Cobalt complex, modified hydroformylation

Cobalt carbonyls are the oldest catalysts for hydroformylation and they have been used in industry for many years. They are used either as unmodified carbonyls, or modified with alkylphosphines (Shell process). For propene hydroformylation, they have been replaced by rhodium (Union Carbide, Mitsubishi, Ruhrchemie-Rhone Poulenc). For higher alkenes, cobalt is still the catalyst of choice. Internal alkenes can be used as the substrate as cobalt has a propensity for causing isomerization under a pressure of CO and high preference for the formation of linear aldehydes. Recently a new process was introduced for the hydroformylation of ethene oxide using a cobalt catalyst modified with a diphosphine. In the following we will focus on relevant complexes that have been identified and recently reported reactions of interest. 

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. 

The use of cobalt carbonyls, modified cobalt catalysts, and ligand-modified rhodium complexes, the three most important types of catalysts in hydroformylation,... 

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 hydroformylation of olefins is one of the largest and most prominent industrial catalytic processes, producing millions of tons of aldehydes annually [102]. Initially, cobalt-carbonyl species were used as catalyst, though rhodium complexes modified by special ligands, usually phosphines, are predominantly used nowadays. Over the last two decades, continued development of new phosphine and phosphite ligands has allowed significant advances in hydroformylation chemistry, especially with respect to catalyst selectivity and stability [103]. 

The cobalt-catalyzed reaction of carbon monoxide and hydrogen with an alkene, hydroformylation, is an extremely important industrial process, but it occurs under vigorous conditions (200-400 bar, 150-200 °C) and is not a particularly selective reaction. In the presence of ligand-modified rhodium catalysts, however, hydroformylation can be carried out under extremely mild conditions (1 bar, 25 C). The catalytic activity of such rhodium complexes is in fact lO -Ky times greater than that of cobalt complexes and side reactions, such as hydrogenation, are significantly reduced. The reactivity of alkenes in hydroformylation follows a similar pattern to that observed in other carbonylation reactions, i.e. linear terminal alkenes react more readily than linear internal alkenes, which in turn are more reactive than branched... 

Use of chirally modified salicylimines is reported in early examples of cobalt complex catalysed asymmetric hydroformylation of styrene. A prominent example is (Aj-A -a-methyl-benzylsalicylaldimine [(+ )-A -SalH, 1] prepared from (5)-l-phenylethylamine and salicyl-aldehyde21-131-134. ( + )-(S)-3-5ec-Butylpyridine (2)164 is one of the few further examples of nonphosphorus-containing chiral ligands used in asymmetric hydroformylation23. 

Cobalt, rhodium and platinum complexes modified with numerous chiral phosphanes have been used in asymmetric hydroformylation of styrene. The results are compiled in Table 4. Iso-product selectivities of >95% and stereoselectivities of >90% ce are reported, in many cases, however, only with low conversion rates and yields. Early results based on optical rotation measurements had to be reevaluated due to wrongly adopted rotation values for hydra tropaldehyde4-. ... 

Asymmetric hydroformylation of prochiral olefins has been investigated both for the elucidation of reaction mechanism and for development of a potentially useful method for asymmetric organic synthesis. Rhodium and platinum complexes have been extensively studied, and cobalt complexes to a lesser extent. A variety of enantiopure or enantiomerically enriched phosphines, diphosphines, phosphites, diphosphites, phosphine-phosphites, thiols, dithiols, P,A-ligands, and P,5-ligands have been developed as chiral modifiers of rhodium and platinum catalysts. - " ... 

Typical examples of different behavior in relation to the metal are trivalent phosphorus ligands. Thus, trials to modify cobalt complexes with PPhg proved rather problematic, due to the shift of the equilibrium to the left-hand side, especially under increased CO pressure (Scheme 1.7). As a consequence, the hydroformylation is catalyzed by the unmodified Co complex. Diphosphines of the type Ph2PZPPh2 (Z = (CH2)2, (CH2)4, CH=CH) cause a dramatic decrease in reactivity [19]. Also, phosphites do not form active hydroformylation catalysts with cobalt. It seems that only basic trialkyl phosphines are suitable for the generation of stable Co phosphine hydroformylation catalysts. 

The modified cobaItoctene hydroformylation using phosphanes derived from limonene was investigated at 170°C and 85bar H2 CO = 2 l. The composition of cobalt complexes in the reaction mixtures was deduced from IR and NMR spectra and correlated to the products of the hydroformylation reaction 7]. 

In the hydroformylation of lower alkenes using a modified cobalt catalyst complex separation is achieved by distillation. The ligands are high-boiling so that they remain with the heavy ends when these are removed from the alcohol product. Distillation is not possible when higher alcohols or aldehydes are produced, because of decomposition of the catalyst ligands at the higher temperatures required. Rhodium complexes can usually also be removed by distillation, since these complexes are relatively stable. 

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. 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydridocomplex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)8 could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

The kinetics of hydroformylation by phosphine- or phosphite-modified complexes is even more complex than that of the cobalt-catalyzed reaction. Depending on the reaction conditions, either alkene complexation (Scheme 7.1, 6 to 7) or oxidative addition of hydrogen (Scheme 7.1, 9 to 10) may be rate-determining. 

Other chirally modified platinum, cobalt and rhodium complexes give lower inductions, although sometimes with higher aldehyde and branched product yields (Table 1). A strong dependance of chemo-, regio- and stereoselectivity on reaction conditions and conversion rates is observed, sometimes with contradictory results. Attempts to optimize the chemo- and re-gioselectivity usually lead to lower asymmetric inductions. Moreover, since double-bond isomerization and racemization take place under hydroformylation conditions, the results reported do not necessarily reflect the primary asymmetric induction of the starting alkene. 

Cobalt compounds are useful chemical catalysts for the synthesis of fuels (Fi-scher-Tropsch process), the synthesis of alcohols and aldehydes from olefins, hydrogen and carbon monoxide at elevated temperatures and pressures ( oxo process , hydroformylation ). They are also used in petroleum refining and the oxidation of organic compounds. In the oxo process, cobalt carbonyl, Co2(CO)g, is employed or generated in situ. For the selective production of n-butanol from propylene, hydrogen and CO, an organophosphine-modified cobalt carbonyl complex is used as the catalyst. Cobalt salts are proven oxidation catalysts examples include the production of terephthalic acid by the oxidation of p-xylene, and the manufacture of phenol by the oxidation of toluene. 

The mechanisms of hydroformylation with rhodium and cobalt catalysts have been studied in detail and are very similar. We have already learnt that for cobalt the active catalyst precursor is [HCo(CO)4] in the case of the modified rhodium catalyst it is the complex [HRh(CO)(PPh3)3]. 

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