Chromium oxidants eliminating

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

However, over Ni-kieselguhr in the absence of solvent or in ether and methylcyclo-hexane 32-33% of a diester, ethyl 3-(3 -hydroxybutyryloxy)butyrate (8), was produced along with 68-67% of ethyl 3-hydroxybutyrate and small quantities of dehydroacetic acid, and over copper-chromium oxide 16% of the diester and 7% of dehydroacetic acid were formed in the absence of solvent. It was suggested that the diester is formed through the hydrogenation of the intermediate 9, which results from 2 mol of acetoacetic ester with elimination of 1 mol of ethanol and that the condensation reaction is reversible (Scheme 5.6). Hence, the formation of the diester is depressed in the hydrogenation in ethanol.121 The reaction pathway in Scheme 5.6 has... 

The dehydrogenation catalyst must be sufhciently active to allow for very short contact times and the use of low temperatures, to minimize thermal cracking reactions. Carbon deposits are eliminated by heatihg in the presence of a gas containing oxygen. -This means that the catalyst must be thermally stable to avoid being deactivated during the oxidation of the deposits. The best catalysts contain alamina and chromium oxide, but these cannot be employed in the presence of steam. Operations are conducted at a temperature between 550 and 700 C, and low pressure, less than 0.1.10 Pa absolute. 

In addition to incorporated oligomers, which produce even-numbered branches, methyl branching is also detected in small amounts in the polymers made with many organochromium (but not chromium oxide) catalysts. Chromocene is especially known for this behavior [303,654,679,680]. It is usually thought to result from (3-hydride elimination to the chromium, followed by reinsertion of the same chain or (perhaps a comonomer) in the backwards 2,1 position. The number of methyl branches formed is usually not large enough to have a significant effect on the resin density. 

One of the most interesting results of this approach of using "two-valent" chromium species is the effect on catalyst activity. Yields as high as 16 kg g 1 h 1 were obtained, which was more than twice that of the chromium oxide parent, and many times more than that of the chromium alkyl when deposited on silica. The induction time of chromium oxide was eliminated, and also the declining kinetics profile of the chromium alkyl catalyst. That is, the hybrid catalyst seemed to have incorporated the best aspects of both parents to yield unusually high polymerization activity. 

Chromium(II) reagents have been used for the deoxygenation of styrene oxide and cyclohexene oxide [36]. The first step of this transformation is thought to be epoxide opening via electron transfer yielding a 6-chromiumoxy radical that is trapped by a second equivalent of chromium(II). Elimination of a chromium oxo species completes the reaction as shown in Scheme 19. 

Chromium can occur in water either in the oxidation state III or VI. Cr(III) possesses significant complex-forming properties. The Cr(VI) forms are stable in aerobic media. In anaerobic media Cr(VI) can be reduced to Cr(III), chromium is eliminated from the liquid phase in the form of low soluble hydrated chromic oxide. 

Hubaut et has studied the liquid phase hydrogenation of polyunsaturated hydrocarbons and carbonyl compounds over mixed copper-chromium oxides. The selectivity of monohydrogenation was almost 100 % for conjugated dienes but much lower for a,p-unsaturated carbonyls. This was due to the adsorption competition between the unsaturated carbonyls and alcohols as primary products. It was suggested that the hydrogenation site was an octahed-rally coordinated Cu ion with two anionic vacancies, and an attached hydride ion. The Cr ion in the same environment was probably the active site for side reactions (hydrodehydroxylation, nucleophilic substitution, bimolecular elimination). 

The mechanism of oxidation for a secondary alcohol with CrOj involves the nucleophilic oxygen reacting with the oxidising agent to produce a charged chromium intermediate. Elimination then takes place where an a-proton is lost along with the chromium moiety to produce the carbonyl group. 

The loop reactors, which are recycled tubular reactors, are used by the Phillips Petroleum Co. and Solvay et Cie. The Phillips process is characterized by the use of a light hydrocarbon diluent such as isopentane or isobutane in loop reactors which consist of four jacketed vertical pipes. Figure 1 shows the schematic flow diagram for the loop reactor polyethylene process. The use of high-activity supported chromium oxide catalyst eliminates the need to deash the product. This reactor is operated at about 35 atm and 85-110° C with an average polymer residence time of 1.5 hr. Solid concentrations in the reactor and effluent are reported as 18 and 50 wt %, respectively. The reactor diameter is 30 in. (O.D.) and the length of the reactor loop is about 450 ft. 

A key property of a passive film that may be manipulated in the future is its semiconductive nature in the context of the cathodic reaction that necessarily occurs on top of the film. It is not always appreciated that chromium oxide is a very good electrical insulator, so that in theory one could enrich chromium to such an extent that the reduction of oxygen on top of the passive film on stainless steel would cease, thus eliminating localized corrosion in salt water. In practice, not much iron content is required to make the film rather conductive, but still, manipulation of cathodic reaction kinetics on passive films has to be considered an important challenge for the future. Many tools and theoretical underpinnings are available for further progress in this area, including a variety of in situ probes, surface-science tools, and classical electrochemistry methods. 

Sinha, A.K. and Suzuki, K. (2005) Three-dimensional mesoporous chromium oxide a highly efficient material for the elimination of volatile organic compounds. Angew. Chem., Int Ed., 117 (2), 275-277. 

Sinha, A. and Suzuki, K. (2007). Novel Mesoporous Chromium Oxide for VOCs Elimination, Appl Catal. B Erwiroru, 70, pp. 417-422. 

The intermediacy of dipolar species such as 186 has been demonstrated by reaction of enamines with 2-hydroxy-1-aldehydes of the aromatic series (129). The enamine (113) reacts in benzene solution at room temperature with 2-hydroxy-1-naphthaldehyde to give the crystalline adduct (188) in 91 % yield. Oxidation with chromium trioxide-pyridine of 188 gave 189 with p elimination of the morpholine moiety. Palladium on charcoal dehydrogenation of 189 gave the known 1,2-benzoxanthone (129). 

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