Cobalt complex hydroformylation

The switch from the conventional cobalt complex catalyst to a new rhodium-based catalyst represents a technical advance for producing aldehydes by olefin hydroformylation with CO, ie, by the oxo process (qv) (82). A 200 t/yr CSTR pilot plant provided scale-up data for the first industrial,... 

Ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [PBuJBr was reported by Knifton as early as in 1987 [2]. The author described a stabilization of the active ruthenium-carbonyl complex by the ionic medium. An increased catalyst lifetime at low synthesis gas pressures and higher temperatures was observed. 

For the phosphine-substituted cobalt carbonyl hydroformylations, it is probable that the mechanism follows the pathway of Heck and Breslow (77, 18), although the possibility of an associative mechanism has been raised (7). The increased stability of the HCo(CO)3PR3 complexes toward loss of CO was cited as being suggestive of a nondissociative pathway. 

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]. 

Modified cobalt complexes of the type frans-Co2(CO)6(phosphine)2 are promising candidates for certain transition metal-catalyzed reactions, in particular for the hydroformylation of long-chained olefins [117]. A series of complexes Co2(CO)6[P(alkyl) (aryl)m]2 (n 0,1,2,3 m S - n) was synthesized and used for solubility measurements. Since the basicity of phosphines affects the catalytic activity, use of fluorous substituents might induce unexpected changes in the activity. Therefore, also derivatives with an additional ethyl spacer between the fluorous group and the phosphine moiety were examined (Sect. 3.1). 

It should be recognized that the stability of cobalt complexes under carbon monoxide can be enhanced by the addition of ligands, as is the case for phosphine-modified cobalt hydroformylation catalysts (57, 58). The stability will also probably depend on properties of the solvent employed. Nevertheless, the plot shown in Fig. 4 appears to be quite useful for assessing long-term cobalt stability under H2/CO in the absence of strongly coordinating solvents or ligands. 

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). 

Relatively few hydroformylations using supported cobalt complexes have been reported. Moffat (78, 79) showed that poly-2-vinylpyridine reversibly reacted with both Co2(CO) and HCo(CO)4, the cobalt carbonyl being displaced by excess carbon monoxide. This enabled the polymer to pick up the cobalt carbonyl at the end of the reaction and, thus, allowed it to be separated from the products by filtration. The polymer acted as a catalyst reservoir by rapidly releasing the cobalt carbonyl into solution in the presence of further carbon monoxide, so that the actual catalysis was a homogeneous process. More recently, cobalt carbonyl has been irreversibly bound to a polystyrene resin... 

The present discussion is limited to the rhodium catalyzed hydroformylation. In the widely accepted mechanism proposed by Wilkinson et ah (36), on the basis of suggestions by Breslow and Heck (37) for the cobalt catalyzed hydroformylation, the reaction steps, with the exception of the hydrogenolysis of the acyl-rhodium complexes, correspond to equilibria (Scheme 10). 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

The fact that 2-phenylbutane, which is the main product formed during 2-phenyl-1-butene hydroformylation, is optically inactive while the aldehydes arising from the same reaction are optically active, has been taken as an indication that the intermediate in hydrogenation is either a configurationally labile a,a-disubstituted benzyl cobalt complex 15) or the corresponding free radical20). 

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 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... 

As early as 1938, Roelen discovered the cobalt-catalyzed hydroformylation of olefins, then known as the oxo reaction, which allowed the synthesis of aldehydes by addition of carbon monoxide and hydrogen to alkenes. Not long after this discovery it was found that cobalt, rhodium, ruthenium and platinum are also suitable as catalysts. However, because of the considerable price advantage for large scale applications in industry, cobalt catalysts are mostly used. Rhodium complexes, however, are... 

---