, that when employing benzonitrile as substrate under similar reaction conditions a multiple arylation is observed, in which nucleophilic arylation on the cyano group and subsequent ortho arylation via C-H bond cleavage is involved. 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

A lot of research has been published on hydroformylation of alkenes, but the vast majority of the effort has been focused on the chemistry of various metal-ligand systems. Quantitative kinetic studies including modeling of rates and selectivities are much more scarce. In this work, we present the approach to modeling of hydroformylation kinetics and gas-solubility. Hydroformylation of 1-butene with a rhodium-based catalyst was selected as a case study. 

The magnitudes of the rate constants for the iridium catalyst were close to those obtained for rhodium 3 and osmium 5 based catalyst systems at similar conditions. However, the unusual dependence on catalyst concentration affects its general utility in comparison to other homogeneous catalysts for the hydrogenation of NBR. 

Rhodium,3 osmium4 and ruthenium5 based catalyst systems are affected by nitrile in a similar way. This arises from the relatively high affinity of complexes of these metals towards nitrile group coordination.11 The resulting equilibrium between free catalyst and catalyst with bound nitrile reduces the effective catalyst concentration and hence reaction rate for a given set of conditions. 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

Given the importance of chiral amines to synthetic chemistry as well as other fields asymmetric hydrogenation of imines has attracted wide interest but limited success compared to C=C and C=0 bond reduction. The first asymmetric hydrogenation of imines was carried out in the seventies with mthenium- and rhodium-based catalysts, followed later by titanium and zirconium systems [82]. Buchwald found that... 

Interesting features in the polymerisation of acetylenic monomers are displayed by rhodium-based catalysts they may be applied in metal akyl or hydride-activated systems, e.g. RhCl3-LiBH4 [49] and [(Cod)Rh]1 [[BPh4]... 

In addition to rhodium-based catalysts, iridium-based eatalysts have also been developed in a process known as the Cativa process. The iridium system follows a cycle similar to the rhodium system in Figure 14-16, beginning with oxidative addition of j CH3I to [Ir(CO)2l2] The first step in the iridium system is much more rapid than in the Monsanto process and the second step is much slower the second step, involving alkyl . migration, is rate determining for the Cativa process. ... 

Hydroformylation refers to the addition of hydrogen and carbon monoxide to unsaturated systems. The hydroformylation of olefins is also known as the oxo synthesis or the Roelen reaction in honor of its inventor. It is one of the major industrial processes. Technical plants use cobalt- or rhodium-based catalysts the active species are supposed to be mononuclear complexes (194). The most desired oxo product is butanal, generated by the hydroformylation of propylene (195). 

In addition to rhodium-based catalysts, iridium-based catalysts have also been developed for carbonylation of methanol. The iridium system, known as the Cativa process, follows a cycle similar to the rhodium system in Figure 14.20, beginning with oxidative addition of... 

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