Selective oxygenation

Figure 2.6. The tetrahedral structures of ice (a), (fc) are planes through sheets of selected oxygen nuclei (open circles), hydrogen nuclei (shotm in the insert as solid circles) are not shown in the main drawing. The insert illustrates the overlap of oxygen line pairs and the hydrogen nuclei, thus forming the hydrogen bonds (dotted lines)...

Salcomine is a useful catalyst for the selective oxygenation of 2,6-disubstituted phenols to the corresponding p-benzoquinones when dimethylformamide is used as the solvent laborious procedures are avoided and high yields of pure p-benzoquinones are obtained. Following the procedure described above, the authors have prepared 2,6-diphenyl-p-benzoquinone (m.p. 134—135°, yield 86%) and 2,6-dimethoxy-p-benzoquinone (m.p. 252°, yield 91%) from the appropriate phenols. 

Convert P, Countanceau C, Cromgneau P, Gloaguen F, Lamy C (2001) Electrodes modified by electrodeposition of CoTAA complexes as selective oxygen cathodes in a direct methanol fuel cell. J Appl Electrochem 31 945-952... 

Their interactions with the more hindered side of an asymmetrical olefin determine the orientation of the substrate during its approach to the metal-oxo bond and the subsequent enantiofacial selective oxygen transfer. 

Table 9-4 Typical Coal Gas Compositions for Selected Oxygen-Blown Gasifiers...

For DMFC systems, Pt cathodes are also used as the catalyst of choice however, given Pt s ability to reduce oxygen and oxidize methanol, this lack of selectivity makes them sensitive to methanol crossover from anode to cathode via the membrane. This methanol crossover can have a depolarizing effect on cathode performance, reducing overall cathode activity. To combat this, an extensive effort has been made to identify and develop selective oxygen/reduction catalysts unaffected by MeOH crossover. 

Ruthenium(in) catalyses the oxidative decarboxylation of n-butyric acid and isobutyric acid by ceric sulfate in aqueous acid. A mechanism for the Ru(III)-catalysed oxidation of o-hydroxybenzoic acid by an acidic solution of bromamine-B (PhS02-NNaBr, BAB) has been proposed based on a kinetic smdy. An ionic mechanism is suggested for the ruthenium(III) analogue of the Udenfriend-type system Ru(III)-EDTA-ascorbate-02, for the selective oxygen-atom transfer to saturated and unsaturated hydrocarbons. The kinetics of the oxidation of p-XC6H4CHPhOH(X =... 

Hemingway, B.S. (1990) Thermodynamic properties for bunsenite, NiO, magnetite, Fe304, and hematite, Fe203, with comments on selected oxygen buffer reactions. Am. Min. 75 781-790... 

TABLE 10.4 Structures, Common Names, Empirical Formulas, and Molecular Weights for Selected Oxygen-Containing Polycyclic Aromatic Compounds (O-PACs) Identified in Ambient Air... 

An isolated, less substituted double bond in trienes that is less reactive does not participate in oxidation allowing selective oxygenation 412... 

Fig. 13.7 Hierarchical structure and overall interrelationships between oxidation mechanisms for simple hydrocarbon fuels and selected oxygenated fuels ([427], modified).

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

Moody, J. S., Kozak, K. R., Ji, C., and Marnett, L.J. (2001). Selective oxygenation of the endocanna-binoid 2-arachidonylglycerol by leukocyte-type 12-lipoxygenase. Biochemistry 40, 861—866. 

Table II. Comparison of Selected Oxygenated Hydrocarbon Compounds in Steam Distilled and LCO2 Extracted Ginger Oil...

Ti-substituted silicalite, used for selective oxygenation of numerous substrate... 

An important claim has been made on the regioselectivity in the ra-alkanc reaction with dioxygen using CuPcC1i7Y as catalyst.1651 A remarkable selectivity for formation of primary alcohols, in other words of selective oxygenation of primary C atoms, has been reported. 

These basic thermodynamic considerations show that intermediate reactions in combustion processes can be very advantageous and that in some cases most or all of the chemical energy could be harnessed as mechanical energy at least theoretically. Important questions of reaction kinetics, actual design and applicability of such a device of the selected oxygen carriers have not been included in these fundamental thermodynamic equilibrium studies. 

Attempts at using C0/02 mixtures with Rh or Ir catalysts for selective oxygenations (cf. Reaction 9) have not proved successful (105). 

Modern variations include the in situ, and thus catalytic, use of this high-valent selective reagent, not only for alcohols but also for ethers (see later). Ru(VII) (perruthenate) in the compounds tetra-n-butylammonium perruthenate (TBAP) and tetra-n-propylammonium perruthenate (TPAP) has found wide application in alcohol oxidation. Ru-oxo complexes with valence states of IV to VI are key intermediates in, for example, the selective oxygen transfer to alkenes, leading to epoxides. On the other hand 16-electron Ru(II) complexes can be used to catalyse hydrogen transfer thus these are excellent catalysts for oxidative dehydrogenation of alcohols. A separate section is included to describe the different mechanisms in more detail. 

Metals such as Zr and Nb have also been proposed for H2 permselective dense membranes [1]. Dense silver membranes have been used for selective oxygen permeation [18],... 

---