Acylcobalt carbonyls

The reaction of the 7i-complexes23 shown in Table 2 may be represented by a mechanism similar to that proposed for the acylcobalt carbonyls. 

If the conjugated diene group is in the acyl chain of the acylcobalt carbonyl, then cyclization is possible. Thus, sorbylcobalt tricarbonyl triphenylphosphine on heating to 80°C., cyclizes to 2-methyl-7r-cyclopentenonylcobalt dicarbonyl triphenylphosphine (41). 

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. 

The detailed chemistry of the alkyl- and acylcobalt carbonyls which are reaction intermediates in many of these catalyses has recently been excellently reviewed by Heck (59). For this reason we have confined ourselves to the catalytic implications so as to supplement that review.2... 

When a large excess of olefin was used, the cobalt hydrocarbonyl was completely used up in Eq. (2), and Eq. (3) did not occur. In this case the products have been recovered as the triphenylphosphine derivatives, or as the esters by reaction of the acylcobalt carbonyls with iodine and an alcohol. 

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... 

The products of the reaction are determined by two factors (1) the initial direction of the addition of hydrocarbonyl (2) isomerization of the resulting alkyl- or acylcobalt carbonyls. 

Heck and Breslow (62) have postulated equilibria between acylcobalt carbonyls and olefin-hydride complexes. 

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.,... 

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. 

Prior to Takegami s studies, the effect of isomerization of acylcobalt carbonyls on the products of the reaction between cobalt hydrocarbonyl and olefins had received little attention. Terminal olefins had been found to give a mixture of linear and branched products at low temperatures under carbon monoxide, and this was taken as reflecting the mode of addition of cobalt hydrocarbonyl (62, 73, 147). In view of the slow rate of isomerization of acylcobalt carbonyls this seems justified. However, it is worth noting that branched products predominated in the reaction of 1-pentene with hydrocarbonyl under nitrogen even when the olefin had isomerized only to the extent of 50% (73). Both isobutylene and alkyl acrylates had been found to produce branched products. It was suggested that isobutylene, with an... 

Takegami et al. (147) reexamined the reaction of olefins with cobalt hydrocarbonyl to determine the effect of reaction variables and the possibility of isomerization on the structure of the acylcobalt carbonyls formed. Products were recovered as their ethyl esters as described previously. Additionally, the uptake of carbon monoxide was measured. This is important since the presence of an acylcobalt tricarbonyl can be inferred when the amount of ester produced exceeds the amount of carbon monoxide absorbed. 

The factors affecting the distribution of products formed in the hydroformylation reaction have already received attention in Section II, A,2. The isomerizations of both olefin and acylcobalt carbonyl can be of importance and the extent of these isomerizations will be dependent on carbon... 

The second mechanism involved the insertion of an olefin into an acylcobalt carbonyl, e.g.,... 

An unusual synthesis of acyldienes from conjugated dienes, carbon monoxide, and alkyl or acyl halides using cobalt carbonylate anion as a catalyst should be mentioned here (57). The reaction apparently involves the addition of an acylcobalt carbonyl to a conjugated diene to produce a l-acylmethyl-7r-allylcobalt tricarbonyl, followed by elimination of cobalt hydrocarbonyl in the presence of base. The reaction can thus be made catalytic. Since the reaction was discussed in detail in the recent review by Heck (59), it will not be pursued further here. 

As with the hydroformylation of olefins, aldehydes are expected by a reduction of acylcobalt carbonyls by cobalt hydrocarbonyl. They are formed in small amounts for a number of epoxides (145). 

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). 

Heck and Breslow (63) obtained evidence relating to Eq. (52) for the case of cobalt. They reacted a number of alkyl and acyl halides carrying functional groups with sodium cobalt tetracarbonylate under carbon monoxide. Normal acylcobalt carbonyls were readily formed except for the case of chloroacetonitrile, where a cyanomethylcobalt carbonyl was isolated. Apparently an a-nitrile group is sufficiently electronegative to stabilize the alkylcobalt carbonyl against carbonylation. 

The isomerization of epoxides is discussed in Section II, D,l. The isomerizations of olefins and of alkyl- and acylcobalt carbonyls have been considered as side reactions to hydroformylation but studies dealing principally with these reactions will now receive attention. 

The isomerization of alkyl- and acylcobalt carbonyls is important in considering the products of the hydroformylation reaction and has been dealt with in part in Section II, A. Equations (9) and (10) give the most likely mechanism for the isomerization. 

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. 

Acylcobalt carbonyls formed from acyl halides appear to isomerize less readily. No isomerization was found using isobutyryl bromide or n-butyryl chloride in nonpolar solvents such as hexane or benzene. Some isomerization was found with diethyl ether and ethyl acetate as solvents, again promoted by absence of carbon monoxide. Curiously, no isomerization... 

This is similar to postulating the various resonance forms for the benzyl anion but does not so easily permit para products. Takegami found that this isomerization was strongly promoted by cobalt hydrocarbonyl. It would be interesting to know if this is also true of the isomerization of aliphatic acylcobalt carbonyls. 

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.,... 

Heck demonstrated Eq. (89) for a number of acylcobalt carbonyls, preparing them from the corresponding alkyl halide and sodium cobalt carbonylate. In the presence of bases, cobalt hydrocarbonyl regenerated cobalt carbonylate ion and a catalytic reaction resulted at atmospheric pressure and at temperatures from 0° to 100° C. Thus the following reaction was reported in 56% yield at 50° C ... 

The base (B) used was dicyclohexylethylamine. At the higher temperatures the isomeric products that one would expect from an isomerization of the acylcobalt carbonyls were formed (see Section II, A). Amines were also used in place of alcohols to give amides. Thus benzyl chloride reacted with carbon monoxide and aniline in tetrahydrofuran solution at 35° C in the presence of sodium cobalt carbonylate catalyst to give a 47% yield of phenylacetanilide. 

A catalytic carboxyalkylation can also occur if the acylcobalt carbonyl is formed from an epoxide and sodium cobalt carbonylate ion. This reaction is discussed in Section II, D,l,a. 

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. 

Treatment of [Co(CO)4] under PT conditions with a mixture of alkyne and Mel (excess) in a CO atmosphere gives hydroxybut-2-enolides (eq. (12)) [148]. When benzyl bromide is used instead of Mel, the but-2-enolides are not formed due to the fast hydrolysis of the acylcobalt carbonyl intermediate [PhCH2COCo(CO)4] before the alkyne complex is formed. This obstacle has been overcome by carrying out the reaction in the absence of water, using solid/liquid PTC [149]. The direction of the PTC carbonylation of alkynes in the presence of Mel changes again when cobalt and ruthenium carbonyls are used simultaneously (1 1) [150]. In this case, y-ketoacids are obtained. 

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. 

Acylmetal complexes generally react with conjugated dienes to produce useful organic compounds. For example, acylcobalt carbonyls add to butadiene, producing 1 -acylmethyl-7r-allylcobalt derivatives, which then undergo elimination of the elements of HCo(CO)3 on treatment with base to give 1-acyl-diene u6). This reaction can be made catalytic with respect to the cobalt catalyst under proper reaction conditions. 

Figure 2.1. The hydrogenolysis of acylcobalt carbonyls by molecular hydrogen. Reprinted with authors permission from Orchin and Rupilius.12...

---