EPR spectra and redox properties of radical cations of dibenzofuran, methylated dibenzofurans and bidibenzofurans: relation to the chemistry of dibenzofuran radical cation

Abstract

The radical cations of dibenzofuran 1, a variety of symmetrically methylated dibenzofurans, viz. 1,9- 2,2,8- 3,3,7- 4 and 4,6-dimethyldibenzofuran 5,2,4,6,8- 6 and 1,3,7,9-tetramethyldibenzofuran 7,1,3,4,6,7,9-hexamethyl- 8 and octamethyl-dibenzofuran 9, the three symmetrical dehydrodimers, 2,2′-10,3,3′-11 and 4,4′-bidibenzofuran 12 and the unsymmetrical dehydrodimer 2,3′-bidibenzofuran 13 have been studied by cyclic voltammetry and EPR spectroscopy. For the latter purpose, the radical cations were generated by oxidation with thallium(III) trifluoroacetate in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) at 22 °C or by DDQ-light oxidation in trifluoroacetic acid at –12 °C. For the dibenzofuran radical cation and its 2-deuteriated isotopomer, a matrix study was performed in CFCl₃ at –196 °C.

The dibenzofuran radical cation could not be generated in fluid solution due to its high reactivity, leading to the formation of oligomeric and polymeric species. Monomeric compounds 4,7,8 and 9, which have the 3,7-positions blocked for further reaction, gave persistent monomeric radical cations, whereas the remaining ones underwent fast transformations to other species (3˙⁺ to a dehydrodimer radical cation, 2˙⁺, 4˙⁺–6˙⁺ to oligomeric species). An analysis of the hfs constants of the monomeric radical cations indicated that the 3,7-positions of 1˙⁺ carry about 70% of the total spin density which is in agreement with the so far known chemistry of this nonalternant radical cation.

Both dehydrodimers 10 and 11 gave fairly persistent radical cations, whereas that of 12 could not be detected, presumably due to oligomerization. The radical cation of the 2,3′-dehydrodimer gave an EPR spectrum showing that the spin density is localized on one of the dibenzofuran moieties.