Catalyst rhenium

In the early 1990s Herrmann and coworkers [56] reported the use of methyl-trioxorhenium (MTO) as a catalyst for epoxidation with anhydrous H202 in tert-butanol. In the initial publication cyclohexene oxide was obtained in 90% yield using 1 mol% MTO at 10 °C for 5 h. At elevated temperatures (82 °C) the corre- 

MTO-catalyzed epoxidations proceed via a peroxometal pathway involving a di-peroxorhenium(VII) complex as the active oxidant (see Fig. 4.25). Major disadvantages of MTO are its limited stability under basic H202 conditions [62] and its rather difficult and, hence, expensive synthesis. 

A novel class of metathesis catalysts derives from methyltrioxorhenium, CH3Re03 (MTO) [22]. Combined with certain solid supports such as Si02/Al203 or Nb205, highly active catalysts are generated [23, 24]. They even tolerate fimctional groups such as ketones, esters, and carboxylic acids. Based upon NMR spectroscopic evidence [25] and in accord with theoretical studies, surface-attached Re=CH2 species initiate metathesis via a novel type of tautomerism [Eq. (3)]. 

The catalyst precursor MTO was previously difEcult to make, but it is now available from cheap starting materials on a multi-kilogram scale [26], It is thus the ideal candidate for industrial uses, be it in metathesis or in oxidation [22], 

Olefin Metathesis, Academic Press, London 1997. 

Dragutan, a. T. Balaban, M. Dimonie, Olefin Metathesis and Ring Opening Polymerization of Cyclo-Olefins, 2nd ed., Wiley-Intersdence, New York 1985. 

US Pat. pending (2004) (d) Compounds 7 have been reported recently in the literature J. Yun, E. R. Marinez, R. H. Grubbs, Organometallics 2004, 23, 4172. 

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Rhenium catalysts are exceptionally resistant to poisoning from nitrogen, sulfur, and phosphorus, and are used for hydrogenation of fine chemicals. 

Platinum-rhenium catalysts have been reduced in one atmosphere of flowing hydrogen and then examined, without exposure to the atmosphere, by ESCA. The spectra indicate that the Group VIII metal is present in a metallic state in the reduced catalyst and that the majority of the rhenium is present in a valence state higher than Re(0). 

The Re peak, present as a doublet in this catalyst, resembles the one obtained for Re20 on tape. This suggests, if rhenium hydroxides may be eliminated from consideration, that the calcined rhenium catalysts may have some rhenium in a valence state lower than 4-7. However, even reduction at 400 C of the uncalcined Pt-Re sample does not produce an observable amount of Re(0). 

Table 1 Comparison of Xylitol and Arabitol feed 4-Hour Sample on 2.5% Nickel / 2.5% Rhenium catalyst, 200°C, 300cc Batch Reactor, 8,300kPa H2...

Figure 3.53 shows the volume of CO produced, and the turnover number of the rhenium catalyst, as a function of the charge passed. Over 14 hours of electrolysis the Re catalyst underwent 300 catalytic cycles without loss of activity and showed no degradation, as seen by isolation and characterisation... 

On the basis of their results the authors postulated two distinct pathways for the electroreduction of C02 by the rhenium catalyst, with the intermediates being (a) (Bipy)Re[CO]3 and (b) the radical (Bipy)Re[CO]3 (see Scheme 3.1). The former is indicated by the fact that production of the green dimer is completely suppressed in CO2-saturated electrolyte, as a result of the C02 intercepting the active species present. 

As can be seen in the scheme below, insertion reactions of aldehydes to the C-H bond of aromatic ketimines by using a rhenium catalyst provided benzo[c]furans via a mechanism involving consecutive steps of C-H bond activation, insertion of aldehyde, intramolecular nucleophilic cyclization, reductive elimination, and elimination of aniline <06JA12376>. 

The first example of a heterogeneous catalyst able to metathesize olefinic esters was recently reported (92). The combination of Re207/ (CH3)4Sn at an olefin/Re/Sn molar ratio of 219/6/1 converted methyl 4-pentenoate at 50°C in 51% conversion to ethylene and the corresponding dimethyl ester of 4-octene-l,8-dioic acid. This reaction exhibited a high degree of selectivity (>99%), and in the absence of (CH3)4Sn the rhenium catalyst was inactive (90). 

However, interesting a case this makes for the examination of mechanism, the point must first be established that it represents a reaction worth conducting, and second that it can be carried out on a laboratory scale to give a useful amount of product. With a suitable rhenium catalyst, nearly quantitative conversions have been realized within a reasonable time. A total of 15 substrates were studied, such that the insensitivity to functional groups, steric and electronic variables were established. All the substrates examined gave essentially complete conversion to product on a scale of 1 g of PyO, irrespective of a wide range of functional groups present (26). 

It should also be noted that the added nucleophilic reagents are not (cannot) themselves be transformed in any sense as a result of this chemistry they serve as promoters of the rhenium catalyst. 

An acid-rhenium catalyst mixture acts on ( )-4-(4-hydroxyphenyl)butan-2-one oxime (44) to produce a high yield of the spiro compound (45), which then rearranges to the substituted quinoline (46)." ° The Beckmann rearrangement product (47)... 

Rhenium is a rare metal with total world production in 2007 estimated at about 50 tonnes. The principal industrial applications of rhenium are in high-temperature alloys used in jet engines and in platinum-rhenium catalysts used in the petroleum industry. In recent years, the demand for rhenium has increased and prices have risen to more than US 10,000 per kg. In late 2008, rhenium was the sixth most expensive traded element (WWW. lipmann.co.u k/fa cts/expe ns i ve. htm I)... 

The same is true of rhenium catalysts rhenium heptoxide [42], rhenium heptasulfide [5i] and rhenimn heptaselenide [54] all require temperatures of 100-300° and pressures of 100-300 atm. Rhenium heptasulfide is not sensitive to sulfur, and is more active than molybdenum and cobalt sulfides in hydrogenating oxygen-containing functions [55,55]. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

Retinal ischemia, lipid hydroperoxides, 612 Rhenium catalysts... 

Primary amines at a primary carbon can be dehydrogenated to nitriles. The reaction has been carried out with a variety of reagents, among others, IF5,"9 lead tetraacetate, 20 nickel peroxide,121 NaOCl in micelles,122 S g-NiSO, 2-1 and CuCl-02-pyridine.124 Several methods have been reported for the dehydrogenation of secondary amines to imines.125 Among them126 are treatment with(l) iodosylbenzene PhIO alone or in the presence of a ruthenium complex, 27 (2) Me2SO and oxalyl chloride, 2" and (3) f-BuOOH and a rhenium catalyst. 29... 

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