The special role of B(C6F5)3 in the single electron reduction of quinones by radicals

Abstract

In the presence of two molar equiv. of B(C₆F₅)₃p-benzoquinone reacts with persistent radicals TEMPO, trityl or decamethylferrocene by single electron transfer to give doubly O-borylated benzosemiquinone radical anions with TEMPO⁺, trityl or ferrocenium counter cations. All three [(C₆F₅)₃B]₂–semiquinone radical anion salts were characterized by X-ray diffraction. The addition of donor reagent THF or DMSO induced rapid back electron transfer, in the case of the [(C₆F₅)₃B]₂–semiquinone radical anion oxoammonium salt giving rise to the formation of the (C₆F₅)₃B–DMSO (or THF) Lewis adduct, p-benzoquinone and the TEMPO radical. The reaction of 9,10-anthraquinone or acenaphthenequinone with either the Gomberg dimer or in 1 : 1 stoichiometry in the presence of two molar equiv. of B(C₆F₅)₃ gave the respective two-fold O–B(C₆F₅)₃ containing 9,10-anthrasemiquinone or acenaphthene-semiquinone radical anion salts with either Ph₃C⁺ or counter cations. These products were also characterized by X-ray diffraction. The salts showed analogous back electron shuttling behavior upon treatment with DMSO. 9,10-Phenanthrenequinone reacted analogously with B(C₆F₅)₃ and the electron rich ferrocene. The [(C₆F₅)₃B]₂–9,10-phenanthrene–semiquinone salt was characterized by X-ray diffraction. The radical anions were characterized by ESR spectroscopy.