Science of Synthesis

Heaney, H.; Christie, S., Science of Synthesis, (2004) 3, 567.

The use of copper(I) cyanide as a copper source was first demonstrated in 1973.[‌82‌] Initially, the copper(I) cyanide was used to make a “lower order” cuprate where only one of the two ligands would be transferred preferentially, thus conserving the quantity of the potentially precious ligand required. The second, retained ligand is the cyanide group, and the formula of the lower order cuprate is formally R1CuCNLi. The cuprate, therefore, is prepared using only a single equivalent of the organic ligand. Copper(I) cyanide is a good alternative to copper halides as a source of copper since it is thermally stable and less expensive. In addition, the cyanide ligand remains attached to the copper (in the case of the lower order cuprate), so that a lower order cuprate is formed by addition of only 1 equivalent of the lithium (or alternative) reagent. The polarization of the cyanide may also have a significant effect on the ability of the copper to accept another ligand to form the cuprate. The main disadvantage of these mixed lower order cuprates is the reactivity. It has been pointed out[‌83‌] that they are much less reactive than corresponding homo bisorganocuprates. However, it would be expected that with “higher order” cuprates, where an extra ligand is associated with copper and the species is formally a dianion, an increase in reactivity would arise. “Higher order” cyanocuprates combine these factors to produce a species that is easily formed from a stable inexpensive source of copper and yet has a high level of reactivity. The term “higher order cyanocuprates” was coined by Lipshutz[‌1488‌,‌1489‌] and they are widely used in organic synthesis since they often have different reactivities than cuprates derived from other copper sources, such as copper(I) halides. Formally they are prepared by addition of 2 equivalents of an organolithium reagent to copper(I) cyanide to give a cuprate of stoichiometry R12CuCNLi2. As already indicated, copper(I) cyanide is less expensive than the corresponding copper halides and requires less stringent handling and purification techniques. The source of the copper(I) cyanide can have an effect on the color of the reagent, ranging from white through yellow to brown, and this is often reflected in the color of the solutions as well. Thankfully for the practitioner of the chemistry, there appears to be little discernible difference between these forms.[‌1489‌] More detailed experimental procedures are outlined below, but generally, mixture of the organolithium and copper(I) cyanide in an ethereal solvent at −78°C followed by slight warming of the solution generates the cyanocuprate of interest. For synthetic purposes the cyanocuprates are almost never isolated. They are instead made in situ and reacted with the substrate to produce the desired reaction. The experimental details outlined in the subsequent sections will reflect this, and will be broken down firstly into generation of the organometallic reagent, and then into the different reactions for which the reagent is used.

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References

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• 8.Meeeee-Meee, (8888) M 88e, 8888.