Metathesis (chemistry)

Metathesis (from Greek: μετάθεσις (metáthesis) "rearrangement, relocation") describes various reactions in chemistry in which groups or partial structures of reactants are exchanged. Metathesis reactions are also known as double conversions . A general reaction equation for metathesis is as follows:

${\displaystyle {\ce {AB + CD -> AD + BC}}}$

Inorganic chemistry

The driving force of these reactions can be the formation of more stable salts or the removal of a product from equilibrium .

In salt metathesis , the cations and anions of two salts mutually exchange. Most of these reactions take place in solutions . In a precipitation reaction, the driving force is the formation of a less soluble salt. Thus, silver chloride is virtually insoluble in aqueous solution. For this reason, when a chloride salt is added to an aqueous silver salt solution, it precipitates as a slightly yellowish sediment:

${\displaystyle {\ce {AgNO3(aq) + NaCl(aq) -> AgCl (v) + NaNO3(aq)}}}$

Organometallic chemistry

Many alkylation reactions in organometallic chemistry are metathesis reactions.

${\displaystyle {\ce {RM + M'X -> RM' + MX}}}$

If an organometallic compound is reacted with a metal halide, the driving force is usually the formation of a stable salt, especially when organometallic compounds of the first and second main groups are involved. Since real salts are formed here, the term salt metathesis is sometimes used for these reactions too .

${\displaystyle {\ce {2 Na(C5H5) + FeBr2 -> 2 NaBr + Fe(C5H5)2}}}$

${\displaystyle {\ce {3 LiCH3 + PCl3 -> 3 LiCl + P(CH3)3}}}$

${\displaystyle {\ce {4 CH3MgCl + SiCl4 ->4 MgCl2 + Si(CH3)4}}}$

In the synthesis of dimethyl zinc from zinc and methyl iodide , methyl zinc iodide is initially formed, which then disproportionates to dimethyl zinc and zinc iodide in a metathesis reaction . Here, too, the driving force is the formation of the stable salt.

${\displaystyle {\ce {2 ICH3 + 2 Zn -> 2 IZnCH3 -> Zn(CH3)2 + ZnI2}}}$

Redox reactions, such as the reduction of halosilanes to silanes, are also often metathesis reactions. The driving force here is the formation of more stable salts, and the gaseous monosilane formed is continuously removed from the reaction mixture.

${\displaystyle {\ce {LiAlH4 + SiCl4 -> LiCl + AlCl3 + SiH4 (^)}}}$

Further examples are comproportionation reactions of organotin compounds according to the Kocheshkov rearrangement :

${\displaystyle {\ce {SnR4 + SnX4 -> 2 SnX2R2}}}$

${\displaystyle {\ce {R= -CH3, -C2H5, -n-C3H7, -Phenyl; X=Cl, Br}}}$

Organic chemistry

In organic chemistry there are a number of metathesis reactions that usually take place with the help of organometallic catalysts . The reactions can proceed either heterogeneously or homogeneously .

Alkene metathesis

General scheme of alkene metathesis

→ Main article : alkene metathesis

In alkene metathesis, also known as olefin metathesis , the alkylidene groups of two alkenes are exchanged in the presence of an organometallic catalyst. As a rule, a static product mixture is created, the composition of which can be controlled by suitable conditions (catalyst, pressure, temperature, etc.).

${\displaystyle {\ce {2 A=B + 2 C=D -> A=C + B=D + A=D + B=C}}}$

Depending on the alkenes used and the way the reaction is carried out, one speaks of various types of alkene metathesis. If two different alkenes react with each other, one speaks of cross metathesis (CM):

${\displaystyle {\ce {R'-CH=CH-R' + R''-CH=CH-R'' <=> R'-HC=CH-R'' + R''-CH=CH-R'}}}$

${\displaystyle {\ce {R', R'' <> H}}}$

An example of the technical implementation of cross metathesis is the Shell Higher Olefin Process (SHOP).

The Phillips-Triolefin process in which propene is converted into ethene and 2-butene is an example of self -metathesis (SM):

${\displaystyle {\ce {2 H2C=CH-CH3 <=> H2C=CH2 + H3C-CH=CH-CH3}}}$

In ring-closing metathesis (RCM), a diene , usually with terminal alkene groups, forms a cycloalkenes with elimination of an alkene. In the reverse reaction, the ring-opening metathesis (ROM), a diene is formed from cycloalkene and alkene. A special case of ring-opening metathesis is the ring-opening metathesis polymerization (ROMP). One example is the Vestenamer process in which cyclooctene is polymerized to form a synthetic rubber .

Alkyne metathesis

General scheme of alkyne metathesis

→ Main article : alkyne metathesis

The first alkyne metathesis was performed by André Mortreux in 1974 . As with alkene metathesis, alkyne metathesis only takes place in the presence of suitable catalysts.

Enyne metathesis

General scheme of cross enyne metathesis

→ Main article : Enyne metathesis

A special case of alkyne metathesis is enyne metathesis, in which an alkene and an alkyne exchange partial structures in the presence of a catalyst. In contrast to alkene and alkyne metathesis, in enyne metathesis the two reactants do not form two products again, but a diene.

Wittig reaction

→ Main article : Wittig reaction

General scheme of the Wittig reaction

According to the definition, the Wittig reaction is also a metathesis reaction. In this, an alkene and a phosphine oxide are formed from an ylide and an aldehyde or ketone via a four-membered heterocycle .

literature

• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 .
• Christoph Elschenbroich: Organometallic chemistry . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 .

Individual evidence

1. ^ Entry on metathesis. In: Römpp Online . Georg Thieme Verlag, accessed on March 26, 2020.
2. ↑ ^{a b} Christoph Elschenbroich: Organometallchemie . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 , pp. 639 .
3. ↑ Christoph Elschenbroich: Organometallchemie . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 , pp. 640 .
4. ↑ Christoph Elschenbroich: Organometallchemie . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 , pp. 642 .
5. ↑ Christoph Elschenbroich: Organometallchemie . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 , pp. 645 .
6. ↑ Christoph Elschenbroich: Organometallchemie . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 , pp. 646 .
7. ↑ André Mortreux, Michel Blanchard: metathesis of alkynes by a molybdenum hexacarbonyl-resorcinol Catalyst. In: J. Chem. Soc., Chem. Commun. 1974, p. 786 , doi : 10.1039 / C39740000786 .
8. ↑ Christoph Elschenbroich: Organometallchemie . 4th edition. Teubner, Stuttgart-Leipzig-Wiesbaden 2003, ISBN 978-3-519-43501-3 , pp. 647 .