Rhenium catalysts alkene metathesis

Leduc A-M, Salameh A, Soulivong D, Chabanas M, Basset J-M, Coperet C, Solans-Monfort X, Clot E, Eisenstein O, Bohm VPW, Roper M (2008) p-H transfer fitnn the metallacyclobutane a key step in the deactivation and byproduct formation for the well-defined silica-supported rhenium alkylidene alkene metathesis catalyst. J Am Chan Soc 130 6288-6297... 

In 1998, Ookoshi and Onaka reported remarkable increase in activity of M0O3 when this was supported on hexagonal mesoporous silica instead of conventional one. With this catalyst (7 wt % Mo) they achieved high conversion of 1-octene into 7-tetradecene at 50°C. Similarly in 2002, Onaka and Oikawa found rhenium oxide dispersed on mesoporous alumina with uniform pore size (7 wt % Re) to be more active in 1-octene metathesis than rhenium oxide on conventional y-alumina. Although both works lacked detailed characterization of supports and prepared catalysts, they clearly showed the positive effect of organized mesoporous siqrport on catalyst activity in alkene metathesis. 

Catalyst deactivation is a serious drawback to exploiting alkene metathesis for the production of olefins. There are many routes for the deactivation of a rhenium-based catalyst. Polar compounds, such as H2O, which might be present as an impurity in the reactants, are catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state, (ii) adsorption of polymeric by-products on the surface of the catalyst, blocking the active sites, and (iii) reductive elimination of the metallacyclobutane intermediate. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. Therefore, reductive elimination of the metallacyclobutane intermediate (to form cyclopropane, or 3-elimination to an alkene) is probably the main cause of deactivation and always seems operative [46,55,56]. 

Supported versions have also appeared, most notably Basset s well-defined siUca-supported rhenium catalyst which is active for the metathesis of both alkenes and alkynes. Basset s concept of surface silica-supported catalysts was also successfully extended to alkane activation by o CdT and C-C bond metathesis, and it is... 

Another facet of surface organometallic chemistry involves modelling of the mechanisms of surface reactions on the basis of the reactivity of molecular models. For example, the reactivity of metal-imine complexes of molybdenum is considered by CHAN, who proposes elementary steps constituting the catalytic cycle of the surface-catalyzed alkene ammoxidation reaction, which is of great industrial importance. HERRMANN provides some very fine examples of molecular models of the rhenium oxide catalysts used commercially in the alkene metathesis reaction. 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

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 ]. 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

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. ... 

---