Amino acids monodentate coordination

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

The complexes of four oxime analogs of amino acids (N-pyruvoyl amino acid oximes) with Bu2SnO were prepared. The ligands are coordinated to the tin centers by monodentate -COO , and oxime N atoms and Oh or Tbp species are formed.Similar studies were performed on the MeSn(IV)-SalGly (salycil-glycine) system. ... 

Amino acid esters act as chelates to Co111 for example, the /3-alanine isopropyl ester is known as both a chelate and as an /V-bonded monodentate,983 and the mechanism of hydrolysis of the ester, which is activated by coordination, to yield chelated /3-alanine has been closely examined. 

L-piperidine-2-carboxylic acid is a non-proteinogenic amino acid that is a metabolite of lysine. The zinc complexes of DL-piperidine-2-carboxylic acid, DL-piperidine-3-carboxylic acid, and piperidine-4-carboxylic acid have been studied. The X-ray crystal structures have been determined for the latter two. [ZnCl2(DL-piperidine 3-carboxylate)2] (42) is monomeric with a tetrahedral metal center and monodentate carboxylates. [Zn2Cl4(piperidine-4-carboxylate)2] (43) contains two bridging carboxylates in a dimeric structure. IR studies suggest that the DL-piperidine-2-carboxy-lato zinc has monodentate carboxylate ligands coordinating.392... 

In 5, coordination through O polarizes the C = O bond producing the incipient carbonium ion, which is more susceptible to nucleophilic attack. In 6, there is intramolecular attack by coordinated OH on the N-bonded monodentate amino acid. In 7, only a small rate enhancement would be anticipated from intermolecular attack on the N-bonded monodentate. In these representations, X is OR, but anticipating the next section, it might also be NHR. These interpretive problems are illustrated in the hydrolysis of complexes of the type 8, where again any of three mechanisms corresponding to 5-7 could apply. 

The enantioselective inverse electron-demand 1,3-dipolar cycloadditions of nitrones with alkenes described so far are catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminium complexes. However, the glyoxylate-derived nitrone 256 favors abidentate coordination to the catalyst, and this nitrone is an interesting substrate, since the products that are obtained from the reaction with alkenes are masked ot-amino acids (Scheme 12.81). 

Essentially three different routes can be considered for the base hydrolysis of an amino acid ester in the presence of a metal ion (equations 8-10). In general terms hydrolysis of the monodentate N-coordinated ester (equation 8) would be expected to be somewhat similar to base hydrolysis... 

Although amino acids and related compounds frequently react in a monodentate fashion with bisperoxovanadate, they can react in a bidentate manner, but the products are often not bisperoxo complexes rather, one peroxide is eliminated in the condensation reaction. This is not necessarily true for all conditions, and bisperoxo-heterobidentate-ligand complexes are known, although in the solid state where a number of x-ray structures have been reported. In this case, one coordination site is an apical position, and a pentagonal bipyramidal product is formed, in the fashion of the oxalato [15] and picolinato (pyridine-2-carboxylato) [16] complexes, as represented diagrammatically in Scheme 6.3. There seems to be no detailed study of these and similar complexes in aqueous solution. Preliminary studies [17,18] of the... 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

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