Cu(0), O₂ and mechanical forces: a saving combination for efficient production of Cu-NHC complexes

Mechanical forces induced by ball-milling agitation enabled the highly efficient and widely applicable synthesis of Cu-carbene complexes from N,N-diaryl-imidazolium salts and metallic copper. The required amount of gaseous dioxygen and insoluble copper could be reduced down to stoichiometric quantities, while reaction rates clearly outperformed those obtained in solution. Utilisation of Cu(0) as the copper source enabled the application of this approach to a wide array of N,N-diaryl-imidazolium salts (Cl^-, BF₄^- and PF₆^-) that transferred their counter anion directly to the organometallic complexes. Cu-NHC complexes could be produced in excellent yields, including utilisation of highly challenging substrates. In addition, five unprecedented organometallic complexes are reported.