Molybdenum catalysis acid

The synthesis of quaternary amino acids 86 have been shown using azlac-tones 85 as nucleophiles and the Trost ligand 39 under palladium [179] or molybdenum catalysis (Scheme 8) [180]. The allylic alkylation of glycine imino esters under biphasic conditions has also been achieved using a chiral phase-transfer catalyst in combination with an achiral Pd catalyst producing the unnatural amino acid derivatives [181]. 

Timofeeva, M. (2003). Acid Catalysis by Heteropoly Acid, Appl. Catal. A Gen,156, pp. 19-35. Jentoft, R, Klokishner, S., Krohnert, J., et al. (2003). The Structure of Molybdenum-heteropoly Acids under Conditions of Gasphase Selective Oxidation Catalysis A Multi-Method In Situ Study, Appl. Catal. A Gen., 256, pp. 291-317. 

Cobalt in Catalysis. Over 40% of the cobalt in nonmetaUic appHcations is used in catalysis. About 80% of those catalysts are employed in three areas (/) hydrotreating/desulfurization in combination with molybdenum for the oil and gas industry (see Sulfurremoval and recovery) (2) homogeneous catalysts used in the production of terphthaUc acid or dimethylterphthalate (see Phthalic acid and otherbenzene polycarboxylic acids) and (i) the high pressure oxo process for the production of aldehydes (qv) and alcohols (see Alcohols, higher aliphatic Alcohols, polyhydric). There are also several smaller scale uses of cobalt as oxidation and polymerization catalysts (44—46). 

The drawback of the CVD method is eliminated in ROMP, which is based on a catalytic (e.g., molybdenum carbene catalyst) reaction, occurring in rather mild conditions (Scheme 2.3). A living ROMP reaction ofp-cyclophanc 3 or bicyclooctadiene 5 results in soluble precursors of PPV, polymers 4 [31] and 6 [32], respectively, with rather low polydispersity. In spite of all cis (for 4) and cis and trans (for 6) configuration, these polymers can be converted into aW-trans PPV by moderate heating under acid-base catalysis. However, the film-forming properties of ROMP precursors are usually rather poor, resulting in poor uniformity of the PPV films. 

Molybdoenzymes other than the nitrogenases are usually termed oxomolybdoenzymes. This prefix relates to the nature of the catalysis effected, i.e. the net effect of the conversion (xanthine to uric acid, sulfite to sulfate, nitrate to nitrite, or aldehyde to carboxylate) corresponds to the transfer of one oxygen atom to or from the substrate. Furthermore, molybdenum X-edge EXAFS studies have established that this metal is coordinated to one or more terminal oxo groups in each enzyme studied by this technique.204... 

These heterogeneous catalysis contain nickel, cobalt, molybdenum, tungsten, platinum, or palladium on acidic aluminum silicate or zeolite supports. As with reforming catalysts, the catalysts here are also believed to be... 

NAD+. They catalyze the hydroxylation of purines. Equation (50) shows the xanthine oxidase-catalyzed oxidation of xanthine to uric acid. Xanthine oxidase is probably the most well-studied molybdenum enzyme. There is good evidence that the molybdenum is the site for binding and reduction of xanthine. The enzyme contains MoVI in the resting form, while MoIV and Mov are implicated during catalysis. 

Finally, the recent development of suitable expression systems for recombinant molybdenum enzymes makes possible the use of site-directed mutagenesis to incisively probe the roles of specific active site residues in catalysis. Although these systems have been somewhat slow in development, presumably due to the intricacies of assembling and inserting the molybdenum center into the overexpressed apoenzyme, at least two such systems have now been developed and more can be expected in the immediate future. Future work can be expected to critically evaluate the involvement of specific amino acid residues in each step of a given catalytic sequence. 

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides and the salts of vanadium, molybdenum and tungsten in their higher oxidation states are used widely as heterogeneous catalysts, for selective oxidation as well as for acid catalysis. Similarly, supported chromia and rhenium oxides find wide application in different catalytic processes. 

Catalysis by heteropoly compounds. III. The structure and properties of 12-heteropoly-acids of molybdenum and tungsten (HjPMOjj and their salts pertinent to heteroge-... 

A wide range of metal ions is present in metalloenzymes as cofactors. Copper zinc snperoxide dismntase is a metalloenzyme that nses copper and zinc to help catalyze the conversion of snperoxide anion to molecnlar oxygen and hydrogen peroxide. Thermolysin is a protease that nses a tightly bonnd zinc ion to activate a water atom, which then attacks a peptide bond. Aconitase is one of the enzymes of the citric acid cycle it contains several iron atoms bonnd in the form of iron-sulfur clusters, which participate directly in the isomerization of citrate to isocitrate. Other metal ions fonnd as cofactors in metalloenzymes include molybdenum (in nitrate rednctase), seleninm (in glutathione peroxidase), nickel (in urease), and vanadinm (in fungal chloroperoxidase). see also Catalysis and Catalysts Coenzymes Denaturation Enzymes Krebs Cycle. 

There have been relatively few examples of the hetero-Diels-Alder reaction where the carbonyl group is part of the 4 tt component. This is mainly due to the instability of both starting materials and products to heat and acid, and consequently Lewis-acid catalysis has been of minor applicability. Yamamoto et al. have now shown, however, that the molybdenum complex [Mo02(acac)2] catalyses the... 

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