Palladium-catalysed dibenzofuran synthesis by dehydrogenative ring closure

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Abstract
Diphenyl ethers readily undergo intramolecular and intermolecular dehydrogenative coupling in the presence of a catalytic quantity of palladium acetate under oxygen pressure to give dibenzofurans and dimers, respectively. The addition of an inert solvent to this system increases the yield of dibenzofurans relative to dimers. Product ratios and an isotope effect in competitive coupling suggest that dibenzofuran is formed by a stepwise coupling mechanism similar to the intermolecular coupling of aryl compounds. The preparation of dibenzofurandicarboxylic acids and the regeneration of the palladium catalyst are described.