Assessing Alkali‐Metal Effects in the Structures and Reactivity of Mixed‐Ligand Alkyl/Alkoxide Alkali‐Metal Magnesiates
Neil R. Judge, Leonie J. Bole, Eva Hevia
Abstract
Abstract Advancing the understanding of using alkali‐metal alkoxides as additives to organomagnesium reagents in Mg−Br exchange reactions, a homologous series of mixed‐ligand alkyl/alkoxide alkali‐metal magnesiates [MMg(CH 2 SiMe 3 ) 2 (dmem)] 2 [dmem=2‐{[2‐(dimethylamino)ethyl]methylamino} ethoxide; M=Li, 1 ; Na, 2 ; (THF)K, 3 ] has been prepared. Structural and spectroscopic studies have established the constitutions of these heteroleptic/heterometallic species, which are retained in arene solution. Evaluation of their reactivity towards 2‐bromoanisole has uncovered a marked alkali‐metal effect with potassium magnesiate 3 being the most efficient of the three ate reagents. Studies probing the constitution of the exchange product from this reaction suggest that the putative [KMgAr 2 (dmem)] 2 (Ar= o ‐OMe−C 6 H 4 ) intermediate undergoes redistribution into its single metal components [KAr] n and [MgAr(dmem)] 2 ( 5 ). This process can be circumvented by using a different potassium alkoxide containing an aliphatic chain such as KOR’ (R’=2‐ethylhexyl) which undergoes co‐complexation with Mg(CH 2 SiMe 3 ) to give [KMg(CH 2 SiMe 3 ) 2 (OR’)] 2 ( 7 ). This ate, in turn, reacts quantitatively with 2‐bromoanisole furnishing [KMgAr 2 (OR’)] 2 ( 9 ) which is stable in solution as a bimetallic compound. Collectively this work highlights the complexity of these alkali‐metal mediated Mg−Br exchange reactions, where each reaction component can have a profound effect not only on the success of the reaction; but also the stability of the final metalated intermediates prior to their electrophilic interception.