Association and Aggregation of Magnesium Organocuprates

Despite the frequent use of magnesium organocuprates derived from Grignard reagents in organic synthesis, the molecular composition of these important reagents is poorly understood. To achieve a better understanding of their speciation in solution, we apply here a combination of electrospray-ionization mass spectrometry, gas-phase fragmentation experiments, and quantum chemical calculations. For solutions of CuCl/2RMgCl (R = Ph, 2-thienyl, Bu, and Me₃SiCH₂) in tetrahydrofuran, we find anions of the type [Cu_nR_n+1]⁻, n = 1–6. Changing the copper precursor, increasing the amount of the Grignard reagent, and adding Me₂S have only relatively minor effects. Gas-phase fragmentation of the [Cu_nR_n+1]⁻ anions results in deaggregation reactions. In addition, [Cu_nBu_n+1]⁻ and [Cu(CH₂SiMe₃)₂]⁻ undergo β-hydrogen and β-methyl eliminations, respectively, as well. Presumably, these decomposition pathways (as well as unknown processes) also occur in solution and explain the depletion of organyl substituents in the observed [Cu_nR_n+1]⁻ anions. The behavior of the magnesium cuprates deviates from that of the well-studied lithium cuprates, which display a higher tendency to form heteronuclear species. Our quantum chemical calculations for LiCuPh₂·LiCl and MgClCuPh₂·MgCl₂ in THF show that the absence of analogous heteronuclear complexes in the case of the magnesium cuprates results from higher relative Gibbs energies of these species.