Oxidative activation initial step

The initiating step in these reactions is the attachment of a group to the sulfoxide oxygen to produce an activated intermediate (5). Suitable groups are proton, acyl, alkyl, or almost any of the groups that also initiate the oxidations of alcohols with DMSO (40,48). In a reaction, eg, the one between DMSO and acetic anhydride, the second step is removal of a proton from an a-carbon to give an yUde (6). Release of an acetate ion generates the sulfur-stabilized carbonium ion (7), and the addition of acetate ion to the carbonium ion (7) results in the product (eq. 15) ... 

H2SO4 = 0.09 M, fi = 2.0 M). Arrhenius parameters are A 10 ° I.mole . sec and E 28.5 kcal.mole . Successive alkylation of the olefinic bond increases the rate of reaction. One unusual feature is the lack of any acidity dependence. This implies that Co(H20)g is the active oxidant and that a radical cation is formed initially the lack of any retardation by added Co(II) means that the initial step is irreversible, viz. 

The experimental evidence, first based on spectroscopic studies of coadsorption and later by STM, indicated that there was a good case to be made for transient oxygen states being able to open up a non-activated route for the oxidation of ammonia at Cu(110) and Cu(lll) surfaces. The theory group at the Technische Universiteit Eindhoven considered5 the energies associated with various elementary steps in ammonia oxidation using density functional calculations with a Cu(8,3) cluster as a computational model of the Cu(lll) surface. At a Cu(lll) surface, the barrier for activation is + 344 k.I mol 1, which is insurmountable copper has a nearly full d-band, which makes it difficult for it to accept electrons or to carry out N-H activation. Four steps were considered as possible pathways for the initial activation (dissociation) of ammonia (Table 5.1). 

The formation of chloramines is an initial step of another mechanism of oxidative modification of LDL. It has been shown that the MPO-hydrogen peroxide-chloride system reacts with L-tyrosine to form p-hydroxyphenylacetaldehyde [163], As activated neutrophils release both MPO and hydrogen peroxide, it was suggested that neutrophils can stimulate the formation of p-hydroxyphenylacetaldehyde by producing chloramines as intermediates during the oxidation of LDL [164],... 

If the initiation step, the activation of H2, is fast, as may be the case on noble metal oxides or highly defective oxide surfaces, the shrinking core or contracting sphere model applies (see Figure 2.3). The essence of this model is that nuclei of reduced metal atoms form rapidly over the entire surface of the particle and grow into a shell of reduced metal. Further reduction is limited by the transport of lattice oxygen out of the particle. The extent of reduction increases rapidly initially, but slows down as the metal shell grows. 

No information is available as to whether metabolism of -hexane in children differs from that of adults. No studies were located comparing metabolism in young and adult animals. The toxicity of -hexane results from biotransformations yielding the active metabolite, 2,5-hexanedione. The initial step is an oxidation to 2-hexanol catalyzed by a cytochrome P-450 enzyme. Some P-450 enzymes are develop-mentally regulated (Leeder and Keams 1997). As the above discussion indicates, it is not completely clear which P-450 enzymes are involved in -hexane metabolism. 

Because many other chemicals can affect the enzymes responsible for n-hexane metabolism (see Section 2.3.3, Metabolism), the possibility of interactions is a significant concern. The initial step in n-hexane metabolism is oxidation to a hexanol by a cytochrome P-450 isozyme other chemicals can induce these enzymes, possibly increasing the rate of metabolism to the neurotoxic 2,5-hexanedione, or competing with M-hexanc and its metabolites at enzyme active sites, reducing the rate of metabolism. Interactive effects can be concentration and/or duration dependent. 

Oxidations dependent on molecular oxygen are of fundamental importance to many chemical and biochemical processes (1.-6). A large number of these processes require the presence of some transition metal ion to "activate" the oxygen. The initial step in many of these reactions is the coordination of O2, and the... 

The herbicide 2,4-D is itself a potent phytotoxin. However, a number of structurally related but inactive compounds may be converted by plants to 2,4-D following the activation process and thus act as herbicides. These phenoxyalkanoic acids are co-(2,4-dichlorophenoxy) alkanoic acids. The transformation may be viewed as shown in Fig. 12 as 6-(2,4-dichlorophenoxy)hexanoic acid as the parent compound. The sequence is called /1-oxidation because the steps in which two carbons are removed initially involve the oxidation of the /1-carbon to the aliphatic acid moiety. 

HS02, a known radical that has been found in H2-02-S02 systems, is suf-hciendy inert to be destroyed without reforming any active chain carrier. In the lean oxidation of the thiols, even at temperatures around 300°C, all the sulfur is converted to S02. At lower temperatures and under rich conditions, disulhdes form and other products such as aldehydes and methanol are found. The presence of the disulfides suggests a chain-initiating step very similar to that of low-temperature hydrocarbon oxidation,... 

Remarkably, the catalytic cycle is not controlled by the presence of phosphine ligands, but it is controlled by the organo group Y at the cobalt the neutral ligand L is displaced by the substrates in the initial step. Oxidative addition of two acetylenes results in a cobaltacycle that reacts with the nitrile to give the pyridine derivative with regeneration of the active [YCo] species. 

Initial dissolution of Fe oxides can be very rapid and is then followed by a slower steady state process. The initial step often corresponds to less than 1% of the total solid (Cornell et al. 1974 Maurice et al. 1995). Samson and Eggleston (1998) subjected hematite to a pH jump experiment and found that when the pH was lowered to 1, a reservoir of dissolution active sites on the surface was depleted, but then regenerated when the pH was raised to 2. The authors suggested that these active sites, which made up -70% of a monolayer, consisted of an adsorbed nutrient Fe . 

The initiation step (as well as the overall oxidation process) can be studied over a wide temperature range, well below ambient temperatures. Thus in 1 it could be studied in the range of 191 -263 K (Table 22) while in 44 the range of 233—303 °K was examined (Table 24). Apparent Arrhenius activation energies,... 

The use of a novel N3O2 ligand set provided an unsaturated five-coordinated Cu complex (19), capable of performing the aerobic oxidation of benzyl alcohol to benzaldehyde in the presence of Cu(CF3S03)2 as initial oxidant, with 44 turnovers in 24 h [163]. The initial step produces the active bis-phenoxy radical species 20 (Eq. 13). Under exclusion of air, greatly reduced catalytic activity was observed, indicative of reoxidation of the active catalyst by O2, although the possibihty for H2O2 (which is a likely side-product) to act as oxidant could not be ruled out. 

Mom+ and Mow+ are written to indicate forms of molybdenum present during the reaction without specifying an exact oxidation level of structure. The number of molybdenum compounds of various valences (Table I) which catalyze this reaction suggest that the initial molybdenum compound may be converted to the same active forms (Step 1). These active forms then complex with the hydroperoxide rapidly (Step 2). 

Cysteine not only is an essential constituent of proteins but also lies on the major route of incorporation of inorganic sulfur into organic compounds.443 Autotrophic organisms carry out the stepwise reduction of sulfate to sulfite and sulfide (H2S). These reduced sulfur compounds are the ones that are incorporated into organic substances. Animals make use of the organic sulfur compounds formed by the autotrophs and have an active oxidative metabolism by which the compounds can be decomposed and the sulfur reoxidized to sulfate. Several aspects of cysteine metabolism are summarized in Fig. 24-25. Some of the chemistry of inorganic sulfur metabolism has been discussed in earlier chapters. Sulfate is reduced to H2S by sulfate-reducing bacteria (Chapter 18). The initial step in assimilative sulfate reduction, used by... 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

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