Propene, alkene metathesis

O Neill and Rooney 90) found that the Mo03-CoO-A1208 catalyst converts diazomethane into nitrogen and ethene under conditions where propene undergoes metathesis. However, because many catalysts are active for this conversion 91), their results cannot be considered as supporting the hypothesis that the metathesis reaction of alkenes proceeds via carbene complexes. 

One of the most curious catalytic reactions of alkenes ever discovered is alkene metathesis or alkene dismutation, in which two alkenes exchange alkyli-dene groups, usually over a tungsten catalyst. The essence of the reaction is illustrated by a commercial process for converting excess propene to a mixture of ethene and butenes ... 

The ability of a p-carbene to react with an unsaturated hydrocarbon and form an enlarged dimetallocycle encourages speculation over their role in such processes as alkene metathesis and Fischer-Tropsch synthesis. In Scheme 6 a possible mechanism for metathesis initiated by a p-carbene is presented, owing much to other workers (T7,22). Reactions of p-carbenes with alkenes are under investigation in our laboratory. Recently Pettit has observed that the p-methylene complex [Fe2(C0)8(p-CH2)] generates propene when subjected to a pressure of ethene and has also suggested the intermediacy of a three-carbon dimetallocycle (23). 

We look at these reactions in detail in Sections 11.3 and 6.6, respectively. For the moment we need only note that the p-C—H of these cyclic dialkyls is held away from the metal and so is not available for p elimination. The P C—C bond is held close to the metal, however, and so these rearrangements are really p eliminations involving a C—C, rather than a C—H, bond. The reaction of Eq. 3.21 is of particular significance because it is the key step of an important catalytic reaction, alkene metathesis, which converts propene to butene and ethylene (Chapter 11). The anion of [Li(tmeda)]2[(CH2)4-Pt(CH2)4] contains two tetramethylene rings bound to square planar Pt(II) and is thermally rather stable. Cyclic diaryls are rare an interesting and very stable example is shown below ... 

The reactions between the Group VI metal hexacarbonyls, representing relatively simple binary carbonyl complexes, and a variety of oxides is illustrative and also derives interest from the active alkene metathesis catalysts provided [20,21]. The initial interaction of Mo(C0)5 with silica has been monitored by infrared [22] and Raman [23] spectroscopies. The complex has been shown to be physisorbed in a similar environment to that adopted in a polar solvent and to have its symmetry lowered. At 44°C, in vacuo, this species is converted to a chemically bound carbonyl which itself is apparently decarbonylated. This is complete by 100 C, giving rise to an active propene metathesis catalyst [22]. Although no C-0 infrared absorptions are evident, only 3 CO groups per metal atom have been released [21], but the nature of the molybdentam in this yellow material is unclear. Some of the metal is EPR active, and this signal indicates a Mo site in an axially symmetric site, possibly square pyramidal [24]. The proportion of Mo increases to a maximum with an activation temperature of 200°C [25]. At that... 

The most thoroughly studied reactions are the metathesis of propene to ethene and 2-butene, and the metathesis of 2-pentene to 2-butene and 3-hexene. Generally, the thermodynamic equilibrium ratio of the trans and cis components of the products is obtained. The reacting alkene molecules need not be identical, two different alkenes react with each other in the same way. 

Recently, a few examples of the metathesis of alkenes carrying functional groups have been reported. According to a patent, acrylonitrile reacts with propene to crotononitrile (cis and trans) and ethene 10) ... 

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. 

A variant of the common Langmuir type models is obtained if it is assumed that both alkene molecules are chemisorbed (Langmuir adsorption) on the same active center. If it is further assumed that there are two different adsorption steps, the following set of reaction equations for the initial stages of the metathesis of propene is obtained ... 

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 ]. 

Note also that, in contrast to classical heterogeneous catalysts, the initiation step of [=SiORe(=CtBu)(=CHtBu)(CH2tBu)] is well defined and corresponds to the cross-metathesis of the alkene with the neopentyhdene ligand. In fact, in the metathesis of propene, 0.7 equiv of a 3 1 mixture of 3,3-dimethyl-l-butene and 4,4-dimethyl-2-pentene is formed (Figure 3.27) the nearly quantitative formation of cross-metathesis products is consistent with a real single-site catalyst. Moreover,... 

ILtSn or ILtPb causes a spectacular improvement of the catalytic performance of Re207/Al203, raising the rate of propene metathesis by 10-100 and also making possible the metathesis of functionalized alkenes where previously no reaction was observed41-43. When R = Me some methane is produced, suggesting that the reaction [Re](CH3)2 — [Re]=CH2 + CH4 is involved in the production of the initiating species. 

Vinylsilanes undergo productive cross-metathesis (CM) and silylative coupling (SC) with allyl-substituted (N, B)functionalized alkenes to yield l-silyl-3,Ar, -substituted propenes with preference (for V-derivatives) and exclusive formation (for boronates) of the f-isomer. 

Olefin metathesis is a catalytic reaction in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. A simple example is the metathesis of propene into ethylene and 2-butene (cis and trans) eq. (1). 

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. ... 

Isotopic labelling studies (see Ch. 5) have shown that with terminal olefins nonproductive metathesis is generally much faster than productive metathesis. Reaction (4) at one time foimd practical application in the petrochemical industry as a means of converting surplus propene into useful products. Reactions of the higher alkenes provide routes to sex pheremones of insects. 

The 2005 Nobel Prize in Chemistry was jointly awarded to Robert H. Grubbs (Caltech), Yves Chauvin (French Petroleum Institute), and Richard R. Schrock (MIT) for establishing olefin metathesis as a reaction of synthetic versatility and contributing to an understanding of the mechanism of this novel process. Olefin metathesis first surfaced in the late 1950s when industrial researchers found that alkenes underwent a novel reaction when passed over a heated bed of mixed metal oxides. Propene, for example, was converted to a mixture of ethylene and 2-butene (cis + trans). 

Among other applications olefin metathesis is useful in the synthesis of cyclic alkenes, the industrial preparation of propene, and in polymerization. 

For organic chemists, the term metathesis is used most often to mean alkene or olefin metathesis. This process, which can be catalysed by a range of transition metals, was discovered accidentally in the petrochemical industry. Its first commercial application was in the Phillips triolefin process in which propene was converted to an equilibrium mixture of ethene, 2-butene and the starting propene at 400 °C in the presence of an unknown tungsten species (Scheme 8.51). The process was in use between 1966 and 1972. Interestingly, with changes in feedstock prices and demands, the process is now run in reverse, producing propene from ethene and 2-butene. 

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