Rhenium-based metathesis catalysts

Many other (cross-) metathesis reactions of functionalized olefins have been shown to be possible in the presence of rhenium-based catalysts, such as self-metathesis (or cross-metathesis with normal olefins) of allyl- and vinylsilanes, unsaturated nitriles, chlorides, bromides etc. The products of these reactions are not yet of use in fine chemistry, but this might be remedied by future developments in this area. 

Catalyst deactivation and regeneration. The activity of a rhenium-based catalyst in the metathesis of unsaturated esters is unavoidably limited by the complexation of the ester group to the active site [8]. Moreover, there are many routes that lead to deactivation of the catalyst. Polar compounds such as H2O or free acids, alcohols and peroxides, which might be present as an impurity in the substrate(s), can act as 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) product molecules on the surface of the catalyst, blocking the active sites (iii) reductive elimination of the metallacyclobutane intermediate [59]. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. 

Catalyst deactivation. The molybdenum-based catalysts deactivate faster than the rhenium-based ones. Studies concerning the stability of the catalyst during continuous metathesis of propene showed a loss of activity due to an intrinsic deactivation mechanism. Because of the high stability of both [Mo]=CH2 and [Mo]=CHCH3, the deactivation of the catalyst is assigned to isomerization of the intermediate metallacyclobutane complexes, leading to inactive 7i-complexes, in a way analogous to that depicted in Scheme 2. This hypothesis is supported by in situ UV/vis spectroscopic studies [67]. 

The development of an olefin metathesis catalyst based on late transition metals was a logical step because they are usually less sensitive to air, moisture, and polar or protic functional groups. Schrock and Toreki [11] created well-defined rhenium... 

The most successful metathesis catalysts are those based on rhenium, molybdenum, or tungsten. An account of these catalyst systems is given in Section 16.3.1 to Section 16.3.3. 

There is a broad agreement that carbene species developed on partly oxidized transition metal ions are necessary to bring about the metathesis reaction. The data reported earlier provide support that the environment of the active site does possess acid properties. It has been proposed that there is a relationship between the Brpnsted acidity of the rhenium-based catalysts and the metathesis activity [52], and that the activity is not related to their Lewis acidity. Dynamic infrared spectroscopic experiments were conducted by poisoning a Re207/Al203 catalyst (8.0 wt% Re) under a propene flow [53], Ammonia adsorption gave rise to coordinated NH3 species, characterized by the bands at 1618, 1326, and... 

Moloy, K.G. Evidence for intrinsically distinct active-sites on rhenium-based metathesis catalysts— Norbomenemetathesis withRe2C>7/Al203.7. Me/. Catal. 1994, 91, 291-302. 

Buffon, R., Schuchardt, U., and Abras, A. Mossbauer and solid-state NMR spectroscopic studies of tin-modified rhenium-based metathesis catalysts. J. Chem. Soc., Faraday Trans. 1995, 91, 3511-3517. 

Buffon, R., Jannini, M.J.D.M., and Abras, A. Effects of the addition of Nb205 to rhenium-based olefin metathesis catalysts. J. Mol. Catal. A Chem. 1997, 155, 173-181. 

When highly pure propene is not available commercially, it can be prepared by the reverse metathesis reaction of ethene and 2-butene [Eq. (1)]. The process is performed either at high temperatures (150-350°C) in the gas phase, over molybdenum or tungsten catalysts (Phillips triolefin process) [4], or at low temperatures (50°C) in the liquid phase, in the presence of rhenium-based catalysts (IFP-CPC process) [13], The raw material may be either ethene and the C4 fraction available from the hydroisomerization unit (previously submitted to an isomerization step to maximize its 2-butene content) or ethene alone, which, before admission to the metathesis unit, is partly dimerized to 1-butene, then isomerized to 2-butene in separate units. The process is useful in the event of a high demand for propene, since the C4 fraction is readily available from a cracking unit. 

The polymerization of 5,5-dimethylnorbornene in the presence of various metathesis catalysts based on molybdenum, tungsten, rhenium, osmium, ruthenium, and iridium compounds to ring-opened polymers having 0-100% CIS content has been reported [138] [Eq. (68)]. 

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). 

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. 

Meta-4 A process for converting ethylene and 2-butene into propylene by metathesis. The process operates in the liquid phase at low temperatures in the presence of heterogeneous catalyst based on rhenium oxide on alumina. The catalyst is constantly regenerated by coke combustion. Developed by IFP and the Chinese Petroleum Corporation of Taiwan. A demonstration plant was operated from 1988 to 1990 and the process was demonstrated at Kaohsiung, Taiwan, in 1999. Now offered by Axens. 

The kinetics of the metathesis of propene over a rhenium oxide-alumina catalyst (5.8% Rea07) have been studied by Kapteijn and Mol. The data correlate with a model based on the carbene mechanism and are in agreement with infrared and adsorption studies. Hsu has developed a kinetic model to express the time-on-stream profile of activity during catalyst break-in and deactivation. 

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