Pd-Catalyzed Synthesis of Dibenzofurans by Phenol-Directed C-H Activation

Significance

Reported is the synthesis of dibenzofurans 2 from 2-arylphenols 1 via palladium-catalyzed phenol-directed C-H activation/C-O cyclization. Control experiments (formation of stoichiometric Pd complexes and isotope labeling studies) showed that C-O reductive elimination was the rate-limiting step. Accordingly, the bulky additive MesCO₂Na was found to increase the yield by promoting reductive elimination. A broad range of functionalities (including acid-sensitive groups, heterocycles, and even sugars) are tolerated, many of which provide synthetic handles for further functionalization. To demonstrate the orthogonality of this method with an established ­related carbazole synthesis (J. A. Jordan-Hore, C. C. C. Johansson, M. Gulias, E. M. Beck, M. J. Gaunt J. Am. Chem. Soc. 2008, 130, 16184), compound 3 was prepared by selective C-O coupling to form the dibenzofuran followed by C-N coupling to afford the carbazole.