Acylcobalt tetracarbonyls olefins

The stoichiometry of Eq. (2) requires the absorption of 1 mole of CO per 2 moles of HCo(CO)4. However, Heck and Breslow (17) showed that, when olefin is used as the solvent, the absorption of CO approaches 1 mole per mole of HCo(CO)4, and they further showed that the 1 1 1 HCo(CO)4 CO olefin complex suggested as a possible intermediate by Kirch and Orchin (16) was in fact an isolable intermediate, namely, an acylcobalt tetracarbonyl, RCOCo(CO)4. Accordingly, the formation of aldehyde and dicobalt octacarbonyl proceeds ... 

Although Eq. (3) indicates that CO absorption is required for aldehyde formation, it has been shown by Karapinka and Orchin 18) that at 25° and with a moderate excess of olefin the rate of reaction and the yield of aldehyde are similar when either 1 atm of CO or 1 atm of Nj is present. Obviously CO is not essential for the reaction and a CO-deficient intermediate, probably an acylcobalt tricarbonyl, can be formed under these conditions. The relative rates of HCo(CO)4 cleavage of tricarbonyl and tetracarbonyl are not known, and thus the stage at which CO is absorbed in the stoichiometric hydroformylation of olefins under CO is not known with certainty. Heck (19) has shown conclusively that acylcobalt tetracarbonyls are in equilibrium with the acylcobalt tricarbonyl ... 

Evidence for the insertion of an olefin group between an acyl group and a cobalt atom has been obtained more directly by analyzing the decomposition products of co-unsaturated acylcobalt tetracarbonyls (CHjp=CH(CH2) COCo(CO)4). The products of thermal decomposition of these complexes depend upon the value of n. When n = 0 or 2 the compounds form relatively stable cyclic olefin 7r-complexes which may be isolated as monotriphenylphosphine derivatives (47). The ir-acrylyl-cobalt tricarbonyl (n o) gives an amorphous polymer on heating (37), whereas the... 

This seems to be the most likely mechanism, both for the isomerization of the acylcobalt carbonyls and excess olefin. The fact that the isomerizing species contains two molecules of carbon monoxide less than the acylcobalt tetracarbonyl would suggest a very slow rate of isomerization. However, it is conceivable that the reaction may be catalyzed by the tricarbonyls, e.g.,... 

Piacenti et al. suggested that the different results at low and high carbon monoxide pressure were due to different catalytic intermediates (A and B) under the two sets of conditions. Thus at low pressures A caused a rapid olefin isomerization and the formation of similar product distributions of aldehydes from 1- and 2-pentene. At high pressures little olefin isomerization occurred and 1-olefin yielded significantly more straight-chain aldehyde than 2-olefin. This would seem consistent with Heck and Breslow s mechanism (62) if A were an acylcobalt tricarbonyl in equilibrium with isomeric olefin-cobalt hydrocarbonyl complexes and B were an acylcobalt tetracarbonyl. 

The present authors feel this point needs further investigation in view of the results of Takegami et al. They found that the isomerization of the acylcobalt tetracarbonyl was very solvent-dependent, and it could well be that conditions in the hydroformylation of olefins and orthoformates were sufficiently different to cause faster isomerization in the former case. Thus, for example, the presence of olefins in the former case may contribute to a faster isomerization, or perhaps orthoformates, like tetrahydrofuran, inhibit the isomerization. A further factor to be considered is the presence of cobalt hydrocarbonyl, which must be present in larger amount in the case of olefin hydroformylation. Takegami et al. (143) have shown that cobalt hydrocarbonyl strongly promotes the isomerization of phenylacetylcobalt car-... 

A useful study has just been completed by Roos and Orchin (125), who have examined the effect of ligands such as benzonitrile on the stoichiometric hydroformylation of olefins. A variety of such reagents (acetonitrile, anisole) were found to act in a similar manner to carbon monoxide by suppressing the formation of branched products and the isomerization of excess olefin. The yield of aldehyde was also increased by increasing ligand concentration up to 2 moles per mole of cobalt hydrocarbonyl. Benzonitrile was not found to affect the rate of the reaction of cobalt hydrocarbonyl with acylcobalt tetracarbonyl, so the ligand must have affected an earlier step in the reaction sequence. It seems most likely that cobalt hydrocarbonyl reacts with olefin in the presence of benzonitrile to form an acylcobalt tricarbonyl-benzonitrile complex which is reduced more rapidly than the acylcobalt tetracarbonyl. 

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). 

Chelated w-olefin complexes are also formed from ,j3-unsaturated acylcobalt tetracarbonyls. Acrylylcobalt carbonyl exists, at least mainly, as... 

In the presence of a large excess of olefin, the carbon monoxide uptake approaches 1 mol/mol of HCo(CO)4 and acylcobalt tetracarbonyl complex is obtained, which in turn can be reduced at room temperature to aldehyde by HCo(CO)4 or by dihydrogen ... 

It has been deduced from results of preparations of acylcobalt carbonyl complexes using sodium tetracarbonylcobaltate(-l) and alkyl halides (101) that in the reaction of an olefin with HCo(CO)4 and carbon monoxide, an alkylcobalt tetracarbonyl is the precursor of the acylcobalt tetracarbonyl. 

The olefin conversion in the reaction with HCo(CO)4 and the acylcobalt tetracarbonyl conversion in the reaction with HCo(CO)4 or with dihydrogen are faster if performed in the presence of dinitrogen or argon instead of in the presence of carbon monoxide. The negative effect of carbon monoxide was explained by the assumption that in these reactions in small equilibrium concentrations highly reactive coordinatively unsaturated 16-electron cobalt complexes are involved according to the dissociation equilibria below ... 

In comparison to aliphatic hydrocarbon olefins, ethyl acrylate and diethyl fumarate show not only exceptionally high reactivity in the Co2(CO)8-catalyzed reaction with HCo(CO)4 but give lower carbonylation to satnrated product ratios as well. A further difference to the behavior of aliphatic olefins is that with ethyl acrylate or diethyl fumarate in large excess not an acylcobalt tetracarbonyl but... 

One explanation would be that in case 1 acylcobalt tricarbonyls, which readily isomerize, would be the main intermediates and that acylcobalt tetracarbonyls, which according to Takegami et al. [70, 917, 918] are much more difficult to isomerize, would be the dominating intermediates in case 2. At low CO partial pressure and high temperature olefin isomerization before hydroformylation is at least partly responsible for the nearly identical isomeric distribution of products from terminal and interdal olefins. 

This reduction is very likely the last step in the industrially important hydroformylation or oxo reaction for converting olefins into aldehydes (4). The catalytic species seems to be cobalt hydrocarbonyl, which first adds to the olefin as in Eq. (2). The alkylcobalt tetracarbonyl so formed then probably isomerizes to the acylcobalt tricarbonyl [Eq. (25)] and is reduced by hydrogen as in Eqs. (45) and (46). 

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. 

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