Cobalt amides bonding

This vitamin possesses the most complex structure of any of the vitamins and is unique in that it has a metallic element, cobalt, in the molecule (Figure 9-19). The molecule is a coordination complex built around a central tervalent cobalt atom and consists of two major parts—a complex cyclic stmcture that closely resembles the porphyrins and a nucleotide-like portion, 5,6-dimethyl-l-(a-D-ribofuranosyl) benzimidazole-3 -phosphate. The phosphate of the nucleotide is esterified with 1-amino-2-propanol this, in turn, is joined by means of an amide bond with the propionic acid side chain of the large cyclic stmcture. A second linkage with the large stmcture is through the coordinate bond between the cobalt atom and one of the nitro-... 

Structure 6-1 has been favored on the basis of NMR and proton-deuteron exchange studies and hydrolysis occurs in about 20% yield. Again the N-terminal first binds to the cobalt ion followed by the carbonyl of the amide bond. The y-carboxylate group of aspartic acid might be implicated in the hydrolysis by contributing to the stability of the complex. 

Cobalt. The rate law for carbonylation of Schiff bases, catalysed by Co2(CO)8, has been reported. Dicobalt octacarbonyl also catalyses reaction between aldehydes, for instance formaldehyde or acetaldehyde, amides, for example acetamide or benzamide, and carbon monoxide. The products are iV-acyl-amino-acids. The main product from the reaction of acetylene with carbon monoxide in the presence of CoH(CO)4 is ethyl acrylate. Characterization of the intermediates permits suggestions to be made as to the mechanism of this reaction. Initial reactions between the acetylene and two molecules of catalyst may give (106), in equilibrium with its isomer (107) the carbon monoxide inserts into the cobalt-carbon bonds of the latter. Further information about Coa(CO)8-catalysed hydro-formylation of acrylonitrile and of 3-methyl[3- H]hex-l-ene has led... 

Almost at the same time Nyokong et al. [125] used a similar method to covalently bind Ni(II)-2 to carboxylic acid functionalized SWCNT. More recently, Nyokong et al. [126] have reported a method of functionalization of SWCNTs, with amine groups using a previously developed diazonium approach. This makes it possible the direct attachment of the Zn(II)-10 by an amide bond to the CNT as illustrated in Fig. 4. Kim and Jeon [129] have reported on the immobilization of a cobalt porphyrin (cobalt tetrakis(o-aminophenyl)porphyrin) of several carbon nanomaterials via the diazonum strategy, but in this case, the diazotation was performed on the macrocycle, by the diazotation of aromatic amine groups of the porphyrin. 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

Coordinated a-amino amides can be formed by the nucleophilic addition of amines to coordinated a-amino esters (see Chapter 7.4). This reaction forms the basis of attempts to use suitable metal coordination to promote peptide synthesis. Again, studies have been carried out using coordination of several metals and an interesting early example is amide formation on an amino acid imine complex of magnesium (equation 75).355 However, cobalt(III) complexes, because of their high kinetic stability, have received most serious investigation. These studies have been closely associated with those previously described for the hydrolysis of esters, amides and peptides. Whereas hydrolysis is observed when reactions are carried out in water, reactions in dimethyl-formamide or dimethyl sulfoxide result in peptide bond formation. These comparative results are illustrated in Scheme 91.356-358 The key intermediate (126) has also been reacted with dipeptide... 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

The above studies indicate that metal ions catalyze the hydrolysis of amides and peptides at pH values where the carbonyl-bonded species (25) is present. At higher pH values where deprotonated complexes (26) can be formed the hydrolysis is inhibited. These conclusions have been amply confirmed in subsequent studies involving inert cobalt(III) complexes (Section 61.4.2.2.2). Zinc(II)-promoted amide ionization is uncommon, and the first example of such a reaction was only reported in 1981.103 Zinc(II) does not inhibit the hydrolysis of glycylglycine at high pH, and amide deprotonation does not appear to occur at quite high pH values. Presumably this is one important reason for the widespread occurrence of zinc(Il) in metallopeptidases. Other metal ions such as copper(II) would induce amide deprotonation at relatively low pH values leading to catalytically inactive complexes. 

Table 16 Rate Constants for the Base Hydrolysis of Ester, Amide and Peptide Bonds in Various Cobalt(III) Complexes (25 °C, / = 1.0 M)a...

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

We saw in Chapter 3 that the hydrolysis of chelated amino acid esters and amides was dramatically accelerated by the nucleophilic attack of external hydroxide ion or water and that cobalt(m) complexes provided an ideal framework for the mechanistic study of these reactions. Some of the earlier studies were concerned with the reactions of the cations [Co(en)2Cl(H2NCH2C02R)]2+, which contained a monodentate amino acid ester. In many respects these proved to be an unfortunate choice in that a number of mechanisms for their hydrolysis may be envisaged. The first involved attack by external hydroxide upon the monodentate A-bonded ester (Fig. 5-62). This process is little accelerated by co-ordination in a monodentate manner. 

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