Olefin metathesis

The alkene metathesis (including olefin metathesis, from Greek Meta: change and Thesis: position), is a chemical reaction, formally the alkylidene groups are exchanged between alkenes in, with statistically distributed product mixtures are formed. The alkene metathesis is an important reaction in synthetic organic chemistry and the petrochemical industry.

• 2.1 reaction conditions

• 3.1 Homogeneous catalysts 3.1.1 Co - Catalysts
• 3.1.2 activators

• 5.1 Application in the Synthesis
• 5.2 Industrial Applications 5.2.1 Phillips triolefin process
• 5.2.2 Shell Higher Olefin Process ( SHOP process )
• 5.2.3 Vestenamer process

History

The first observations

RL Banks and GC Bailey from the Phillips Petroleum Company discovered in 1964 that propene is converted to heterogeneous tungsten and molybdenum contacts to ethene and butene and called the reaction olefin disproportionation. Three years later, a homogeneous catalyst system consisting of tungsten hexachloride and ethyl -dimethyl- aluminum in ethanol / benzene from the Goodyear Tire & Rubber Company has developed and refers to the process as olefin metathesis.

Since the early 1970s, the alkene metathesis held as a field of research on industrial processes also entered the academic organic chemical research.

There new catalyst systems have been developed. However, have been reported only by a few successful olefin metathesis of functionalized substrates up to the 1980s. Since the catalysts are still based on the combination of strongly Lewis acidic transition metal halides and organometallic compounds, the tolerance was reversed very low compared with functional groups.

Was first described in detail the olefin metathesis in 1970 by the French chemist Yves Chauvin. His proposal for the mechanism of the reaction is valid up to today. In 1990, Richard R. Schrock published a defined and highly reactive in the alkene metathesis catalyst based on molybdenum. This is very sensitive to atmospheric oxygen and water and tolerates only a few unprotected functional groups, which significantly reduces its use as Synthesereagenz.

Two years later, Robert H. Grubbs has also published a catalyst defined on the basis of ruthenium. This catalyst and its subsequent developments also proved to be highly reactive in the metathesis reaction, however, were much more resistant to oxygen and water through.

All three chemists, Chauvin, Grubbs and Schrock received in 2005 for their discoveries and the great progress in the development of the metathesis the Nobel Prize for Chemistry.

In addition to the successes in research, the industry was looking for other applications for olefin metathesis. End of 1980 successes have been achieved with the synthesis of new polymers using the ADMET ( acyclic diene metathesis ).

Reaction mechanism

The general reaction equation in abstract form as follows:

The reaction and the composition of the resulting product mixture can be obtained by the choice of reaction conditions, such as the addition of ethylene in excess or by removal of a reaction component from the product mixture can be directed in a certain direction.

The starting point of the reaction mechanism forms a metal -carbene complex, which forms a metal - cyclobutane complex with another alkene and plays a central role in the catalytic cycle. Of the metal - cyclobutane complex elimination of the statistically distributed olefin and a metal -carbene complex is.

Reaction conditions

The reaction conditions are very mild in modern catalysts. During the heterogeneous WO3/SiO2-Kontakt still required temperatures of about 300 ° C, both the heterogeneous Re2O7/Al2O3-Kontakt and the Grubbs catalysts are active at room temperature.

Catalysts

The homogeneous systems used often consist of three components:

• A metal salt or an organometallic complex
• A co-catalyst
• An activator.

The catalytically particularly effective salts and complexes of molybdenum, rhenium and tungsten have been found. As a co- catalyst are used mostly metal- organic complexes of main-group elements, especially of tin and aluminum. The activators used in such systems oxygen-containing compounds such as alcohols or ethers have been applied.

Homogeneous catalysts

• WCl6
• CH3Re (CO) 5
• Grubbs catalysts

Co - catalysts

• Aluminum alkyls
• Aluminiumalkylchloride
• Tin alkyls

Activators

• Ethanol
• Diethyl ether
• Oxygen

Heterogeneous catalysts

• WO3 SiO2
• Re.sub.2O.sub.7 on Al2O3

Catalyst poisons

As a catalyst poisons occur mainly impurities in the olefin such as traces of water and hydrogen sulfide on. And olefins with conjugated double bonds can disable homogeneous catalysts effective. Heterogeneous catalysts may be blocked by polymeric reaction products.

Substrates

Almost all olefins having isolated double bonds may be used in the metathesis reaction. Olefins containing functional groups can be used, wherein any (hetero - ) may not be in conjugation with the double bond, the double bonds of the functional group.

The alkene metathesis of cycloalkenes usually leads to unsaturated polymers.

Application

Application in the synthesis of

We distinguish the following metathesis reactions:

• Ring-closing metathesis ( ring -closing metathesis, RCM)
• Enynes metathesis ( EM)
• Ring-opening metathesis ( ring opening metathesis, ROM)
• Ring-opening metathesis polymerization ( ring opening metathesis polymerization, ROMP)
• Acyclic diene metathesis ( Acyclic serve metathesis, ADMET )

In the ring- closing metathesis ω -terminal diolefins are α, implemented with the release of ethylene in large, difficult to access by other methods cyclic olefins. By ring -closing metathesis previously entirely unknown heterocycles were first synthesized.

One study described the enantioselective ROM by means of a Hoveyda -Grubbs catalyst:

In the enyne metathesis reaction of an alkene with an alkyne to a conjugated diene. The two groups intramolecularly as shown in the RCM reaction and provide cyclic olefins having an additional double bond in conjugation with the ring double bond. The reaction is called a ring-closing metathesis enynes ( RCEYM ).

In the ring-opening metathesis polymerization ( ROMP) of cyclic olefins react with ring opening to unsaturated polymers. As a substrate of this reaction are mainly cyclic olefins, which have a certain ring strain, as norbornene or cyclopentenes, as the driving force of the reaction is the reduction of ring strain.

The acyclic diene metathesis is used to terminal diolefins polymerize to polyenes. The reaction was 1991 KB Wagener found that 1,5-hexadiene successfully polymerized polybutadiene, 70 % trans double bonds and an average molecular weight of 28000.

Industrial Applications

In industry, the alkene metathesis is mainly used in the petrochemical and polymer chemistry.

Phillips triolefin process

When triolefin process propylene is converted to ethylene and butene. So far, however, only one plant was built after this process, but which is now closed.

Shell Higher Olefin Process ( SHOP process )

In the SHOP process, the olefin metathesis is used industrially. With the arising in the SHOP process α -olefins are mixtures which are separated by distillation. The higher molecular weight fraction is isomerized and the produced internal olefins are converted with ethylene by metathesis again α -olefins.

Vestenamer process

When Vestenamer process cyclooctene is polymerized by means of metathesis. The method is referred to as ring -opening metathesis polymerization (ROMP ). The resulting polyoctenamer is a semi-crystalline rubber which is used as a processing aid and a plasticizer for other rubbers.