PREPARATION OF ETHEREAL LITHIUM DIMETHYLCUPRATES (ME(2)CULI)(2) AND ME(2)CULI.LII DISPLAYING NARROW-LINE WIDTH C-13 NMR RESONANCES

Halide-free lithium dimethylcuprate (Me(2)CuLi)(2) (1) and the Gilman reagent Me(2)CuLi.LiI (2) have been generated from an ether-soluble form of solid MeLi obtained by transmetalation between BuLi and Mel. The C-13 NMR spectra of these cuprates show single narrow resonances at -9.25 and -9.28 ppm (W-1/2 < 4 Hz), respectively. The preparative procedure can be adapted to allow for C-13 enrichment and is thus suitable for mechanistic studies using C-13 NMR. Ab initio geometry optimization and subsequent calculation of the cuprate C-13 chemical shifts indicates that the structural environment around the methyl carbons in the two compounds is remarkably similar. Modeling of the LiI-mediated equilibrium between Me(2)CuLi.LiI and (Me(2)CuLi)(2) suggests strongly that the Gilman reagent consists primarily of dimer (Me(2)CuLi)(2) and free LiI. The virtually identical C-13 NMR shifts observed for 1 and 2 are thereby explained, along with the effect of lithium halide on line width.