Alkyl cobalt carbonyls

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

M 1)>CH3 (366 M 1)>C6H5 (66 M ) k7 varied in a similar manner, R = CH3CH2>C6H5 CH3>C6H5CH2, CFj. Thus the tendency for styrene and fluoroolefins to be hydrogenated could result from either the thermodynamic or kinetic difficulty with which the corresponding alkyl-cobalt carbonyls undergo carbonylation. 

If the total pressure is low or the olefin excess high, the concentration of cobalt hydrocarbonyl and hydrogen will be reduced and, according to a mechanism proposed by Bertrand et al. [320], the acyl compound can react with the alkyl cobalt carbonyl instead of with hydrogen or cobalt hydrocarbonyl. The ketone is then formed as a result of this reaction. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

In hydrocarboxylation, the Reppe reaction, the catalyst can be nickel or cobalt carbonyl or a palladium complex where R = H or alkyl. 

Organomercury compounds undergo a similar reaction. Alkyl and aryl Grignard reagents can be converted to carboxylic esters with Fe(CO)5 instead of CO. Amides have been prepared by the treatment of trialkyl or triarylboranes with CO and an imine, in the presence of catalytic amounts of cobalt carbonyl ... 

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... 

The introduction of alkyl phosphine complexes of cobalt carbonyl as hydroformylation catalysts was reported to have a significant effect on product composition (50, 51). Slaugh and Mullineaux (52) reported that hexanol with a 91% linear distribution was formed by the hydroformylation of 1-pentene at 150°C, 500 psi, H2/CO 2.0, catalyst [Co2(CO)8 + 2(n-C4H9)3P]. Under the same conditions except at a temperature of 190°C, the n-hexanol was 84% of the hexyl alcohol produced. 

Selected examples of the conversion of alkyl halides into ethyl esters using cobalt carbonyl complexes... 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

In the sixties it was recognised that ligand substitution on the cobalt carbonyl complex might influence the performance of the catalyst. Tertiary alkyl phosphines have a profound influence ... 

The behaviour of the ruthenium catalysts is quite different from that previously reported for cobalt carbonyl catalysts, which give a mixture of aldehydes and their acetals by formylation of the alkyl group of the orthoformate (19). The activity of rhodium catalysts, with and without iodide promoters,is limited to the first step of the hydrogenation to diethoxymethane and to a simple carbonylation or formylation of the ethyl groups to propionates and propionaldehyde derivatives (20). 

The 2-methylenecyclopentanone initially formed presumably rearranges into 2-methyl-2-cyclopentenone under the reaction conditions. The final step of the mechanism, elimination of the cobalt carbonyl group, is not well understood but the same kind of elimination and reduction reactions occur with known 3-ketocobalt complexes. As mentioned above, crotonaldehyde, acrolein (27), and glyddaldehyde (38) react rapidly with cobalt hydrocarbonvl under similar conditions to give reduction products, rather than forming stable alkyl- or acyl-cobalt tetracarbonyl derivatives. 

A number of metal alkyls add readily to double bonds. These include the titanium alkyls, chromium aryls and alkyls, the alkylmanganese carbonyls, acyl-cobalt carbonyls, alkali metal alkyls, the magnesium alkyls, and aluminum alkyls. 

Compared with the analogous hydrogenation of aldehydes, the reaction requires somewhat more drastic conditions (about 200°C and 6 hrs), but the temperature is still within the stability range of the cobalt carbonyl phosphine complexes containing tertiary alkyl phosphines as ligands. If aryl phosphines are used, a more or less pronounced decomposition of the carbonyl complexes can be observed (as indicated by the IR... 

Neither the palladium nor nickel catalyst described will promote the carbonylation of saturated aliphatic halides as noted above. However, this reaction can be catalyzed with cobalt (17) or iron (77) and probably with manganese (18) carbonyl anion salts. These carbonyl anions are strongly nucleophilic species and readily displace halide or other good leaving groups from primary or secondary positions giving alkyl metal carbonyl complexes. 

Reductive carbonylation and alkylation of imines.3 This cobalt carbonyl catalyzes a reaction of imines with organoboranes and carbon monoxide that results in amides. / -Keto amides are obtained from a-keto imines by this reaction. 

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. 

The isomerizations have been studied by Takegami et al. by reacting the isomeric alkyl or acyl halides with an alkali metal cobalt carbonylate (142, 148,149). 

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 ... 

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

A nucleophilic attack by 4.7 on CH3I produces 4.8 and I. Conversion of 4.8 to 4.9 is an example of a carbonyl insertion into a metal alkyl bond. Another CO group adds onto the 16-electron species 4.9 to give 4.10, which in turn reacts with I to eliminate acetyl iodide. Formation of acetic acid and recycling of water occur by reactions already discussed for the rhodium cycle. Apart from these basic reactions there are a few other reactions that lead to product and by-product formations. As shown in Fig. 4.4, both 4.9 and 4.10 react with water to give acetic acid. The hydrido cobalt carbonyl 4.11 produced in these reactions catalyzes Fischer-Tropsch-type reactions and the formation of byproducts. Reactions 4.6 and 4.7 ensure that there is equilibrium between 4.7 and 4.11. 

In the sixties it was recognized that ligand substitution on the cobalt carbonyl might influence the performance of the catalyst. It has been found that aryl phosphines or phosphites have little influence in fact they may not even coordinate to cobalt under such high CO pressures. Tertiary alkyl phosphines, however, have a profound influence [5] the reaction is much slower, the selectivity to linear products increases, the carbonyl complex formed, HCoL(CO)3, is much more stable, and the catalyst acquires activity for hydrogenation. This process has been commercialized by Shell. As a result of the higher stability of the cobalt complex, the Shell process can be operated at lower pressures and higher temperatures (50-100 bar vs 200-300 bar for HCo(CO)4, 170°C vs 140°C). 

Mechanistically, alcohol carbonylation reactions catalyzed by the HCo(CO)4/ Co(CO)4 system appear to be governed by several features which are unique to this system. In particular, the high inherent acidity of the HCo(CO)4 species (45), coupled with the nucleophilicity of the conjugate base (55), is responsible for the activation of the substrate and formation of the alkyl-cobalt bond. In addition, the facility of homolytic cleavage of cobalt-carbon bonds (46, 47) may be responsible for the complications in selectivity not normally observed with other systems. 

The Co1 arene complex [Co(C6Me6)2]+ has a sandwich structure, with two ij6-bound arene ligands. The complex is paramagnetic, with two unpaired electrons.39 Alkyl cobalt(I) carbonyls, generally made by the reactions... 

Substituted cobalt carbonyls of the type Co2(CO)6L2 have also been compared to binary cobalt carbonyls as hydroformylation catalysts for linear aldehydes. One study compared four Co2(CO)6L2 complexes (L = tertiary phosphines with functionahzed alkyl groups) to the well-known complexes Co2(CO)6(P n-Bu 3)2 and Co2(CO)g. The bulky phosphine substituent P(CH2CH2CH20CH2CH2)3 showed lower activity than the others, but analogous selectivity. 

Many industrially important catalytic processes use alkyl and acyl cobalt carbonyl complexes, which can be synthesized from (2 ). Acyl complexes produced after formation of... 

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

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