Alkylcobalt tetracarbonyl

Conjugated dienes can be acylated by treatment with acyl- or alkylcobalt tetracarbonyls, followed by base-catalyzed cleavage of the resulting jt-allyl carbonyl derivatives. The reaction is very general. With unsymmetrical dienes, the acyl... 

The alkylcobalt tetracarbonyls react completely analogously with carbon monoxide, forming acyl cobalt tetracarbonyls (43). 

Similarly, alkylcobalt tetracarbonyls react with triphenylphosphine (44, 45) or with phosphites (36) to give high yields of acylcobalt tricarbonyl triphenylphos-phines or phosphites. 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

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). 

Their view that cobalt hydrotricarbonyl, instead of die hydrotetra-carbonyl, is the reactive species is based on evidence that the formation of alkylcobalt tetracarbonyl is inhibited by carbon monoxide more fundamentally, initial complexing with olefin would presumably require the participation of a coordinately unsaturated carbonyl. 

The first alkylcobalt tetracarbonyl, and indeed the first simple alkylcobalt compound of any type, was prepared by Hieber and his co-workers (20). The compound prepared, methylcobalt tetracarbonyl, was obtained in 2%... 

A second method for preparing alkylcobalt tetracarbonyls involves the addition of cobalt hydrocarbonyl to olefins (5). With unsymmetrical olefins, two isomeric alkylcobalt tetracarbonyls are possible, depending upon the direction of addition of the cobalt hydrocarbonyl. Both possible isomers are often obtained. 1-Pentene and cobalt hydrocarbonyl at 0°C give a 50 50 mixture of the two possible pentylcobalt tetracarbonyls. Methyl acrylate... 

A third method for preparing alkylcobalt tetracarbonyls is the reaction of cobalt hydrocarbonyl with epoxides. The products are 2-hydroxyalkyl-cobalt tetracarbonyl derivatives (9). Ethylene oxide produces 2-hydroxy-ethylcobalt tetracarbonyl. 

The coordinated carbonyl groups of the alkylcobalt and acylcobalt tetracarbonyls may be replaced by other ligands. When alkylcobalt tetracarbonyls react with ligands, they generally form acylcobalt tricarbonyl derivatives. 

The formation of acylcobalt tetracarbonyls from alkylcobalt tetracarbonyls and carbon monoxide described above is an example of this type of reaction. The similar reactions with triarylphosphines and phosphite esters have been thoroughly studied because the equilibria are far on the side of the acyl compounds and the products are convenient derivatives to prepare from the alkylcobalt tetracarbonyls (7,10), The triarylphosphine and phosphite ester derivatives are much more thermally and oxidatively stable than the alkylcobalt tetracarbonyls themselves. 

The acylcobalt tetracarbonyls react with ligands by losing carbon monoxide, producing the same acylcobalt tricarbonyl derivatives as obtained from the alkylcobalt tetracarbonyls and the same ligands (7). 

Allylic cobalt tetracarbonyls are less stable than saturated alkylcobalt tetracarbonyls because they very readily evolve carbon monoxide and form 7T-aIlylcobalt tricarbonyls. Nuclear magnetic resonance studies have shown that these 7r-allylcobalt complexes possess symmetrical rather than un-symmetrical structures (6, 2S). 

The intermediate acylcobalt tricarbonyl is very probably also in equilibrium with the corresponding alkylcobalt tetracarbonyl and this equilibrium would explain why the acylcobalt and alkylcobalt tetracarbonyls react so similarly. 

This alkoxycarbonylation reaction is also catalytic, if the alkylcobalt tetracarbonyl is formed from an epoxide and cobalt carbonyl anion in a hydroxylic solvent (9). A stoichiometric amount of base is not required in this reaction. The initial product, a derivative of the anion of 2-hydroxy-ethylcobalt tetracarbonyl, may undergo three reactions (a) react with more epoxide to give polymer, (b) undergo an internal hydride shift to form aldehyde or ketone, or (c) undergo protonation, carbon monoxide insertion, and alcoholoysis (or hydrolysis) to form ester (or acid). Varying amounts of... 

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). 

The alkylcobalt tetracarbonyls decompose fairly rapidly at 0° C even in dilute solution. One of the decomposition products seems to be the corresponding acylcobalt tetracarbonyl 22). This product was first thought to be the isomeric form of the alkylcobalt tetracarbonyl, the acylcobalt tricarbonyl 4, 10, 22). The formation of acylcobalt tetracarbonyls from alkylcobalt tetracarbonyls probably involves a disproportionation reaction forming, at least as a transitory intermediate, an alkylcobalt tricarbonyl. 

Warming pure alkylcobalt tetracarbonyls, or their solutions, to room temperature or above causes a further reaction to take place, producing dialkyl ketones (J). These products can be accounted for by assuming that the acylcobalt tetracarbonyl adds to the alkylcobalt tricarbonyl as cobalt hydrocarbonyl probably adds to an acylcobalt tricarbonyl [Eq. (48)]. The cobalt(III) intermediate could then decompose and form ketone and a mixture of cobalt carbonyls. 

The one example where the simple insertion product is a stable product is the carbon monoxide insertion reaction. The addition of carbon monoxide to alkylcobalt tetracarbonyls to form acylcobalt tetracarbonyls has already been discussed above because of its basic importance in alkylcobalt and acylcobalt carbonyl chemistry. The mechanism of this insertion is thought to involve a 1 2 shift of the alkyl group from cobalt to carbon followed by reaction of the intermediate acylcobalt tricarbonyl with another external carbon monoxide. Although there is no conclusive evidence for or against this mechanism in the carbonylation of cobalt compounds, there is evidence for it in the related carbonylation of alkylmanganese pentacarbonyls (2, 24). 

The same reaction occurs much more rapidly and without gas evolution with alkylcobalt tetracarbonyls and conjugated dienes (14). Thus, the reaction probably involves the addition of an acylcobalt tricarbonyl to the diene, perhaps by way of a w complex, either 1 2 or 1 4 and then a cyclization to the TT-allyl derivative. 

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 acylcobalt tetracarbonyl is formed from the alkylcobalt tetracarbonyl and carbon monoxide in an equilibrium reaction ... 

In the rate-determining step, a geminate radical pair is formed by H-atom transfer from HCo(CO)4 to styrene. In subsequent fast reactions, the a-phenylethyl and tetracarbonylcobalt radicals can escape to free radicals or combine to form a branched chain alkylcobalt tetracarbonyl. 

Using the isolable model alkylcobalt tetracarbonyl, CH3CH2 0(C=0)CH2Co(CO)4, the kinetics of both types of reductions has been determined (164). The rate of ethyl acetate formation is first order in CH3CH2 0(C=0)CH2Co(CO)4 and close to negative first order in carbon monoxide. 

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. 

Some alkylcobalt tetracarbonyl derivatives of general formula RCo(CO)4 have been obtained. These pentacoordinate derivatives are much less stable than the corresponding hexacoordinate alkylmanganese pentacarbonyl derivatives RMn(CO)5. Treatment of Na[Co(CO)4] with a deficiency of methyl iodide under carefully controlled conditions gives a 2 to 4% yield of the very unstable CH3Co(CO)4, a volatile material which decomposes above — 35° C (55). Similar unstable ethyl and benzyl RCo(CO)4 derivatives have been obtained by treatment of Na[Co(CO)4] with [(C2H5)30][Bp4] and benzyl bromide, respectively (231). 

Ethyl cobalt tetracarbonyl is formed smoothly and reversibly under mild conditions from HCo(CO)4 and ethylene. Infrared studies show that alkylcobalt tetracarbonyls are in equilibrium with the acyl tricarbonyl derivatives in solution at room temperature. Further evidence for these 16-electron intermediates comes from kinetic studies on the reduction of acetylcobalt tetracarbonyls by hydrogen or by HCo(CO)4. In both cases the reduction is strongly inhibited by CO, suggesting that it is not acetylcobalt tetracarbonyl itself but the dissociated complex RCOCo(CO)3 which is reacting ... 

In ref. 127) alkylcobalt tetracarbonyls are specified rather than the more general term alkyl metal compound used by us. However, there seems to be no reason why for the sake of discussion the suggested equilibrium proposed for alkylcobalt tetracarbonyls might not be extended to include alkyl transition metal compounds as a whole. 

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