Alkene metathesis

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General scheme of alkene metathesis; Alkylidene groups are shown in color

The alkene metathesis (also olefin metathesis ; from the Greek meta: change and thesis: position) is a chemical reaction in which the alkylidene groups (colored in the reaction scheme) are exchanged between alkenes , resulting in statistically distributed product mixtures. Alkene metathesis is an important reaction in synthetic organic chemistry and petrochemistry .

history

First observations

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

Since the early 1970s, alkene metathesis has found its way into academic organic chemical research as a research area that goes beyond industrial processes.

New catalyst systems have been developed. However, only a few successful olefin metatheses on functionalized substrates were reported until the 1980s. Since the catalysts were still based on the combination of strongly Lewis acidic transition metal halides and organometallic compounds, the tolerance towards functional groups was conversely very low.

Systematic investigations by Chauvin, Schrock and Grubbs

Olefin metathesis was first described in detail in 1970 by the French chemist Yves Chauvin (1930–2015). His suggestion on the mechanism of the reaction is still valid today. In 1990 Richard R. Schrock published a well-defined and highly reactive catalyst in alkene metathesis based on molybdenum . This is very sensitive to air, oxygen and water and only tolerates a few unprotected functional groups, which significantly reduces its use as a synthesis reagent.

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

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

In addition to the successes in research, the industry was looking for other uses for olefin metathesis. At the end of 1980 successes were achieved with the synthesis of new polymers with the help of ADMET (Acyclic Diene Metathesis).

Reaction mechanism

The general reaction equation in abstract form is:

The reaction and the composition of the resulting product mixture can be steered in a certain direction through the choice of the reaction conditions, for example the addition of excess ethylene or by removing a reaction component from the product mixture.

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

Catalytic cycles of the metathesis reaction

Reaction conditions

The reaction conditions are very mild with modern catalysts. While the heterogeneous WO 3 / SiO 2 contact still required temperatures of over 300 ° C., both the heterogeneous Re 2 O 7 / Al 2 O 3 contact and the Grubbs catalysts are already 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.

Salts and complexes of molybdenum, rhenium and tungsten have proven to be particularly effective catalytically. Organometallic complexes of main group elements, especially tin and aluminum, are usually used as cocatalysts. Oxygen-containing compounds such as alcohols or ethers have been used as activators in such systems.

Homogeneous catalysts

Co-catalysts

  • Aluminum alkyls
  • Aluminum alkyl chlorides
  • Tin alkyls

Activators

  • Ethanol
  • Diethyl ether
  • oxygen

Heterogeneous catalysts

  • WO 3 on SiO 2
  • Re 2 O 7 on Al 2 O 3

Catalyst poisons

The main catalyst poisons are olefin impurities such as traces of water and hydrogen sulfide. Even olefins with conjugated double bonds can effectively deactivate homogeneous catalysts. Heterogeneous catalysts can be blocked by polymeric reaction products.

Substrates

Almost all olefins with isolated double bonds can be used in metathesis. Olefins with functional groups can be used, and any (hetero) double bonds of the functional group that may be present may not be in conjugation to the double bond.

The alkene metathesis of cycloalkenes usually leads to unsaturated polymers.

application

Application in synthesis

A distinction is made between the following metathesis reactions:

  • Alkene cross metathesis
  • Ring-closing metathesis (RCM)
  • Enin metathesis (EM)
  • Ring-opening metathesis ( ring-opening metathesis , ROM)
  • Ring-opening metathesis polymerization ( ring opening metathesis polymerization , ROMP)
  • Acyclic diene metathesis ( acyclic diene metathesis , ADMET)

In the ring-closing metathesis, α, ω-terminal diolefins are converted into large cyclic olefins, which are difficult to access with other methods, with the release of ethylene. By ring-closing metathesis, previously completely unknown heterocycles could be synthesized for the first time.

One study described enantioselective ROM using a Hoveyda-Grubbs catalyst :

Ring-opening metathesis

In ring-opening metathetic polymerization (ROMP), cycloolefins react with ring opening to form unsaturated polymers. The substrate for this reaction is primarily cycloolefins which have a certain ring strain, such as norbornenes or cyclopentenes, since the driving force of the reaction is the reduction in ring strain.

Acyclic diene metathesis is used to polymerize terminal diolefins to polyenes. The reaction was found in 1991 by KB Wagener, who successfully polymerized 1,5-hexadiene to polybutadiene with 70% trans double bonds and an average molecular weight of 28,000.

Industrial applications

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

Phillips-Triolefin process

In the triolefin process, propylene is converted into ethylene and butene. So far, however, only one system has been built using this process, but it has now been shut down.

Shell higher olefin process (SHOP process)

In the SHOP process , olefin metathesis is used on an industrial scale. The α-olefins formed in the SHOP process are mixtures that are separated by distillation. The higher molecular weight fraction is isomerized and the resulting internal olefins are converted back into α-olefins with ethylene by metathesis.

Vestenamer process

In the Vestenamer process, cyclooctene is polymerized by means of metathesis. The process is known as ring-opening metathesis polymerization (ROMP). The resulting polyoctenamer is a semi-crystalline rubber that is used as a processing aid and plasticizer for other rubbers.

Web links

Commons : Olefin metathesis catalysts  - Collection of pictures, videos and audio files

Individual evidence

  1. ^ RL Banks, GC Bailey: Olefin Disproportionation. A New Catalytic Process In: Ind. Eng. Chem. Prod. Res. Dev. 3 (3), 1964, pp. 170-173, doi: 10.1021 / i360011a002 .
  2. ↑ N. Calderon, HY Chen, KW Scott: Olefin metathesis - A novel reaction for skeletal transformations of unsaturated hydrocarbons. In: Tetrahedron Letters . 34, 1967, pp. 3327-3329.
  3. ^ PB van Dam, MC Mittelmeijer, C. Boelhouver: Metathesis of unsaturated fatty acid esters by a homogeneous tungsten hexachloride-tetramethyltin catalyst. In: Chem. Commun. 1972, pp. 1221-1222, doi: 10.1039 / C39720001221 .
  4. ↑ E. Verkuijen, C. Boelhouver: Formation of cyclohexa-1,4-diene by metathesis of linoleic and linolenic acid ester. In: Chem. Commun. 1974, pp. 793-794, doi: 10.1039 / C39740000793 .
  5. ↑ R. Nakamura, S. Fukahara, S. Matsumoto, K. Komatsu: disproportionation of the Unsaturated ester. In: Chem. Let. 1976, pp. 253-256.
  6. ↑ HG Alt, FPD Sanzo, MD Rausch, PC Uden: Automated thermal degradation studies on solid σ-organotransition metal complexes: Dimethyl-titanocene, -zirconocene and -hafnocene. In: Organomet. Chem. 107, 1976, p. 257.
  7. ↑ D. Villemin: Synthesis de macrolides par methathese. In: Tetrahedron Letters . 21, 1980, pp. 1715-1718.
  8. ↑ J. Tsuji, S. Hashiguchi: Application of olefin metathesis to organic synthesis. Syntheses of civetone and macrolides. In: Tetrahedron Letters . 21, 1980, pp. 2955-2958.
  9. ^ RR Schrock: Living ring-opening metathesis polymerization catalyzed by well-characterized transition-metal alkylidene complexes. In: Acc. Chem. Res. 23, 1990, pp. 158-165, doi: 10.1021 / ar00173a007 .
  10. ↑ RR Schrock, JS Murdzek, GC Bazan, J. Robbins, M. Di Mare, M. O'Regan: Synthesis of molybdenum imido alkylidene complexes and some reactions Involving acyclic olefin. In: J. Am. Chem. Soc. 112, 1990, pp. 3875-3886, doi: 10.1021 / ja00166a023 .
  11. ↑ RR Schrock, GC Bazan, E. Khosravi, WJ Feast: Living ring-opening metathesis polymerization of 2,3-difunctionalized norbornadienes by Mo (: CHBu- tert ) (NC 6 H 3 Pr-iso 2 -2.6) (OBU tert ) 2 . In: J. Am. Chem. Soc. 112, 1990, pp. 8378-8387, doi: 10.1021 / ja00179a023 .
  12. ↑ RR Schrock, GC Bazan, JH Oskam: Living ring-opening metathesis polymerization of 2,3-difunctionalized 7-oxanorbornenes and 7-oxanorbornadienes by Mo (CHCMe 2 R) (NC 6 H 3 - iso -Pr2-2,6) ( O-tert -Bu) 2 and Mo (CHCMe 2 R) (NC 6 H 3 - iso -Pr2-2,6) (OCMe 2 CF 3 ) 2 . In: J. Am. Chem. Soc. 113, 1991, pp. 6899-6907, doi: 10.1021 / ja00018a028 .
  13. ↑ M. Lindmar-Hamberg, KB Wagener: acyclic metathesis polymerization: the olefin metathesis reaction of 1,5-hexadiene and 1,9-decadienes. In: Macromolecules . 20, 1987, pp. 2949-2951, doi: 10.1021 / ma00177a053 .
  14. ↑ Katharina Johannes, Martin Watzke, Jürgen Martens: Synthesis of α, β-Unsaturated Caprolactams Starting from Heterocyclic Imines. In: J. Heterocyclic Chem. 47, 2010, pp. 697-702.
  15. ↑ Martin Watzke, Knut Schulz, Katharina Johannes, Pasqual Ulrich, Jürgen Martens: First Synthesis of Bi- and Tricyclic α, β, -Unsaturated δ-Oxacaprolactams from Cyclic Imines via Ring-Closing Metathesis. In: Eur. J. Org. Chem. 2008, pp. 3859-3867.
  16. ^ Almuth Schwäblein, Jürgen Martens: First Synthesis of α, β-Unsaturated Lactones with High Diversity through the Passerini Reaction and Ring-Closing Metathesis (RCM). In: Eur. J. Org. Chem. 2011, pp. 4335-4344.
  17. ^ JJ van Veldhuizen, SB Garber, JS Kingsbury, Amir H. Hoveyda : A Recyclable Chiral Ru Catalyst for Enantioselective Olefin Metathesis. Efficient Catalytic Asymmetric Ring-Opening / Cross Metathesis in Air. In: J. Am. Chem. Soc. 124, 2002, pp. 4954-4955, doi: 10.1021 / ja020259c .
  18. ↑ Product description of Vestenameren .

See also