Rhodium Catalyzed Olefin Hydroformylation

Conventional triorganophosphite ligands, such as triphenylphosphite, form highly active hydroformylation catalysts (95—99) however, they suffer from poor durabiUty because of decomposition. Diorganophosphite-modified rhodium catalysts (94,100,101), have overcome this stabiUty deficiency and provide a low pressure, rhodium catalyzed process for the hydroformylation of low reactivity olefins, thus making lower cost amyl alcohols from butenes readily accessible. The new diorganophosphite-modified rhodium catalysts increase hydroformylation rates by more than 100 times and provide selectivities not available with standard phosphine catalysts. For example, hydroformylation of 2-butene with l,l -biphenyl-2,2 -diyl... 

Our approach is to use the inexpensive ligands that are already used industrially as well as conventional solvents. The goal of this project is to develop a thermomorphic approach to the rhodium-catalyzed hydroformylation of higher olefins (>Ce) that enhances conversion rates and ease of product recovery while minimizing catalyst degradation and loss. 

Optically active aldehydes are important precursors for biologically active compounds, and much effort has been applied to their asymmetric synthesis. Asymmetric hydroformylation has attracted much attention as a potential route to enantiomerically pure aldehyde because this method starts from inexpensive olefins and synthesis gas (CO/H2). Although rhodium-catalyzed hydrogenation has been one of the most important applications of homogeneous catalysis in industry, rhodium-mediated hydroformylation has also been extensively studied as a route to aldehydes. 

Hydroformylation of hetero olefins such as carbonyl compounds is not known to proceed with significant levels of efficiency, whereas the hydroformylation of olefins has been developed to a sophisticated stage. Generally, aldehydes resultant from the latter process exhibit a low propensity to undergo further hydroformylation, with the exception of some activated aldehydes. The rhodium-catalyzed hydroformylation of formaldehyde is the key step in the synthesis of ethyleneglycol from synthesis gas. Chan et al. found... 

Rhodium-catalyzed biphasic hydroformylation of olefins. The Ruhrchemie-Rhone Poulenc process for manufacturing... 

The discovery and use of fluorophosphites and chlorophosphites as trivalent phosphorus ligands in the rhodium catalyzed, low-pressure hydroformylation reaction are described. The hydroformylation reaction with halophosphite ligands has been demonstrated with terminal and internal olefins. For the hydroformylation of propylene, the linear to branched ratio of the butyraldehyde product shows a strong dependency on the ligand to rhodium molar ratios, the reaction temperature, and the carbon monoxide partial pressure. 

Both the rhodium and the cobalt complexes catalyze olefin isomerization as well as olefin hydroformylation. In the case of the rhodium(I) catalysts, the amount of isomerization decreases as the ligands are altered in the order CO > NR3 > S > PR3. When homogeneous and supported amine-rhodium complexes were compared, it was found that they both gave similar amounts of isomerization, whereas with the tertiary phosphine complexes the supported catalysts gave rather less olefin isomerization than their homogeneous counterparts (44, 45). 

Scheme 8.1 Rhodium-catalyzed hydroformylation of alkenes leading to linear (L) and branched (B) aldehydes and isomerized (IS) olefins.

Figu re 5.3. Rhodium-catalyzed hydroformylation of long-chain olefins under FBS conditions (a) and in sc C02 (b). 

An example is the rhodium catalyzed hydroformylation reaction, which is an industrially important homogenous catalytic process [3]. In contrast, it is amazing that such an important transition-metal catalyzed C/C bond-forming process has been employed only rarely in organic synthesis [4]. Part of the reason stems from the difficulty in controlling stereoselectivity. Even though some recently developed chiral rhodium catalysts allow for enantio- and diastereoselective hydroformylation of certain specific classes of alkenes [5, 6], only little is known about the diastereoselective hydroformylation of acyclic olefins [7, 8]. 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

Hydroformylation, also known as the oxo process, is the transition metal catalyzed conversion of olefins into aldehydes by the addition of synthesis gas (H2/CO) and is the second largest homogeneous process in the world in which more than 12 billion pounds of aldehydes are produced each year.222 Aldehydes are important intermediates in a variety of processes such as the production of alcohols, lubricants, detergents, and plasticizers. Several transition metals can catalyze the hydroformylation of olefins, but rhodium and cobalt have been used extensively. 

Considerable progress has been made recently by using phosphites (general formula (R0)3P) as ligands in rhodium-catalyzed oxo synthesis. Rhodium catalysts with phosphites such as 2 and 3 showed high activities in the hydroformylation of long-chain olefins [39, 40]. 

Only limited data are available for the kinetics of oxo synthesis with the water-soluble catalyst HRh(CO)(TPPTS)3. The hydroformylation of 1-octene was studied in a two-phase system in presence of ethanol as a co-solvent to enhance the solubility of the olefin in the aqueous phase [115]. A rate expression was developed which was nearly identical to that of the homogeneous system, the exception being a slight correction for low hydrogen partial pressures. The lack of data is obvious and surprising at this time, when the Ruhrchemie/ Rhone-Pou-lenc process has been in operation for more than ten years [116]. Other kinetic studies on rhodium-catalyzed hydroformylation have been published, too. They involve rhodium catalysts such as [Rh(nbd)Cl]2 (nbd = norbomadiene) [117] or [Rh(SBu )(CO)P(OMe)3]2 [118], or phosphites as ligands [119, 120]. 

Scheme 3.4 Synthesis ofthe chiral fluorous (R,S)-3-H F -Binaphos (16) ligand for the rhodium-catalyzed enantioselective hydroformylation of olefins in supercritical carbon dioxide (seCOj) [19] (Rf = (CH2)2(CF2)6F).

This hydrocarboration method is a valuable tool in industrial and laboratory synthesis, since it allows introduction of the one-carbon unit of carbon monoxide into unsaturated substrates and construction of new carbon skeletons with aldehyde functions or derivatives thereof formed by reduction, oxidation, condensation and other conversions. Hydroformylation, mainly catalyzed by cobalt, rhodium, or platinum complexes is an unsymmetrical 1,2-addition leading to linear and branched products if terminal olefins are used as the substrate. Since linear products are normally the industrial products wanted54, considerable efforts have concentrated on the control of regiochemistry. Other problems of the hydroformylation method arise from side reactions such as hydrogenation, double bond migration, and subsequent reactions of the products (e.g., condensation, reduction, dccarbonylation)54. 

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