Acetyl cobalt complex

Next, the migratory CO insertion, which is considered as the rate-determining step (rds), followed by coordination of a CO ligand gives the acetyl-cobalt complex [Co(COCH3)(CO)4] (Eq. 20.5) [5, 6]. 

Tetracarbonylalkyl derivatives of cobalt(I) have low stability. As early as 1964 it had been noted that ketones are formed in the thermal decomposition of CoR(CO)4 (R = Me, Et) presumably involving a binuclear intermediate or an intermolecular mechanism. The mechanism of acetone formation was studied for other cobalt systems that are more easily handled, namely, Co(>/ -C5H5)Me2(PMe3) and Co2(ti -CsH )2Me2(fi2-CO)2 . Upon carbonylation, in the former case, the transient carbonyl derivative Co()j -C5H5)Me2(CO) was observed spectroscopically, whereupon it underwent carbon monoxide insertion to give an acetyl-methyl complex, followed by reductive elimination of acetone ... 

The tris(Ar-hydroxethyIethylenediamine)cobalt(III) chloride that was used in these reactions was reported to be an orange crystalline solid (111). In a subsequent investigation (69), attempts to prepare this compound by the air oxidation of a mixture of cobalt (II) and the amine failed. The compound was, however, prepared by the displacement of ammonia from [Co(NH3)e]Cl3 by iV-hydroxyethylethylenediamine and a dark red compound was obtained. Attempts to react the hydroxy groups in this red complex with a variety of reagents (nitric acid, thionyl chloride, benzoyl chloride, and acetyl chloride) were as unsuccessful as the previously reported attempts to react the hydroxy groups in the orange cobalt complex. 

Specific complexing between metal ions and pairs of amino- and hydroxy-groups in aminoglycoside antibiotics has allowed the preparation of a range of specifically substituted derivatives thus, a cobalt complex of sisomicin had the sites paired in (3) blocked and on acetylation afforded 3,2, 6 -tri-A-acetyl... 

Acetylation occurs at the 2-position of allene systems (Scheme 8.14). The intermediate 7t-allyl complex breaks down via the nucleophilic displacement of the cobalt carbonyl group by the hydroxide ion to produce the hydroxyketone (7) [ 11 ]. An alternative oxygen-initiated radical decomposition of the complex cannot, however, be totally precluded. The formation of a second major product, the divinyl ketone (8), probably arises from direct interaction of the dicobalt octacarbonyl with the allene and does not require the basic conditions. 

Friedel—Crafts acetylation can be achieved, but the introduction of one acetyl group deactivates the remaining chelate rings so that mixtures of mono-, di- and tri-acetyl products are formed in the case of chiomium(III) and cobalt(III) complexes, but only mono- and di-acetyl products from rhodium(III) acetylacetonate. 

The acetyl iodide formed reacts with water or methanol to give acetic acid or methyl acetate. The latter is subsequently hydrolyzed to acetic acid. Methyl iodide is regenerated according to equation (78). The synthesis of acetic acid from methanol catalyzed by complexes of cobalt, rhodium and iridium has been reviewed.469... 

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