Alkylcobalt derivatives

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... 

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). 

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.,... 

Alkali Metal Derivatives of Metal Carbonyls, 2, 157 Alkyl and Aryl Derivatives of Transition Metals, 7, 157 Alkylcobalt and Acylcobalt Tetracarbonyls, 4, 243 Allyl Metal Complexes, 2, 325... 

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q < —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

This chapter reviews the methods of preparation and the chemistry of the alkylcobalt and acylcobalt tetracarbonyis. These compounds are among the most thoroughly studied of the organo-substituted transition metal carbonyl derivatives. Recent evidence suggests that the mechanisms of reaction of the various transition metal compounds are closely related. Consequently, the information obtained from the alkylcobalt and acylcobalt tetracarbonyis should provide a basis for predicting the reactions of other transition metal compounds. 

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). 

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... 

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. 

Insertion reactions of alkylcobalt or acylcobalt tetracarbonyls with saturated aldehydes or ketones have not been observed. Carbonyl insertions do occur in some unsaturated carbonyl systems, however. The cyclization of the intermediate acylacrylylcobalt tricarbonyls, formed from acetylenes and alkylcobalt or acylcobalt tetracarbonyls, to butenolactone derivatives, as described above, is one example of the reaction. Another example is the addition of alkylcobalt or acylcobalt tetracarbonyls to a, -unsaturated aldehydes or ketones. In this reaction an acyl group from the cobalt compound is added to a carbonyl oxygen and the cobalt carbonyl group forms a iT-allyl system with the carbonyl carbon and the double bond 19). 

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... 

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. 

Other alkylcobalt and alkyltitanium compounds, especially those with branched alkyl substituents, also decompose by olefin elimination. The isobutyl complex, [(CH3)2CHCH2Co(CN)s] , slowly loses isobutene at room temperature (115). The unique stability of methyl derivatives of the transition metals relative to other alkyl derivatives is partly due to the absence of a j8-hydrogen atom which can be transferred to the metal. The... 

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). 

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. 

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 ... 

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