Borocyclic Radicals Prepared from Orthoquinone-Containing Polycyclic Aromatics by Photoirradiation

Borocyclic radicals with highly conjugated aromatics were generated from orthoquinone-containing polycyclic aromatic compounds by trapping the photoinduced triplet state with simple boron halide under irradiation with light of appropriate wavelength. The picene-based borocyclic radical was remarkably stable when stored at 23 °C in a desiccator for over 1 year. The crystal structure of this stable radical had a stacking structure of a planar π-conjugated system, and the electrical conductivity was higher than those of ordinary organic radical systems.

Complexes of Metal Halides with Unreduced o-(Imino)quinones

A series of complexes of metal halides with unreduced quinone-type ligands have been synthesized and characterized in detail. The 3,6-di-tert-butyl-o-benzoquinone (1) and 4,6-di-tert-butyl-N-aryl-substituted o-iminobenzoquinones (2-5) (aryl is 2,6-dimethylphenyl in 2, 2-methyl-6-ethylphenyl in 3, 2,6-diethylphenyl in 4, and 2,6-diisopropylphenyl in 5) were used to obtain the molecular complexes with metal 12 group halides as well as with indium(III) iodide. The molecular structures of five complexes, bearing an unreduced form of redox-active ligand, have been established by single-crystal X-ray analysis. The spectral data, electrochemical measurements, and DFT calculations indicate the significant transformations of the molecular orbitals of 1-5 upon complexation with Lewis acids. The reduction potentials of o-(imino)quinones in complexes with metal halides shift into the anodic region versus uncoordinated ones. The choice of metal halide allows varying the shift magnitude up to 1.7 V in 2·CdI₂. The change of the oxidizing ability of the 1-5 upon coordination with Lewis acids enables the oxidation of mercury and ferrocene, infeasible for free ligands.

Isolable Anionic, Neutral and Cationic Radicals from Reactions of N,N'-Dimesityldiamidocarbene and Lewis Acids

B(C₆ F₅ )₃ undergoes nucleophilic attack by N,N'-dimesityldiamidocarbene (DAC) with fluoride transfer to the boron center, resulting in a new zwitterion (1). This B-F fluoride can be replaced or abstracted to give the corresponding hydride (2) or triflate (3) derivatives or the corresponding cation (4). These species are reduced with KC₈ or Cp₂ Co to give isolable anionic and neutral radicals (5-8). Similarly, the [Ph₃ C] cation undergoes nucleophilic attack by DAC resulting in the spontaneous formation of the radical cation (9).

Radicals derived from Lewis acid/base pairs

While conventional approaches to stabilizing main group radicals have involved the use of Lewis acids or bases, this tutorial review focuses on new avenues to main group radicals derived from combinations of donor and acceptor molecules.

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)₃p-benzoquinone reacts with persistent radicals TEMPO, trityl or decamethylferrocene by single electron transfer to give the doubly O-borylated benzosemiquinone radical anion with TEMPO⁺, trityl cation or ferrocenium counter cations.

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.

Homolytic cleavage of peroxide bonds via a single electron transfer of a frustrated Lewis pair

Single electron transfer (SET) reactions are effected by the combination of a Mes₃P/B(C₆F₅)₃ frustrated Lewis pair with benzoyl peroxide derivatives. The resulting homolytic cleavage of the peroxide bonds affords the radical salts [Mes₃P˙]⁺[RCOOB(C₆F₅)₃]^- (R = Ph 1, p-BrC₆H₄2, p-MeC₆H₄3). These species react with Ph₃SnH to give [Mes₃PH]⁺[RCOOB(C₆F₅)₃]^- (R = Ph 10, p-BrC₆H₄11, p-MeC₆H₄12) and (Ph₃Sn)₂. The mechanism of these SET reactions is discussed in light of control experiments and DFT calculations.