Cobalt catalyzed reactions

1 Cross-Coupling with C(sp )-Electrophiles Gobalt salts are competent catalysts to promote the ahcenylation and the arylation of G(sp )-organomagnesium hahdes under mild conditions by reaction with alkenyl, aryl, or heteroaryl hahdes. Alkynylation with alkynyl hahdes has also been reported, but generahy low yields are obtained [166]. 

The use of aromatic electrophiles is less developed. Halopyridines are viable partners [171]. More significantly, coupling of aryl bromides with primary Grignard reagents takes place in Et20 at room temperature, but requires the presence 

---

A similar synthesis starts from commercially available 1,5-hexadiyne and 2-methyl-cyclopent-2-enone. The benzocyclobutene is obtained from a bis-acetylene in a cobalt-catalyzed reaction. It rearranges regio- and stereoselectively to a 3-deoxy steroid derivative. The overall yield from the cyclopentenone was 40% (R.L. Funk, 1977). 

An example illustrating the synthesis of condensed oxepins by the cobalt-catalyzed reaction of bistrimethylsilylacetylene with a hexa-l,5-diyne derivative is shown in Scheme 175.234 This type of process has been discussed earlier in the context of pyran synthesis (see Scheme 158 in Section V,B,2). 

Conflicting results have been reported for the effects of catalyst concentration in the cobalt-catalyzed reaction. In early work, Hughes and Kirshenbaum (31) reported that these parameters were very influential in determining product composition high temperatures and high catalyst concentrations resulted in products containing decreased amounts of the... 

In addition to the increased proportion of linear product, other differences from the unmodified cobalt-catalyzed reaction may be noted. The... 

The cobalt-catalyzed reaction was studied by isolation of the lactones formed by hydrogenation and lactonization at higher temperatures (73). The hydroformylation was conducted at 140°C and 300 atm, followed by hydrogenation and cyclization at 200°-240°C, Eq. (33). 

Figure 2 shows the generally accepted dissociative mechanism for rhodium hydroformylation as proposed by Wilkinson [2], a modification of Heck and Breslow s reaction mechanism for the cobalt-catalyzed reaction [3]. With this mechanism, the selectivity for the linear or branched product is determined in the alkene-insertion step, provided that this is irreversible. Therefore, the alkene complex can lead either to linear or to branched Rh-alkyl complexes, which, in the subsequent catalytic steps, generate linear and branched aldehydes, respectively. 

Hydrogenation of acetic anhydride to acetaldehyde (equation 23) has been demonstrated utilizing cobalt carbonyl under one atmosphere of hydrogen. However, the cobalt complex is short lived. A more efficient cobalt catalyzed reaction with substantial catalyst longevity was realized at a temperature of 190 and 3000 psi pressure CO and hydrogen. The main products were equal amounts of EDA and acetic acid. Upon investigation, this reaction was found exceptionally efficient at a more reasonable 1500 psi pressure provided that the temperature was maintained... 

In all cases, the oxidation rate was smallest for experiments involving thiophenol and ferf-butanethiol. The oxygen uptake vs. time curves for cobalt-catalyzed reactions showed an initial high slope followed by a decrease in slope after ca. 30% reaction to a final steady value. 

Metal-Nitrogen Compounds The cobalt catalyzed reaction of primary and secondary amines with carbon monoxide leads to the formation of formamide derivatives. Dimethylamine, for example, gives iV,i T-dimethvlformamide in 60% yield (90, 91). Very likely cobalt-nitrogen compounds are intermediates which undergo a CO insertion and then reduction. The following mechanism has been suggested for the reaction (90). 

Formate esters of the various alcohols formed are observed as major products in these cobalt-catalyzed reactions, and the mole ratio of formates to alcohols remains constant throughout a reaction. This observation would be consistent with the occurrence of a rapid carbonylation equilibrium process,... 

Fahey presents the products of (17) as uncomplexed formaldehyde and HCo(CO)3 rather than a bound-formaldehyde species (43). Free formaldehyde is a thermodynamically unfavorable product from H2 and CO (8), and significant stabilization may be expected as the result of coordination in a metal complex. However, thermodynamic calculations are presented which indicate that small equilibrium concentrations of formaldehyde could be present under the conditions of these cobalt-catalyzed reactions (43). Although small amounts of uncoordinated formaldehyde are indeed expected as a result of the following endothermic (36, 37) equilibrium ... 

Regarding the cobalt-catalyzed reactions, the electrochemical analyses of the involved processes allowed the discovery of a chemical way for the synthesis of these organozinc compounds. This chemical way will be evoked in this chapter. 

CoH(CO)4], formed in situ when cobalt metal or cobalt(II) salts are reacted with synthesis gas at elevated temperature under pressure, is the active species in the cobalt-catalyzed reaction.14,17,18 It is also capable of stoichiometric... 

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. 

Lanthanide ions (La3+, Ce3+) were found to promote the cobalt-catalyzed reaction in a two-phase system with p-cyclodextrin or PEG-400 (polyethylene glycol 400) as phase-transfer catalysts.134... 

Hydroformylation is a multistep catalytic process from the data on the cobalt-catalyzed reaction it has generally been proposed that the reaction occurs according to the scheme in Fig. 12 21). 

Shortly after Footes initial report on cobalt catalyzed reactions (see Sect. 5.8), Noyori and coworkers found that Ru(PPh)3Cl2 catalyzes the reductive cleavage of endoperoxides (Fig. 47) [152, 259]. While 1,2-dioxanes provided a mixture of... 

Noyori and colleagues investigated the ring opening of unsaturated mono- and bicyclic endoperoxides catalyzed by 5-10 mol% of Pd(PPh3)4 [226, 227]. Similarly to the cobalt-catalyzed reactions, (Z)-4-hydroxy enones resulted as the main products, which were accompanied by (Z)-2-ene-l, 4-diols and diepoxides. The latter are formed as the major products under either ruthenium or cobalt catalysis (see Part 2, Sects. 3.5 and 5.8). Both two-electron and radical mechanisms were considered for this transformation. Saturated bicyclic endoperoxides gave mixtures of cyclic 4-hydroxy ketones and 1,4-diols and their formation may be a result of a radical process [227, 228]. 

By far the most important synthesis gas reaction is its conversion into methanol, using copper/zinc oxide catalysts under relatively mild conditions (50 bar, 100-250°C). Methanol is further carbonylated to acetic acid (see Section 22-7), so that CH3C02H, methyl acetate, and acetic anhydride can all be made from simple CO and H2 feedstocks. Possible pathways to oxygenates in cobalt catalyzed reactions are shown in Fig. 22-6. 

The cobalt-catalyzed reaction of olefins with CO and hydrogen to give aldehydes (Equation 2) was discovered by Roelen at Ruhrchemie in 1938, and is now known as hydroformylation (or, sometimes, as the 0x0 synthesis, Section 4.6) and is the basis of another major industrial process. 

Similar yield and enantioselectivity were obtained with chalcones when Co(acac)2 was substituted for Ni(acac)2 catalysts under otherwise identical reaction conditions. However, the cobalt-catalyzed reaction was significantly slower and produced a significant amount of reduced by-product (5%) compared to reactions catalyzed by nickel. For both the cobalt- and nickel-catalyzed reactions, both (—)DAIB and (+)DAIB were shown to be superior to several chiral aminoalcohols for enantioselectivity. 

In the absence of oxygen, these radicals can be trapped by addition to olefins. Co , on the other hand, generates primarily acyl radicals from aldehydes i.e., the cobalt catalyzed reaction does not proceed through the enol mechanism. 

The first calculation of the complete hydroformylation cycle with Rh-phosphine catalysts (substrate = ethylene, model ligand = PH3) was published in 1997 [3]. The QM methods used are HF and MP2, respectively (cf. Section 3.1.2.1). Hybrid DFT methods such as B3LYP [4], however, are more appropriate in terms of both accuracy and efficiency [5, 6] (cf. Section 3.1.2.1). Therefore, the same model system was recalculated [7] on the level B3LYP functional/DZVP basis set [8]/quasi-relativistic pseudopotentials on rhodium [9]. Since homologous Ir catalysts are interesting alternatives from an economic point of view [10], calculations with the central metal Ir were also made. This comparative treatment is supported by the experimental assumption of a common mechanism [11], which equals the Heck-Breslow mechanism of the cobalt-catalyzed reaction [12],... 

Starting from optically active nitriles, Botteghi and co-workers [32] have applied the cobalt-catalyzed reaction for the prepartion of optically active 2-substituted pyridines (eq. (8)). The chiral center is maintained during the alkyne-nitrile co-cyclization reaction. This reaction has recently been extended to the synthesis of bipyridyl compounds having optically active substituents [33] and provides an access to chiral ligands of potential interest in transition metal-catalyzed asymmetric synthesis. 

The regiochemical product distribution of the co-cyclization of two or three different alkynes occurs statistically. In some cases carefully controlled reaction conditions allow isolation of a main product from mixed cyclotrimerizations. For example, l,2,3,4-tetraphenyl-5,6-diethylbenzene can be obtained from cobalt-catalyzed reaction of tolane and 3-hexyne in good yield [62]. The first example of an intermolecular, regiospecific cross-benzannulation reaction catalyzed by Pd(PPh3)4 was reported by Yamamoto [63]. The reaction of 2-alky 1-but-l-ene-3-yne with disubstituted diynes leads exclusively in high yields to... 

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... 

The process based on the cobalt-catalyzed reaction was commercialized by BASF [3,4] but proved not to be so selective as subsequent processes, with an acetic acid yield of 90% (based on methanol feedstock) and 70% (based on CO). The major organic by-products were higher alcohols, aldehydes, and carboxylic acids that required demanding and expensive separation procedures to give acetic acid of sufficient purity. 

Mechanistic studies of rhodium-catalyzed hydroformylation of olefins have shown that the basic feature of the catalyst cycle is more or less the same as that of the cobalt-catalyzed reaction.When unmodified rhodium carbonyls, e.g., Rh4(CO)i2 and RhefCOlie, are used as catalysts, there is an equilibrium among Rh4(CO)i2, Rh6(CO)i6, and HRh(CO)n (n = 3 or 4) in the presence of carbon monoxide and hydrogen, which complicates the mechanistic study. Nevertheless, HRh(CO)n (n = 3) is postulated as the active catalyst species, and the... 

The data in Figure 4.8-3, which shows the Arrhenius plots for cobalt-catalyzed reactions in the three reaction phases, reveal that the reaction rate fell in the order gas phase > supercritical phase > liquid phase. 

The 3-cyanomethyl and 4-ethynyl substituents of an indole can be linked in a cobalt-catalyzed reaction with alkynes, to generateergoline derivatives (397) in modest yield (Equation (112)) <94SL487>. 

---