A Gilman reagent is a lithium

Lithium

Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

 and copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 (diorganocopper

Organocopper compound

Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions...

) reagent

Reagent

A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...

 compound, R₂CuLi, where R is an organic radical. These are useful because they react with organic chloride

Chloride

The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

s, bromide

Bromide

A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...

s, and iodide

Iodide

An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...

s 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

Henry Gilman

Henry Gilman was an American organic chemist known as the father of organometallic chemistry, the field within which his most notable work was done. He discovered the Gilman reagent, which bears his name....

. Lithium dimethylcopper (CH₃)₂CuLi can be prepared by adding copper(I) iodide

Copper(I) iodide

Copper iodide is the inorganic compound with the formula CuI. It is also known as cuprous iodide. It is useful in a variety of applications ranging from organic synthesis to cloud seeding....

 to methyllithium in tetrahydrofuran

Tetrahydrofuran

Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...

 at −78 °C.