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

Abstract

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.

Low‐Valent Ate Complexes Formed in Cobalt‐Catalyzed Cross‐Coupling Reactions with 1,3‐Dienes as Additives

“…Being highly electropositive, lithium and magnesium readily form cations, whereas more electronegative cobalt strongly prefers anionic ate complexes. Very similar behavior has previously been observed for the transmetalation of iron, copper, and zinc precursors as well.…”

Reactivity of Copper(III)–Oxo Complexes in the Gas Phase

“…[21][22][23] Copper(III) intermediates are predicted to play an important role in many copper-catalyzed reactions. [31,32] As far as the application of copper(III) complexes for oxidation catalysis is concerned, To lman and co-workersde-velopeda na pproachb ased on electrochemical oxidation of a[ ( L-2H)Cu II -OH] À complex, in which (L-2H) was ab is-deprotonated pincerl igand. The stable copper(III) complexes that have been prepared so far have relied on highly stabilizing planart etradentate ligands.…”

Association and Dissociation of Grignard Reagents RMgCl and Their Turbo Variant RMgCl⋅LiCl

“…The analysis of these challenging analytes therefore also serves as a rigorous test for the applied methods. The qualitative insight afforded by ESI mass spectrometry is ideally complemented by electrical conductivity measurements, which can provide more quantitative information on the ionic constituents in solution ,. NMR spectroscopy, in turn, has the advantage of probing both neutral and charged species.…”