Cations chelated

As seen in the preceding chapters, macrocyclic polyamines such as [18]aneN6 can be cation chelators, anion chelators, or both depending on the conditions. It was expected that with these dual properties, the macrocyclic polyamines might serve as litholytic agents by removing Ca2+ and phosphate or oxalate anions from insoluble calculi. 

Dagnall and West8 have described the formation and extraction of a blue ternary complex, Ag(I)-l,10-phenanthroline-bromopyrogallol red (BPR), as the basis of a highly sensitive spectrophotometric procedure for the determination of traces of silver (Section 6.16). The reaction mechanism for the formation of the blue complex in aqueous solution was investigated by photometric and potentiometric methods and these studies led to the conclusion that the complex is an ion association system, (Ag(phen)2)2BPR2, i.e. involving a cationic chelate complex of a metal ion (Ag + ) associated with an anionic counter ion derived from the dyestuff (BPR). Ternary complexes have been reviewed by Babko.9... 

The monolithium salt of 4-hydroxy-4-(phenylethynyl)-2.5-cyclohexadienone (12), prepared in situ by the addition of lithium acetylide to /7-benzoquinone, was treated with methylmagnesium chloride in l HF-TMEDA or in THF —DMPU. The syn-, 4-addition adduct 13, derived from intramolecular delivery of the carbon nucleophile by the hydroxy oxygen, as well as the <7s-1,4-diol 14, obtained via intermolecular 1,2-addition, were obtained in varying amounts depending on the conditions. The selectivity on 1,4- to 1,2-addition increased by the addition of cation chelating agents such as DMPU, TMEDA, and 15-crown-5. Although the 1,4 to 1,2... 

Roshal AD, Grigorovich AV, Doroshenko AO et al (1998) Flavonols and crown-flavonols as metal cation chelators. The different nature of Ba2+ and Mg2+ complexes. J Phys Chem A 102 5907-5914... 

Separation is based on the reversible chelate-complex formation between the chiral selector covalently bonded to the chromatographic support, and the chiral solute with transition metal cations. Chelation properties of both the chiral selector and the chiral solute are required. Compounds therefore need to have two polar functional groups in a favorable arrangement to each other, like a )3-amino acids, amino alcohols and a-hydroxy acids, which can form rings membered with central chelating metal ions, like Cu(II), Zn(II), Cyclic... 

The sodium cation chelation by the bis(enone) anion-radicals shown in Scheme 3.52 controls their further transformations although they proceed at the expense of other reaction centers (Yang et al. 2004). This kind of intramolecular cyclobutanation is characterized with the pronounced cis-stereoselectivity. However, this stereoselectivity disappears if the reaction proceeds in the presence of the tetrabutylammonium cation, when such a chelation is impossible. 

Addition of butyl lithium to a suspension of palladium(II) diiodide and methylene bridged bisimidazolium salts leads to the in situ formation and complexation of the NHC resulting in the cationic [(chelate)2Pd]l2 in low yield. Higher yields... 

The solvolysis of tetrabenzyl pyrophosphate catalyzed by lutidine (see above) is further catalyzed a thousandfold by calcium ions. In the presence of 0.02M calcium ion and 0.2M lutidine, the solvolysis of tetrabenzyl pyrophosphate (with cleavage of the P—O bond) is increased by a factor of the order of 1,COO,000 over the uncatalyzed reaction (63). The bivalent cation chelates with two oxygen atoms of the pyrophosphate ester, and the positive charge in the chelate greatly enhances the susceptibility of the phosphorus atom toward nucleophilic attack. 

Fluoroquinolines likewise furnish 5- and 8-TMS haloquinolines, respectively [79 JOM(171)273] (Scheme 8). Finally, 8-fluoroquinoline resists LDA metalation, a result which may be explained by the presence of a strong cation chelating site in the vicinity of the fluorine and nitrogen atoms. 

Although some of the cationic chelate complexes have long been known,141 most of the cationic boron complexes have been synthesized in the last three decades following the... 

For the determination of metal cations, chelation complexes formed with oxine (8-hydroxyquinolein), alizarine or benzoine are used. These complexes are extractable with organic solvents. 

Metalloenzyme is represented by E(Mg2 ), C is chelator, E(Mg2 C) represents chelator reversibly bound to the metalloenzyme with dissociation constant KD and with characteristic enhancement of the protein fluorescence, and k is the first-order rate constant for irreversible formation of inactive protein P and the cation-chelator complex, Mg2+C. Values of both Kd and k may vary according to the nature of the chelator, the nature of the metal ion (if other cations can replace Mg2 ), and the intrinsic stability of the cation-chelator complex, as well as pH, temperature, and protein concentration. 

The unique benefit of recombinant microbes for screening is that target specificity is evaluated early in the process. Compounds that nonspecifically affect enzyme function (by sulfhydryl modification, cation chelation, alkylation, etc.) will not survive the comparative testing phase. The hit rate in our laboratories in these types of screens has been under 0.01% (unpublished observations), which means that additional attention can be focused only on the most promising candidates. In contrast, screens that employ purified recombinant enzymes for HTS often have hit rates >2.5% (unpublished observations). Further characterization of so many candidates requires a considerable effort in both biology and chemistry laboratories. 

Similar insoluble salts form with many cationic chelates of heavy metal atoms. Thus, 12-molybdosilicic acid precipitates Cu+2, Ag+, Cd+2, Zn+2, Sn+2, Cr+3, Ni+2, and Co+2 in the presence of ethylenediamine, thiourea, hexamethylenetetramine, dithiooxamide, or similar chelating agents 106 ... 

In the course of evolution from prokaryotes to eukaryotes, the energetic role of PolyP decreased. However, other functions came to the fore, such as phosphate storage, cation chelation, regulation of enzyme activities, gene expression and membrane transport (Figure 10.6). The significance of the regulatory functions of PolyP increased in eukaryotes. 

Phosphate reserve Phosphate reserve Cationes chelation... 

Cationes chelation Cationes chelation Enzyme activities regulation... 

Scheme 8.24 Preparation of cationic chelating ruthenium(II) carbenes 89 by protonation of neutral ruthenium(II) vinylidenes 87 via ruthenium carbynes 88 as intermediates (L = P Pr3 R = H, Me, Ph ...

Further evidence for a dissociative pathway is given by the rates of carbonylation of the Pd—Me bond in cationic chelate complexes [(P—P)PdMe(S)]+ which are at least 10 times higher than those of the corresponding neutral chloride complexes.158... 

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