Catalysis selenium

Indeed, one can easily conclude that selective allylic oxidation of olefins, in the context of fine chemicals, is a largely underdeveloped area of catalysis. Selenium dioxide catalyzes the allylic oxidation of a variety of olefins with TBHP, affording the corresponding allylic alcohols, but the system is homogeneous [3] and, hence, falls outside the scope of this book. The only heterogeneous catalysts for allylic oxidation which seem to have synthetic utility are palladium-based [4]. 

Susac et al. [33] showed that the cobalt-selenium (Co-Se) system prepared by sputtering and chemical methods was catalytically active toward the ORR in an acidic medium. Lee et al. [34] synthesized ternary non-noble selenides based on W and Co by the reaction of the metal carbonyls and elemental Se in xylenes. These W-Co-Se systems showed catalytic activity toward ORR in acidic media, albeit lower than with Pt/C and seemingly proceeding as a two-electron process. It was pointed out that non-noble metals too can serve as active sites for catalysis, in fact generating sufficient activity to be comparable to that of a noble metal, provided that electronic effects have been induced by the chalcogen modification. 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

The critical discovery that acetyl phosphate is generated and the information gained from several studies of each of the components of GR allowed an enzyme mechanism to be proposed (Arkowitz and Abeles 1991). However, with the current knowledge that one of the subunits of protein B also contains selenium, further work is needed to characterize the intermediates of the reaction and to explain the role of an additional selenocysteine residue. Whether this additional selenocysteine residue in protein B might serve as a direct reductant of the postulated thioselenide derivative of selenoprotein A and possibly serve as a link to the Trx-TrxR system is unknown. It should also be noted that the selenium-limited cultures that were initially studied during analysis of selenoprotein A (Turner and Stadtman 1973) apparently contained active fractions of proteins B and C, suggesting the role for selenium in protein B may not prove to be absolutely necessary for enzyme catalysis. 

Schuler E, Haring D, Boss B, Herderich M, Schreier P, Adam W, Mock-Knoblauch C, Renz M, Saha-Moller CR, Weichold O (1998) The potential of selenium-containing peroxidases in asymmetric catalysis glutathione peroxidase and seleno subtUisin. In Werner H, Schreier P (eds) Selective reactions of metal-activated molecules. Proceedings of the 3rd international symposium SEB 347. Vieweg, Braunschweig, p 35... 

Many proteins, including many enzymes, contain hghtly bound metal ions. These may be inhmately involved in enzyme catalysis or may serve a purely structural role. The most common tightly bound metal ions found in metalloproteins include copper (Cu+ and Cu +), zinc (Zn +), iron (Fe + and Fe +), and manganese (Mn +). Other proteins may contain weakly bound metal ions that generally serve as modulators of enzyme activity. These include sodium (Na+), potassium (K+), calcium (Ca +), and magnesium (Mg +). There are also exotic cases for which enzymes may depend on nickel, selenium, molybdenum, or silicon for activity. These account for the very small requirements for these metals in the human diet. 

With other electrophiles, ferrocenes 12 and 13 could be obtained, bearing a selenium group [19] or a silanol moiety [20], respectively, in the ortho position. Those compounds proved to be catalytically active as well, and in particular 13 was of interest, since - to the best of our knowledge - it was the first silanol ever used as a chiral ligand in asymmetric catalysis. Details of this study will be discussed below. 

The air oxidation of 2-methylpropene to methacrolein was investigated at atmospheric pressure and temperatures ranging between 200° and 460°C. over pumice-supported copper oxide catalyst in the presence of selenium dioxide in an integral isothermal flow reactor. The reaction products were analyzed quantitatively by gas chromatography, and the effects of several process variables on conversion and yield were determined. The experimental results are explained by the electron theory of catalysis on semiconductors, and a reaction mechanism is proposed. It is postulated that while at low selenium-copper ratios, the rate-determining step in the oxidation of 2-methylpropene to methacrolein is a p-type, it is n-type at higher ratios. 

Dorman G (2000) Photoaffinity Labeling in Biological Signal Transduction. 211 169-225 Drabowicz J, Mikolajczyk M (2000) Selenium at Higher Oxidation States. 208 143-176 Famulok M, Jenne A (1999) Catalysis Based on Nudeid Acid Structures. 202 101 -131 Frey H, Schlenk C (2000) Silicon-Based Dendrimers. 210 69-129... 

Molybdenum Inorganic Coordination Chemistry Nifro-genase Catalysis Assembly Nitrogenase Metal Cluster Models Selenium Proteins Containing Selenocysteine Tungsten Proteins. 

Chloriuatton of alkenes, Allylic chlorination is usually conducted by a free-radical reaction with NCS. Chlorination can be effected with catalysis by C6H5SeCl, in which case the main product is usually the rearranged allylic chloride. However, the formation of rearranged allylic chloride is sensitive to the structure of the alkenes and also to the particular selenium compound used as catalyst. See also N-phenylselenosuccinimide, this volume. 

The nature of the reactive species in selenium(IV) dioxide oxidations in ethanol and TBHP has been investigated. A solution of selenium(lV) dioxide/ethanol (1 2) in deuteriochloroform, obtained by ultrasonic treatment as described above, gave two resonances in the 77Sc-NMR spectrum, one consistent with the resonance obtained for pure 36 (S = 1344) the second signal (<5 = 1323) was not consistent with selenic(IV) acid. Thus, oxidations performed with solutions of selenium(IV) oxide in ethanol contain no selenium(IV) oxide but diethyl selenate(IV) (36) and ethyl hydrogenselenate(IV) (37), the former being the more reactive due to acid catalysis in the ene step, vide infra. 

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