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Issue 14, 2016
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Assessing the reactivity of sodium alkyl-magnesiates towards quinoxaline: single electron transfer (SET) vs. nucleophilic alkylation processes

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Abstract

By exploring the reactivity of sodium butyl-magnesiate Image ID:c5dt04044b-t1.gif (1) supported by the bulky chelating silyl(bisamido) ligand {Ph2Si(NAr*)2}2− (Ar* = 2,6-iPr2-C6H3) towards Quinoxaline (Qx), the ability of this bimetallic system to effectively promote SET processes has been disclosed. Thus 1 executes the single-electron reduction of Qx affording complex Image ID:c5dt04044b-t2.gif (2) whose structure in the solid state contains two quinaxolyl radical anions Qx˙ stabilised within a dimeric magnesiate framework. Combining multinuclear NMR and EPR measurements with DFT calculations, new insights into the constitution of 2 in solution and its magnetic behaviour have been gained. Further evidence on the SET reactivity of 1 was found when it was reacted with nitroxyl radical TEMPO which furnished contacted ion pair sodium magnesiate [(Ph2Si(NAr*)2)Mg(TEMPO)Na(THF)3] (4) where both metals are connected by an alkoxide bridge, resulting from reduction of TEMPO. The role that the different ligands present in 1 can play in these new SET reactions has also been assessed. Using an amination approach, the Bu group in 1 can be replaced by the more basic amide TMP allowing the isolation of Image ID:c5dt04044b-t3.gif (3) which was characterised by multinuclear NMR and X-ray crystallography. 1H NMR monitoring of the reaction of 3 with Qx showed its conversion to 2, leaving the hydrogen atoms of the heterocycle untouched. Contrastingly, using sodium homoalkyl magnesiate [NaMg(CH2SiMe3)3] (5) led to the chemoselective C2 alkylation of this heterocycle, suggesting that the presence of the steric stabiliser {Ph2Si(NAr*)2}2− on the mixed-metal reagent is required in order to facilitate the Qx reduction.

Graphical abstract: Assessing the reactivity of sodium alkyl-magnesiates towards quinoxaline: single electron transfer (SET) vs. nucleophilic alkylation processes

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Publication details

The article was received on 15 Oct 2015, accepted on 18 Nov 2015 and first published on 20 Nov 2015


Article type: Paper
DOI: 10.1039/C5DT04044B
Citation: Dalton Trans., 2016,45, 6175-6182
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    Assessing the reactivity of sodium alkyl-magnesiates towards quinoxaline: single electron transfer (SET) vs. nucleophilic alkylation processes

    Z. Livingstone, A. Hernán-Gómez, S. E. Baillie, D. R. Armstrong, L. M. Carrella, W. Clegg, R. W. Harrington, A. R. Kennedy, E. Rentschler and E. Hevia, Dalton Trans., 2016, 45, 6175
    DOI: 10.1039/C5DT04044B

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