Cobalt carbonyl elimination

Many research groups have attributed the isomerization to a series of additions and eliminations of a cobalt carbonyl hydride. However, it has been shown that aldehydes may be found with formyl groups attached to a carbon atom other than the two of the double bond even under non-isomerizing conditions. Piacenti and co-workers (44, 45) studied the hydroformylation of [l-14C]propylene and of a>-deuterated a-olefins. Even for a-olefins with chain lengths up to C6, the formyl group was attached to all possible carbon atoms in the product mixture. However, in the deuterated experiments, deuterium was present only on carbons 2, 3, and a) of the resulting aldehydes. These results were explained by pro-... 

The alkyne-cobalt carbonyl complex 3 formed from the alkyne 1 and dicobalt octacarbonyl 2 should lose at least one of the GOs on the metal to provide the vacancy for the incoming olefins. Subsequently, an olefin-bound complex 5 rearranged oxidatively to yield a metallacyclic intermediate 6. Migratory insertion of GO of 6 would provide the homologated ring intermediate 7, and the following two successive reductive eliminations afford the cyclopentenone... 

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. 

Step 2 Extraction of the Catalyst from the Aqueous Solution. It is not feasible technically to charge the aqueous solution of cobalt carbonyl hydride directly into the hydroformylation reactor because two phases may form, especially with the long chain olefins. The most direct and most efficient way to eliminate water while permitting full use of the carbonyl catalyst is to extract it from the water phase with the olefin intended for hydroformylation. The extraction is carried out between... 

The cobalt carbonyl phosphine complexes dissolved in the samples, and the products were detected by IR. Ketones, alcohols, and other organic components were determined after distillation (to eliminate cobalt) by GLC. 

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. 

Alternatively, he suggested attack by cobalt carbonyl anion, followed by a hydride shift and elimination of cobalt carbonyl anion. 

Reports of nucleophilic vinylic photosubstitution reactions, which occur via the S l mechanism, are conspicuously scarce. One such example is the cobalt carbonyl catalysed photostimulated carbonylation of vinylic halides339. By this method 1-bromo and 1-chlorocyclohexene are converted into 1-cyclohexenecarboxylic acid in 98 and 97% yield, respectively. In a prototype vinylic S l reaction, of / -bromostyrene with the enolate anion CH2COCMe3, an ionic elimination-addition route seems to be followed along with the S l route340. 

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. 

The mechanism of hydrocyanation of alkenes catalyzed by soluble complexes is closely related to the mechanism of hydrogenation and hydrosilation. Hydrocyanation occurs by a sequence consisting of oxidative addition of HCN, olefin insertion into the M-H bond, and reductive elimination to form the new C-C bond. The mechanism of the original hydrocyanation catalyzed by cobalt carbonyl has not been studied in depth, but the mechanism of the reactions catalyzed by nickel complexes has been studied in depth and is better defined. 

Alternatively, the aldehyde could form from the reaction of HCo(CO) with the coordinatively unsaturated RC(0)Co(CO)j by an oxidative addition of the H-Co bond, followed by reductive elimination to form aldehyde and a dinudear cobalt-carbonyl product. 

Heck has formulated a mechanism which accounts for hydroformylation of olefins catalyzed by cobalt carbonyl (68). A modification of this mechanism is presented in Fig. 5. Cobalt octacarbonyl reacts with hydrogen to form the tetracarbonyl hydride. It is proposed that this coordinatively saturated complex loses a CO group to form the four-coordinate hydride (LX). Coordination of an olefin yields the olefin complex (LXI). Migration of hydride yields an unsaturated alkyl complex (LXII). Further insertion of a CO group (undoubtedly by a migration mechanism) affords the four-coordinate acyl cobalt(I) complex (LXIII). Oxidative addition of hydrogen affords the hypothetical dihydride (LXIV), which eliminates the product aldehyde and regenerates the cobalt(I) hydride catalyst (LX). This latter... 

The insertion of carbon monoxide into a C—M bond is a common reaction for alkyl derivatives of the late transition elements. It has been reported for Mo, Mn, Fe, Co, Ni, Pd, and Pt compounds. The insertion of CO into the C —Co bond of cobalt carbonyl derivatives is a key step in the 0x0 reaction and catalytic carboxylation processes (see Section IV,D). The CO insertion reaction is frequently reversible but sometimes only the reverse reaction, CO elimination, is known, e.g.,... 

Radical mechanisms are also found in reductive elimination. An important example is the radical-chain mechanism of the dehydrogenation of cobalt carbonyl hydride ... 

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