Chelates stability

The ligand pATa values and transition metal chelate stability constants of arylisoxazoles were detected photometrically and the stability of the complexes studied (79JlCi25i). 

EDTA and NTA are essentially the only two chelants formulated into BW deposit control programs. The rationale for selection of one product over another is often a point of contention. Looking first at the relative chelation stability constants (chelate log values, Ks) ... 

Vahrenkamp and co-workers formed a number of chelate-stabilized ketone complexes with pyridylphenyl ketone. Octahedral complexes formed with a 2 1 ratio of ligand to zinc were formed with trifluoromethanesulfonate, chloride, and bromide anions. 1 1 reactions resulted in the formation of square pyramidal [ZnL2X]+ with X = Br, I, and a trigonal-bipyramidal dimer [L2Zn(NCS)2]2 with thiocyanate bridges.350... 

There are a number of important N/O bidentate ligands including the chelate stabilized ketone and aldehyde complexes, and amino acid complexes. 

Li, M., and Meares, C.F. (1993) Synthesis, metal chelate stability studies, and enzyme digestion of a peptide-liked DOTA derivatives and its corresponding radiolabeled immunoconjugates. Bioconjugate Chem. 4, 275-283. 

Co(en)2(/3-AlaOi-Pr)](C104)3 in aqueous solution (49). We add some further information on these two studies here. Both esters are bound via terminal amino-iV as well as carbonyl-O. Such chelation stabilizes carbonyl-0 coordination and the O-donor is not displaced from the metal at any stage. The other amine ligands surrounding Co(III) provide the necessary octahedral ligand field (27), and we have found the... 

The stereochemical outcome of the Wittig reaction can depend on the presence or absence of lithium salts. This may be due to a betaine intermediate stabilized by lithium cation. A stable adduct of this type has now been observed during a Wittig reaction. When Ph3P=CH2 is treated with 2,2 -dipyridyl ketone, P NMR shows the formation of an oxaphosphetane (72) and addition of lithium bromide gives the chelation-stabilized betaine lithium adduct (73). 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

The donor element is an important factor in chelate stability. Generally, certain metals form more stable complexes when bonded to a preferred donor... 

In cases where the various factors controlling regiospecificity are in conflict, mixtures of products may arise. Thus 1,3,5-trihydroxyxanthone affords a mixture of the chromenes (79) and (80) <70JCS(C)1662). The transition state leading to the linear chromene allows retention of the chelate stabilization with loss of pyranone resonance, whilst the reverse situation obtains in the formation of the angular molecule. 

The regioseledivity of the last reaction in Scheme 5.13 is not only because of the greater acidity of the methylene group, but also because some secondary and tertiary amides (e.g. /3-arylamides, /3-vinylamides, or /3-(phenylthio)amides, or borane complexes of /3-phosphino propionamides [132, 133]) are deprotonated at the /3 position under kinetic control to yield chelate-stabilized carbanions [58, 134], Illustrative examples of such remarkable metalations are shown in Scheme 5.14. 

Such reactive carbanions can also act as reducing agents or as strong bases, and can lead to numerous unexpected reactions [197]. Few solvents are sufficiently inert to withstand them [198], Chelate-stabilized, a-heteroatom-substituted carbanions can, on the other hand, be quite stable, and in recent years numerous useful transformations involving these intermediates have been developed. 

Scheme 5.31. Formation and alkylation of chelate-stabilized a-oxygen and a-sulfur carbanions [55, 272-274].

Not only alky] groups, but also aryl [492, 493], vinyl [494], acyl [276, 495—497], alkoxycarbonyl [498], aminocarbonyl [499-501], silyl [502-504], or phosphoryl groups [279, 280] can migrate to a vicinal carbanion (Scheme 5.68). Because some of these groups can be used to stabilize a-heteroatom-substituted carbanions by chelate formation, migration of these groups to the carbanion is a potential side reaction in the generation and alkylation of chelate-stabilized carbanions. 

Some heteroatom-substituted or chelate-stabilized organolithium compounds, on the other hand, can be sufficiently stable toward racemization to enable their use in stereoselective reactions with electrophiles [223, 225, 271, 531, 543, 552-554] (Scheme 5.75). This increased configurational stability of a-heteroatom-substituted carbanions might be due to the stronger pyramidalization of such carbanions [261,555] and fixation of the metal by chelate formation. 

C(6)H causes steric hindrance of metal coordination in all flavin redox states to further prevent unfavorably high chelate stability. 

This lack of ring strain in the six-membered chelate stabilizes the system towards hydrolysis at... 

A model for such a reaction sequence is (21-XLIV).184 In the case of Pd11 complexes of rigid bidentate nitrogen ligands, products of multiple successive insertions of alkenes and CO have proved isolable.185 The insertion of an alkene into the Pd—acyl bond of a neutral acyl chloro complex leads to displacement of the halide ligand and formation of a chelate-stabilized product (21-XLV).186... 

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