Association\nand Aggregation of Magnesium Organocuprates

Despite the frequent use of magnesium organocuprates\nderived from\nGrignard reagents in organic synthesis, the molecular composition\nof these important reagents is poorly understood. To achieve a better\nunderstanding of their speciation in solution, we apply here a combination\nof electrospray-ionization mass spectrometry, gas-phase fragmentation\nexperiments, and quantum chemical calculations. For solutions of CuCl/2RMgCl\n(R = Ph, 2-thienyl, Bu, and Me<sub>3</sub>SiCH<sub>2</sub>) in tetrahydrofuran,\nwe find anions of the type [Cu<sub><i>n</i></sub>R<sub><i>n</i>+1</sub>]<sup>−</sup>, <i>n</i> = 1–6.\nChanging the copper precursor, increasing the amount of the Grignard\nreagent, and adding Me<sub>2</sub>S have only relatively minor effects.\nGas-phase fragmentation of the [Cu<sub><i>n</i></sub>R<sub><i>n</i>+1</sub>]<sup>−</sup> anions results in\ndeaggregation reactions. In addition, [Cu<sub><i>n</i></sub>Bu<sub><i>n</i>+1</sub>]<sup>−</sup> and [Cu(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>]<sup>−</sup> undergo\nβ-hydrogen and β-methyl eliminations, respectively, as\nwell. Presumably, these decomposition pathways (as well as unknown\n processes) also occur in solution and explain the depletion of organyl\nsubstituents in the observed [Cu<sub><i>n</i></sub>R<sub><i>n</i>+1</sub>]<sup>−</sup> anions. The behavior\nof the magnesium cuprates deviates from that of the well-studied lithium\ncuprates, which display a higher tendency to form heteronuclear species.\nOur quantum chemical calculations for LiCuPh<sub>2</sub>·LiCl\nand MgClCuPh<sub>2</sub>·MgCl<sub>2</sub> in THF show that the\nabsence of analogous heteronuclear complexes in the case of the magnesium\ncuprates results from higher relative Gibbs energies of these species.