When lithium dialkylcopper compounds, also known as Gilman reagents (after Henry Gilman), are converted by alkyl bromides, chlorides, or iodides, the cross-coupling products are obtained in good yields. Alkyl fluorides do not react with Gilman reagents. Gilman reagents can be synthesized through the treatment of the corresponding alkyllithium compound with copper(I) iodide in diethyl ether.
Synthesis of (CH3)2CuLi (a Gilman reagent).
Fig.1Synthesis of (CH3)2CuLi (a Gilman reagent).
In the reaction between a Gilman reagent and an alkyl halide, the alkyl halide's halide is exchanged for an alkyl group of the Gilman reagent. Thus, a new carbon-carbon bond is formed. The reaction through which "asymmetric" alkanes may be produced yields a product that is composed of the Gilman reagent's and alkyl halide's alkyl groups (cross-coupling). The reaction between (CH3)2CuLi and 1-iododecane in diethyl ether at 0 C, for instance, results in a 90% yield of undecane.
Alkyl cross-coupling with Gilman reagents.
Fig.2Alkyl cross-coupling with Gilman reagents.
The alkyl cross-coupling reaction with Gilman reagents may also be carried out with aryl, vinyl, allyl, and benzyl halides. However, in these cases, one equivalent of the Gilman reagent's alkyl groups disappears.
Examples of cross-couplings with aryl and vinyl halides.
Fig.3Examples of cross-couplings with aryl and vinyl halides.
The precise mechanism of the cross-coupling reaction is not known. However, it has been reasoned that radical intermediates appear during the reaction. |
