Acyclic 1,2-dimagnesioethanes/-ethene derived from magnesium(i) compounds: multipurpose reagents for organometallic synthesis

Abstract

Reactions of three magnesium(I) dimers, [{(^ArNacnac)Mg–}₂] (^ArNacnac = [(ArNCMe)₂CH]⁻; Ar = xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), α-methylstyrene (MS), trans-stilbene (TS) or diphenylacetylene (DPA) led to the 1,2-addition of the Mg–Mg bond across the substrate, giving rise to the 1,2-dimagnesioethanes, [{(^XylNacnac)Mg}₂(μ-DPE)], [{(^DepNacnac)Mg}₂(μ-MS)], [{(^ArNacnac)Mg}₂(μ-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{(^MesNacnac)Mg}₂(μ-DPA)]. The reactions involving the 1,1-substituted alkenes are shown to be readily redox reversible, in that the reaction products are in equilibrium with a significant proportion of the starting materials at room temperature. Variable temperature NMR spectroscopy and a van't Hoff analysis point to low kinetic barriers to these weakly exergonic reactions. [{(^MesNacnac)Mg}₂(μ-DPE)] and [{(^MesNacnac)Mg}₂(μ-DPA)] behave as 1,2-di-Grignard reagents in their reactions with very bulky amido-zinc bromides, yielding the first examples of a 1,2-dizincioethane, [(L*Zn)₂(μ-DPE)] (L* = –N(Ar*)(SiPrⁱ₃); Ar* = C₆H₂Me{C(H)Ph₂}₂-4,2,6), and a 1,2-dizincioethene, [(^TBoLZn)₂(μ-DPA)] (^TBoL = –N(SiMe₃){B(DipNCH)₂}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{(^MesNacnac)Mg}₂(μ-DPE)], which behaves as a two-electron reducing agent when treated with amido-cadmium and amido-magnesium halide precursors, yielding the cadmium(I) and magnesium(I) dimers, [^PhBoLCdCd^PhBoL] (^PhBoL = –N(SiPh₃){B(DipNCH)₂}) and [L^†MgMgL^†] (L^† = –N(Ar^†)(SiMe₃); Ar^† = C₆H₂Prⁱ{C(H)Ph₂}₂-4,2,6), respectively. A further class of reactivity for [{(^MesNacnac)Mg}₂(μ-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCl, which gives a magnesio-germane, presumably via intramolecular C–H activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg(^MesNacnac)}]. [{(^MesNacnac)Mg}₂(μ-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in this reaction.