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Active intermediates summary

SUMMARY - Different forms of adsorbed molecular and atomic oxygen on an Ag(UO) surface are analyzed from a theoretical point of view. It is proposed that the active intermediate for the epoxidation of ethylene is an Ag-O surface species. A parallel is made to discrete silver complexes as catalysts for alkene epoxidation as it is experimentally shown that these have similar properties at the silver surface. 180 Labelling studies indicate an Ag-O complex as the reactive catalyst. [Pg.377]

Conversion ranges 6.4-20.0% focus of this paper is a reaction mechanism study the results suggested that the substituted heteropo-lyoxometalate reacts with H202 to form an intermediate, and the substituted transition metal ion seems to be the center of activating H202 Summary of reactions with this POM and... [Pg.713]

In summary, the isotopic transient studies have shown the existence of two intermediates and their parallel conversion is involved in the production of methane from adsorbed CO. These intermediates can be connected in various overall reaction schemes which cannot be distinguished from each other by isotopic transient studies alone. The ratio of the rate constant of the more active intermediate to the less active intermediate lies between two and seven for the catalysts studied and is dependent on the catalyst. The absolute values of the rate constants are also dependent on the catalyst used. [Pg.121]

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. [Pg.170]

One of the first applications of the then newly developed Ru-binap catalysts for a,/ -unsaturated acids was an alternative process to produce (S)-naproxen. (S)-Naproxen is a large-scale anti-inflammatory drug and is actually produced via the resolution of a racemate. For some time it was considered to be one of the most attractive goals for asymmetric catalysis. Indeed, several catalytic syntheses have been developed for the synthesis of (S)-naproxen intermediates in recent years (for a summary see [14]). The best results for the hydrogenation route were obtained by Takasago [69] (Fig. 37.15), who recently reported that a Ru-H8-binap catalyst achieved even higher activities (TON 5000, TOF 600 h 1 at 15 °C, 50 bar) [16]. [Pg.1296]

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. [Pg.170]

Multidomain synthetases can include specialized domains to modify the amino acids of peptide intermediates during chain elongation. The chemistry carried out by these domains introduces specific structural motifs, which are often important for biological activity, into the peptide natural product. A summary of additional NRPS enzyme chemistry is described below and illustrated in Figure 9. [Pg.635]

In summary, the copper ion transfers an electron from the unsaturated substrate to the diazo-nium cation, and the newly formed diazonium radical quickly loses nitrogen. The aryl radical formed attacks the ethylenic bond within the active complexes that originated from aryldiazo-nium tetrachlorocuprate(II)-olefin or initial arydiazonium salt-catalyst-olefln associates and yields >C(Ar)-C < radical. The latter was detected by the spin-trap ESR spectroscopy. The formation of both the cation-radical [>C=C<] and radical >C(Ar)-C < as intermediates indicates that the reaction involves two catalytic cycles. In the other case, radical >C(Ar)-C < will not be formed, being consumed in the following reaction ... [Pg.263]

In summary, the chemistry of the donor/acceptor-substituted carbenoids represents a new avenue of research for metal-catalyzed decomposition of diazo compounds. The resulting carbenoids are more chemoselective than the conventional carbenoids, which allows reactions to be achieved that were previously inaccessible. The discovery of pan-tolactone as an effective chiral auxiliary, and rhodium prolinates as exceptional chiral catalysts for this class of rhodium-carbenoid intermediate, broadens the synthetic utility of this chemistry. The successful development of the asymmetric intermolecular C-H activation process underscores the potential of this class of carbenoids for organic synthesis. [Pg.337]

Besides their obvious role as reactive intermediates in a powerful synthetic approach the 4a,4b-dihydrophenanthrenes offer a fascinating combination of unusual chemical and physical properties. Over the past 15 years these topics were investigated at length at the Weizmann Institute in Rehovot and elsewhere, and the present review is intended to provide an up-to-date summary of the activity in this field. [Pg.40]

In summary, the results with pepsin extend the renin data reported by Szelke and Boger and strongly support the postulate of Boger that statine is an analog of a dipeptide tetrahedral intermediate.(20) The C-3 hydroxyl group hydrogen bonds to Asp-213 (220) and Asp-33(35) and displaces a "bound" water molecule from the active site. The isobutyl side chain of statine corresponds to the PI substituent that binds to the SI subsite on the enzyme. The C-1 and C-2 atoms of statine serve to span... [Pg.220]

Partial summary of lipoprotein metabolism in humans. I to VII are sites of action of hypolipidemic drugs. I, stimulation of bile acid and/or cholesterol fecal excretion II, stimulation of lipoprotein lipase activity III, inhibition of VLDL production and secretion IV, inhibition of cholesterol biosynthesis V, stimulation of cholesterol secretion into bile fluid VI, stimulation of cholesterol conversion to bile acids VII, increased plasma clearance of LDL due either to increased LDL receptor activity or altered lipoprotein composition. CHOL, cholesterol IDL, intermediate-density lipoprotein. [Pg.270]

In summary our study of a-substituted nitrosamines has given us a very interesting view of the chemical reactivity of this class of compounds. We have been able to show that the chemical behavior is governed by the intermediate existence of N-alkyl N-nitrosoimmonium ions in the thermal and solvolytic reactions. The significance of these N-alkyl N-nitrosoimmonium ions in the biological activity of dialkyInitrosamines is not clear at the moment. [Pg.73]

In summary, oxygen associated with Bi is responsible for allylic hydrogen abstraction, whereas oxygen polyhedra around Mo are inserted into the allylic intermediate.962-964 Nitrogen insertion in ammoxidation takes place in a similar way965 on an active site containing Mo=NH instead of Mo=0. [Pg.512]

An additional pathway of similar energy was found to proceed via a concerted oxygen atom insertion [45]. In summary, it appears as if different substrates can be differently activated and the reaction pathway hence proceeds via different intermediates. Clearly, more studies are necessary before the mechanism of methane hydroxylation by MMOH can be fully understood. [Pg.43]

In summary, our photophysical studies indicate that the thermally activated relaxation pathways of (2E)Cr(III) very likely involve 2E-to- (intermediate) surface crossing. These (intermediates) can be associated with some, not necessarily the lowest energy, transition state (or transition states) for ground state substitution. The Arrhenius activation barriers for thermally activated relaxation are remarkably similar from complex to complex, but they can be altered in systems with highly strained ligands. Some of this work indicates that the steric and electronic perturbations of the ligands dictate the choice among possible relaxation channels. [Pg.99]


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See also in sourсe #XX -- [ Pg.447 , Pg.448 ]

See also in sourсe #XX -- [ Pg.392 ]




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Activated intermediate

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