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

Reactions of three magnesium(i) dimers, [{(Nacnac)Mg-}] (Nacnac = [(ArNCMe)CH]; Ar = xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), α-methylstyrene (MS), -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, [{(Nacnac)Mg}(μ-DPE)], [{(Nacnac)Mg}(μ-MS)], [{(Nacnac)Mg}(μ-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{(Nacnac)Mg}(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. [{(Nacnac)Mg}(μ-DPE)] and [{(Nacnac)Mg}(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)(μ-DPE)] (L* = -N(Ar*)(SiPr ); Ar* = CHMe{C(H)Ph}-4,2,6), and a 1,2-dizincioethene, [(LZn)(μ-DPA)] (L = -N(SiMe){B(DipNCH)}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{(Nacnac)Mg}(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, [LCdCdL] (L = -N(SiPh){B(DipNCH)}) and [LMgMgL] (L = -N(Ar)(SiMe); Ar = CHPr{C(H)Ph}-4,2,6), respectively. A further class of reactivity for [{(Nacnac)Mg}(μ-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCl, which gives a magnesio-germane, presumably intramolecular C-H activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg(Nacnac)}]. [{(Nacnac)Mg}(μ-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in this reaction.