Organocopper chemistry is the study of the physical properties, reactions, and synthesis of organocopper compounds, which are organometallic compounds containing a carbon to copper chemical bond. They are reagents in organic chemistry. The first organocopper compound, the explosive copper(I) acetylide Cu2C2 (Cu+[−C≡C−]Cu+), was synthesized by Rudolf Christian Böttger in 1859 by passing acetylene gas through a solution of copper(I) chloride in ammonia:
C2H2 + 2 [Cu(NH3)2]Cl → Cu2C2 + 2 NH4Cl + 2 NH3
Structure and bonding Organocopper compounds are diverse in structure and reactivity, but almost all are based on copper with an oxidation state of +1, sometimes denoted Cu(I) or Cu+. With 10 electrons in its valence shell, the bonding behavior of Cu(I) is similar to Ni(0), but owing to its higher oxidation state, it engages in less pi-backbonding. Organic derivatives of copper's higher oxidation states of +2 and +3 are sometimes encountered as reaction intermediates, but rarely isolated or even observed. Organocopper compounds form complexes with a variety of soft ligands such as alkylphosphines (R3P), thioethers (R2S), and cyanide (CN−). Due to the spherical electronic shell of Cu+, copper(I) complexes have symmetrical structures - either linear, trigonal planar or tetrahedral, depending on the number of ligands.
Simple complexes with CO, alkene, and Cp ligands Copper(I) salts have long been known to bind CO, albeit weakly. A representative complex is the polymeric carbonylcopper(I) chloride CuCl(CO). Unlike classical metal carbonyls, these compounds exhibit little π backbonding.
Alkenes bind to copper(I), although again generally weakly. Dewar-Chatt-Duncanson backbonding between Cu+ and alkenes is a consequence of electron correlation, and so invisible to Hartree-Fock simulation. The binding of ethylene to Cu in proteins has a broad significance in plant biology so much so that ethylene is classified as a plant hormone. Its presence, detected by the Cu-protein, affects fruit ripening and many other developments. Copper forms no metallocene, and adds cyclopentadiene only in the presence of "soft" Lewis bases to give half-sandwich complexes. One such derivative is π-cyclopentadienyl(triethylphosphine)copper(I), [C5H5]−[Cu(P(CH2CH3)3)]+.
Alkyl- and arylcopper compounds
Copper(I) Copper halides react with organolithium reagents to give organocopper compounds. The area was pioneered by Henry Gilman, who reported methylcopper in 1936. Thus, phenylcopper is prepared by reaction of phenyllithium with copper(I) bromide in diethyl ether. Grignard reagents can be used in place of organolithium compounds. Gilman also investigated the dialkylcuprates. These are obtained by combining two equivalent of RLi with Cu(I) salts. Alternatively, these cuprates are prepared from oligomeric neutral organocopper compounds by treatment with one equivalent of organolithium reagent. Compounds of the type [CuRn](n−1)− are reactive towards oxygen and water, forming copper(I) oxide. They also tend to be thermally unstable, which can be useful in certain coupling reactions. Despite or because of these difficulties, organocopper reagents are frequently generated and consumed in situ with no attempt to isolate them. They are used in organic synthesis as alkylating reagents because they exhibit greater functional group tolerance than corresponding Grignard and organolithium reagents. The electronegativity of copper is much higher than its next-door neighbor in the group 12 elements, zinc, suggesting diminished nucleophilicity for its carbon ligands. Copper salts react with terminal alkynes to form the acetylides. Alkyl halides react with organocopper compounds with inversion of configuration. On the other hand, reactions of organocopper compound with alkenyl halides proceed with retention of subtrate's configuration. Organocopper compounds couple with aryl halides (see Ullmann condensation and Ullmann reaction):
ArX + (Ar')2CuLi ⇌ ArAr'CuLi + Ar'X 2 ArAr'CuLi ⇌ (Ar)2CuLi + (Ar')2CuLi ArAr'CuLi + O2 → Ar−Ar'
Aggregation Alkyl and aryl copper complexes aggregate both in crystalline form and in solution. Aggregation is especially evident for charge-neutral organocopper compounds, i.e. species with the empirical formula (RCu), which adopt cyclic structures. Since each copper center requires at least two ligands, the organic group is a bridging ligand. This effect is illustrated by the structure of mesitylcopper, which is a pentamer. A cyclic structure is also seen for CuCH2SiMe3, where Me stands for methyl group CH3, the first 1:1 organocopper compound to be analyzed by X-ray crystallography (1972 by Lappert). This compound is relatively stable because the bulky trimethylsilyl groups provide steric protection. It is a tetramer, forming an 8-membered ring with alternating Cu-C bonds. In addition the four copper atoms form a planar Cu4 ring based on three-center two-electron bonds. The copper to copper bond length is 242 pm compared to 256 pm in bulk copper. In pentamesitylpentacopper a 5-membered copper ring is formed, similar to (2,4,6-trimethylphenyl)gold, and pentafluorophenylcopper is a tetramer.
Lithium dimethylcuprate(I) is a dimer in diethyl ether, forming an 8-membered ring with two lithium atoms linking two methyl groups, (Li+[Cu(CH3)2]−)2. Similarly, lithium diphenylcuprate(I) forms a dimeric etherate, ([Li(O(CH2CH3)2)]+[CuPh2]−)2, in the solid state.
Copper(III) The involvement of the otherwise rare Cu(III) oxidation state has been demonstrated in the conjugate addition of the Gilman reagent to an enone: In a so-called rapid-injection NMR experiment at −100 °C, the Gilman reagent Li+[Cu(CH3)2]− (stabilized by lithium iodide) was introduced to cyclohexenone (1) enabling the detection of the copper-alkene pi complex 2. On subsequent addition of trimethylsilyl cyanide the Cu(III) species 3 is formed (indefinitely stable at that temperature) and on increasing the temperature to −80 °C the conjugate addition product 4. According to an accompanying in silico experiments the Cu(III) intermediate has a square planar molecular geometry with the cyano group in cis orientation with respect to the cyclohexenyl methine group and anti-parallel to the methine proton. With other ligands than the cyano group this study predicts room temperature stable Cu(III) compounds.
Reactions of organocuprates
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