Hydroformylation reaction cycle

Fig. 6 Ligand-modified rhodium-catalyzed hydroformylation reaction cycle...

The first catalyst used in hydroformylation was cobalt. Under hydroformylation conditions at high pressure of carbon monoxide and hydrogen, a hydrido-cobalt-tetracarbonyl complex (HCo(CO)4) is formed from precursors like cobalt acetate (Fig. 4). This complex is commonly accepted as the catalytic active species in the cobalt-catalyzed hydroformylation entering the reaction cycle according to Heck and Breslow (1960) (Fig. 5) [20-23]. 

In the late 1960s, Wilkinson postulated the reaction cycle of the ligand-modified rhodium catalyzed hydroformylation (Fig. 6). 

A method for observing intermediates directly in the reaction cycle is in situ IR spectroscopy under reaction conditions. As early as 1975, Penninger published a contribution concerning in situ IR spectroscopic studies of cobalt carbonyl modified by tri-u-butylphosphine as a hydroformylation catalyst [58] at relatively low catalyst concentrations of 2 mmoll-1. The observed carbonyl... 

In almost all kinetic investigations it is found that hydroformylation is first order in substrate and hydrogen concentration. This suggests slow steps in the reaction cycle involving olefin and hydrogen, and the reaction rate ta becomes... 

The catalytic cycle for hydroformylation reactions has also been established for certain homogeneous catalysts. Scheme 8.4 illustrates that for HRh(CO)2(PPh3)2, although the cycle is the same for the analogous cobalt catalyst. 

A key issue in the hydroformylation reaction is the ratio of linear and branched product being produced (Figure 7.1). Scientifically it is an interesting question how the linearity can be influenced and maximised by influencing the kinetics and changing the ligands. The catalytic cycle for the formation of linear aldehyde is shown in Figure 7.2. The first processes for... 

Sketch plausible catalytic cycles for (a) the OXO [i.e., HCo(C0)4-catalyzed] and (b) the Union Carbide hydroformylation reactions. 

T,he hydroformylation reaction or oxo synthesis has been used on an industrial scale for 30 years, and during this time it has developed into one of the most important homogeneously-catalyzed technical processes (I). A variety of technical processes have been developed to prepare the real catalyst cobalt tetracarbonyl hydride from its inactive precursors, e.g., a cobalt salt or metallic cobalt, to separate the dissolved cobalt carbonyl catalyst from the reaction products (decobaltation) and to recycle it to the oxo reactor. The efficiency of each step is of great economical importance to the total process. Therefore many patents and papers have been published concerning the problem of making the catalyst cycle as simple as possible. Another important problem in the oxo synthesis is the formation of undesired branched isomers. Many efforts have been made to keep the yield of these by-products at a minimum. 

Although the overall reaction mechanisms (catalytic cycles) written for hydroformylation reactions with an unmodified cobalt catalyst (Scheme 1) and the rhodium catalyst (Scheme 2) serve as working models for the reaction, the details of many of the steps are missing and there are many aspects of the reaction that are not well understood. 

Much less is known concerning the platinum-catalyzed hydroformylations. However, a reasonable catalytic cycle can be constructed (Scheme 3) from the available information on the generation and reactions of many of the intermediate complexes shown.6,8,9,15 The ability of platinum to catalyze hydroformylation reactions while palladium is not a good catalyst could be due to the ability of platinum to achieve the +4 oxidation state more readily. 

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

In an industrial hydroformylation reaction with a rhodium catalyst in the presence of excess phosphine and high pressures of CO, what would probably be the minimum number of catalytic cycles and intermediates ... 

A hypothetical catalytic cycle for asymmetric hydroformylation reaction is shown in Fig. 9.13. The precatalyst Rh(acac)(P-P) reacts with H2 and CO to give the square planar catalytic intermediate 9.47. Alkene addition to 9.47 can lead to the formation of 9.48, 9.49, and 9.50. The steric requirements of the chelating ligand would have to be such that the formation of 9.50 is avoided. This is because alkene insertion into the Rh-H bond in this case would lead to the formation of the linear rather than the branched alkyl. Both 9.48 and 9.49, which differ in the coordination positions of the phosphorus atoms, can give 9.51, which has the desired branched alkyl ligand. 

Figure 9.13 Proposed catalytic cycle for asymmetric hydroformylation reaction with [Rh(acac)(P-P)] as the precatalyst. The chelating chiral phosphorus ligand could be 9.45 or 9.46. acac = acetylacetonato anion.

The active metal complex could be quantitatively stripped from the silica support material by washing with methanol, and consequently, the receptor (the silica support) could be used in several, different reaction cycles. The rhodium catalyst was reused in 11 consecutive runs in batch mode for the hydroformylation of oct-l-ene (80 °C, 20-50 bar) there was no noticeable loss of activity (<0.1% per run) when a xantphos derivative was used as a ligand, and there was an approximate loss of 1% per run when a triphenyl-phosphine-derived monodentate ligand was used (Figure 32). Turnover... 

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

An example is the hydroformylation reaction of cyclohexene catalyzed by the unsaturated compound HCo(CO)3 which is formed under reaction conditions from the precursor HCo(CO)4. Following the usual mechanism (see, e. g., [18]), the catalytic cycle is depicted in Scheme 1. Since the oxidative addition of H2 to the acylcobalt complex is the rate-determining step in this case the rate equation follows eq. (2) (cf. Section 2.1.1) ... 

Scheme 1. Catalytic cycle of the cobalt-catalyzed cyclohexene hydroformylation reaction.

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