Rhenium catalysts, review

The implementation of enantioselectivity issues in heterogenized oxidation catalysts has been a key strategy for a number of years which has been successful only with certain classes of catalysts. For example, molybdenum and rhenium catalysts are among the most efficient in epoxidation reactions, however, attempts to modify them with chiral ligands in order to achieve enantioselective transformations, either in solution or anchoring the resulting species on solid supports such as the MCM family, has met with only moderate success. The activity is generally decreased and only poor to moderate enantioselectivities have been observed in some cases. This topic has been recently reviewed. ... 

The oxidation chemistry of methylrhenium trioxide (MTO) has been reviewed.72 The oxidation of thiophenes by hydrogen peroxide has also been studied, using MTO as a catalyst.73 The latter reacts with H2O2 to generate 1 1 and 1 2 rhenium peroxides, which are able to transfer an oxygen atom to the sulfur of the substrate, to give first the sulfoxide and then the sulfone. Whilst electron-donating substituents accelerate the first oxidation, the reverse trend is observed for oxidation of the sulfoxide. 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

A timeline for the development of olefin metathesis, adapted from a review by Grubbs, is shown in Figure 21.3. Olefin metathesis is more than 50 years old. " It was first conducted with ill-defined rhenium, molybdenum, and tungsten systems generated from perrhenate, aluminum oxide, - and tetraethyl lead as additive, from molybdenum oxide on p-TiO and tetramethyltin as additive, ° or from tungsten phenoxides supported on niobium oxide and silicon oxide activated with alkylaluminum reagents. The temperatures for these processes are hi, but the catalysts are relatively inexpensive and can be long lived. These are the types of catalysts that have been used for the synthesis of commodity chemicals by olefin metathesis. 

A special volume is dedicated to the papers from the 9th International Symposium on Olefin Metathesis.274 xhe development of well-defined metathesis catalysts is the subject of a review s and some theoretical calculations. 276 However, the development of mixed component systems continues with molybdenum and rhenium carbonyls and nitrosyl precursors enjoying topical popularity. 

A new generation of bifunctional catalysts was introduced in 1967. The catalyst containing rhenium in addition to platinum provides greater stability.In 1975, the process using a catalyst containing platinum and iridium was commercialized. These catalysts are called bimetallic catalysts. The bimettillic catalysts are typically 3 to 4 times more active than the all-platinum catalyst. A bimetallic catalyst with rhenium typically contains about 0.3% platinum and 0.3% rhenium. The reasons for the effectiveness of these bimetallic catalysts are beyond the scope of this volume and the readers should refer to the appropriate monographs or reviews. ... 

Besides the use of vanadium-based catalysts, a wide variety of other catalyst compositions were reported. A recent review focussed on FeSbO based catalysts promoted by appropriate additives as suitable for the ammoxidation of alkyl-substituted aromatics and hetero aromatic compounds. A unique preparation method of a fluid-bed catalyst is presented using nitric acid oxidation of antimony trioxide catalyzed with iron ions. The catalysts thus prepared have superior catalytic and physical properties. [78]. In addition, some unique compositions were reported by different research groups. For instance, new ammoxidation catalysts based on rhenium carbonyl cluster complexes containing antimony and bismuth ligands were reported by Adam et al. [79]. Single-site multifunctional catalysts based on [Cu RUj C ] nanocluster anchored to inner walls of mesoporous silica were also used in the ammoxidation of 3P [80]. 

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