Cobalt hydrides catalysts

Most likely the cobalt catalyst is HCo(CO)2(L), which has a very electron rich metal centre and dissociation of CO does not occur under the reaction conditions. The first step is a reaction of the cobalt hydride with ethylene oxide forming a hydroxyethylcobalt species, which does not require dissociation of... 

As recently reported, cobalt-catalyzed addition of olefins to butadiene is probably an example of the addition of cobalt alkyls to butadiene (106). The catalyst was the type prepared by reaction of cobalt chloride with an aluminum alkyl in the presence of a diene. A bis-7r-allylcobalt derivative is probably formed. The unstable 7r-allylcobalt compounds probably decompose (reversibly) into cobalt hydride. The hydride would add to the olefin present to form a dialkyl, which could then add again to the diene. 

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. 

The metal carboxylate insertion mechanism has also been demonstrated in the dicobaltoctacarbonyl-catalyzed carbomethoxylation of butadiene to methyl 3-pentenoate.66,72 The reaction of independently synthesized cobalt-carboxylate complex (19) with butadiene (Scheme 8) produced ii3-cobalt complex (20) via the insertion reaction. Reaction of (20) with cobalt hydride gives the product. The pyridine-CO catalyst promotes the reaction of methanol with dicobalt octacarbonyl to give (19) and HCo(CO)4. 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

General articles concerning transition metal hydrides,366 their crystallography,368 and on the preparation and properties of borohydride complexes369 are available. An overview on the use of HCo(CO)4 and related cobalt hydrides as catalysts in the hydroformylation of alkenes is available.367... 

However, only alkyl formates are formed in the conventional reactions of alcohols, CO2 and H2 using transition metal complexes, because intermediary hydride complexes generally react with CO2 to give formate complexes. On the other hand, we have found that mthenium cluster anions effectively catalyze the hydrogenation of CO2 to CO, methanol, and methane without forming formate derivatives [2-4]. Ethanol was also directly formed from CO2 and H2 with ruthenium-cobalt bimetallic catalyst [5]. In this paper, we report that this bimetallic catalytic... 

The first generation of hydroformylation processes (e.g., by BASF, ICI, Kuhlmann, Ruhrchemie) was exclusively based on cobalt as catalyst metal. As a consequence of the well-known stability diagram for cobalt carbonyl hydrides, the reaction conditions had to be rather harsh the pressure ranged between 20 and 35 MPa to avoid decomposition of the catalyst and deposition of metallic cobalt, and the temperature was adjusted according to the pressure and the concentration of the catalyst between 150 and 180 °C to ensure an acceptable rate of reaction. As the reaction conditions were quite similar, the processes differed only in the solution of the problem of how to separate product and catalyst, in order to recover and to recycle the catalyst [4]. Various modes were developed they largely yielded comparable results, and enabled hydroformylation processes to grow rapidly in capacity and importance (see Section 2.1.1.4.3). 

For the cobalt catalyst the story appears somewhat clearer. Both experiment and theory have been shown to be in agreement and support a stepwise pathway for the reaction of the cobalt(I) alkyl complexes with 1-alkenes, reacting by (3-hydride transfer via a cobalt hydride intermediate [135]. Such cobalt alkyls have also been shown to contain low-spin cobalt(II) antiferromagnetically coupled to a hgand radical anion. The lowest triplet state is thermaUy accessible and accounts for the observed H NMR chemical shifts at room temperature [66]. 

The use of organometallic compounds as chain-transfer catalysts in free-radical polymerization has been widely studied. One objective is the production of polymers with terminal vinyl groups and lower molecular weight components compared with polymerization in the absence of chain-transfer catalysts. Gomplexes of cobalt(ii) have been used as effective catalysts, but the instability of the intermediate cobalt hydride does not permit firm establishment of the reaction mechanism. To address this issue, several chromium compounds have been applied as catalysts for the polymerization of methylmethacrylate (MMA) and styrene. The temperature dependence of the rate constant for free-radical polymerization of MMA for catalyzed chain transfer by (GsPh5)Gr(GO)3 has been determined using the Mayo equation. ... 

Iguchi catalyst will reduce only activated alkenes, such as cinnamate ion, in which the radical is benzylic. Finally, the organic radical abstracts H from a second molecule of the cobalt hydride to give the final product. 

A quite different process, called Reppe carbonylation, has been used to convert acetylene to acrylic acid esters. The catalysts are carbonyls of iron, cobalt or nickel and the hydrogen source is a hydrogen halide, HX. The process is thought to involve oxidative addition of HX to the metal carbonyl, followed by coordination and insertion of alkyne into the M—H bond and insertion of CO into the M—C bond. The resulting acyl complex is cleaved by alcohol to produce the ester and the metal hydride catalyst. De Angelis et al. have reported a theoretical analysis of the Ni(CO)4 system. 

Heck has formulated a mechanism which accounts for hydroformylation of olefins catalyzed by cobalt carbonyl (68). A modification of this mechanism is presented in Fig. 5. Cobalt octacarbonyl reacts with hydrogen to form the tetracarbonyl hydride. It is proposed that this coordinatively saturated complex loses a CO group to form the four-coordinate hydride (LX). Coordination of an olefin yields the olefin complex (LXI). Migration of hydride yields an unsaturated alkyl complex (LXII). Further insertion of a CO group (undoubtedly by a migration mechanism) affords the four-coordinate acyl cobalt(I) complex (LXIII). Oxidative addition of hydrogen affords the hypothetical dihydride (LXIV), which eliminates the product aldehyde and regenerates the cobalt(I) hydride catalyst (LX). This latter... 

Dinitrogen)hydrodotris(triphenylphosphine)Co(I) has also been used to dimerize ethylene [143]. Here the dimerization of ethylene takes place at room temperature without the use of a Lewis acid. Ethylene conversion decreases with time, presumably due to partial decomposition of the catalyst. However, the decomposition is slow at 0°C. A mechanism has been proposed in which the olefin is first inserted in the cobalt-hydride bond and then a second molecule of the olefin is inserted in the cobalt-alkyl bond. Displacement of the dimer by the olefin regenerates the cyclic process. 

Very interesting are the results of recent investigations on the mechanisms of Co(II) mediated reductions of nitriles, alkenes and alkyl halides by LiAlH4 and NaBH4. Those studies have unambiguously identified borides and aluminides of cobalt as catalysts in all three reductions, a finding clearly at odds with commonly held notions about the mechanisms of such processes and which could also be relevant to other transition-metal—hydride systems [12]. 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

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