Rhodium catalysts hydroformylation

A method has been developed for the continuous removal and reuse of a homogeneous rhodium hydroformylation catalyst. This is done using solvent mixtures that become miscible at reaction temperature and phase separate at lower temperatures. Such behavior is referred to as thermomorphic, and it can be used separate the expensive rhodium catalysts from the aldehydes before they are distilled. In this process, the reaction mixture phase separates into an organic phase that contains the aldehyde product and an aqueous phase that contains the rhodium catalyst. The organic phase is separated and sent to purification, and the aqueous rhodium catalyst phase is simply recycled. 

A method to reduce degradation/deactivation of a phosphite modified rhodium hydroformylation catalyst in the separation system involves feeding a diene such as butadiene to the vaporizer to convert the phosphite-modified rhodium catalyst to a more stable form. [34] In the reactor, the diene is hydrogenated and catalyst activity is restored. 

Two-Phase (Water-Soluble) Rhodium Hydroformylation Catalysts... 

Rhodium Hydroformylation Catalysts with Bidentate Ligands... 

J. Feldman and M. Orchin, Membrane-supported rhodium hydroformylation catalysts. /. Mol. Catal, 63 (1990) 213. 

A similar effect on activity and selectivity is observed upon addition of co-sol-vents to the dimeric rhodium hydroformylation catalyst, [ Ru2(/<-S- Bu) (CO) -(TPPTS)2] [12]. Specifically, there is an inverse relationship between solvent solvo-phobicity [13] and the log of the ./iso ratio of the aldehyde products for the hydroformylation of 1-octene. The catalyst is most selective to linear products in water. 

This result is in accordance with virtually all other phosphine- or phosphite-coordinated rhodium hydroformylation catalysts, where an excess of phosphine... 

Fig. 5.3-2 Schematic representation of a supported ionic liquid phase (SILP) catalyst exemplified for a typical rhodium hydroformylation catalyst.

It has been suggested that the acyl complex is [RCO-Co(CO)4], or [RCO Co(CO)3PBu3l for modified catalysts, but a site of unsaturation cis to the acyl ligand may be required (c.f.. Reference 60). A more probable formulation is therefore [RCO Co(CO)3], or [RCO Co(CO)2PBu3]. A binuclear, free-radical mechanism for the cobalt-catalyzed hydroformyla-tion of styrene or other conjugated substrates has also been proposed. These studies are far-reaching, especially because similar binuclear elimination steps have not received much consideration in studies of rhodium hydroformylation catalysts. ... 

Oswald, A. A. Hendriksen, D. E. Kastrup, R. V. Mozeleski, E. J. Electronic Effects on the Synthesis, Structure, Reactivity, and Selectivity of Rhodium Hydroformylation Catalysts. In Homogeneous Transition Metal Catalyzed Reaction Moser, W. R., Slocum, D. W., Eds. American Chemical Society Washington, DC, 1992 pp 395-418. 

Scavenging of free phosphines by electrophiles such as protons, other metals, conjugated enones, etc. presents a potential route to phosphine loss in catalytic systems. As yet, the participation of phosphonium intermediates has not been reported for rhodium hydroformylation catalysts, but they could be easily conceived, especially when dienes or enones are also present. 

Currently, a wide range of methods are available to generate active rhodium hydroformylation catalysts from catalyst precursors based on rhodium in oxidation states of O-III. Because of the almost unmanageable amount of protocols concerning the rhodium-based hydroformylation in the literature, a clear conclusion about the efficiency and duration of catalyst formation processes prior to the hydroformylation is hard to draw. A deeper understanding of these processes occurring prior to the hydroformylation would be of interest in order to distinguish between different catalyst precursors. 

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