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

Livingstone, Zoe and Hernán-Gómez, Alberto and Baillie, Sharon E. and Armstrong, David R. and Carrella, Luca M. and Clegg, William and Harrington, Ross W. and Kennedy, Alan R. and Rentschler, Eva and Hevia, Eva (2016) Assessing the reactivity of sodium alkyl-magnesiates towards quinoxaline : single electron transfer (SET) vs. nucleophilic alkylation processes. Dalton Transactions, 45 (14). pp. 6175-6182. ISSN 1477-9234 (https://doi.org/10.1039/c5dt04044b)

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Abstract

By exploring the reactivity of sodium butyl-magnesiate (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 (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 (3) which was characterised by multinuclear NMR and X-ray crystallography. (1)H 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.

ORCID iDs

Livingstone, Zoe, Hernán-Gómez, Alberto ORCID logoORCID: https://orcid.org/0000-0001-8150-7249, Baillie, Sharon E., Armstrong, David R., Carrella, Luca M., Clegg, William, Harrington, Ross W., Kennedy, Alan R. ORCID logoORCID: https://orcid.org/0000-0003-3652-6015, Rentschler, Eva and Hevia, Eva ORCID logoORCID: https://orcid.org/0000-0002-3998-7506;