Acylcobalt tetracarbonyl

Alkyl- and acylcobalt tetracarbonyls were the subject of a comprehensive review by Heck 115). Since then (1966), they have been given some attention in review articles concerned primarily with catalysis 62, 117, 118). Because of an extensive coverage afforded the early developments, emphasis herein will be on the more recent findings. 

Alkali Metal Derivatives of Metal Carbonyls, 2, 1S7 Alkyl and Aryl Derivatives of Transition Metals, 7, 1S7 Alkyl cobalt and Acylcobalt Tetracarbonyls, 4, 243 Allyl Metal Complexes, 2, 32S... 

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. 

The reaction of acylcobalt tetracarbonyls and re-allyl cobalt tricarbonyls in the presence of trimethylolpropane phosphite can be represented by the following mechanism25. 

Although the acylcobalt tetracarbonyls react with hydroxide ion under phase-transfer conditions, in the presence of alkenes and alkynes they form o-adducts rapidly via an initial interaction with the ir-electron system. Subsequent extrusion of the organometallic group as the cobalt tetracarbonyl anion leads to a,(J-unsaturated ketones (see Section 8.4). In contrast, the cobalt carbonyl catalysed reaction of phenylethyne in the presence of iodomethane forms the hydroxybut-2-enolide (5) in... 

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 ... 

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° ... 

The hydrocarbonyl is readily trapped by a base such as dicyclohexyl-amine. Since the acylcobalt tetracarbonyl can be generated by the... 

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.,... 

Of the two acylcobalt tetracarbonyls, the linear isomer is likely to be the more stable thermally at any given pressure of carbon monoxide since the dissociation of the acylcobalt tetracarbonyl to the acylcobalt tricarbonyl is promoted by steric effects (52, 118). Increasing temperature should therefore favor the linear product if the carbon monoxide pressure is sufficient to form mainly acylcobalt tetracarbonyls. The stability of the tricarbonyls, however, is more likely to be determined by the electronic effects which determine the initial addition of cobalt hydrocarbonyl. 

For a more detailed consideration of the isomerization of alkyl- and acylcobalt carbonyls, Section V,B should be consulted. It is sufficient to say here that this isomerization is normally slow at room temperature, especially for the linear acylcobalt tetracarbonyls. The reaction also appears to be quite sensitive to solvent for reasons which have not yet been adequately explained. Alkylcobalt carbonyls are rapidly converted to the acylcobalt carbonyls and do not appear to give rise to any significantly faster isomerization. 

In support of the existence of an acylcobalt tricarbonyl, Heck and Breslow cited the appearance of an infrared band at 5.8 p, similar to that occurring in acylcobalt tetracarbonyls when alkylcobalt tetracarbonyls are examined in solution. They postulated the equilibrium, Eq. (20). There is now some doubt of the value of this evidence since the 5.8 /x band is due in part at least to the acylcobalt tetracarbonyl formed by some kind of disproportionation, or decomposition during the preparation (53). However, evidence for Eq. (23) has since been found in a study of the reaction of acylcobalt tetracarbonyls with triphenylphosphine, where a first-order dissociation was indicated (52). 

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. 

Piacenti s results for 1- and 2-pentene at high pressure would then fit in quite well with the fact that Takegami et al. (147-149) found that linear acylcobalt tetracarbonyls were much more difficult to isomerize than their branched-chain isomers. However, Piacenti et al. reject the possibility of an isomerization of alkylcobalt carbonyls in view of their work on the hydro-formylation of orthoformic esters (Section II, D,2). 

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). 

Evidence for the existence of the coordinatively unsaturated tricarbonyls has already been discussed. Equation (50) is similar to the corresponding decarbonylation of acylcobalt tetracarbonyls. 

This reaction was studied by Heck (52), who found that in the reaction of acylcobalt tetracarbonyls with triphenylphosphine, which proceeds via the acylcobalt tricarbonyl, the rate increases nearly two orders of magnitude as R goes from CH3 to (CH3)3C. This steric acceleration of dissociation would be expected to affect ks to an even greater extent, thus explaining the increase of k5/k4 with increasing branching of R. 

Synthesis and Reactions of Alkylcobalt and Acylcobalt Tetracarbonyls R. F. Heck... 

When cobalt hydrotetracarbonyl was used as the catalyst, and the reaction temperature was 0 , epoxides were readily carbonylated to give high yields of unstable (2-hydroxyalkyl)cobalt tetracarbonyls (see equation 3) which were readily carbonylated to aflFord acylcobalt tetracarbonyls (see equation 4), Heck later showed that the complex can be stabilized by replacing one of the carbonyl ligands by triphenylphosphine (see equation 5). 

The discovery that carbon monoxide can be inserted into alkyl-cobalt tetracarbonyl to yield unstable acylcobalt tetracarbonyl (which... 

Although butadiene reacts with Co2(CO)8 to yield the diene complexes (diene)C02(CO)o and (diene)2Co2(CO)4 (268), with alkyl- or acylcobalt tetracarbonyls it produces only the Tr-allylic species, 1-alkyl- or 1-acylmethyl-TT-allylcobalt tricarbonyls (281). These will react, in turn, with P(C3Hb)3 which displaces one CO ligand to form monotriphenyl-phosphine derivatives (281). 

Acylcobalt tetracarbonyl complexes, formed from Na[Co(CO)4] and alkyl or acyl halides, react with alkynes to give 2,4-pentadieno-4-lactones (Scheme 20). The reaction is catalytic in cobalt and yields are around 60% for a variety of substituted alkynes and alkyl halides. 

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