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

作者：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^*

来源：Dalton Transactions, 2016, 45(14): 6175-6182.

DOI：10.1039/c5dt04044b

By exploring the reactivity of sodium butyl-magnesiate [Na(THF)(5)](+) [{Ph2Si(NAr*)(2)}Mg(Bu)(THF)(-) (1) supported by the bulky chelating silyl(bisamido) ligand (Ph2Si(NAr*)(2))(2-) (Ar* = 2,6 -iPr(2)-C6H3) towards Quinoxaline (Ox), 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 Na(THF)(6)](2)(+) {Ph2Si(NAr*)(2)}(2)Mg-2(Qx)(2)](2-) (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*)z)Mg(TEMPO-)Na(THF)31 (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 IMP allowing the isolation of [Na(THF)(6)](+) [{Ph2Si(NAr*)(2)}Mg(TMP)](-) (3) which was characterised by multinuclear NMR and X-ray crystallography. H-1 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.

• 出版日期2016