Cobalt carbon bond, reactivity

The strength of the cobalt-carbon bond is very much influenced by the nature of the other five ligands. If these ligands are varied, cobalt complexes may be obtained having different reactivities of the Co-C bond (45). 

A striking feature of the B12 complexes (la, Ib, and Ic) is the presence of a naturally-occurring cobalt-carbon bond. The rarity of a water-stable example of such a bond led Abeles to predict that in this bond probably lies the secret of its reactivity [11]. Indeed, as we shall see later, there is now substantial support for this insightful prediction. 

It is generally believed that the cobalt-carbon a-bond is rather unstable and reactive. However, cobaloximes, model complexes of vitamin B12, form simple alkylcobaloximes by several ways, such as reductive alkylation, addition of olefins, and substitution reaction. The cobalt-carbon bonds thus formed are surprisingly stable, and undergo a variety of cleavage reactions, such as/3-elimination, coupling, substitution with nucleophiles, and other reactions. The effect of this specific ligand is remarkable. 

The mechanism of this reaction was investigated to find out if carbon monoxide dissociation is the rate-determining step (16). The rate of the reaction of acetylcobalt tetracarbonyl with iodine is too fast to measure under conditions which allow dissociation rate to be measured easily. Thus, dissociation is not rate-determining, and the acylcobalt tetracarbonyl and the iodine must be reacting directly. Further studies with the less reactive acyl(triphenylphosphine)cobalt tricarbonyls showed that the first step in the reaction with iodine is a rapid cleavage of the cobalt-carbon bond to form acyl iodide and iodo(triphenylphosphine)cobalt tricarbonyl. 

In CMRP, all chains are - ideally - terminated by a cobalt-carbon bond, and can be reactivated at a moderate temperature so as to allow the release of macroradicals. When carried out in the presence of radical traps, reactivation of the protected chains can be used not only for the end-functionalization of polymers but also for removing the cobalt complex that originally was attached to the chains. As a rule, the treatment of polymers prepared by CMRP with nitroxides and thiols produced cobalt-free polymers that were terminated by an alkoxyamine and hydrogen, respectively [44]. Both, fullerenes [45, 46] and nanotubes [47], which are prone to radical addition, were similarly used as radical traps, leading to carbon nanoobjects grafted with polymers. 

In the Pauson-Khand reaction one acetylene, one olefin and cobaltcarbonyl (as a carbon monoxide source) are used and cyclopentenone is obtained by a [2 + 2 -t-1 ] cyclization addition [56-61]. As shown in Scheme 17.5, the Pauson-Khand reaction is, at first, two -electrons of the reactive acetylene bond to two cobalt atoms (Figure 17.6), then the olefin coordinates to one cobalt atom and then inserts into the cobalt-carbon bond. Further, one carbonyl inserts into the new cobalt-carbon bond, and cyclopentenone is obtained by elimination of Co2(CO)6 [56]. 

We shall report here the synthesis of cobalt-carbon bond involved in coenzyme B12 and cobalamine, cobaloxime, and Schiff base analog models, together with some aspects of their reactivity. 

In marked contrast to the majority of activated metals prepared by the reduction process, cobalt showed limited reactivity toward oxidative addition with carbon halogen bonds. Iodopentafluorobenzene reacted with 2 to give the solvated oxidative addition products CoL and Co(C,F5)2 or Co(C F )L The compound CoiOJF 2PEt, was isolated in 54% yield by addition of triethylphosphine to tne solvated materials. This compound was also prepared in comparable yield from 1 by a similar procedure. This compound had previously been prepared by the reaction of cobalt atom vapor with C6F5I(81). 

Since the secondary reactions involved in alcohol homologation reactions, strictly speaking, are outside the scope of this review, the effects of various promoters on this reactivity will not be discussed. The presented material is given only to serve as a comparison with the analogous cobalt systems, in which the weakness of the metal-carbon bonds governs this aspect of reactivity. 

G. Costa Pure Appl. Chem. 30, 335 (1972) The effect of the nature of ligands on the reactivity of the metal-carbon bond in cobalt chelates 18 (61)... 

One important property of CF3-transition metal complexes became apparent almost immediately when all of the low-valent, late transition metal trifluoromethylated compounds then known were found to be significantly more thermally and oxidatively stable than the analogous methylated species. Tetracarbonyl(trifluoromethyl)cobalt(I), for example, was isolated by distillation at 91°C, whereas the hydrocarbyl Co(CO)4(CH3) decomposes at subambient temperatures (72). Additionally, while the reverse of the decarbonylation reaction, CO insertion, is commonly observed in methylated transition metal species, these reactions are essentially unknown for trifluoromethyl metal complexes (13). Prior to 1980, evidence for CO insertion into an M—CF3 bond had been reported in only one case. That reaction employed the photolysis of Mn(C05)(CF3) in an argon matrix at 17 K, and the identity of the product was not determined (14). The clear implication of the above results is that MCF3 metal—carbon bonds are significantly less reactive and thus presumably stronger than MCH3 metal—carbon bonds. 

The catalysis of carbon-carbon bond formation remains as one of the most active areas of research in catalytic antibody technology and organic synthesis [18]. Mosbach and coworkers reported the preparation and evaluation ofavinylpyridine-styrene-divinylbenzene copolymer imprinted with an aldol condensation reactive intermediate analog (Scheme 13). Dibenzoylmethane (DMB, 28) was chosen as the imprinted template based on molecular modeling studies for the cobalt (II) ion-mediated aldol condensation of acetophenone, 29, and benzaldehyde, 30, to produce chalcone, 31. 

In marked contrast to the majority of activated metals prepared by the reduction process, cobalt showed limited reactivity toward oxidative addition with carbon-halogen bonds. lodopentafluorobenzene reacted with 2 to give the solvated oxidative addition products C0I2 and Co(C6F5)2 or CofCeFs) . 

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