Amino acids bidentate 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 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 a bidentate coordination to the catalyst, and this nitrone is an interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids (Scheme 12.81). 

Bis-aquo complexes were also reported with amino acids such as a- and jS-alanine (MeCH(NH2)C02H, CH2(NH2)CH2C02H) and o -aminobutyric acid, EtCHfNH OjH-2558 The anhydrous complexes [NiLj (L = glycinate, alaninate, carboxylate groups both bidentate and bridging.2558,2559... 

Because of its biological and therapeutic importance L-Dopa and its coordination properties have been extensively studied. The weight of potentiometric and spectroscopic evidence points to the ligand chelating either as a normal bidentate amino acid (N,0) or through the two phenolate oxygens (0,0) the ligand is therefore classed as ambidentate (a similar situation occurs for tyrosine in which it is sterically impossible to achieve tridentate behaviour). 

Complexes of pyridine-2-carboxylic acid (picolinic acid, picH) and its substituted derivatives commonly exhibit the N—O bidentate nature of this aromatic amino acid. From aqueous solutions chelates are obtained with the coordinated carboxylic group deprotonated, or neutral ligand forms may be isolated from non-aqueous media.26 Bis chelates are common in either case with bivalent metal ions. The tris chelates of trivalent cobalt27 and manganese28 have been structurally characterized recently. The latter is tetragonally distorted in a structure similar to Mnin(oxine)3. 

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