Cobalt complexes Vitamin

Similar cyclizations can be performed using cobalt complexes (vitamin B12 and Bi2 analogs) as mediators [97-100] (Scheme 68). 

The electrochemical processes involving cobalt complexes have already been thoroughly investigated [62, 66], Additional results have been reported with cobalt complexes different from vitamin Bxj and also with nickel ligated to tetradentated macrocyclic ligands. Both series of complexes lead to radicals. 

Cobalt complexes with square planar tetradentate ligands, including salen, cor-rin, and porphyrin types, all catalyse the reduction of alkyl bromides and iodides. Most preparative and mechanistic work with these reactions has used cobalamines, including vitamin-B,. A generalised catalytic cycle is depicted in Scheme 4.10 [219]. At potentials around -0.9 V vs. see, the parent ligated Co(lll) compound un-... 

Vitamin B12 Vitamin B12 is an important cobalt complex. The vitamin was isolated from liver after it was found that eating large quantities of raw liver was an effective treatment for pernicious anaemia. The term vitamin B12, refers to cyanocobalamin. Vitamin B12 is a coenzyme, and serves as a... 

It would be interesting to know whether this enzyme system works with cobalt only when the latter is held in the vitamin B12 structure, or whether it is possible to produce cobalt-carbon compounds from other cobalt complexes as well. 

A special mention13 may be made of cobalt complexes the [Co(HDMG)2] (w = 0, 1) has been named cobaloxime by analogy with cobalamin , which is another name of vitamin B,2. 

Cyanocobalamin (vitamin B12), a cobalt complex first isolated from liver but now produced commercially from microbiological culture, is needed to maintain normal synthesis of red blood cells in man and animals. Ruminants obtain cyanocobalamin from their symbiotic rumen flora while in other herbivores such as the... 

With the name corrinoid we will refer to a class of compounds which have a molecular skeleton similar to that of the cobalt complex present in Vitamin B12 of which the main characteristics are the direct link between two pyrroles and the fully saturated p positions. These major features afford a macrocycle which is more contracted than a porphyrin and without an aromatic n system. 

The catalytic isomerization of meso-epoxides to allylic alcohols has been achieved with chiral cobalt complexes, in particular with cobalamin (vitamin B12) [47, 48]. 

The same radical cyclization can be performed using a more simplified vitamin B12 model, such as Z w(dimethylglyoximato)(pyridine)Cobalt chloride Co3+ Cl -Py complex (B), as shown in eq. 11.3. Treatment of propargyl P-bromoethyl ether (3) with NaBH4 and a catalytic amount of the cobalt complex (B) provides fi-exo-methylene tetrahydrofuran via 5-exo-dig manner [2-7]. 

Nature demonstrates that transition metals can be very effective in catalyzing transformations, which are impossible to accomplish otherwise under physiological conditions. The prime example is vitamin B12, whose resting state is adenosylco-balamine(III) (reviews [267-273]). On homolysis it triggers a variety of radical reactions crucial to the living world. This inspired the interest of chemists and led to a number of applications. More recently, interest shifted to catalysis by low-valent cobalt complexes. 

Radical reactions involving cobalt(I) catalysis find their model in Nature, which uses vitamin B12 247 extensively for catalyzing difficult transformations [267-273]. This served as an early inspiration to use cobalt complexes as mediators and catalysts for radical reactions [268, 298]. 

Radical additions of primary, secondary, and tertiary alkyl bromides 249 to diethyl mesaconate 248 catalyzed by 5 mol% vitamin B12a 247 (X=OH2) proceeded in yields of 63-90% [301]. Deuteration experiments and comparison to similar addition processes support that 247 is initially reduced to cobalt complex 253A. This reacts with 249 giving an alkylcobalamin(III) intermediate... 

The thermal stability of benzylcobalamin is significantly lower than that of the benzyl derivatives of various other cobalt complexes for example [C6H5CH2—Co(DMGH)2(py)] and [C6H5CH2— Co(CN)5]3, that have been invoked as vitamin Bi2 models (9, 10, 12, 13). 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

The second-order rate constants for reactions of Co(I)(BDHC) with alkyl halides were determined spectrophotometrically at 400 nm (17). These rate constants are listed in Table VII along with those for Co(I)(corrinoid)(vitamin Bi2s) in methanol at 25°C (35). These data indicate that the SN2 mechanism is operative in the reaction of Co(I)(BDHC) the iodides are more reactive with the cobalt complex than the bromides, and the rate decreases with increasing bulkiness of the alkyl donor. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B12s, which is confirmed by the rate ratios for... 

But the champion is vitamin B12, a complicated cobalt complex with a three-dimensional structure of great intricacy. No chemist would learn this structure but would look it up in an advanced textbook of organic chemistry. You will find it in such books in the index under vitamin B12 and not under its systematic name. We do not even know what its systematic name might be and we are not very interested. 

Vitamin B12 (1) was discovered some 55 years ago as the (extrinsic) antipemicious anemia factor, important for human and animal metabolism The cobalt complex (1) was structurally characterized by X-ray analysis in the laboratory of D.C. Hodgkin. In these studies, the unique buildup of the tetrapyrrolic corrin ligand and the intramolecularly coordinating pseudo-nucleotide function of the vitamin were discovered. ... 

The unusual nature of the red cobalt complex (I) induced considerable activity in the field of coordination chemistry vitamin B12 derivatives were found to represent unique examples of kinetically labile Co complexes, the analysis of whose model type coordination chemistry itself proved to be of particular value.Interestingly, for the transition metal cobalt, no other cofactor function has been clearly established, except for that in the B -derivatives. ... 

Bis (dimethylglyoximato) cobalt complexes are of interest as models of vitamin Bi2. The cobalt atom in the square planar ligand field of the four sp -hybridized nitrogen atoms of dimethyl-glyoxime has a pronounced tendency to form stable organo-cobalt derivatives, in analogy to the cobalt atom in the... 

Finally, a history of organometallic chemistry would be incomplete without mention of what surely is the oldest known organometallic compound, vitamin B12 coenzyme. This naturally occurring cobalt complex, whose structure is illustrated in Figure 13-6, contains a cobalt-carbon n bond. It is a cofactor in a number of enzymes that catalyze 1,2 shifts in biochemical systems ... 

Albert Esohenmoser (1925-), working at the ETH In Zurich, synthesized vitamin B12, a cobalt complex and at the time the most complicated molecule yet made, In an unusual International collaboration with Woodward at Harvard. 

Cobalt complexes of biologic importance, specifically vitamin B,2 and its derivatives, undergo electron transfer ... 

Interest in the study of the oxidative addition reactions of low-spin cobalt complexes is enhanced by parallels with corresponding reactions of vitamin B,2 derivatives and hence by their possible relevance as Bi2-model systems . 

Hush NS, Woolsey IS (1974) Optical and electron spin resonance spectra of cobalt complexes related to vitamin B12. J Chem Soc Dalton Trans 24-34... 

One of the articles describes F-430, a nickel complex with a cyclic ligand related to porphyrins. In strict anaerobes and in anaerobes able to withstand a low level of dioxygen, e.g. sulphur bacteria, both vitamin B12, a cobalt complex, and heme, an iron complex, are present. They too are related to porphyrins. It is then of interest to compare and contrast their functions. Vitamin B12 has been described in volume 20 (Hogenkamp HPC, Sando GN (1974) Struct Bond 20 23). In this volume the value of heme to anaerobes is stressed. Putting these three articles together we begin to see that early life had a requirement for trace elements in highly specified forms. 

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