Oxygen, activation

Over gold catalysts, the activation of molecular oxygen is the key for the selective oxidation of alkanes, alkenes, and alcohols. Usually, the dissociation of molecular 

By contrast, there is a wealth of mechanistic information available in biochemistry and homogenous catalysis concerning oxygen activation mechanisms on first-row transition metals. It is therefore appropriate to provide a brief overview of this field here. However, this is a very large and active research field hence, it is not possible to do full justice to this subject here. For more extensive overviews, see references [50-60] in Section 4.5.6. 

1 Oxygen Activation in Nature Nature is extremely efficient in activating dioxygen without releasing biologically harmful intermediates such as the hydroxyl 

The COX mechanism serves as a paradigm for oxygen activation at binuclear active sites. A key feature is that the active site is constructed such that a peroxide molecule (0-0 distance 1.4 A) fits comfortably between the two metal centers. 

One particular feature of this mechanism is that the two metals in the active site can provide three electrons, which means that a fourth electron has to come from a different source, in this case a redox active tyrosine residue. A different solution to the same problem has been found in the enzyme methane monoxygenase, which involves a binuclear iron center and will be discussed subsequently. 

The characteristic feature that becomes apparent is nature s use of high-valent metal-oxo species that form during dioxygen activation. Such species are aggressive oxidants. Hence, their creation is tightly coupled to the presence of substrate in a confined space, which is absolutely critical for the reactions to proceed in a controlled manner. 

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Protonation of the carbonyl oxygen activates the carbonyl group toward nucleophilic addition Addition of an alcohol gives a tetrahedral inter mediate (shown m the box m the preceding equation) which has the capacity to revert to starting materials or to undergo dehydration to yield an ester... 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

A. E. Martell and D. T. Sawyer (eds.). Oxygen Complexes and Oxygen Activation by Transition Metals, Plenum, New York, 1988, 341 pp. 

Studies on the oxygen activation mechanisms by new heme enzymes using hemoprotein mutants and synthetic heme models 96YGK1046. 

Fig. 7.46 The stable phases in the Al-O-S system in molten Na2S04 at 1 000°C as a function of acidity and oxygen activity (after Goebel and Pettit )...

Fig. 7.72 A thermodynamic phase stability diagram for Al-O-S species in the eqilibrium with liquid Na2S04 at 1 000°C as a function of the oxygen activity and the acidity of the salt (after...

O Protonation of the carbonyl oxygen activates the carboxylic acid. .. 

The other method for quality assurance inspection of pellet wt in the primer eliminates the need for a comparator oxygen-containing standard. Here, the Cu in the cup-anvil combination in the primer is used as an internal standard by comparing the 0.511 MeV positron annihilation radiation from 62Cu produced by the 63Cu(n,2n)62Cu reaction to the 6.1 MeV 7 from l6N produced by oxygen activation. In this case the actual determination of pellet wt is not required the ratio of Cu to O, which should be fixed for a pro-... 

This feasibility study shows that determination of pellet wt by fast neutron oxygen activation analysis can be used for quality assurance inspection of M34 primers. Either direct oxygen analysis, where a comparison standard (such as lucite) is used, or a ratio method, utilizing the Cu in the cup-anvil combination as an internal standard, can be applied. In general, the uniformity of production primers is quite satisfactory, as is usually the case where production procedures are standardized. It seems likely that the light pellet is one which has been improperly manufd and will probably be well below specifications in pellet wt. Production experience with such primers indicates that only one in 3x10s primers is expected to show low pellet wt therefore, one would not expect to find a reject in a small sampling. Nevertheless, detection and rejection of this one bad unit is critical for the prevention of weapon malfunctions and possible injuries to personnel... 

This methodology has provided an alternative, highly enantioselective route to sertraline 1 [94]. Insertion into the oxygen-activated CH2 position of allyl ethers yields syn-aldol products with high stereocontrol (Eq. 13) [97], and... 

The measured potential difference U R is related to the oxygen activity, do, on the catalyst surface via 22 27... 

C.G. Vayenas, C.Georgakis, J. Michaels, and J. Tormo, The role of PtOx in the isothermal rate and oxygen activity oscillations of the Ethylene Oxidation on Pt, J. 

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

Atkinson, D.E. (1977). In Cellular Energy Metabolism and its Regulation. Academic Press, New York. Babcock, G.T. Wikstrom, M. (1992). Oxygen activation and the conservation of energy in cell respiration. Nature 356, 301-309. 

Elstrer, E.F., Saran, H., Bors, W. Longfelder, E. (1978). Oxygen activation in isolated choroplasts. European Journal of Biochemistry, 89, 61-6. 

Babcock GT, Wikstrom M Oxygen activation and the conservation of energy in cell respiration. Nature 1992 356 301. 

Scheme 14 Oxygen activation and transfer by dinitrosyliron complexes [35]...

Oxidation is initiated by formation of radicals which may be the result of enzyme catalysed reactions like oxygen activation by xanthine oxidase in... 

Matsui T, M Furukawa, M Unno, T Tomita, M Ikeda-Saito (2005) Roles of distal Asp in heme oxygenase from Corynebacterium diphtheriae, HmuO. A water-driven oxygen activation system. J Biol Chem 280 2981-2989. 

Figure 1.120. Probable ranges of sulfur and oxygen activities for the Te-type and Se-type deposits. The diagram was constructed mainly based on Heald et al. (1987). Temperature = 250°C (Shikazono et al., 1990).

The Mukaiyama aldol reaction refers to Lewis acid-catalyzed aldol addition reactions of silyl enol ethers, silyl ketene acetals, and similar enolate equivalents,48 Silyl enol ethers are not sufficiently nucleophilic to react directly with aldehydes or ketones. However, Lewis acids cause reaction to occur by coordination at the carbonyl oxygen, activating the carbonyl group to nucleophilic attack. 

Oxygen activation of molecules at metal surfaces was first established in the 1970s by surface spectroscopies (XPS and UPS) over a wide temperature range (80-400 K). Furthermore, the distinction was made between the reactivity of partially covered surfaces and the relative inactivity of the oxide monolayer. 

Oxygen activation is a central theme in biochemistry and is performed by a wide range of different iron and copper enzymes. In addition to our studies of the dinuclear non-heme iron enzymes MMO and RNR, we also studied oxygen activation in the mononuclear non-heme iron enzyme isopenicillin N synthase (IPNS). This enzyme uses O2 to transform its substrate ACV to the penicillin precursor isopenicillin N [53], a key step in the synthesis of the important P-lactam antibiotics penicillins and cephalosporins [54, 55],... 

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