Well over a decade ago, two reviews were contributed to the Organic Reactions series covering substitution and conjugate addition reactions in organocopper chemistry. Their appearance, which highlighted most of the early work in this field, served not only as a source of invaluable references to original literature reports, but also stimulated a vast number of subsequent studies on the properties and uses of organocopper complexes.
The work cited in this chapter, which dates from ca. 1975, concerns in large measure uses of organocopper complexes originating from either catalytic or stoichiometric quantities of a copper(I) halide together with a Grignard (RMgX) or organolithium (RLi) reagent. These combinations form either neutral organocopper reagents RCu (1) or copper(I) monoanionic salts R2CuM (M = Li or MgX), commonly referred to as “lower-order” species. The latter ate complexes with lithium as gegenion (2) are also known as “Gilman reagents” in recognition of their origins. Copper(I) cyanide is also an excellent precursor, affording homogeneous mixtures of lower order cyanocuprates RCu(CN)Li, (3), upon treatment with an equivalent of an organolithium. The strength of the CuCN linkage presumably accounts for direct cuprate formation with 1 equivalent of the organolithium, rather than the metathesis that occurs with copper(I) chloride, bromide, or iodide.
While use of reagents (1), (2) and (3) alone can be aptly classified as broad-based and intense, their importance has further encouraged extensive development of variations on these themes (i.e., their composition and reactivity profiles). Reagents (1–3) have been found to be unexpectedly compatible with certain electrophilic additives at low temperatures, which substantially alter their reactivity. Rather than forming (2) from 2 equivalents of the same RLi, different organolithiums can be utilized to give RTRRCuLi, (4), conserving potentially valuable RLi. This scenario raises the question of controlling the selectivity of transfer of the desired ligand RT rather than the anticipated residual (or “dummy”) group RR from copper to electrophilic carbon. Fortunately, many solutions to this problem now exist.
Admixture of 2RLi (or RTLi + RRLi) with copper(I) cyanide proceeds beyond the stage of (3) to ultimately arrive at copper(I) dianionic complexes 5, the so-called “higher-order” cyanocuprates. Undoubtedly it is the cyano ligand, with its π-acidic nature, which enables copper to accept a third negatively charged ligand. Although reagents (5) do not yet share in all of the benefits offered by time in comparison with their lower-order counterparts, they nicely complement prior art. Moreover, as with species (1–4), they continue to evolve, providing the synthetic community with alternatives for highly selective and efficient means of making key carbon–carbon bonds.