No abstract

Cuprophilic interactions in neutral perpendicular model dimers of the type (CH3CuX)2 (X = OH2, NH3, SH2, PH3, N2, CO, CS, CNH, CNLi) were analyzed by ab initio quantumchemical methods. The basis set superposition error for the weakly interacting CH3CuX subunits is significant and is discussed in detail. A new correlation-consistent pseudopotential valence basis set for Cu. derived at the second-order Møller-Plesset level suppresses considerably the basis set superposition error in Cu-Cu interactions compared to the standard Hartree-Fock optimized valence basis set. This allowed the first accurate predictions of cuprophilicity, which has been the subject of considerable debate in the past. The dependence of the strength of cuprophilic interactions on the nature of the ligand X was addressed. The Cu-Cu interaction increases with increasing sigma-donor and pi-acceptor capability of the ligand and is approximately one-third of the well-documented aurophilic interactions. By fitting our potential-energy data to the Hershbach-Laurie equation, we determined a relation between the Cu-Cu bond length and the Cu-Cu force constant; this is important for future studies on vibrational behaviour. The role of relativistic effects on the structure and the interaction energy is also discussed. Finally we investigated cuprophilic interactions in (CH3Cu)4 as a model species for compounds isolated and characterized by X-ray diffraction.

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.

No abstract

Acceptor-substituted lithio compounds LiA-CR'R2, in which the acceptor A is an RC(O), N=C, RS02, RS(O)NR, RSO, RS, 02N, or RC(N-NR2) group, have long played an important role in organic synthesis. Their significance has grown still further in the last fifteen years, as one has increasingly learnt to employ them successfully in chemo-, regio-, diastereoand enantioselective reactions. Remarkably, little if anything was known of the structures of these compounds. It is therefore not surprising that interest in the structures of this class of compounds has greatly increased in recent years. In the following review, we shall summarize recent research into the structures of the lithio compounds of sulfones, sulfoximides, sulfoxides, thioethers and 1,3-dithianes, nitriles, nitro compounds, and hydrazones. Crystal structure determinations from recent years are central to this study. They are supplemented by solution studies and by calculations of structures. Structural similarities and differences between the individual states of aggregation are pointed out wherever possible, as is also the relationship between structure and reactivity.