Tetraalkylcuprates(III): Formation, Association, and Intrinsic Reactivity
Jacs, 2012, DOI: 10.1021/ol203437a, 2012, 134 (1), pp 613–622 published on 01.01.2012
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.