Alkylcobalt carbonyls

Interception of the reaction sequence at the alkylcobalt carbonyl stage before carbonyl insertion, and hydrogenation of this intermediate, produces an alkane. This undesired side reaction is only minor (1-3%) in cobalt-catalyzed hydroformylation of a nonfunctional olefin, but may become predominant with phenyl- or acyl-substituted olefins. Ethylbenzene has been obtained in >50% yield from styrene (37), and even more alkane was obtained from a-methylstyrene (35). 

Rearrangement of Alkylcobalt Carbonyls Takegami et al. [25) reported that when ethyl ot-bromopropionate is treated with KCo(CO)4 in toluene at 0° in the presence of 1 atm of CO and the product cleaved with iodine, both the expected ester, ethyl methyl-malonate, and the rearranged ester, diethyl succinate, are formed, the latter in smaller quantity. At 25°, however, the succinate predominates. 

This scheme is particularly attractive because Heck and Breslow (22a) reacted methyl acrylate with HCo(CO)4 at 0° in pentane in 1 atm of CO and obtained both products, the malonate in 25% yield and the succinate in 6% yield. In view of the coincidence of yield and of distribution of products, one must consider the possibility that a dehydrohalogenation to acrylate occurred prior to the formation of alkylcobalt carbonyls ... 

Alkylcobalt carbonyl isomerization via the formation of olefincobalt-carbonyl hydride complexes with retention of the olefin attached to the cobalt atom has been suggested as the mechanism of formation, of the necessary precursors of the products with high stereospecificity. 

Considering the results obtained in the reaction of orthoformic esters with CO and H2 (13) and the postulated instability of secondary and tertiary alkylcobalt carbonyls, the suggested role of alkylcobalt carbonyls in the course of this reaction seems questionable. Although a small amount of olefin is detected among the reaction products of sec-butyl-orthoformate with CO and H2, probably because of the low stability of secondary alkylcobalt carbonyls, and an 80 20 ratio is found between 2-methylbutanal and n-pentanal formed (13), it is still to be explained why the hydroformylation of 2-butene under the same conditions involving the same alkylcobalt intermediates gives a 29 71 ratio (4) of the same aldehydes. 

The main reaction product, 2-phenylbutane, shows no detectable optical activity between 590 and 350 m. The 2-phenylbutane does not arise from the alkylcobalt carbonyls as reported (10) since these give rise to the optically active 3-phenylpentanal or 4-phenylpentanal. Taking into account the peculiar behavior under oxo conditions of double bonds conjugated with phenyl rings (14), it might be assumed that electronic effects produce predominately the disubstituted benzylcobalt complex CH3... 

It is not easy to distinguish between these alternatives since they give rise to such a similar kinetic pattern. It is possible that each may occur under appropriate conditions analogies for both possibilities exist in metal carbonyl chemistry (96). Subsequent steps such as the insertion of olefin to give the alkylcobalt carbonyl are also susceptible to SN1 or SN2 interpretations. Carbon monoxide insertion does occur in the absence of an atmosphere of carbon monoxide but the reaction could be assisted here by the presence of olefin (73). 

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. 

The stoichiometric hydroformylation of olefins with cobalt hydrocarbonyl is also inhibited by an atmosphere of carbon monoxide (62, 73) (Section II, A) and this has been shown to involve a CO inhibition of alkylcobalt carbonyl formation (Eq. (18)). 

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). 

Heck discussed two possible mechanisms. The first involved a reaction between acyl and alkylcobalt carbonyls. This should produce saturated ketones, e.g.,... 

This isomerization to ketones also occurs under the milder conditions under which cobalt hydrocarbonyl is reacted with epoxides, however, and it seems likely that cobalt hydrocarbonyl was also present under the conditions of Eisenmann s experiment. Heck has therefore suggested that the mechanism could involve the formation of a hydroxyalkylcobalt carbonyl followed by elimination to produce the enol form of the ketone in the same way that alkylcobalt carbonyls can give olefins. 

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. 

It was suggested that such derivatives would have unusual stability like the corresponding 7r-allyl complexes and would be further reduced by hydrocarbonyl rather than undergoing carbonylation. Heck (63) has, however, attempted the preparation of similar derivatives from chloroacetone and phenacyl bromide with sodium cobalt tetracarbonylate, Instead of finding unusually stable complexes he reported an unusual instability. It seems likely that in fact normal alkylcobalt carbonyls are formed, e.g.,... 

These are probably not sufficiently stable to be isolable as such, but probably carbonylate sufficiently more slowly than unsubstituted alkylcobalt carbonyls that decomposition competes with carbonylation. 

Olefin isomerization was found to occur as a side reaction. This was presumed to occur via the alkylcobalt carbonyls formed by Eq. (71) as discussed in Section V and elsewhere. 

The possible sources of isomeric aldehyde formation include olefin isomerization, regioselectivity of the addition of the hydridocobalt carbonyl to the olefin, isomerization of the alkylcobalt carbonyl, and isomerization of the acylco-balt carbonyl species. There is no evidence for an isomerization of the alkylcobalt carbonyl species under the conditions of industrial oxo synthesis (high pressure) [96]. In contrast, the isomerization of a coordinated olefin is well known and a plethora of studies have proven this behavior [4]. 

The selective reaction of the hydridocobalt carbonyl with the olefin via Mar-kovnikov and anti-Markovnikov addition gives rise to the branched and linear alkylcobalt carbonyl isomers. It is believed that the sterically less demanding nature of HCo(CO)3 favors the formation of the branched isomer, whereas HCo(CO)4 generates predominantly the linear isomer. This is in accordance with the increased selectivity observed at higher carbon monoxide partial pressures. As HCo(CO)4 is the less reactive catalyst, the catalytic activity drops at the same time. 

From dimethyl fumarate l,2-bis(methoxycarbonyl)ethylcobalt-tetracarbonyl was obtained in 65% isolated yield (145,150). This seems to be a convincing evidence for the intermediacy of an alkylcobalt carbonyl in the hydroformylation of an olefin. Apparently, the COOR-group stabilizes the carbon-cobalt bond much more in the alkyl- than in the acyl-complex and... 

A detailed kinetic, preparative, and spectroscopic investigation has shown that the reaction of HCo(CO)4 with dimethyl fumarate to form the corresponding alkylcobalt carbonyl is reversible and stereospecific (151). 

Reductive Cleavage of Alkylcobalt Carbonyls. Saturated product-formation accompanies not only the catalytic aldehyde-formation as a side reaction but the stoichiometric aldehyde-formation as well. Beside carbonylated products substantial amounts (20-40%) of saturated product is the result of the reaction of various olefins and HCo(CO)4 using a low olefin HCo(CO)4 molar ratio. Under such conditions with 1-substituted vinylarenes and with conjugated diolefins almost exclusively the saturation of the carbon-carbon double bond occurs. See Table 13 for characteristic examples. 

Obvious explanation of the saturated hydrocarbon-formation is the reduction of the alkylcobalt carbonyl intermediate by HCo(CO)4 or by dihydrogen. 

Equilibrium between Acyl- and Alkylcobalt Carbonyls. The insertion of carbon monoxide into the carbon-metal bond of alkylmetal carbonyls to form the corresponding acyl derivatives is one of the most important and most investigated organometallic reactions (169-173). In this reaction, a new carbon-carbon... 

Some interesting syntheses 236) have been carried out utilizing alternately the reactions of alkylcobalt carbonyl derivatives with tricovalent phosphorus derivatives and the cleavage of acylcobalt carbonyl derivatives to the corresponding anions with methanolic sodium methoxide, e.g. 

We present here preparative and structural studies on alkylcobalt carbonyl phosphine eomplexes with centres of configurational chirality in the alkyl-or in the phosphine ligand. 

In the course of previous studies with alkylcobalt carbonyl tert-phosphine complexes, similar to type 3, we discovered ... 

The considerations described earlier in this paper on the stmcture of complex 3a are reflecting the situation in the condensed P2i2i2i phase. Since most of the reactions where alkylcobalt carbonyls are important intermediates are... 

The volatile methylcobalt tetracarbonyl is thermally very unstable, m.p. —44°, and it decomposes at —35° [60]. It is readily oxidiz. The acylation reactions and the role of alkylcobalt carbonyl complexes in catalytic hydroformylation reactions are discussed on p. 334 and in chapter 9 respectively. 

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