Magnesium, chelates

When a solution of magnesium methoxide (prepared by the reaction of magnesium with methanol) is saturated with carbon dioxide, an active carboxylating agent, MMC, is produced. The reagent carboxylates substrates capable of enolization apparently by promoting formation of the magnesium chelate of the a-adduct. The reaction has been... 

The affinity of chelants for particular substrates differs, but copper, chromium, and iron are more attracted than calcium or magnesium. Chelation is also affected by pH levels and other conditions, so it is necessary when formulating BW compounds to ensure that the correct chelant is selected for the particular purpose. 

These reactions presumably involve formation of a magnesium chelate of the keto acid. The (3-ketoacid is liberated when the reaction mixture is acidified during workup. 

To achieve the synthesis of all the chlorophylls as magnesium chelates we shall return later to the way the magnesium was inserted while simultaneously the metals iron, cobalt and nickel were prevented from entering the chlorin centre despite the facts that all these metals bind more strongly to chlorin, and they themselves have to be handled specifically and separately to make coenzymes based on them. We note first the general advantages of metal insertion and once again observe the sophistication of the... 

Magnesium ion is usually involved (for charge neutralization ) where high-energy phosphate is moved from one molecule to another by an enzyme, i.e., the metabolically active form of ATP is usually the magnesium chelate. 

All prepared magnesium enolates 17 are stable in refluxing diethyl ether. Deuteriation, and reactions with various electrophiles confirm their structure (see section HI). It is noteworthy that the lithiated carbanion-enolate analogue, directly obtained by deprotonation of an a-ketoester 18 with lithiated bases (LDA, for example), is not stable and immediately degrades in the medium, whatever the temperature. Comparatively, the magnesium chelate 17 shows a higher stability, which allows its preparation and synthetic applications. 

Carbanions of enantiomerically pure sulfoxides have been investigated as precursors for asymmetric synthesis. However, they react with carbonyl compounds with modest selectivity. This selectivity is increased by the incorporation of an ester group adjacent to the carbanionic center. The observed stereochemistry in this reaction is consistent with chelated intermediates, where magnesium chelate is particularly efficient, as illustrated in equation 91 " . ... 

Prepared from (MeO)2Mg/MeOH, the magnesium chelates of /3-ketoesters or /9-diketones show high stability and sometimes a particular reactivity (see Section II). 

Base-catalyzed transformations can be carried out elsewhere on a complex molecule in the presence of such protected -dicarbonyl magnesium chelate. For example, the chelated magnesium enolate of a /3-ketoester such as 71 prevents the carbonyl keto group becoming an acceptor in aldol condensations. However, in the presence of excess of magnesium methanolate, exchange of the acetyl methyl protons can occur via a carbanion 72 stabilized by delocalization into the adjacent chelate system (equation 99). 

Conseqnently, the magnesinm chelate 71 can also react as a nucleophilic donor in aldol reactions. In the chemistry involving magnesium chelates, these two aspects model their mode of action as nucleophilic partners in aldol condensations. This is exemplified in aldol condensations of y-diketones . Thus, sodium hydroxyde catalyzed cyclization of diketone 73 to give a mixtnre of 3,5,5-trimethyl-cyclopent-2-enone 74 and 3,4,4-trimethyl-cyclopent-2-enone 75 in a 2.2/1 isomeric ratio (equation 100). When treated with magnesinm methanolate, the insertion of a a-methoxy carbonyl group as control element, as in 76, allows the formation of a chelated magnesium enolate 77, and the major prodnct is now mainly the aldol 78. This latter treated with aqueous NaOH provides the trimethylcyclopent-2-enones 74 and 75 in a 1/49 ratio. 

The chlorophylls are responsible for the green colour of nearly all plants. The isomers, chlorophyll a and chlorophyll b exist in a 3 1 ratio in plants. Chlorophylls are magnesium-chelated tetrapyrroles with an esterified 20-carbon alcohol, phytol. Acidic conditions can cause the replacement of magnesium for hydrogen and change the chlorophyll into pheophytins. The pheophytins are brown in colour, and are normally undesirable in most foods. 

Koga reports that a.P-unsaturated aldimines (89), derived from L-r-leucine r-butyl esters, undergo asymmetric additions with Grignard reagents which is due to formation of a highly ordered magnesium chelate (Scheme 29).75... 

The tetranuclear magnesium chelate complexes [(NH4)4n Mg4(L10 n)6 ] (29a,b) were first synthesized by reaction of dialkyl malonate 28, methylmagnesium iodide, and oxalyl chloride, followed by workup in aqueous ammonium chloride solution [102-105]. Now methyllithium/magnesium chloride instead of methylmagnesium iodide (direct method) is used, which by mere replacement of magnesium chloride by the chlorides of manganese, cobalt, and nickel also allows the synthesis of the corresponding tetranuclear complexes 29 (with Mn = Mn2+, Co2+, Ni2+) [103, 105]. 

Since the development of chelate chemistry has taken place almost wholly within the last three decades, the number of applications that have been found for this class of compounds is quite remarkable. Mail may not, however, take credit for the two most important uses of chelate compounds the use of the magnesium chelate, chlorophyll, as a catalyst for the incorporation of the hydrogen of water into plant systems and the use of the iron chelate, heme, as an oxygen carrier in human respiration. The ring system in both heme and chlorophyll is essentially that shown for copper porphyrin in Figure 22-4, but there are organic substituents on... 

Magnesium-Chelating Substances Passes test. Nitrilotriacetic Acid Not more than 0.1%. pH of a 1 100 Solution Between 6.5 and 7.5. 

Magnesium-Chelating Substances Transfer 1 g of sample, accurately weighed, into a small beaker, and dissolve it in 5 mL of water. Add 5 mL of a buffer solution prepared by dissolving 67.5 g of ammonium chloride in 200 mL of water, adding 570 mL of ammonium hydroxide, and diluting with water to 1000 mL. Then add 5 drops of eriochrome black TS to the buffered solution, and titrate with 0.1 M magnesium acetate to the appearance of a deep wine red color. Not more than 2.0 mL is required. 

The addition of vinyl and aryl Grignard reagents to propargyl alcohols followed by reaction with a nitrile provides access to furans and butenolides in a one-pot procedure. These reactions are believed to involve a magnesium-chelate intermediate. Highly substituted furans can be prepared with control over the substitution pattern by the judicious choice of substrates and reagents (Scheme 26) <2000TL17>. 

Lithium and magnesium chelates have no effect on the stereoselection of this reduction. Triisobutylborane is somewhat more effective than n-butylboranc no reduction occurs if tri-.sec-butylborane is used. 

Since EIMS is suitable only for relatively volatile materials, most metals must be converted to volatile chelates before they can be analysed by this technique. The magnesium chelate found most amenable to EIMS is the diketonate Mg(2,2, 6,6 -tetra-methyl-3,5-heptanedione)2 (Mg(THD>2) (19). The chelate can be formed in aqueous solutions at pH >9 in the presence of excess THD (16). Extraction into ethyl ether provided a simple method for separating the chelate from excess THD which otherwise interferes in the MS analysis of Mg(THD)2 The ether layer was transferred to a 10 ml glass tube and allowed to stand at room temperature. 

The MgBr2-promoted additions to the (5)-a-benzyloxy aldehyde proceed via a chelation-controlled transition state (Fig. 18). Approach to the carbonyl face of the nearly planar five-membered magnesium chelate is directed by the methyl substituent resulting in re attack by the allenylstannane through the antiperiplanar geometry for the (P) stannane and the synclinal geometry for the (M) stannane. An antiperiplanar orientation of the (A/) stannane would place the vinylic methyl substituent in close proximity to the chelate ring. 

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