Synthesis of Dibenzofuran

Dibenzofuran is a tricyclic heterocyclic compound with a 14π electron ring system comprised of two benzene rings fused with 2,3- and 4,5-positions of the furan ring. The chemistry of dibenzofuran has not been extensively studied and explored. It has been isolated from coal tar. The structural unit is present as a substructure in various naturally occurring compounds such as morphine and codeine. Exposure to dibenzofuran either by inhalation of polluted air or drinking of contaminated water causes acute or chronic reproductive disorders and cancer. Chlorinated dibenzofuran is a highly toxic air pollutant.

Synthesis

(a) Dibenzofuran337 has been prepared through intramolecular cyclization of 2-phenoxybenzene- diazonium tetrafluoroborate with hydroquinone in 70% yields. However, when the same reaction was carried out in the presence of FeSO4, the yield of dibenzofuran was improved to 79%. 

(b) A variant of this reaction is through intramolecular cyclization of ortho-diazonium salts of diaryl ether in the presence of palladium acetate as a catalyst in ethanol.

(c) Cyclization of biphenyl ethers in the presence of equimolar use of palladium acetate in boiling acidic solvents such as acetic acid or TFA or a mixture of acetic acid and methanesulfonic acid (15:1) afforded dibenzofuran. Best results are obtained when pivolic acid is used as solvent.339 Irradiation of a dilute solution of diphenyl ethers in cyclohexane using iodine as oxidant gave dibenzofuran.

(d) An efficient synthesis of dibenzofurans has been reported through Pd/C-catalyzed intramolecular cyclization of o-iododiaryl ethers in DMA under ligand-free conditions.