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)₃ <i>p</i>-benzoquinone reacts with persistent radicals TEMPO, trityl or decamethylferrocene by single electron transfer to give doubly <i>O</i>-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, <i>p</i>-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.