Glycine, cobalt complexes

NO2C2H5, Glycine cobalt complex, 25 135 cobalt complexes, 23 75, 92 N02CjH,7, Alanine cobalt complex, 25 137... 

O2IOSC7HS, Osmium, dicarbonyI(Ti -cycIo-pentadienyI)iodo-, 25 191 02lOsC,2H,s, Osmium, dicarbonyliodo(Ti -pentamethylcyclopentadienyl)-, 25 191 O2NC2HS, Glycine cobalt complex, 25 135 02NC3Ht, Alanine cobalt complex, 25 137 02N4P2QH,2, 1 W,5W-[1, 4,2,3]Diazadiphos-phoIo[2,3-b][l,4,2,3]diazadiphosphoIe-2,6-(3W,7W)-dione, 1,3,5,7-tetrame-thyl-, 24 122... 

Physical techniques can be used to investigate first order reactions because the absolute concentrations of the reactants or products are not required. Dixon et. al [3] studied the base hydrolysis of cobalt complex, [Co(NH3)5L]3+, where L = (CH3)2SO, (NH2)2C = O, (CH3)03P = O in glycine buffers. 

Now we will look at some examples of metal-ion-catalyzed reactions. Co catalyzes the condensation of two molecules of the ethyl ester of glycine to form the ethyl ester of glycylglycine. The actual catalyst is a cobalt complex, [Co(ethylenediamine)2]. ... 

Cobalt(ll) forms many complexes which can exhibit oxygen-carrying properties (2,19). Reversible oxygen uptake in solutions of cobalt (ll)-histidine (33-36), and cobalt (II) in the presence of a-amino acids and peptides (37—39) has been known for some time. The reaction of cobalt (II) with dipeptide was first observed in enzymic studies involving glycyl-glycine (40). 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

Grigg and co-workers (383) found that chiral cobalt and manganese complexes are capable of inducing enantioselectivity in 1,3-dipolar cycloadditions of azomethine ylides derived from arylidene imines of glycine (Scheme 12.91). This work was published in 1991 and is the first example of a metal-catalyzed asymmetric 1,3-dipolar cycloaddition. The reaction of the azomethine yhde 284a with methyl acrylate 285 required a stoichiometric amount of cobalt and 2 equiv of the chiral ephedrine ligand. Up to 96% ee was obtained for the 1,3-dipolar cycloaddition product 286a. 

A similar reaction is observed with the Cu+2 complex. No reaction occurs with glycine esters. A similar reaction occurs when cobalt (III) complexes are prepared from solutions of hydroxyethylethylenediamine and similar ligands (24, 25). The chelate ring-forming portion of the complex remains intact however, the products derived from the oxidized hydroxyethyl group appear complicated. 

Amino acid complexes are also nucleophilic towards the Vilsmeier reagent. The cobalt(II) glycine complex (36) gives rise initially to an unusually stable iminium complex, which can be hydrolyzed with concentrated sulfuric acid to the complex of formylglycine (Scheme 13). The formyl group... 

It has been known for many years that the rate of hydrolysis of a-amino acid esters is enhanced by a variety of metal ions such as copper(II), nickel(II), magnesium(H), manganese(II), cobalt(II) and zinc(II).338 Early studies showed that glycine ester hydrolysis can be promoted by a tridentate copper(II) complex coupled by coordination of the amino group and hydrolysis by external hydroxide ion (Scheme 88).339 Also, bis(salicylaldehyde)copper(II) promotes terminal hydrolysis of the tripeptide glycylglycylglycine (equation 73).340 In this case the iV-terminal dipeptide fragment... 

Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

Peptide bond formation using non-labile cobalt(III) complexes has now been developed to a useful synthetic level (Section 61.4.2.2.4), but few attempts have been made to use other metal centres. The formation of glycine peptide esters in the presence of copper(II) has been noted.133 Treatment of glycine esters with copper(II) (other metal ions can also be used) in a non-aqueous solvent at room temperature gave di-, tri- and tetra-glycine peptide esters. After carbobenzyloxyla-tion, the peptide esters were separated by column chromatography, and no evidence was obtained... 

A variety of N-O-chelated glycine amide and peptide complexes of the type [CoN4(GlyNR R2)]3+ have been prepared and their rates of base hydrolysis studied.169 The kinetics are consistent with Scheme 8. Attack of solvent hydroxide occurs at the carbonyl carbon of the chelated amide or peptide. Amide deprotonation gives an unreactive complex. Rate constants kOH are summarized in Table 16. Direct activation of the carbonyl group by cobalt(III) leads to rate accelerations of ca. 104-106-fold. More recent investigations160-161 have dealt with... 

The activation of the normally inert methylene group of glycine by coordination to a transition ion has been recognized for many years. The substantial increase in the acidity of the methylene hydrogens has been directly confirmed by deuterium labelling experiments,451 and has also been shown to occur in other a-aminocarboxylate complexes of cobalt(III)451 456 and platinum(II).457... 

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