Metal-chelated intermediates

McLafferty rearrangement 133, 163 Meisenheimer complexes 699, 702 Metal-chelated intermediates 838 Metal-halogen exchange 781, 784 Methionine, oxidation of 852-855 Methionine sulphone 853 Methionine sulphoxide 851-869 reduction of 1063 residues of... 

In contrast to a, -ethylenic ketones or even a, -ethylenic sulfones, a, ) -ethylenic sulfoxides generally are not sufficiently electrophilic to undergo successful nucleophilic j8-addition . a-Carbonyl-a, j8-ethylenic sulfoxides, however, are potent, doubly activated alkenes which undergo rapid and complete -addition of various types of nucleophiles even at — 78 °C. A brief account summarizing this area is available . The stereochemical outcome of such asymmetric conjugate additions to enantiomerically pure 2-sulfmyl 2-cycloalkenones and 2-sulfinyl-2-alkenolides has been rationalized in terms of a metal-chelated intermediate in which a metal ion locks the -carbonyl sulfoxide into a rigid conformation (36 cf. 33). In this fixed conformation, one diastereoface of the cyclic n... 

The iodoalkoxide Sml2(Ol-Bu) promotes the intramolecular Tishchenko reaction via a metal-chelate intermediate to form perfectly stereocontrolled lactones (eq. (24)) [189]. Ethyllanthanide iodide complexes, EtLnl (Ln = Pr, Nd, Sm) etc. [25 b, 186], also serve as catalyst precursors for the Tishchenko condensation reaction [190]. 

High levels of regio- and stereoselectivity were observed in metal-catalyzed 1,3-DC of acyclic nitrones with a -hydroxy enones. The reaction probably occurs through the formation of reactive 1,4-metal-chelated intermediates. The remarkable diastereo- and enantiocontrol could be obtained through two complementary approaches by using a camphor-derived a -hydroxy enone in combination with Cu(OTf)2 or an achiral enone such as 81 in combination with the bis(oxazoline)-Cu(II) catalyst 83. At the end, the hydroxylated auxiUaiy could be easily removed. For example, treatment with periodic acid released a carboxylic moiety as in 86 and 87 <05AG(E)6187>. 

The above example demonstrates the manner in which a study of model nonenzymatic reactions can elucidate enzymatic reactions. Another example of such a model reaction that has been studied is the decomposition of 6-succinoaminopurine and its derivatives in the presence of metal ions (8). The results of this study indicated that a side-chain carboxyl group is essential for the reaction, and that in the a-position it was much more effective than in the jS-position. Therefore a metal chelate intermediate for the reaction was postulated in which cleavage of the C—N bond could occur quite readily to give adenine, a product which was identified, and fumaric acid. The latter compound was not identified but is obtained in analogous enzymatic reactions involving adenylsuccinic acid. The most effective metal ions were copper(II) and manganese(II), although protons in the absence of metal ions were also able to carry out this reaction at a slow but measurable rate. 

One of the most familiar examples of an oxidation reaction in which a metal chelate intermediate has been postulated, is the conversion of the oxalate ion to carbon dioxide by permanganate ion. An induction period... 

In all reactions, no over-reduction was observed, confirming the formation of the stable metal-chelate intermediate described by Nahm and Weinreb (49). Furthermore, during the reduction, no aldehyde derivative could be... 

Pentanedione is widely used in extraction processes for the separation and purification of metals because of its abiUty to form covalent metal chelates. It is also used as an intermediate in the production of heterocycHc substances and dyes, as a fuel additive (324), and in metal plating and resin modification. 

Several important classes of polar monomers have so far eluded copolymerization by the Pd(II) system. Vinyl chloride insertion, for example, leads to catalyst deactivation following P-halide elimination to form inert chloride species such as 1.32, as shown by Jordan [90], Similarly, attempted vinyl acetate copolymerization results in deactivation by an analogous acetate elimination process, although the ester chelate intermediate that forms after insertion also effectively shuts down the reaction [90], Therefore, -elimination of polar groups represents a significant and unresolved problem for late transition metal polymerization systems unless access of the metal to it is restricted. 

Mobashery and co-workers also reported the synthesis and inhibitory apph-cation of 2 -N02 derivatives of neamine and kanamycin (Scheme 4.28). Using the metal-chelating method, the 2 -NH2 was selectively unmasked and then oxidized into 2 -N02, which will increase the acidity of 2 -H. Upon phosphorylation at the 3 -0H, elimination of phosphate will lead to the formation of a nitroalkene intermediate, 198, that can function as a Michael acceptor and... 

Uses. Chemical intermediate, metal chelator, and lubricant additive... 

In the search for a reactive functional group which could be substituted on the acetylacetonate ring, chloromethylation of these chelates was attempted. The initially formed products were too reactive to be characterized directly. Treatment of rhodium acetylacetonate with chloromethyl methyl ether in the presence of boron trifluoride etherate afforded a solution of a very reactive species, apparently the chloromethyl chelate (XXX) (26). Hydrolytic workup of this intermediate yielded a polymeric mixture of rhodium chelates, but these did not contain chlorine On the basis of evidence discussed later on electrophilic cleavage of carbon from metal chelate rings and on the basis of their NMR spectra, these polymers may be of the type shown below. Reaction of the intermediate with dry ethanol afforded an impure chelate which is apparently the trisethyl ether (XXXI). Treatment of the reactive intermediate with other nucleophiles gave intractable mixtures. 

The technical and economic aspects of wet flue gas simultaneous desulfurization and denitrification systems are presented so that their practicality for utilization by utility industry can be assessed. The emphasis is on the kinetics of the systems based on the employment of ferrous chelates to promote the solubility of NO and the reactivity of NO with SO2 in scrubbing liquors. Analytical techniques are developed for characterizing reaction intermediates and products. Alternative approaches and novel ideas that could develop into a more efficient and cost-effective scrubber system employing metal chelate additives are discussed. 

In both catalytic, asymmetric Michael reactions and nitroaldol reactions, enones and/or aldehydes appear to coordinate to the lanthanoid metal. Why, then, is LSB more effective for catalytic, asymmetric Michael reactions, whereas LLB is more effective for catalytic, asymmetric nitroaldol reactions This disparity might arise from slight differences in bond lengths in the chelated intermediate, as well as slight differences in bite angle for the BINOL moiety caused by varying the alkali metal. 

Members of a second prominent group of mechanism-based inhibitors contain a peptide isostere, an element mimicking an intermediate formed during amide bond cleavage. Inhibitors of metalloproteases often contain a metal-chelating unit such as hydroxamate or phosphonic acids, which act as a bidentate ligand. 

A more favourable approach is the incorporation of the active species in an electrically conducting polymer layer which then acts as an (electrical) intermediate between the electrode surface and the catalyst. Polypyrrole is considered to be especially suitable because it is acceptably stable under ambient conditions (2), has a high conductivity and can be easily prepared electrochemically from a great variety of solvent systems, including aqueous solutions (3-5). The catalytic species that have been applied in such polypyrrole-based systems comprise metal particles (6-9), metal chelates (10-13) (with anionic side groups) and enzymes (14-18). 

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