Organic copper compounds

Organo-copper compounds or organo- copper compounds are organometallic compounds of the chemical element copper . They are most frequently used in organic chemistry for syntheses . Due to their high reactivity, they are usually not bought as a ready-to-use reagent , but either prepared directly before the desired reaction and used immediately or they are created during this ( in situ ).

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Copper forms with organic residues , i.e. H. Molecular parts that mainly consist of hydrocarbons (abbreviated here with R), a series of compound types that are differentiated according to their stoichiometric composition and other metals that are involved in addition to copper . A large number of compounds with different organic radicals such as methyl or ethyl radicals belong to each of these types of compounds . The more complex organic copper compounds with more than one organic residue are also known as organic cuprates (from Latin cuprum = copper, at: anion ending, cf. sulfate ) due to their R ₂ Cu ^- anions .

The most important classes of compounds are:

• Gilman cuprates (after Henry Gilman ) with the general formula R ₂ CuLi
• Normant cuprates (according to J. F. Normant) with the general formula R ₂ CuMgX
• Knochel cuprates (according to P. Knochel ) with the general formula RCu (CN) ZnX
• Monoalkyl copper compounds with the general formula RCu
• Cyano cuprates with the general formula R ₂ CuLi · LiCN

In addition to the “normal” cuprates, which are also referred to as lower-order cuprates , there are also higher-order cuprates with the general formula Me _n Cu _m R _{n + m} (Me = metal).

Examples of all important compounds with n - butyl as an organic part of the molecule

history

As the first copper-carbon compound, Rudolf Christian Böttger produced the copper acetylide Cu ₂ C ₂ , an explosive compound of copper and ethyne , in 1859 . The actual research began in 1936 with Henry Gilman, who first researched the reaction of monoalkyl copper compounds with haloalkanes . In 1941, Kharash recognized that Grignard compounds with 2-cyclohexen-1-one in the presence of copper do not salt to the 1,2-product (as would have been normal in reactions of Grignard compounds with ketones ), but to the 1,4- Product react. In 1952 Gilman was able to synthesize the first organic cuprate with (CH ₃ ) ₂ CuLi.

Manufacturing

Organic copper compounds are produced by transmetallation, ie by exchanging the metal in the molecule, from other organometallic compounds and copper halides , usually copper (I) iodide or copper (I) bromide . Also pseudohalides , such as copper (I) cyanide are possible as copper donor for the synthesis of these compounds.

${\displaystyle \mathrm {CuI+2\ BuLi\longrightarrow Bu_{2}CuLi+\ LiI} }$

Reaction of copper (I) iodide and butyllithium (Bu = butyl radical)

${\displaystyle \mathrm {CuBr+2\ BuMgBr\longrightarrow Bu_{2}CuMgBr+MgBr_{2}} }$

Reaction of copper salt and Grignard compound (Bu = butyl radical)

Different precursor reagents are required for the individual reagent classes. The Gilman cuprates are organic lithium compounds such as butyllithium or methyllithium . In the case of the normant cuprates, Grignard compounds , i.e. organic compounds of magnesium , are used as starting substances, in the case of the ankle cuprates, organic zinc compounds. The correct stoichiometry must be observed: with a ratio of 1: 1 monoalkyl copper compounds are formed, with 2: 1 from organic compounds to copper salt cuprates and with even larger excesses of organic reagents cuprates of a higher order.

It is also possible to use two organometallic compounds which differ in their organic radicals. One of these two is usually an unreactive compound, often thiophene . Since only one organic residue reacts in a reaction of a cuprate, this has the advantage of saving reagents that would otherwise be lost during the reaction. This is especially important with more complicated and therefore more difficult to manufacture molecules.

${\displaystyle \mathrm {CuI+\ BuLi\longrightarrow BuCu+LiI} }$

${\displaystyle \mathrm {BuCu+\ Thio{-}Li\longrightarrow (Bu\ Thio\ CuLi)} }$

Production of organic copper compounds with different residues (Bu = butyl residue, thio = thiophene )

properties

So far only organic copper compounds are known in which copper has the oxidation state + I, in intermediate stages also + III. Organic copper (II) compounds disintegrate immediately, breaking the copper-carbon bond. Since the copper- oxygen bond is much stronger than the copper- carbon bond, most organic copper compounds are very sensitive to oxygen and water. Reactions with them must therefore be carried out in inert organic solvents such as diethyl ether or 1,4-dioxane and under exclusion of air (e.g. argon ).

Monoalkyl copper compounds

Monoalkyl copper compounds have the general formula RCu; R is an organic residue . The remainder can be an alkyl group , an aromatic , an alkene or an alkyne . More complicated or larger residues are also possible. However, no nucleophilic functional groups , such as ester or alcohol groups , may be present in the molecule, since these react both with other organic copper compounds and with the reagents required for this (e.g. Grignard compounds ) during manufacture .

Like other organometallic compounds, organic copper compounds are nucleophilic; that is, they transfer their organic residue to electrophilic sites in other molecules during the reaction . This creates a carbon-carbon single bond .

Monoalkyl copper compounds are often explosive and readily decompose. They are therefore difficult to handle and are rarely used or only used in catalytic quantities. This applies above all to the compounds whose organic residue is small. For example, methyl copper (CH ₃ Cu) already decomposes from −15 ° C, while phenyl copper (C ₆ H ₅ Cu) is still stable at 100 ° C in an inert atmosphere.

The structure of many organic copper compounds is tetrameric , they are each made up of four bridged CuR units. This results in a square structure, with organic molecular parts on the corners and copper on the side edges. An exception is methyl copper, this compound is polymer composed of long Cu-Me-Cu chains.

Cuprates

Cuprates differ significantly in their properties from the monoalkyl copper compounds. They are much more stable and nucleophilic than these. That is why they are used much more frequently and also in stoichiometric amounts in reactions. The individual types of cuprates hardly differ in their properties and reactions and can therefore usually be used alternatively. An exception are the ankle cuprates. These are significantly less reactive than the others and must be activated by adding Lewis acids such as boron trifluoride before the reaction . This makes it possible to carry out reactions with molecules that are normally not accessible. These are mainly those in which certain functional groups, such as an ester group , are present.

In solutions , Gilman cuprates exist in an equilibrium of dimer composed of two R ₂ CuLi and monomers , depending on the solvent .

Reactions

Cuprates play a role in some reactions in organic chemistry . They are used both as a catalyst and stoichiometrically. Cuprates are usually not used directly, but are only formed from organometallic compounds and copper halides during the reaction. The most important reactions are the 1,4- addition to α, β-unsaturated ketones , carboxylic acids or esters , nucleophilic substitutions and coupling reactions .

1,4 addition

The 1,4-addition (also called Michael-addition ) to α, β-unsaturated ketones, carboxylic acids or esters is the most important reaction of organic copper compounds. α, β-Unsaturated ketones are compounds which have a double bond on the carbon atom which is adjacent to the carbonyl group . The numbers 1 and 4 refer to a count in which the oxygen atom of the carbonyl group is given the number 1 and the following carbon atoms are numbered sequentially. In the 1,4-addition a CC bond is thus formed on the double bond, which becomes a single bond .

1,4 Addition of 2-cyclohexen-1-one and a Gilman cuprate

Nucleophilic substitution

In addition to 1,4-addition, nucleophilic substitution is another important reaction of cuprates. In this reaction, the cuprate transfers an organic residue to a carbon atom that carries a suitable leaving group and forms a CC bond. The leaving group, for example an iodide or bromide ion, is split off. The mechanism proceeds according to the so-called S _N 2 mechanism , ie the attack of the nucleophilic carbon and the splitting off of the leaving group take place simultaneously.

${\displaystyle \mathrm {R_{2}CuLi+Me{-}I\longrightarrow R{-}Me+LiI+RCu} }$

Nucleophilic substitution of methyl iodide and formation of a CC bond

Couplings

Copper catalyzes some coupling reactions of organic molecules. In the process, organic copper compounds are formed in intermediate stages . A distinction is made between some couplings with different reagents and reaction conditions. An important coupling is the Sonogashira coupling , in which a terminal alkyne (i.e. an alkyne with a hydrogen atom on one side ) reacts with the copper salt to form a copper-alkyne compound. This is then palladium - catalysis with a haloalkane coupled.

Sonogashira coupling

In addition to this coupling, which requires a palladium catalyst in addition to copper, there are also some palladium-free coupling reactions. These are above all the Ullmann and Glaser couplings . In the Ullmann coupling, symmetrical biaryls are synthesized at elevated temperature with the help of copper (I) iodide . In the Glaser coupling, terminal alkynes are linked with one another with the help of copper halides and oxygen .

Ullmann coupling

Individual evidence

1. ↑ ^{a b} Chr. Elschenbroich: Organometallchemie , 5th edition, 2005, p. 234.
2. ^ H. Heaney, S. Christie: Product class 4: Organometallic complexes of copper. In: Josef Houben, Theodor Weyl, IA O'Neil: Science of Synthesis Volume 3, Thieme Verlag, 2004, p. 529.
3. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1452.
4. ↑ Reinhard Brückner: reaction mechanisms: organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-1579-9 , pp. 715 .
5. ↑ Reinhard Brückner : reaction mechanisms: organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-1579-9 , pp. 689 .
6. ↑ Christoph Elschenbroich: Organometallchemie . 5th edition. Teubner BG GmbH, Wiesbaden 2005, ISBN 3-519-53501-7 , p. 241 .

literature

• H.Heaney, S.Chistie: Product class 4: organometallic complexes of copper , Science of Synthesis, 2004 (pp. 305–662) (engl.)
• Christoph Elschenbroich: Organometallic chemistry . 5th edition. Teubner BG GmbH, Wiesbaden 2005, ISBN 3-519-53501-7 , p. 234-245 .
• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , pp. 1451-1452.