Acylcobalt

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

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

Phase-transfer catalysed formation of the acylcobalt tricarbonyl complexes using a polymer-supported ammonium salt produces lower yields of the complexes [4],... 

A second interfacial exchange reaction of the o-acylcobalt complex with hydroxide ion leads to the production of the alkanecarboxylate anion, which migrates into the aqueous phase, leaving the cobalt tetracarbonyl anion in the organic phase for subsequent reaction (Scheme 8.2). Optimum yields of the carboxylic acids are obtained with ca. 40 1 ratio of the alkyl halide to dicobalt octacarbonyl. Co(Ph,P)2Cl2 can also be used and has the advantage that the cobalt can be recycled easily [5]. 

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... 

It is well known that, although CH3Co(CO)4 is unstable, the acylcobalt... 

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... 

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

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... 

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). 

This hydride then may add an acetylene molecule to form the vinyl derivative. A carbon monoxide insertion will produce the acrylyl nickel compound which can yield acrylate esters by either of two routes. Direct alcoholysis of the acyl nickel group may take place, as occurs with acylcobalt compounds (42) or, an acyl halide (or other acyl derivative, e.g., acyl alkanoate) may be eliminated. Alcoholysis of the acyl halide would then complete the catalytic cycle (35). 

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 n complex 1 has not been isolated or observed directly, but its involvement is strongly supported by indirect evidence. In the second step the alkene inserts into the cobalt-hydrogen bond to yield an alkylcobalt complex (2), which is transformed via the migratory insertion of CO into a coordinatively unsaturated acylcobalt complex (3). 

The basic steps in the hydroformylation mechanism do not change in the presence of ligand-modified cobalt, but the kinetics of the reaction is affected.30 First-order dependence of the rate on hydrogen partial pressure and an inverse first-order dependence on CO partial pressure were observed in the unmodified system.36 At high CO pressures the rate is first-order in both alkene and cobalt concentrations. The last, product-forming, step—the cleavage of the acylcobalt... 

Acylcobaltate complexes, [RCCo(NO)(CO)P(C6H5)3] Li+ (2). These are prepared by reaction of RLi with 1 in THE They are stable at -40° but decompose at 25°. The acyl group is transferred to a, 3-enones, quinones, and allylic halides.1... 

Metal carbonyls Acylcobaltate complexes, 101 Arene(tricarbonyl)chromium complexes, 19... 

Tricarbonyl(naphthalene)chromium, 19 Trimethylsilyl chlorochromate, 327 Cobalt Compounds Acylcobaltate complexes, 101 [Bis(salicylidene-y-iminopropyl)methyl-amine]cobalt(II), 41 Cobalt(II) chloride, 249 Cobalt zeolites, 296 Dicarbonylcyclopentadienylcobalt, 96 Di- x-carbonylhexacarbonyldicobalt,... 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

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... 

Aldehyde was also produced under nitrogen (73), but the ratio of branched to straight-chain aldehyde was much greater than in the presence of 1 atm of carbon monoxide. Under nitrogen the carbon monoxide must have come from the cobalt hydrocarbonyl so that in this case an acylcobalt tricarbonyl would be formed in Eq. (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. 

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