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 sBu2Mg 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‐C6H4)2(OR)}2] (FG=OMe; NMe2) or a homoaryl [(THF)4Li2Mg(4‐FG‐C6H4)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 [Li2MgsBu4] and Mg(OR)2, with [Li2MgsBu4] being the active species performing the Br/Mg exchange process.

Assessing Alkali‐Metal Effects in the Structures and Reactivity of Mixed‐Ligand Alkyl/Alkoxide Alkali‐Metal Magnesiates

“…This motif is closely related to the one we have previously reported for the product of Mg−Br exchange obtained from treating 2‐bromoanisole with an equimolar mixture of Mg s Bu 2 and LiOR’ [14] . However on this occasion this compound is obtained by direct exchange with Li 2 Mg s Bu 4 to give a Li 2 MgAr 4 intermediate which in turn undergoes co‐complexation with Mg(OR’) 2 also present in the reaction media via the bimetallic Schlenk equilibrium depicted in Figure 1 [15] . As aforementioned, while we do not observe the presence of this equilibrium for potassium magnesiate 3 , we decided to benchmark its reactivity towards to 2‐bromoanisole against that of higher order potassium magnesiate K 2 Mg(CH 2 SiMe 3 ) 4 which can be isolated as its TMEDA solvate by combining TMEDA, KCH 2 SiMe 3 and Mg(CH 2 SiMe 3 ) 2 in a 2 : 2 : 1 ratio [51] .…”

Ambient Temperature Polymerization of Methyl Methacrylate Mediated by Ate Complexes

“…Young NMR tube in THF-d 8 , and gave rise to a Schlenk equilibrium presented in Figure 1. [31] The proton signals of the starting materials all disappeared, leading to three new products: the expected LiMg[N(TMS) 2 ] 2 nBu (signal of TMS at À 0.02 ppm, signal of Mg-CH 2 -R at À 0.62 ppm), and the products of the Schlenk equilibrium Mg[N(TMS) 2 ] 4 Li 2 (signal of TMS at À 0.17 ppm) and Mg(nBu) 2 (signal of Mg-CH 2 -R at À 0.68 ppm, as confirmed by the 1 H NMR of commercial Mg(nBu) 2 ).…”

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

“…Equivalent atoms of 2 b generated by (1Àx, 1Ày, 1Àz) symmetry operation. [20] To ascertain whether the different composition of 2 a,b and 3 a,b could have an influence on their further functionalization, electrophilic interception studies were carried out (Scheme 2). [10] Thus, transmetallation of 2 a, 3 a and 3 b with ZnCl 2 followed by Pd catalysed cross-coupling with 4-iodo-trifluoromethylbenzene or ethyl-3-iodobenzoate led to the formation of asymmetric bis(arenes) 4 a, 5 a and 5 b in good yields ranging from 62 to 83 %.…”

“…Left: Synthesis of [LiMgsBu 2 (OR')] (8); [R' = CH 2 CH 2 N-(CH 3 )CH 2 CH 2 N(CH 3 ) 2 ] via co-complexation of LiOR' with sBu 2 Mg and Right: molecular structure of 8 with displacement ellipsoids at 30 % probability, all H atoms omitted, and with C atoms drawn as wire frames (except for CH group of bridging sBu groups) for clarity. [20] Molecular structure of 9 with displacement ellipsoids at 50 % probability, all H atoms omitted, and with C atoms in THF molecules drawn as wire frames for clarity. [20] Scheme 3.…”

“…[20] Molecular structure of 9 with displacement ellipsoids at 50 % probability, all H atoms omitted, and with C atoms in THF molecules drawn as wire frames for clarity. [20] Scheme 3. Proposed organometallic species involved in lithium alkoxide mediated Mg/Br exchange of bromoarenes.…”