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 chlorides, bromides, and iodides to replace the halide group with an R group. This is extremely useful in creating larger molecules from smaller ones.
These reagents were discovered by Henry Gilman. Lithium dimethylcopper (CH3)2CuLi can be prepared by adding copper(I) iodide to methyl lithium in tetrahydrofuran at -78°C. In the reaction depicted in scheme 1  the Gilman reagent is a methylating reagent reacting with an alkyne in an conjugate addition and the negative charge is trapped in a nucleophilic acyl substitution with the ester group forming a cyclic enone.
- Modern Organocopper Chemistry, N. Krause Ed. Wiley-VCH, 2002.