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

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 SiMe ) (dmem)] [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 (dmem)] (Ar=o-OMe-C H ) intermediate undergoes redistribution into its single metal components [KAr] and [MgAr(dmem)] (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 SiMe ) to give [KMg(CH SiMe ) (OR')] (7). This ate, in turn, reacts quantitatively with 2-bromoanisole furnishing [KMgAr (OR')] (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.