Rhenium Oxide Catalysts

Cyclodimerhation of isoprene to 1,5-dimethylcycloocta-1,5-diene and disproportion with a rhenium oxide catalyst and isobutene produce 2,6-dimethyUiepta-l,5-diene. The diene is hydroformylated to citroneUal, which after hydrogenation produces citroneUol (137). 

B. Mitra, X. Gao, I. E. Wachs, A. M. Hirt, and G. Deo, Characterization of supported rhenium oxide catalysts, effect of loading, support and additives, Phys. Chem. Chem. Phys. 3, 1144-1152 (2001). 

Warwel et al. [27] synthesized 1,8-nonadiene in 75 % yield via ethenolysis of cy-cloheptene over a Rc207/Al203 catalyst (at 60 °C and an ethene pressure of 80 bar). The diene was subsequently subjected to intermolecular metathesis (release of ethene), again over a rhenium oxide catalyst (at 35 °C and 15 torr vacuum), giving 80% yield of 1,8,15-hexadecatriene. The latter was converted, via oxidation, intramolecular aldol condensation, and hydrogenation, to muscone (3-methylcyclo-pentadecanone), an important perfume ingredient. 

The selectivity of cross-metathesis reactions has been demonstrated by the reaction of trans-3-hexene with vinyl acetate or a,yff-unsaturated esters (e. g. methyl rran -crotonate) in the presence of a supported rhenium oxide catalyst (Eq. 14). 

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

Supported rhenium oxide catalysts have attracted much attention because of practical applications (see Ch. 17). One of the potential advantages of rhenium catalysts is that they are more tolerant of functional groups than tungsten or molybdenum catalysts (see Ch. 7). 

Unpromoted supported rhenium oxide catalysts can be regenerated many times without any loss of activity. However, the activity of a deactivated tetraalkyltin-... 

Heterogeneous catalysts for the metathesis of acyclic unsaturated esters are mainly based on rhenium. The rhenium oxide catalysts (Table 7.2) not only have the advantage that they are active at room temperature, but they are also highly selective, and can be easily recovered (Mol 1991). Mo03/Si02, that has been photoreduced in a CO atmosphere and subsequently treated with cyclopropane, has an even higher activity (Berezin 1991, Mol 1994b). Several well-defined metal... 

Although rhenium is a precious metal, promoted Re207-based catalysts are to be preferred over molybdenum ones their activation is simpler and regenerability is much better. Rhenium oxide catalysts can be regenerated many times, while the rhenium can be recovered relatively easily from these catalysts after total deactivation. 

Mol, J.C. Olefin metathesis over snpported rhenium oxide catalysts. Catal. Today 1999, 51, 2S9-299. 

Despite their relatively fast deactivation, supported rhenium oxide catalysts are sometimes preferred for practical applications because of their low operation temperature. Unpromoted Re207/Al203 catalysts can easily be regenerated by oxidation in an air stream at 823 K under atmospheric pressure without any loss of activity. 

During a joint venture with Naarden International, Shell International developed a new route to citronellol, but this was never commercialized. The route starts with cyclodimerization of isoprene (1) to give 1,5-dimethylcycloocta-1,5-diene (137). Olefin metathesis with isobutene in the presence of a rhenium oxide catalyst produces 2,6-dimethylhepta-1,5-diene (138). The diene is hydroformylated to citronel-lal (139), which after hydrogenation produces citronellol (85) (169). Alternatively, the 1,5-dimethylcycloocta-1,5-diene can be partially hydrogenated and the monoene (140) then pyrolyzed to 2,6-dimethylocta-l,7-diene (141), which can be converted subsequently to citronellol in several steps (170). These processes are shown in Fig. 8.28. 

BP has developed an even more active rhenium oxide catalyst supported on alumina that operates at 20°-50°C, with an equilibrium conversion higher than 60%. This follows from an observation by Philhps that alumina is a more suitable support than silica. 

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. 

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