The relation between ion pair structures and reactivities of lithium cuprates

From Li+ well-solvating solvents or complex ligands such as THE [12]crown-4 , amines etc., lithium cuprates R2CuLi((LiX)-Li-.) crystallise in a solvent -separated ion pair (SSTP) structural type (e.g. 10). In contrast, solvents with little donor qualities for Lit such as diethyl ether or dimethyl sulf ide lead to solid-state structures of the contact ion pair (CIP) type (e.g. II). H-1,Li-6 HOESY NMR investigations in solutions of R2CuLi((LiX)-Li-.) (15, 16) are in agreement with these findings: in THF the SSIP 18 is strong ly favoured in the equilibrium with the CTP 17 and in diethyl ether one obs erves essentially only the CLP 17 Salts LiX (X = CN, Cl, Br, I, SPh) have o nly a minor effect on the ion pair equilibrium. These structural investigat ions correspond perfectly with Bertz's logarithmic reactivity profiles (LRP s) of reactions of R2CuLi with enones in diethyl ether and THF, the faster reaction in diethyl ether is due to the predominance of the CIP 17 in this solvent, which is the reacting species: in THF only little CIP 17 is presen t in a fast equilibrium with the SSIP 18. A kinetic analysis of the LRPs qu antifies these findings. Recent quantum-chemical studies are also in agreem ent with the CIP 17 being the reacting species. Thus a uniform picture of s tructure and reactivity of lithium cuprates emerges.