Gilman reagent

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General structure of a Gilman reagent

A Gilman reagent is a lithium and copper (diorganocopper) reagent compound, R2CuLi, where R is an organic radical. These are useful because they react with organic chlorides, bromides, and iodides to replace the halide group with an R group. This is extremely useful in creating larger molecules from smaller ones.[1]

Generalized chemical reaction showing Gilman reagent reacting with organic halide to form products and showing Cu(III) reaction intermediate

These reagents were discovered by Henry Gilman.[2] Lithium dimethylcopper (CH3)2CuLi can be prepared by adding copper(I) iodide to methyllithium in tetrahydrofuran at −78 °C. In the reaction depicted below,[3] the Gilman reagent is a methylating reagent reacting with an alkyne in a conjugate addition, and the negative charge is trapped in a nucleophilic acyl substitution with the ester group forming a cyclic enone.

Scheme 1. Example Gilman reagent reaction

Gilman reagents have complicated structures in crystalline form and in solution. Lithium dimethylcuprate is a dimer in diethyl ether forming an 8-membered ring with two lithium atoms coordinating between two methyl groups. Similarly, lithium diphenylcuprate forms a dimeric etherate, [{Li(OEt2)}(CuPh2)]2, in the solid state.[4]

Lithium diphenylcuprate etherate dimer from crystal structure - 3D stick model Skeletal formula of lithium diphenylcuprate etherate dimer

If the Li+ ions are rendered inert by complexation with the crown ether 12-crown-4, the isolated diorganylcuprate anions that remain adopt a linear coordination geometry at copper.[5]

Dimethylcuprate anion from crystal structure Diphenylcuprate anion from crystal structure

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References[edit]

  1. ^ J. F. Normant (1972). "Organocopper(I) Compounds and Organocuprates in Synthesis". Synthesis 1972 (02): 63–80. doi:10.1055/s-1972-21833. 
  2. ^ Henry Gilman, Reuben G. Jones, and L. A. Woods (1952). "The Preparation of Methylcopper and some Observations on the Decomposition of Organocopper Compounds". Journal of Organic Chemistry 17 (12): 1630–1634. doi:10.1021/jo50012a009. 
  3. ^ Modern Organocopper Chemistry, N. Krause Ed. Wiley-VCH, 2002.
  4. ^ N. P. Lorenzen, E. Weiss (1990). "Synthesis and Structure of a Dimeric Lithium Diphenylcuprate:[{Li(OEt)2}(CuPh2)]2". Angew. Chem. Int. Ed. 29 (3): 300–302. doi:10.1002/anie.199003001. 
  5. ^ H. Hope, M. M. Olmstead, P. P. Power, J. Sandell, X. Xu (1985). "Isolation and x-ray crystal structures of the mononuclear cuprates [CuMe2]-, [CuPh2]-, and [Cu(Br)CH(SiMe3)2]-". J. Am. Chem. Soc. 107 (14): 4337–4338. doi:10.1021/ja00300a047.