Tetraalkylcuprates are prototypical examples of organocopper(III) species, which remained elusive until their recent detection by NMR spectroscopy. In agreement with the NMR studies, the present electrospray ionization mass spectrometric experiments, as well as supporting electrical conductivity measurements, indicate that LiCuMe(2)·LiCN reacts with a series of alkyl halides RX. The resulting Li(+)Me(2)CuR(CN)(-) intermediates then afford the observable Me(3)CuR(-) tetraalkylcuprate anions upon Me/CN exchanges with added MeLi. In contrast, the reactions of LiCuMe(2)·LiCN with neopentyl iodide and various aryl halides give rise to halogen-copper exchanges. Concentration- and solvent-dependent studies suggest that lithium tetraalkylcuprates are not fully dissociated in ethereal solvents, but partly form Li(+)Me(3)CuR(-) contact ion pairs and presumably also triple ions LiMe(6)Cu(2)R(2)(-). According to theoretical calculations, these triple ions consist of two square-planar Me(3)CuR(-) subunits binding to a central Li(+) ion. Upon fragmentation in the gas phase, the mass-selected Me(3)CuR(-) anions undergo reductive elimination, yielding both the cross-coupling products MeR and the homocoupling product Me(2). The branching between these two fragmentation channels markedly depends on the nature of the alkyl substituent R. The triple ions LiMe(6)Cu(2)R(2)(-) (as well as their mixed analogues LiMe(6)Cu(2)R(R')(-)) also afford both cross-coupling and homocoupling products upon fragmentation, but strongly favor the former. On the basis of theoretical calculations, we rationalize this prevalence of cross-coupling by the preferential interaction of the central Li(+) ion of the triple ions with two Me groups of each Me(3)CuR(-) subunit, which thereby effectively blocks the homocoupling channel. Our results thus show how a Li(+) counterion can alter the reactivity of an organocopper species at the molecular level.

Supporting InformationTable of Contents 1.) Anion-mode ESI mass spectra of Li 0.8 Cu n,s Bu 0.8 ( 13 CN) solutions (Figures S1 -S4) S2 2.) Mass spectra of mass-selected Li 2 Cu 3 n,s Bu 2 (OH)( 13 CN) 3 − and Li 3 Cu 4 n,s Bu 3 (OH)( 13 CN) 4 − anions and their fragment ions (Figures S5 -S8) S4

We use a combination of electrospray ionization mass spectrometry and electrical conductivity measurements to probe the ions present in ethereal solutions of lithium cyanocuprates. Electrospray ionization mass spectrometry shows that solutions of LiCuR2·LiCN and Li0.8CuR0.8(CN) (R = Me, Et, n Bu, s Bu, t Bu, and Ph) in diethyl ether contain Li n–1Cu n R2n – and Li n–1Cu n R n (CN) n – anions, respectively. Analogous species are also observed for solutions of LiCu t Bu2·LiCN and Li0.8Cu t Bu0.8(CN) in 2-methyltetrahydrofuran, cyclopentyl methyl ether, and methyl tert-butyl ether and were previously found for solutions of lithium cyanocuprates in tetrahydrofuran. Although the change of solvent thus does not lead to the formation of any major new cuprate anions, it has a strong effect on the association/dissociation equilibria. As directly confirmed by the conductivity experiments, contact ion pairs strongly predominate in solutions of lithium cyanocuprates in diethyl ether, whereas the more polar tetrahydrofuran gives rise to larger amounts of solvent-separated ion pairs; a particularly high dissociation tendency is observed for the LiCu t Bu2·LiCN reagent. Temperature-variant conductivity measurements of LiCuPh2·LiCN solutions in tetrahydrofuran do not show a significant temperature dependence of the association/dissociation equilibria for this system. The present findings largely support the results of previous NMR spectroscopic studies and help to explain how the solvent affects the reactivity of lithium cyanocuprates.

We have used a combination of electrospray ionization mass spectrometry and electrical conductivity measurements to analyze solutions of the Gilman cuprates LiCuR2·LiX, with R=Ph, Bu and X=Cl, Br, I, in tetrahydrofuran and have compared our findings with previous results on cyanocuprates LiCuR2·LiCN. Among the various polynuclear organocuprate ions observed, Li2Cu3Ph6(-), LiCu4Ph6(-), and Cu5Ph6(-) are of particular interest because aggregates of the same composition are known from X-ray crystal structures. Control experiments have indicated that the polynuclear organocuprate anions detected in solution are indeed identical to those formed in the solid state. As abundant ions of the type Li2Cu3R6(-) are found in solutions of Gilman cuprates and cyanocuprates alike, their possible involvement in organocuprate reactions should be considered. For comparison, we have also included solutions of LiCu(R)I, LiCuX2·LiX, LiCuX2, and CuCN·2LiX in the present study.

Supporting InformationTable of Contents 1.) Anion-mode ESI mass spectra recorded with the TSQ 7000 instrument (Figures S1 -S4) S2 2.) Characterization of the HCT ion trap (Figures S5 -S8) S4 3.) Comparison of measured and simulated isotope patterns (Figures S9 -S10) S6

Conjugate additions of organocuprates are of outstanding importance for organic synthesis. To improve our mechanistic understanding of these reactions, we have used electrospray ionization mass spectrometry for the identification of the ionic intermediates formed upon the treatment of LiCuR2 ⋅LiCN (R=Me, Bu, Ph) with a series of α,β-unsaturated nitriles. Acrylonitrile, the weakest Michael acceptor included, did not afford any detectable intermediates. Fumaronitrile (FN) yielded adducts of the type Lin-1 Cun R2n (FN)n (-) , n=1-3. When subjected to fragmentation in the gas phase, these adducts were not converted into the conjugate addition products, but re-dissociated into the reactants. In contrast, the reaction with 1,1-dicyanoethylene furnished the products of the conjugate addition without any observable intermediates. Tri- and tetracyanoethylene proved to be quite reactive as well. The presence of several cyano groups in these substrates opened up reaction pathways different from simple conjugate additions, however, and led to dimerization and substitution reactions. Moreover, the gas-phase fragmentation behavior of the species formed from these substrates indicated the occurrence of single-electron transfer processes. Additional quantum-chemical calculations provided insight into the structures and stabilities of the observed intermediates and their consecutive reactions.