Cobalt-Based Hydroformylation

The catalytic cycle for the cobalt-based hydroformylation is shown in Fig. 5.7. Most cobalt salts under the reaction conditions of hydroformylation are converted into an equilibrium mixture of Co2(CO)8 and HCo(CO)4. The latter undergoes CO dissociation to give 5.20, a catalytically active 16-electron intermediate. Propylene coordination followed by olefin insertion into the metal-hydrogen bond in a Markovnikov or anti-Markovnikov fashion gives the branched or the linear metal alkyl complex 5.24 or 5.22, respectively. These 

The cobalt and rhodium catalysts have one important difference between their respective mechanisms. Unlike in the rhodium-catalyzed process, there is no oxidative addition or reductive elimination step in the cobalt-catalyzed hy-droformylation reaction. This is reminiscent of the mechanistic difference between rhodium- and cobalt-based carbonylation reactions (see Section 4.2.3). The basic mechanism is well established on the basis of in situ IR spectroscopy, kinetic and theoretical analysis of individual reaction steps, and structural characterization of model complexes. 

Both Co(CO)4(COPr ) and Co(CO)4(COPr ) have been isolated, and their reactions with H2 as well as HCo(CO)4 have been studied. On the basis of such studies participation by intermediates such as 5.23 and 5.25 in the catalytic cycle is firmly established. The reactions of HCo(CO)4 with Co(CO)4(COPr ) and Co(CO)4(COPr ) are about 20-30 times faster than the corresponding reactions with dihydrogen at 25°C. However, as already mentioned, at high temperature ( 100°C) and pressure ( 100 bar), reaction with dihydrogen is the main product-forming step. 

For the other catalytic intermediates, there are spectroscopic data and/or strong theoretical arguments in favor of their existence. Thus Co(CO)3(COMe), an analogue of 5.23 and 5.25 has actually been observed spectroscopically at low temperature by the matrix isolation technique. A similar experimental technique has also established the formation of Co(CO)3(Me), an analogue of 5.22 and 5.24. 

In the Shell process (SHOP) phosphine-modified cobalt-catalyzed hydrofor-mylation is one of the steps in the synthesis of linear alcohols with 12-15 carbon atoms (see Section 7.4.1). Two important characteristics of this reaction should be noted. First, the phosphine-modified precatalyst HCo(CO)3(PBu3) is less active for hydroformylation than HCo(CO)4 but more active for the subsequent hydrogenation of the aldehyde. In this catalytic system both hydroformylation and hydrogenation of the aldehyde are catalyzed by the same catalytic species. Second, the phosphorus ligand-substituted derivatives are more stable than their carbonyl analogues at higher temperatures and lower pressures (see Table 5.1). 

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Precipitation of the catalyst from the reaction medium, followed by filtration, as in the cobalt-based hydroformylation process (see Section 5.4). Here cobalt is removed from the reaction products in the form of one of its salts or as the sodium salt of the active carbonyl catalyst. The aqueous salts can be recycled directly, but sometimes they are first converted into an oil-soluble long-chain carboxylic acid salt, such as the corresponding naphthenate, oleate, or 2-ethylhexanoate. 

An example of a cobalt-based hydroformylation process for long-chain, highly branched olefins is a process developed by Exxon to produce IDA from isomer mixtures of the nonene. There are several recently published patents that describe a further development of this process, thus illustrating that continuous development of the older cobalt technology keeps it competitive for selected challenging hydroformylation processes, such as for complex, highly branched, longer chain olefins. 

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

Amidocarbonylation converts aldehydes into amido-substituted amino acids, which have many important industrial applications ranging from pharmaceuticals to detergents and metal-chelating agents.588 Two catalyst systems have been developed, a cobalt-based system and, more recently a palladium-based system. In the cobalt system, alkenes can be used as the starting material, thus conducting alkene-hydroformylation, formation of hemi-amidal and carbonylation in one pot as... 

All metals in the neighborhood of rhodium on the periodic table are known to be active in hydroformylation. Rhodium is by far the most active metal being used in concentrations of 10-100 mg/kg, usually at temperatures below 140 °C. Typical concentrations of cobalt-based catalysts are in the range of 1 -10 g/kg at temperatures up to 190 °C to get sufficient space-time yield. Apart from some specialized applications, other metals are only of scientific interest because of their low activity. A proper comparison of metal activity is difficult because of the different requirements on the reaction conditions. A generally accepted rough order of activity is given in Table 1. 

Rhodium-phosphine catalysts are unable to hydroformylate internal olefins, so much that in a mixture of butenes only the terminal isomer is transformed into valeraldehydes (see 4.1.1.2). This is a field still for using cobalt-based catalysts. Indeed, [Co2(CO)6(TPPTS)2] -i-lO TPPTS catalyzed the hydroformylation of 2-pentenes in a two-phase reaction with good yields (up to 70%, but typically between 10 and 20 %). The major products were 1-hexanal and 2-methylpentanal, and n/i selectivity up to 75/25 was observed (Scheme 4.12). The catalyst was recycled in four mns with an increase in activity (from 13 to 19 %), while the selectivity remained constant (n/i = 64/36). 

Three commercial homogeneous catalytic processes for the hydroformyla-tion reaction deserve a comparative study. Two of these involve the use of cobalt complexes as catalysts. In the old process a cobalt salt was used. In the modihed current version, a cobalt salt plus a tertiary phosphine are used as the catalyst precursors. The third process uses a rhodium salt with a tertiary phosphine as the catalyst precursor. Ruhrchemie/Rhone-Poulenc, Mitsubishi-Kasei, Union Carbide, and Celanese use the rhodium-based hydroformylation process. The phosphine-modihed cobalt-based system was developed by Shell specih-cally for linear alcohol synthesis (see Section 7.4.1). The old unmodihed cobalt process is of interest mainly for comparison. Some of the process parameters are compared in Table 5.1. 

The alternative processes include cobalt-catalyzed hydroformylation and similar rhodium-based processes. Hydroformylation with cobalt requires much higher temperatures (140-170°C) and pressures (70-200 bar). The activity ratio of rhodium and cobalt may be of the order of 1000 but the costs of the metals... 

For a more detailed outline of the various cobalt- and rhodium-based hydroformylation catalysts and for additional related references, see G. O. Spessard and G. L. Miessler, Organometallic Chemistry, Prentice Hall, Upper Saddle River, NJ, 1997, pp. 257-265. 

The hydroformylation of olefins is the most widely used homogeneous catalytic process using CO gas. It involves the addition of one molecule of CO and H2 to an olefin in the presence of a transition metal catalyst, most frequently based on cobalt or rhodium, resulting in the formation of an aldehyde. Generally, it is believed that the activation of H2 in cobalt-catalysed hydroformylation occurs on the unsaturated species Co2(CO)7 or Co(acylXCO)3 formed by the following reactions ... 

Rhodium-based processes dominate in the hydroformylation of propene. On the other hand, the production capacity of cobalt-based processes has remained at a virtually constant level in the C4 field for years (Figure 7). 

Several explanations may be taken into account for only a small decrease in importance an existing high-pressure infrastructure which is also used for the hydroformylation of olefins higher than propene, where cobalt-based processes are advantageous a combined cobalt recovery and recycle system for the hydroformylation of different olefins and the use of tail gases from low-pressure hydroformylation units. 

Rhodium carbonyl complexes are used as catalyst precursors in the hydroformyla-tion reaction Since rhodium catalysts are two to four orders of magnitude more efficient than cobalt-based ones, they are used to carry out hydroformylation of kineti-cally inert olefins, such as styrene and nitroalkenes. ... 

A co-catalyst is added to the hydroformylation catalyst to favor the aldolization reaction. This co-catalyst may be zinc acetyiacetosste or magnesium ethylate, for unmodified cobalt base systems. Shell employs potassmm for cohak/phosphine catalysts. 

The first investigations concerning the hydroformylation of fatty compounds were accomplished by Ucciani and co-workers with cobalt catalysts such as cobalt bislaurate and dicobalt octacarbonyl [29]. Later, Frankel and co-workers found that the cobalt-catalyzed hydroformylation of methyl oleate also leads to the corresponding fatty alcohols [30]. In recent investigations on the hydroformylation of fatty compounds, the preferred catalyst is based on rhodium. For instance, the hydroformylation of methyl oleate catalyzed by [Rh(acac)(CO)2]/biphephos yields an isomeric mixture of formylstearic add methyl esters [31]. 

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