Oxidation reactions, transition-metal

Several catalytic systems have been recently studied for selective oxidation reactions. Transition metals on oxide supports have shown high activity and selectivity toward CO oxidation [59-63]. In general, a high activity and selectivity are achieved when the affinity of the metal toward CO is higher than that toward H2, and the support is able to supply mobile oxygen. In such cases, the selective... 

Besides the oxidative and transition-metal-catalyzed condensation reactions discussed above, several other syntheses were developed to generate PPP and PPP derivatives. 

There are many transition metal ion oxidants used in organic chemistry for the interconversion of functional groups. Those which have been used for the preparation of sulphones from sulphoxides will be discussed below. It is very interesting to note that this type of oxidant often reacts more rapidly with sulphoxides than with sulphides and so sulphoxides may be selectively oxidized with transition metal ion oxidants in the presence of sulphides. This is in direct contrast to the oxidation of sulphides and sulphoxides with peracids and periodate, for example, where the rate of reaction of the sulphide is more than 100 times that for the corresponding sulphoxide. 

Bonnesen PV, Puckett CL, Honeychuck RV, Hersh WH (1989) Catalysis of Diels-Alder reactions by low oxidation state transition-metal Lewis acids fact and fiction. J Am Chem Soc 111 6070-6081... 

At higher reaction temperatures (>300°C), micro- or meso-porous materials and/or oxides containing transition metals are preferable. The performances are considerably dependent on the type of reductant, besides the characteristics of the catalyst and the type of transition metal. Although all possible combinations have been explored, including the usage of high-throughput methods, identification of a suitable catalyst formulation active in HC-SCR under practical conditions, especially to decrease by more than... 

Ford, P.C. and Lorkovic, I.M. (2002) Mechanistic aspects of the reactions of nitric oxide with transition-metal complexes, Chem. Rev., 102, 993, and references therein. 

With the advance of three-way catalysis for pollution control, used mainly in automobile catalytic conversion but also for the purification of gas exhausts from stationary sources, a need has arisen to develop a basic understanding of the reactions associated with the reduction of nitrogen oxides on transition metal catalytic surfaces [1,2]. That conversion is typically carried out by using rhodium-based catalysts [3], which makes the process quite expensive. Consequently, extensive effort has been placed on trying to minimize the amount of the metal needed and/or to replace it with an alternatively cheaper and more durable active phase. However, there is still ample room for improvement in this direction. By building a molecular-level picture of theprocesses involved,... 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Peroxyl radicals with a strong oxidative effect along with ROOH are continuously generated in oxidized organic compounds. They rapidly react with ion-reducing agents such as transition metal cations. Hydroxyl radicals react with transition metal ions in an aqueous solution extremely rapidly. Alkyl radicals are oxidized by transition metal ions in the higher valence state. The rate constants of these reactions are collected in Table 10.5. 

In contrast to superoxide, which participates in one-electron transfer reactions as a reductant, nitric oxide is apparently able to oxidize various transition metal-containing proteins and enzymes. The study of NO reaction with hemoglobin has been started many years ago when... 

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these will be assessed with regard to kinetic insight and quantification. An attempt will be made to identify questions that are ripe for investigation. Although not limited to it, the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction C l) will be useful to readers who are interested in a more comprehensive view. 

The reaction mechanisms of these transition metal mediated oxidations have been the subject of several computational studies, especially in the case of osmium tetraoxide [7-10], where the controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally [11-20]. Based on the early proposal of Sharpless [12], that metallaoxetanes should be involved in alkene oxidation reactions of metal-oxo compounds like Cr02Cl2, 0s04 and Mn04" the question arose whether the reaction proceeds via a concerted [3+2] route as originally proposed by Criegee [11] or via a stepwise [2+2] process with a metallaoxetane intermediate [12] (Figure 2). 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

In terms of electron transfer reactions, transition metal ions can be the one- or two-electron type. The two-electron ions transform into unstable states on unit change of the metal oxidation number. In the outer-sphere mechanism, two-electron transfer is a combination of two one-electron steps. 

Transition metah—found in the groups located in the center of the periodic table, plus the lanthanide and actinide series. They are all solids, except mercury, and are the only elements whose shells other than their outer shells give up or share electrons in chemical reactions. Transition metals include the 38 elements from groups 3 through 12. They exhibit several oxidation states (oxidation numbers) and various levels of electronegativity, depending on their size and valence. 

Chelants are not biocides on their own, but can boost the performance of many biocides so that lower doses are required in personal care and other liquid formulations. This is believed to be due to removal of calcium by chelants from bacterial cell walls, thus allowing the biocide to pass through the cell wall more efficiently [7]. Chelants can also act as antioxidants by binding transition metal ions and preventing oxidation reactions of metal ions with radicals and oxygen. 

The performance of a number of single oxides of transition metals was studied by Skorbilina et al. [295] using a differential reactor. As usual, o-tolualdehyde, phthalic anhydride and carbon oxides are the main reaction products. The initial selectivity with respect to partial oxidation products decreases in the order Co > Ti > V > Mo > Ni > Mn > Fe > Cu from 71% to 33%. The relatively high initial selectivities demonstrated by the deep oxidation catalysts (e.g. Co, Ni, Mn) indicates that the primary activation is probably the same for all these catalysts, while the differences that actually determine the character of the catalyst are connected with the stability of intermediates and products. 

As discussed above, iodosylbenzene (Phl=0) oxidizes various transition-metal ions (Mn+) such as manganese, iron, ruthenium, and chromium ions to the corresponding oxo-metal species (0=Mtransfer agents. Likewise, fM-(p-toluenesulfonyl)imino]-phenyliodinane(PhI=NTs) also oxidizes these metal ions to give the corresponding tosylimino-metal species (TsN=M(n+2)+) that undergo nitrene-transfer reaction such as aziridination (Scheme 6B.28) [73],... 

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