Structural effects in ion pairs from 1,3-diarylbenzo[c]furans and alkali metals: an electron paramagnetic resonance and ENDOR study

Abstract

The radical anions of 1,3-diarylbenzo[c]furans (1–4) obtained by reduction with alkali metals have been studied by EPR, ENDOR and TRIPLE resonance. For 1˙^– and 2˙^–, the inequivalence of both ortho hydrogens of each aryl substituent was established, and a small dihedral angle between the benzo[c]furan system and each aryl ring. For 3˙^– and 4˙^–, the four ortho methyl groups of the aryl substituents in positions 1 and 3 were found to be magnetically equivalent, and the dihedral angle was estimated to be ca. 70°. From the distribution of spin density and excess negative charge for each of the anions and the nature of the alkali-metal counter-ion, conclusions can be drawn about the position of the cation in each case. The lifetimes of the radical anions increase in the order of decreasing electron spin density and excess negative charge density on the furan ring.