Requirements chelates

For the late metals where M-C bonds are less strong, CC bond activation seems always to need some special driving force, such as relief of strain, chelate assistance, or attainment of an aromatic product. For early metals, M-C bonds can be much stronger and simple CC bonds can be more easily cleaved. A classic early example from Watson (equation 1) requires chelate assistance but C-C cleavage can be competitive with the alternate CH bond cleavage (beta-elimination) that normally dominates ... 

Iron may be removed by repeated venesection in haemochromatosis where there is no anaemia. A single vensection of 450 ml of blood, in the absence of anaemia, removes 200-250 mg of iron and can be repeated weekly in individuals with haemochromatosis until the ferritin reaches the normal range. After complete removal of the iron load, maintenance therapy in the form of venesection every 3-4 months is required. A small number of patients with haemosiderosis and cardiac failure may require chelator therapy. 

A 27-year-old female, who worked primarily in a room with a TWA mercury air concentration of 0.709 g/m3 and who had been on the job for 1.5 years, showed a variety of symptoms, including gum pain, dizziness, poor attention, bad temper, some numbness, hypersalivation, hyperhidrosis, dizziness, and fatigue. She had initial urine and blood mercury levels of 408 g/L and 105 g/L, respectively, but did not require chelation the symptoms abated fully approximately 2 months following discontinuation of exposure (Yang et al. 1994). 

In all enantioselective hydrogenations the ability of the substrate to form a chelate ring with the catalyst is extremely important. For this reason the enantioselective reductive ami-nation of ketones is always particularly difficult, because these compounds usually do not have a structure suitable for the required chelation. Burk et al. circumvent this problem by reversible derivatization. The ketones are converted into A -acetylhydrazones, whose structures resemble those of enamides. [11] The C-N double bond can then be hydrogenated by nPr-DuPHOS-rhodium with ee values almost as high as those for C-C double bonds of enamides. The A-acetylhydrazines obtained thus can either be transformed into... 

This year has seen several studies directed towards the use of carbohydrates as auxiliaries for Diels-Alder, 1,3-dipolar cycloadditions and related pericyclic processes. The arabino and ribo derivatives 125 and 126 have both been evaluated as dieneophiles in the Diels-Alder reaction and shown to provide efficient asymmetric induction. The xylitol, ribitol and arabinitol-based auxiliaries 127-129 also provide good levels of induction by a mechanism that requires chelation involving the oxygen centres of the dioxane rings. ... 

Pb appears to be bound in two different compartments, a relatively mobile and a relatively nonmobile, or stable, compartment. With the administration of chelators there is an initial rapid phase of Pb removal that may represent removal of extracellular Pb. This appears to shift the equilibrium concentration between cellular and extracellular compartments. Continued chelation then produces a slower phase of Pb elimination. The Center for Disease Control warns that children who require chelation therapy will also require long-term medical surveillance and care. Rebound of blood lead levels resulting from release of lead from tissue pools after an apparently successful course of chelation therapy should be anticipated." It is apparent, therefore, that although some tis-... 

Treatment for children with lead poisoning involves removal of the source of lead and careful clinical and laboratory surveillance of the child. Because lead is excreted so slowly, many children with elevated levels of blood lead require chelation therapy to lower their lead levels. This involves giving the child one of several different chemicals intravenously, through injection, or orally, depending on the chemical used, which binds with the lead, causing the body to excrete it more rapidly. 

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

Careful examination of literature reporting Lewis-acid catalysis of Diels-Alder reactions in combination with kinetic investigations indicate that bidentate (or multidentate) reactants are required in order to ensure efficient catalysis in water. Moreover, studies of a number of model dienophiles revealed that a potentially chelating character is not a guarantee for coordination and subsequent catalysis. Consequently extension of the scope in this direction does not seem feasible. 

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

A problem common to produced water appHcations is the tendency for oil fouling of the resin. If weak acid or chelate resins are used, a two-step regeneration process is required which uses acid to remove calcium and magnesium from the resin, foUowed by a dilute NaOH solution to convert the resin to the sodium form. 

Metal Deactivators. The abiUty of metal ions to catalyse oxidation can be inhibited by metal deactivators (19). These additives chelate metal ions and increase the potential difference between the oxidised and reduced states of the metal ions. This decreases the abiUty of the metal to produce radicals from hydroperoxides by oxidation and reduction (eqs. 15 and 16). Complexation of the metal by the metal deactivator also blocks its abiUty to associate with a hydroperoxide, a requirement for catalysis (20). 

Experimentally deterrnined equiUbrium constants are usually calculated from concentrations rather than from the activities of the species involved. Thermodynamic constants, based on ion activities, require activity coefficients. Because of the inadequacy of present theory for either calculating or determining activity coefficients for the compHcated ionic stmctures involved, the relatively few known thermodynamic constants have usually been obtained by extrapolation of results to infinite dilution. The constants based on concentration have usually been deterrnined in dilute solution in the presence of excess inert ions to maintain constant ionic strength. Thus concentration constants are accurate only under conditions reasonably close to those used for their deterrnination. Beyond these conditions, concentration constants may be useful in estimating probable effects and relative behaviors, and chelation process designers need to make allowances for these differences in conditions. 

The more stable the chelate, the higher the pM that it can maintain, and the higher the pH required to precipitate the metal hydroxide. From equation 22 it can be seen that the smaller the solubihty product ie, the more iusoluble the metal hydroxide, the higher the pM that a chelant must maintain to prevent precipitation. The stabiUty constant of the Fe(III)—EHPG complex (/2), is so large (10 ) that iron is not precipitated even ia strongly alkaline solutions. 

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