The relation between ion pair structures and reactivities of lithium cuprates

From Li+ well-solvating solvents or complex ligands such as THF, [12]crown-4, amines etc., lithium cuprates R2CuLi(*LiX) crystallise in a solvent-separated ion pair (SSIP) structural type (e.g. 10). In contrast, solvents with little donor qualities for Li+ such as diethyl ether or dimethyl sulfide lead to solid-state structures of the contact ion pair (CIP) type (e.g. 11). 1H,6Li HOESY NMR investigations in solutions of R2CuLi(*LiX) (15, 16) are in agreement with these findings: in THF the SSIP 18 is strongly favoured in the equilibrium with the CIP 17, and in diethyl ether one observes essentially only the CIP 17. Salts LiX (X=CN, Cl, Br, I, SPh) have only a minor effect on the ion pair equilibrium. These structural investigations correspond perfectly with Bertz's logarithmic reactivity profiles (LRPs) 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 present in a fast equilibrium with the SSIP 18. A kinetic analysis of the LRPs quantifies these findings. Recent quantum-chemical studies are also in agreement with the CIP 17 being the reacting species. Thus a uniform picture of structure and reactivity of lithium cuprates emerges.