The Relation between Ion Pair Structures and Reactivities of Lithium Cuprates

Abstract

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.

Theory meets experiment: Gas-phase chemistry of coinage metals

“…Despite the huge success in the application of organocuprates as reagents in chemical synthesis, the precise nature of the reactive species at a molecular level represents still a hot topic [146][147][148][149][150][151].…”

Influence of Copper Salts, Solvents, and Ligands on the Structures of Precatalytic Phosphoramidite Copper Complexes for Conjugate Addition Reactions

“…[21] This new structural motif of the ground state of catalytically active copper complexes offers an explanation for the results of earlier synthetic optimization procedures, for example, the strong ligand acceleration and the minimum ligand-tocopper salt ratio of 1.5:1. However, the supramolecular structures of organocuprate complexes, [23][24][25][26] as well as the ee values and yields of copper-catalyzed conjugate addition reactions, [4] are both known to be highly salt-and solvent-dependent. Moreover, the size of the amine moiety of a phosphoramidite ligand seems to be decisive for its enantioselectivity.…”

Structure of (Me2Cu)Li · 3DME and its oligomers [(Me2Cu)Li · 3DME] n (n = 2–5): a theoretical study

“…In the crystal struc ture of (Me 2 Cu)Li•3DME retrieved from the Camb ridge Structural Database (CSD), 40 the difference be tween C-Cu bond lengths is even larger (0.009 Å). Different С-Cu bond lengths in the same di organocuprate anion were also reported for {(Ph 2 Cu) -[Li(12 crown 4) 2 ] + } 71 (Δd(C-Cu) = 0.009 Å) and [{[(Me 3 Si)CH 2 ] 2 Cu} -(Li•3DME) + ] 9 (Δd(C-Cu) = 0.003 Å); the geometric parameters of these compounds were also retrieved from the CSD. 40 These differences revealed by X ray analysis can be con sidered as measurements errors that fall within the limits of standard deviations (~0.010 Å).…”

“…2) shows that the C-Li distances in the gas phase are about 0.5 Å shorter than the shortest dis tance C(21)-Li(3) in the crystal (5.806 Å). 9 Even larger are the differences between the calculated Cu-Li distance in 4 (see Table 2) and the Cu-Li distances determined by X ray analysis 9 (6.034 Å, Cu(2)-Li(3); 6.106 Å, Cu(2)-Li(4); 5.936 Å, Cu(2)-Li(5) and 6.584 Å, Cu(2)-Li (6)). The bond angle ϕ(C(1)-Cu-C(2)) in monomer 4 and in the crys tal are also noticeably different (173.3 vs. 179.2°, respec tively).…”

“…These differences can be due to the fact that the PBE method overestimates the interaction of (Me 2 Cu) -with (Li•3DME) + in the gas phase and underestimates the interaction between the copper atom and carbon at oms in (Me 2 Cu) -; this is the reason why the internuclear distances C-Cu in 4 (1.957 and 1.958 Å) are larger than in the crystal (1.929 and 1.938 Å). 9 21 3 phase (see Table 2) and (Me 2 Cu)Li•3DME in the crys tal 9 is that in the latter case each (Me 2 Cu)Li•3DME molecule is surrounded by other same type molecules (see Fig. 2, a) and each (Me 2 Cu) -anion actually interacts with several (Li•3DME) + cations rather than a sin gle (Li•3DME) + cation (see Fig.…”