Magnesium ligands

Magnesium binding in COMT. The magnesium ligands are Aspl41, Aspl69, Asnl70, both hydroxyls of 3, 5-dinitrocatechol (DNC) and a water molecule (W). 

These can be prepared by electrolytic oxidation of chlorates(V) or by neutralisation of the acid with metals. Many chlorates(VII) are very soluble in water and indeed barium and magnesium chlorates-(VII) form hydrates of such low vapour pressure that they can be used as desiccants. The chlorate(VII) ion shows the least tendency of any negative ion to behave as a ligand, i.e. to form complexes with cations, and hence solutions of chlorates (VII) are used when it is desired to avoid complex formation in solution. 

Recently Desimoni et used the same bis(oxazoline) ligand in the magnesium(II) catalysed Diels-Alder reaction of the N-acyloxazolidinone depicted in Scheme 3.4. In dichloromethane a modest preference was observed for the formation of the S-enantiomer. Interestingly, upon addition of two equivalents of water, the R-enantiomer was obtained in excess. This remarkable observation was interpreted in terms of a change from tetrahedral to octahedral coordination upon the introduction of the strongly coordinating water molecules. 

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

In total, 29 pbospborus-containing chiral ligands of various structures were screened under tlie optimized ji-selective condhions, but most of tlieni gave little or no chiral induction. Hie four ligands 38a-d, all derived from i-)-TADDOL, depicted in Fig. 8.4 gave tts in excess of 3096 in the reaction between etliyl magnesium bromide and cinnamyl cliloride. 

The solvent used plays an important role, since it can stabilize the organomag-nesium species through complexation. Nucleophilic solvents such as ethers—e.g. diethyl ether or tetrahydrofuran—are especially useful. The magnesium center gets coordinated by two ether molecules as ligands. 

The mechanism of chelation involves the donation of a pair of electrons to a substrate species (such as iron, copper, calcium, or magnesium) and a reconfiguration of the shape of the new molecule to provide a minimum of bond strain. The chelant or ligand bites the substrate at several points, forming a stable, but not necessarily particularly strong, coordinate bond. 

Can a chiral catalyst containing the same ligand/metal components promote the formation of both enantiomers enantioselectively The bis(oxazoline)magnesium perchlorate-catalyzed asymmetric Diels-Alder reaction [103]... 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

The coordination chemistry of ancillary amidinate ligands with a pyridine functionality has been described. Magnesium, aluminum, zirconium, and lanthanum complexes have been prepared in which the amidinate anions act as tridentate, six-electron-donor ligands Amidinate ligands containing quinolyl substituents were constructed in the coordination sphere of lanthanide... 

N,N -Chelation is also exhibited by the dianionic P(III)/P(V) ligands (25) in the MejSn complex (31) [39] and in the magnesium complex (32) [40], which is prepared by oxidation of [Mg(thf)2[ BuNP(p-N Bu)2PN Bu] by elemental tellurium [40]. One of the endocychc N Bu groups in (32) is also weakly coordinated to magnesium, thus providing an intramolecular base-stabihzation similar to that observed for complexes of type (8). 

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