Rhenium olefin metathesis

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

Although the number of applications of olefin metathesis to transition metal complexes is small compared to the number of applications in organic synthesis, this field is becoming increasingly important. Spectacular examples are the double RCM reactions of copper phenanthroline complexes as a synthetic route to catenanes [113] or a recently reported approach to steric shielding of rhenium complex terminated sp-carbon chains [114]. 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

The first catalysis of an olefin metathesis reaction was reported by Banks and Bailey in 1964 (56). They reported that activated molybdenum hexacarbonyl on alumina converted propylene, for example, into ethylene and 2-butene at 150°C and 30 atm. Oxides of rhenium are also powerful heterogeneous catalysts. 

Rhenium alkoxocomplexes are of interest as homogeneous catalysts of olefine metathesis [519], They have also been used for the preparation of fine powders ofrhenium and rhenium alloys for catalytical and powder metallurgical use [1452]. 

Chabanas, M., Baudouin, A., Coperet, C. and Basset, J.-M. (2001) A highly active well-defined rhenium heterogeneous catalyst for olefin metathesis prepared via surface organometallic chemistry. J. Am. Chem. Soc., 123, 2062. 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

McKinney and Stone have isolated a rhenium hydroxycarbene species from the reaction of [Re(R)(CO)5] with a Lewis acid (159) [Eq. (48)]. The hydroxycarbene compound was found to function as a catalyst for olefin metathesis. 

Rhenium oxide-alumina catalysts are reduced at ambient temperatures and sub-atmospheric pressure by propene and higher alkenes, generating metathesis activity. Ethylene at these conditions did not show any reduction capabilities. Reduction with CO or NH3 at 300-500° C did not result in metathesis activity. At room temperature CO did not adsorb on reduced catalysts however, NO adsorbs and is a poison for the olefin metathesis reaction. Water generated in reducing catalysts with alkenes is mainly associatively adsorbed and, at ambient temperatures, exchanges hydrogen atoms with propene and butene. Activity for double-bond isomerization is partly accounted for by associatively adsorbed water, which generates acidity. ... 

Mol, J.C. (1999) Olefin Metathesis over Supported Rhenium Oxide Catalysts, Catalysis Today 51, 289-299 52. 377. 

When olefins are combined with a variety of catalysts, most commonly containing tungsten, molybdenum, or rhenium, they undergo the transformation generalized as Eq. (1) and known as the olefin metathesis... 

In the 1970 s an intense interest in the olefin metathesis reaction [35] served as a driving force for research concerning metal-carbon double bonds. In its simplest form the olefin metathesis reaction consists of a redistribution of alkylidene components of olefins (Eq. 6). It was known to be promoted by tungsten, molybdenum, and rhenium, although at the time the catalytic reactions were black boxes, with nothing known about the mechanism or the detailed nature of the catalyst. Among the proposals was one that consisted of a reaction between an alkylidene complex (or carbene complex at that time) and an olefin to give a metallacyclobutane intermediate, from which an olefin could be lost and a new alkylidene complex formed, [36] as shown in... 

The pentacarbonylrhenium halides, first prepared by Hieber, are starting materials for the syntheses of many novel rhenium carbonyl compounds. Photochemical, vibrational, and kinetic " properties of these molecules have been studied. A rhenium carbonyl halide-alkyl aluminum halide system polymerizes acetylene and is a useful olefin-metathesis catalyst. " ... 

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. 

In the last decade, impressive advances have been made in the synthesis of alkylidene and alkylidyne complexes that contain metals from groups 4 [5] and 5 [5i,j,m, 6], especially Ti and V, but - except for the ROMP of norbornene by vanadium complexes - group 4 and 5 metals have not shown wide-ranging activity for olefin metathesis. Well-characterized rhenium(VII) complexes are known to be active for metathesis, and many rhenium(Vll) alkylidene and alkylidyne complexes have been isolated [2a], but little attention has been paid to the syntheses of rhenium alkylidene or alkylidyne complexes in the last decade. Theoretical calculations have been carried out on M(X)(CHR )(Y)(Z) complexes... 

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