Untangling the Complexity of Mixed Lithium/Magnesium Alkyl/Alkoxy Combinations Utilised in Bromine/Magnesium Exchange...

While it is known that the addition of Group 1 alkoxides to s-block organometallics can have an activating effect on reactivity, the exact nature of this effect is not that well understood. Here we describe the activation of sBu₂ Mg towards substituted bromoarenes by adding one equivalent of LiOR (R=2-ethylhexyl), where unusually both sBu groups can undergo efficient Br/Mg exchange. Depending on the substitution pattern on the bromoarene two different types of organometallic intermediates have been isolated, either a mixed aryl/alkoxide [{LiMg(2-FG-C₆ H₄ )₂ (OR)}₂ ] (FG=OMe; NMe₂ ) or a homoaryl [(THF)₄ Li₂ Mg(4-FG-C₆ H₄ )₄ ] (FG=OMe, F). Detailed NMR spectroscopic studies have revealed that these exchange reactions and the formation of their intermediates are controlled by a new type of bimetallic Schlenk-type equilibrium between heteroleptic [LiMgsBu₂ (OR)], alkyl rich [Li₂ MgsBu₄ ] and Mg(OR)₂ , with [Li₂ MgsBu₄ ] being the active species performing the Br/Mg exchange process.