Acylcobalt carbonyls formation

No indications of any acylcobalt carbonyl formation from the added olefin were found by NMR and infrared spectroscopy (184). The values of the equilibrium constant in Table 17 show that the normal/iso ratio favors the n-butyrylcobalt tetracarbonyl and from the equilibrium constants between 25 and 85°C AH = 2.0 0.85 kJ/mol and AS = 9.1 2.6 J/(mol K) were calculated. 

The final product can be isolated easily as the triphenylphosphine complex. This reaction is also general as far as the acylcobalt carbonyl is concerned, but the yields vary widely depending upon which acetylene is used (34). Presumably, the presence of substituents on the acetylene favors the cyclization step rather than the formation of linear products. The larger the substituents the more favorable the cyclization becomes. If cyclization does not take place relatively rapidly, linear compounds and polymers of acetylene, or of acetylene and CO are probably formed. Thus, these reactions demonstrate the insertion reaction of both acetylenes and ketonic carbonyl groups. 

For 1-pentene, Karapinka and Orchin (73) found that Eq. (2) was strongly inhibited by carbon monoxide at 0° C. This was confirmed by Heck and Breslow (62) who noted that the inhibition also retarded the formation of alkyl- and acylcobalt carbonyls as well as aldehydes. Thus, about 30% less alkyl- and acylcobalt carbonyls were formed in 15 minutes under 1 atm of carbon monoxide than under 1 atm of nitrogen. These results should not, of course, be taken as implying that nitrogen promotes the reaction. Takegami et al. (147) have noted that under nitrogen a side reaction consumes cobalt... 

The results of Takegami et al. (147) on the formation of acylcobalt carbonyls from olefins include curves of carbon monoxide uptake against time which are autocatalytic. This implies that the intermediate suppressed by carbon monoxide is also formed as the product of a subsequent reaction. This intermediate is likely to be HCo(CO)3, which might be formed by a reaction such as... 

Originally, Piacenti et al. explained the formation of isomeric products in terms of an equilibrium of alkylcobalt carbonyls with olefin-hydrocarbonyl complexes as in the Oxo reaction. More recently, however, they have noted that the conditions under which n-propyl orthoformate gave no isomeric products (below 150° C, carbon monoxide pressure 10 atm) are conditions under which isomerization occurs readily in the hydroformylation of olefins (115). Since alkylcobalt carbonyls were formed in both reactions they dismissed the possibility that this isomerization was due to alkyl- or acylcobalt carbonyls. The fact that Takegami et al. have found that branched-chain acylcobalt tetracarbonyls isomerize more readily than straight-chain acylcobalt tetracarbonyls would seem to fit in quite well with the results of Piacenti et al., however, and suggests that the two findings may not be so irreconcilable as might at first appear (see Section II, B,2). 

The products were isolated as esters by reaction of the acylcobalt carbonyls with an alcohol and iodine. In the case of the alkyl halides, carbon monoxide was normally absorbed, but under nitrogen, acylcobalt tricarbonyls must be formed. The reaction with alkyl halides was slow and some isomerization was noted using M-propyl iodide (formation of n-butyrates and isobutyrates). Absence of carbon monoxide promoted the isomerization. Isopropyl iodide gave no reaction. When ethyl a-bromopropionate was used, no isomerization was found at — 25 °C under carbon monoxide, but the isomerized product, diethyl succinate, was the major product at 25° C under carbon monoxide or nitrogen. Under the conditions of the experiments no isomerization of the alkyl halide itself was found. 

Heck (59) has suggested that the first step in the carboxylation reaction is the formation of cobalt hydrocarbonyl, which can be formed from dicobalt octacarbonyl and solvent (55). Alkylation and carbonylation then produce an acylcobalt carbonyl. Reaction of the acylcobalt carbonyl with the compound containing active hydrogen then regenerates cobalt hydrocarbonyl, e.g.,... 

When nonconjugated dienes react with carbon monoxide and water in the presence of dicobalt octacarbonyl, saturated and unsaturated cyclic ketones are produced (55, 77). This appears to be due to the formation of unsaturated acylcobalt carbonyls followed by cyclization, as discussed in Section II, B,3. 

The mechanism involves the elimination of and readdition of HCo(CO>3 as shown in Scheme 1. Starting with either acyl isomer, a 50% yield of the mixed aldehydes is obtained in addition to the mixture of isomeric acylcobalt compounds. Formation of aldehyde and olefin from the acylcobalt carbonyl is thus a disproportionation reaction that competes with the skeletal rearrangement. 

Support of the above conclusion was provided by calculations, which show that the actual rate of acylcobalt carbonyl reduction by dihydrogen is indeed higher than that by HCo(CO)4 under hydroformylation conditions. These calculations were based on the actual concentration of the cobalt complexes and dihydrogen in the reaction mixtures and the observed rates of aldehyde formation in those solutions. Assuming a simplified four-step mechanism of olefin hydroformylation not accounting for the effect of carbon monoxide on the proposed steps, under the conditions presented in Table 6 the rate constants for the reactions in Scheme 7 were calculated (130) ... 

By the addition of a phosphine to an alkylcobalt tetracarbonyl, the phosphine-substituted acylcobalt carbonyl can be prepared (182,183). In the case of (methoxycarbonyl)methylcobalt tetracarbonyl as a model compound, some details of the reaction have been revealed. Adding a phosphine to this complex, the kinetically controlled formation of a phosphine-substituted acylcobalt carbonyl is observed that can be converted to the thermodynamically more stable phosphine -substituted derivative. 

It was stated, therefore, that mainly dihydrogen will be responsible for aldehyde formation from acylcobalt carbonyls under the industrial conditions of hy-droformylation and the reaction with HCo(CO)4 will only play a minor role (188). This conclusion was found to be in agreement with measurements performed under catalytic conditions (131) and confirms the original proposal of Heck (99) put forward long time ago. 

Besides nickel and cobalt, almost all of the catalysts discussed in the last chapter which were suited for the formation of free acids can be applied, e. g. rhodium, palladium and, with certain restrictions, iron. Cobalt hydrocarbonyl catalyzes the stoichiometric ester synthesis at mild reaction conditions [35, 121]. The initially formed acylcobalt carbonyls react rapidly with alcohols even at 50 °C and, in the presence of Na-alcoholate, even at 0 °C to give esters [121]. Dienes with isolated double bonds react with carbon monoxide and alcohols at mild reaction conditions in the presence of Pd/HCl to give unsaturated monocarboxylic acid esters and at more severe conditions to give saturated dicarboxylic acid esters [508]. 

Another important line of investigation concerned the carbonyl insertion reaction, which was best defined in manganese chemistry (75, 16) and extended to acylcobalt tetracarbonyls by Heck and Breslow. The insertion may be through three-membered ring formation or by nucleophilic attack of an alkyl group on a coordinated CO group. 

One final interesting isomerization achieved in the cobalt carbonyl system should be mentioned. Heck and Breslow (22b) found that acylcobalt tetracarbonyl compounds undergo alcoholysis with the formation of HCo(CO)4. With methanol, the reaction proceeds at 50° ... 

Acetylene and monosubstituted acetylenes appear to give some of the 7T-(penteno-4-lactonyl)cobalt tricarbonyl complexes on reaction with alkylcobalt or acylcobalt tetracarbonyls also but other products are formed too. These other products have not been characterized but are thought to be linear, low molecular weight polymers of the acetylene or of the acetylene and carbon monoxide with an acyl group at one end of the polymer chain and a cobalt carbonyl group at the other. The formation of cyclic products from the more-substituted compounds and cyclic and linear ones from the less-substituted compounds is explainable because substitution is known to improve many cyclization reactions. 

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