Organocuprate aggregation and reactivity

A broad range of organocopper intermediates in different aggregation states were characterized by electrospray ionization (ESI) mass spectrometry, which provided valuable information on these fluxional species. To complement the mass spectrometric data, electrical conductivity measurements and theoretical calculations were employed. Tetrahydrofuran (THF) solutions of CuCN/(RLi)m stoichiometry (m = 0.5, 0.8, 1.0, and 2.0 and R = Me, Et, nBu, sBu, tBu, Ph) were analyzed by ESI mass spectrometry, and organocuprate anions were detected for all cases. The composition of these species showed clear dependence on the amount of RLi used. Thus, while cyanide-free Lin–1CunR2n– anions completely predominated for CuCN/(RLi)2 solutions, cyanide-containing Lin–1CunRn(CN)n– complexes prevailed for CuCN/(RLi)m reagents with m ≤ 1. Ligand mixing studies on LiCuMe2•LiCN and LiCuR2•LiCN systems (R = Et, nBu, sBu, tBu, Ph) revealed fast exchange equilibria operating in solution. When THF was substituted for the less polar diethyl ether (Et2O), no major new species were observed. However, the proportion of higher nuclearity anions was consistently greater in the latter solvent than in the former. Further experiments with 2-methyltetrahydrofuran (MeTHF), cyclopentyl methyl ether (CPME) and methyl tert-butyl ether (MTBE) solutions confirmed the suggestion that higher aggregation states are favored by lower polarity solvents. Additional conductivity experiments indicated that contact ion pairs strongly predominate for solutions in Et2O, whereas the more polar THF gives rise to larger amounts of solvent-separated ion pairs. Following the detection of organocuprate ions, their gas- and condensed-phase reactions were investigated. Collision-induced dissociation (CID) experiments were used to study intrinsic reactivities in the gas phase. Higher aggregates were found to break apart into fragments of lower nuclearity, whereas monomeric species decomposed by beta-H elimination when possible. In some CID spectra, the presence of hydroxyl-containing signals led to the conclusion that a reaction with background water inside the mass spectrometer was taking place. This bimolecular reaction was then studied in detail for many different systems. The results indicate that lithium centers seem to be a necessary (but not only) pre-requisite for hydrolysis. For example, no reaction was observed for monomeric CuMe2– anions, whereas the reactions of LiCu2Me4– and Li2Cu3Me6– were much faster. Following the successful characterization of organocuprates, their synthetically useful coupling reactions with alkyl halides were probed. ESI mass spectrometric experiments, supported by electrical conductivity measurements, indicated that LiCuMe2•LiCN reacts with a series of alkyl halides RX (R = Me, Et, nPr, nBu, PhCH2CH2, CH2=CHCH2, and CF3CH2CH2). The resulting Li+Me2CuR(CN)− intermediates then afford the observable Me3CuR− tetraalkylcuprate anions upon Me/CN exchanges with added MeLi. In contrast, the reactions of LiCuMe2•LiCN with neopentyl iodide and various aryl halides gave rise to halogen-copper exchanges. Concentration- and solvent-dependent studies suggested that lithium tetraalkylcuprates partly form Li+Me3CuR− contact ion pairs and presumably also triple ions LiMe6Cu2R2−. According to theoretical calculations, these triple ions consist of two square-planar Me3CuR− subunits binding to a central Li+ ion. Upon fragmentation in the gas phase, the Me3CuR− anions undergo reductive elimination, yielding both cross- (MeR) and homo-coupling products (Me2). The branching between these channels showed a marked dependence on the nature of R. The fragmentation of LiMe6Cu2R2− also affords both cross- and homo-coupling products, but strongly favors the former. This was rationalized by the preferential interaction of the central Li+ ion with two Me groups of each Me3CuR− subunit, which thereby block the homo-coupling channel. Finally, the reactivity of organocuprates in conjugate addition reactions was investigated, with cyano-substituted ethylenes C2Hn–4(CN)n, n = 1 – 4 as Michael acceptors. In the case of acrylonitrile, n = 1, polymerization was induced, but no reactive intermediates were detected. In contrast, the reaction with fumaronitrile, n = 2, permitted the detection of π-complexes in different aggregation states. The identities of the latter were confirmed by the release of intact fumaronitrile upon their fragmentation in the gas phase. The reactions with 1,1-dicyanoethylene, n = 2, did not halt at the stage of the π-complexes, but proceeded all the way to Michael adducts. In the case of tricyanoethylene, n = 3, dimeric polycyano carbanions were formed. For tetracyanoethylene, n = 4, the reaction instead leads to Cu(III) species, which undergo reductive eliminations. Thus, all intermediates commonly proposed for the conjugate addition of organocuprates to Michael acceptors were detected, providing strong evidence for the currently accepted mechanism

Tetraalkylcuprates(III): Formation, Association, and Intrinsic Reactivity

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 LiCuMe₂·LiCN reacts with a series of alkyl halides RX. The resulting Li⁺Me₂CuR­(CN)⁻ intermediates then afford the observable Me₃CuR^– tetraalkylcuprate anions upon Me/CN exchanges with added MeLi. In contrast, the reactions of LiCuMe₂·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⁺Me₃CuR^– contact ion pairs and presumably also triple ions LiMe₆Cu₂R₂^–. According to theoretical calculations, these triple ions consist of two square-planar Me₃CuR^– subunits binding to a central Li⁺ ion. Upon fragmentation in the gas phase, the mass-selected Me₃CuR^– anions undergo reductive elimination, yielding both the cross-coupling products MeR and the homocoupling product Me₂. The branching between these two fragmentation channels markedly depends on the nature of the alkyl substituent R. The triple ions LiMe₆Cu₂R₂^– (as well as their mixed analogues LiMe₆Cu₂R­(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 Me₃CuR^– 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