Science of Synthesis

Heaney, H.; Christie, S., Science of Synthesis, (2004) 3, 600.

Homocuprate reagents of the type R12CuM, where M is Li or MgX (X = halide), are probably the most common class of organocopper reagents. In comparison to reagents made from only 1 equivalent of organometallic reagent, R1Cu, these reagents are more nucleophilic and have enhanced stability. In addition, the homogeneity of the reagents often makes for more reproducible reactions when compared to those using catalytic quantities of copper. Gilman and co-workers first introduced these reagents during a study into the synthesis of organocopper reagents.[‌153‌] The bright yellow insoluble polymeric methylcopper was found to redissolve in diethyl ether when another equivalent of methyllithium was added to the reaction. The clear colorless solution so formed was attributed to the cuprate Me2CuLi. This family of reagents is now commonly referred to as Gilman cuprates. For the purposes of this section, a Gilman cuprate is that derived from the addition of 2 equivalents of an organometallic reagent to a copper(I) source, providing a copper reagent with the formal stoichiometry R12CuM, where M is the metal from the original organometallic. In the vast majority of cases, M is lithium, and the original organometallic is an organolithium. Grignard reagents are also of use (M = MgX, X = halogen); however, these are not as common as might be expected. Also, it is not wise to assume that the two counterions are interchangeable. Different results may be obtained when starting from a lithium source or a Grignard reagent. As usual, the best advice is to try both lithium and Grignard sources, as well as different types of copper. The copper source has traditionally been a copper(I) halide, and these have been discussed at length in the introduction and preceding pages of this chapter. For cuprates in particular, Bertz has suggested that the copper(I) bromide–dimethyl sulfide adduct is superior to other copper(I) halides, but that copper trifluoromethanesulfonate and copper isothiocyanate are respectable alternatives.[‌147‌] Nevertheless, the best advice is again to screen a number of different copper sources for each new reaction that is being undertaken.

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References

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• 8.Meeeee-Meee, (8888) 88/8, 888.
• 8.Meeeee-Meee, (8888) M 88e, 8888.