Synthesis of flake-shaped [3]cyclo-4,6-dibenzofuranylene

The cyclic trimer of dibenzofuran linked at the 4,6-positions, named [3]cyclo-4,6-dibenzofuranylene, was synthesized by Ni(0)-mediated reductive coupling of 4,6-dibromodibenzofuran. DFT calculation indicated that the flake-shaped C₂ symmetric structure is the most stable conformer and the propeller-shaped D₃ symmetric structure is the minor conformer with a higher energy (+19.1 kcal mol^-1). The calculated inversion energy of the C₂ conformer through a C_s symmetric transition state was +4.1 kcal mol^-1, showing rapid fluctuation, which is consistent with the observation of only one set of ¹HNMR signals of dibenzofuran of [3]cyclo-4,6-dibenzofuranylene.