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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2016-03, Vol.45 (14), p.6175-6182
• Main Authors: Livingstone, Zoe, Hernan-Gomez, Alberto, Baillie, Sharon E, Armstrong, David R, Carrella, Luca M, Clegg, William, Harrington, Ross W, Kennedy, Alan R, Rentschler, Eva, Hevia, Eva
• Format: Article
• Language: English
• Subjects: Alkylation; Ligands; Nuclear magnetic resonance; Quinoxalines; Radicals; Reduction; Single electrons; Sodium
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c5dt04044b

By exploring the reactivity of sodium butyl-magnesiate (1) supported by the bulky chelating silyl(bisamido) ligand {Ph sub(2)Si(NAr*) sub(2)} super(2-) (Ar* = 2,6-iPr sub(2)-C sub(6)H sub(3)) 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&z.rad; 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 [(Ph sub(2)Si(NAr*) sub(2))Mg(TEM PO super(-))Na(THF) sub(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. super(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(CH sub(2)SiMe sub(3)) sub(3)] (5) led to the chemoselective C2 alkylation of this heterocycle, suggesting that the presence of the steric stabiliser {Ph sub(2)Si(NAr*) sub(2)} super(2-) on the mixed-metal reagent is required in order to facilitate the Qx reduction.