Intermediates cobalt macrocycle

B. D. Curtis, C. J. Dubois, D. L. Electrochemical Reduction of CO2 Catalyzed by a Dinuclear Palladium Cooperativity OrganometalUcs 1995, 14, 4937. (k) Ogata, T. Yanagida, S. Brunschwig, B. S. Fujita, E. Mechanistic and Kinetic-Studies of Cobalt Macrocycles in a Photochemical CO2 Reduction System - Evidence of C0-CO2 Adducts as Intermediates /. Am. Chem. Soc. 1995,117, 6708. (1) Arana, C. Keshavarz, M. Potts, K. T. Abruna, H. D. Electrocatalytic Reduction of CO2 and 0-2 with Electropolymerized Films of Vinyl-Terpyridine Complexes of Fe, Ni and Co Inorg. Chim. Acta 1994, 225, 285. 

Ogata T, Yanagida S, Brunshwig BS, Fujita E (1995) Mechanistic and kinetic studies of cobalt macrocycles in a photochtaiucal CO2 reduction system evidence of C0-CO2 adducts as intermediates. J Am Chem Soc 117 6708-6716... 

Scheme 1 outlines the retrosynthetic analysis of the Woodward-Eschenmoser A-B variant of the vitamin B12 (1) synthesis. The analysis begins with cobyric acid (4) because it was demonstrated in 1960 that this compound can be smoothly converted to vitamin B12.5 In two exploratory corrin model syntheses to both approaches to the synthesis of cobyric acid,6 the ability of secocorrinoid structures (e. g. 5) to bind metal atoms was found to be central to the success of the macrocyclization reaction to give intact corrinoid structures. In the Woodward-Eschenmoser synthesis of cobyric acid, the cobalt atom situated in the center of intermediate 5 organizes the structure of the secocorrin, and promotes the cyclization... 

The third class of metal catalysts includes nickel and cobalt complexes of Schiff bases and nitrogen macrocyclic ligands, which can form on electroreduction cobalt(I) and nickel(I) reactive intermediates for the activation of organic halides. 

Template reactions between malonaldehydes and diamines in the presence of copper(II), nickel(II) or cobalt(II) salts yield neutral macrocyclic complexes (equation 15).99-102 Both aliphatic102 and aromatic101 diamines can be used. In certain cases, non-macrocyclic intermediates can be isolated and subsequently converted into unsymmetrical macrocyclic complexes by reaction with a different diamine (Scheme ll).101 These methods are more versatile and more convenient than an earlier template reaction in which propynal replaces the malonaldehyde (equation 16).103 This latter method can also be used for the non-template synthesis of the macrocyclic ligand in relatively poor yield. A further variation on this reaction type allows the use of an enol ether (vinylogous ester), which provides more flexibility with respect to substituents (equation 17).104 The approach illustrated in equation (15), and Scheme 11 can be extended to include reactions of (3-diketones. The benzodiazepines, which result from reaction between 1,2-diaminobenzenes and (3-diketones, can also serve as precursors in the metal template reaction (Scheme 12).101 105 106 The macrocyclic complex product (46) in this sequence, being unsubstituted on the meso carbon atom, has been shown to undergo an electrochemical oxidative dimerization (equation 18).107... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

Perhaps the best-characterized example of this mechanism involves the synthesis of heme cofactors and their subsequent incorporation into various hemoproteins (see Iron Heme Proteins Electron Transport). Succinctly, enzyme-catalyzed reactions convert either succinyl-CoA or glutamate into 5-ammolevulinic acid. This molecule is further converted through a series of intermediates to form protoporphyrin IX, the metal-ffee cofactor, into which Fe is inserted by ferrochelatase. Analogous reactions are required for the synthesis of other tetrapyrrole macrocycles such as the cobalamins (see Cobalt Bu Enzymes Coenzymes), various types of chlorophylls, and the methanogen coenzyme F430 (containing Co, Mg, or Ni, respectively). Co- and Mg-chelatases have been described for insertion of these metals into the appropriate tetrapyrrolic ring structures. ... 

The availability of only one coordination site due to the large rigid equatorial ligand of the cobalt porphyrin macrocycle would appear to make such an intermediate unlikely. Viewed as a transition state with a large component of C—H bond formation, the Co—C bond formation would occur later in the process. In the extreme of complete C—H bond formation prior to Co—C formation, the intermediate is indistinguishable from a radical mechanism. 

Progress of biological action in enzymatic methyl transfer and rearrangement reactions, medicinal aspects, structure and reactivity, and biosynthesis of vitamin B12 and B 12-coenzymes was very recently comprehensively presented at the 4 European Symposium on Vitamin Bi2 and B 12-Proteins and reviewed in an excellent monograph (70). Therefore it is intended with this contribution to address mainly results from research in the field of vitamin B12 biosynthesis. The procedure makes sense because of two reasons. First, many of the biosynthetic intermediates are closely related to hydroporphyrinoid structures discussed in previous sections and second, reactions of the biosynthetic pathway concern the chemistry of the hydroporphyrinoid and corrinoid frameworks involved, whereas the biochemical reactivity of vitamin B12 is mainly restricted to the central cobalt ion of the corrin macrocycle. 

Carbon-Cobalt(iii) Bonds.—These bonds are readily formed in reactions of aliphatic free radicals with Co complexes. Reaction of CHgOH, MeCHOH, HOCHCH2OH, or CHaCHO with the macrocyclic complex [Co K2)J produces the unstable intermediate [Co (2)—RH] containing a cr-carbon-cobalt bond ... 

Other Reagents. Cobalt(i) complexes have been proposed as intermediates in the chromium(ii) reduction of macrocyclic cobalt(m) complexes. The reaction [CoLd(NH8)2] + + Cr + [Ld=(22)] is autocatalytic, and it is also strongly catalysed by the addition of [CoLt(HaO)2] [Lt=ligand (23)]. In the proposed mechanism (44)... 

In an atmosphere of dioxygen, the catalytic oxidation of 1,2-diaminobenzene takes place in the presence of the dinuclear cobalt complex of the 24-member macrocyclic ligand OBISDIEN In water at 25°C the only oxidation product is 2,3-diaminophenazine, which is formed via a complex mechanism involving o-benzoquinonediimine and a p.-peroxo complex as intermediates. The mechanism shown in Figure 50 has been proposed to interpret the observed kinetic behavior. 

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