Molybdenum oxide, catalyst olefin metathesis

Such immobilized molybdenum oxide catalysts are active in selective oxidation reactions. For example, methanol can be oxidized with air to methyl formate at ca. 500 K with 90-95 % selectivity [T22]. The catalyst obtained from y-Al203 and tet-rakis(t -allyl)dimolybdenum (Eq. 6-7) is considerably more active in ethylene hydrogenation and olefin metathesis than the catalysts prepared by conventional fixation of [Mo(CO)6] followed by calcination. 

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 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. 

Molybdenum oxide supported on amorphous 3-Ti02 and reduced with hydrogen or carbon monoxide, followed by reoxidation with a mixture of N2O and H2, is a novel olefin metathesis catalyst. Tanaka et al. report that the metathesis reaction proceeds over this catalyst without hydrogen scrambling or isomerization through alkyl or carbonium intermediates. Degenerate metathesis is ten- to fourteen-fold faster than non-degenerate metathesis. ... 

Some metal oxide catalysts are activated by thermal reduction with hydrogen or carbon monoxide. For example, the catalytic activity of molybdenum oxide and tungsten oxide for the metathesis reaction of olefins is very much enhanced by their slight reduction (1). The catalytic activity for butene isomerization and ethene oligomerization appears on niobium oxide by its... 

Reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes led to very active bimetallic, heterogeneous olefin metathesis catalysts. Surface metal ions might be involved in bonding interactions with the organometallic complex, possibly leading to heterometallic species on inorganic oxides. ... 

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. 

Hoveyda et al. reported the enantioselective synthesis of (+)-africanol through asymmetric olefin metathesis followed by one-carbon contraction via decarbonylation (Scheme 8.16) [65]. When symmetrical norbornene 84 was treated with a chiral molybdenum catalyst, desymmetrization took place to afford the chiral bicycle 85 in an enantioselective fashion. The resulting vinyl terminus was subsequently transformed into a methyl group via hydroboration, oxidation to aldehydes, and decarbonylation. Further manipulations of the functional groups gave (+)-africanol. 

Apart Ifom the molybdenum carbene complexes already listed in Tables 2.1 and 2.2 Mo-based catalysts are of three main types (i) other Mo complexes, activated by a suitable cocatalyst (ii) M0CI5, also activated by a cocatalyst and (iii) supported oxides, generated in various ways. For the metathesis of terminal olefins higher than propene. Mo-based catalysts are generally more effective than the corresponding W-based catalysts. 

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