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

Abstract

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 sBu 2 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-C 6 H 4) 2 (OR)} 2 ] (FG = OMe; NMe 2) or a homoaryl [(THF) 4 Li 2 Mg(4-FG-C 6 H 4) 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 [LiMgsBu 2-(OR)], alkyl rich [Li 2 MgsBu 4 ] and Mg(OR) 2 , with [Li 2 MgsBu 4 ] being the active species performing the Br/Mg exchange process.