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

In the presence of two molar equiv. of B(C6F5)(3) p-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 Cp*Fe-2(+) ferrocenium counter cations. All three [(C6F5)(3)B](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)(3)B](2)-semiquinone radical anion oxoammonium salt giving rise to the formation of the (C6F5)(3)B-DMSO (or THF) Lewis adduct, pbenzoquinone and the TEMPO radical. The reaction of 9,10-anthraquinone or acenaphthenequinone with either the Gomberg dimer or Cp*Fe-2 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 Cp*(2)Fe(+)counter cations. These products were also characterized by X-ray diffraction. The Cp*Fe-2(+) 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 Cp*Fe-2(+) [(C6F5)(3)B] 2-9,10-phenanthrene-semiquinone salt was characterized by X-ray diffraction. The radical anions were characterized by ESR spectroscopy.