Cobalt carbon bond

B12 and related organocobalt chemistry formation and cleavage of cobalt-carbon bonds. P. J. Toscano and L. G. Marzillo, Prog. Inorg. Chem., 1984, 31,105-204 (677). 

Investigations of the cobalt-carbon bond energies in organometallic cobalt porphyrins continue to attract interest, originally because of their similarity to coenzyme B 2, and more recently because of their role in the catalysis of free radical... 

Niu, S., Hall, M. B., 1997, Comparison of Hartree-Fock, Density Functional, Mpller-Plesset Perturbation, Coupled Cluster, and Configuration Interaction Methods for the Migratory Insertion of Nitric Oxide into a Cobalt-Carbon Bond , J. Phys. Chem. A, 101, 1360. 

General descriptive/synthetic chemistry Cobalt carbon-bonded compounds10-12 coordination compounds2-9,13-15... 

Fig. 28. Routes of cobalt-carbon bond cleavage showing possible electronic arrangements of the resulting spin labeled derivative...

The results of this work leads one to the conclusion that the catalytic intermediate is, a cobalt(II) species. The cobalt-carbon bond cleaves homolytically to give an organic free radical. If the initial cleavage gave a carbanion and cobalt(III), the ESR signal would not disappear, and if... 

Chemistry Formation and Cleavage of Cobalt Carbon Bonds. 31 105... 

The substitution behavior of coenzyme B12, in which a cobalt-carbon bond is present between the Co(III) center and the 5 -desoxy-... 

Complexation of the nitrile to the cobalt atom (end-on or side-on) can occur followed by insertion into the cobalt-carbon bond to give a seven-membered intermediate (Fig. 4b). 

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). 

Cobalt-carbon bonds may be formed or cleaved by reactions in which the organic group may be present as ... 

Metal Hydrides. It is likely that the reduction of aldehydes to alcohols by cobalt hydrocarbonyl (27) is an example of a carbonyl insertion reaction with a metal hydride. It is not clear which way the hydrocarbonyl adds to the carbonyl groups —whether it forms a cobalt-carbon bond (2), or a cobalt-oxygen bond (90). 

RCH2OH + Co2(CO)7 (72) A known reaction of cobalt hydrocarbonyl suggests that the cobalt-carbon bond may be preferred. It has been reported that, under rather vigorous conditions, acetaldehyde or formaldehyde react with CO and a cobalt catalyst to give o -hydroxy acids or esters in alcohol solution (7). The intermediate with the carbon-cobalt bond probably is undergoing a CO insertion reaction, folllwed by a hydrolysis or... 

If the formation of formate esters under hydroformylation conditions involves the carbonylation of an alkoxycobalt carbonyl as suggested previously (90), this would be evidence that cobalt hydrocarbonyl adds the reverse way to acyl groups. Since the formation of formate esters can be explained as well by a CO insertion into a cobalt-hydrogen group followed by alcoholysis, however, the data would be explained best if a cobalt-carbon bond was formed in the hydride reduction of acyl compounds. 

The nature of the axial ligand in the trans position to the R group. For a series of cobalt methyl complexes it was shown, that ligands in the trans position affect the cobalt-carbon bond... 

It is believed that the reaction starts with homolytic cleavage of the cobalt-carbon bond (at a cost of perhaps 100 kJ mol-1)8 to yield a Co(ll) atom and a 5 -decxy-adenosyl radical. This radical then abstracts a hydrogen atom (in Eq. 19.35 from the methyl group). Migration of the —GO)SR group takes place, followed by return of the hydrogen atom from 5 -deoxyadenosine to the substrate. This regenerates the 5 -deoxyadenosyl radical, which can recombine with the Co( I) atom to form the coenzyme. 

Both 23 and the B12 coenzyme, 24, are compounds of Co(III) and both substances have all electrons paired. B,2 can be reduced to a form with Co(II) which has an unpaired electron and gives an esr signal (Section 27-9). The cobalt-carbon bond of 24 appears to be formed from 23 by removal of the cyano group and a two-electron reduction to Co(I). The reduced cobalt is... 

These complexes can exist in a triangular peroxo form (7a) for early d° transition metals, or in a bridged (7b) or linear (7c) form for Group VIII metals. They can be obtained from the reaction of alkyl hydroperoxides with transition metal complexes (equations 9 and 10),42-46 from the insertion of 02 into a cobalt-carbon bond (equation ll),43 from the alkylation of a platinum-peroxo complex (equation 12),44 or from the reaction of a cobalt-superoxo complex with a substituted phenol (equation 13).45 Some well-characterized alkylperoxo complexes are shown (22-24). 

Styrene derivatives can be selectively converted to the corresponding benzyl alcohols by molecular oxygen in the presence of bis(dimethylglyoximato)chloro(pyridine)cobalt(III) and sodium tetrahydroborate (equation 242).559 A likely mechanism for this reaction involves insertion of the alkene into the cobalt-hydride bond, followed by 02 insertion into the cobalt-carbon bond, as in equation (11), and decomposition of the peroxide adduct (168) to the ketone, which is reduced to alcohol by NaBH4 (equation 243). 

Cobalt-Carbon Bond Homolysis Studies with coenzyme Bi2 model compounds and TEMPO as a radical trap have allowed the determination of rate constants and activation parameters, which in turn led to estimates of the Co-C bond dissociation energies (BDEs). The key role of the homolytic Co-C cleaving step in these reactions was established by the following observations (Co11) is produced addition of external... 

AS = —25 29 J/ (mol K), from which Keq = 1.5 0.1 and AH0 0 were obtained. The identical rate constant for the homolysis of C-R in the presence of TEMPO and for reversion to (Co)-R in the absence of the scavenger shows that the two processes have the same rate-limiting step, that is, cobalt-carbon bond homolysis. The principle of microscopic reversibility requires that the conversion of (Co)-R to C-R have the same transition state. Scheme 8.20 rationalizes all of the observations. 

The stereochemical course of the cleavage of cobalt-carbon bonds in alkyl... 

More recently, van der Donk and coworkers reported radical cyclizations catalyzed by vitamin B12 using titanium(III) citrate as a stoichiometric reducing agent (Fig. 69, entry 17) [330]. Here /V-allylic 2-(isopropenyl) pyrroles 290 or allyl 2-phenylallyl ethers serve as the starting material in tandem hydrocobaltation/ radical 5-exo cyclization sequences giving dihydropyrrolizine derivatives 291 or 292. The mechanistic course is not completely clear. However, it is assumed that the reactions start with an initial hydrocobaltation of the isopropenyl unit in the presence of the allylic alkene (see Sect. 5.7). The benzylic cobalt intermediate is subject to homolysis of the very weak cobalt-carbon bond and initiates the radical 5-exo cyclization. Interestingly, the fate of the cyclized radical is dependent on the... 

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