Solid- and Solution-Phase Structures of Zinc Enolates of Amides and Ketones

Solid-phase structural data for five Zn enolate derivatives are presented. Included are the amide enolates BrZn[CH2C(O)NEt2](thf) (1a), BrZn[CH2C(O)NEt2](dmso) (1b), and BrZn[CH2C(O)NEt2](tBu-py) (1c), which are all dimeric in the solid phase with each of the enolate ligands binding in a C,O-bridging fashion. Solution molecular weight data for 1a, determined using diffusion-order NMR spectroscopy, indicate a degree of association of 3.8 in C6D6 solution. In the presence of added Lewis base, however, dimeric (in 1:9 D8-thf/C6D6) and monomeric (in D6-dmso) species are formed. Similar solution behavior is observed for the Reformatsky ester BrZn[CH2C(O)OtBu](thf). Structural data are also presented for the Zn ketone enolates BrZn[OC(CMe2)iPr](dmf) (2b) and BrZn[OC(CH2)Mes](dmf) (3b). These are dimeric in the solid phase, with O-bridging enolate ligands. This nuclearlity is maintained for 2b in noncoordinating solvents and in the presence of 10% thf. Dissolution of 2b in D6-dmso, however, appears to form a mononuclear species.