Friedel-Crafts Benzoylation of Dibenzofuran

Abstract

Friedel-Crafts benzoylation of dibenzofuran has been studied in detail to elucidate the isomer distributions and the reactivity of each position of dibenzofuran. The reaction of dibenzofuran with benzoyl chloride in the presence of aluminum chloride (molar ratio; 1: 1: 1) in nitrobenzene at 15°C for 1 hour gave benzoyldibenzofuran (BD) in yield of 93.3% which was composed with 1-BD of O.6%, 2-BD of 90.5%, 3-BD of 8.7%, and 4-BD of 0.2%. Carrying out the reactions in chlorinated hydrocarbons or at high temperature, the yield of 3-BD increased. The competitive benzoylation of dibenzofuran and benzene gave the following partial rate factors (ft), f, : 34.6, f, : 4520, f, : 371, and f4: 19.8. Using an excess of aluminum chloride to benzoyl chloride, f f2, and especially f decreased. It was assumed that the low reactivities of 1- and 4-positions were attributed to the steric hindrance by hydrogen at 9-position and the n-orbital of oxygen at 5-position in dibenzofuran against the acylium complex.
The, CND0/2 calculations for the intermediates assumed in protonation of dibenzofuran were carried out to obtain the reactivity order of each position of dibenzofuran. The result was consistent with one in protodetritiation of dibenzofuran. The difference of results in the protonation and the acylation were interpreted on the basis of the difference in the steric hindrances between the n-orbital of oxygen of dibenzofuran and the attacking agents.