Synthesis of 1,1-Bicarbazoles by Sequential Iron(III)- and Palladium(II)-Catalyzed Oxidative Coupling Reactions

The iron(III)-catalyzed oxidative coupling of diarylamines to 2,2 '-bis(arylamino)-1,1 '-biaryls and subsequent twofold palladium(II)-catalyzed oxidative cyclization provide a convergent synthetic route to 1,1 '-bicarbazoles. Screening a range of different palladium(II) salts led to palladium(II) acetate, pivalate, and hexafluoroacetylacetonate as the most efficient catalysts. Remarkably, the twofold palladium(II)-catalyzed oxidative cyclization can also be performed under argon. The mechanism for the oxidative cyclization under an inert gas presumably involves regeneration of the catalytically active palladium(II) species by oxidative addition of pivalic acid. Domino Iron and Palladium Catalysis: An elegant synthetic approach to 1,1 '-bicarbazoles via activation of six C-H bonds is described. The strategy consists of an iron(III)-catalyzed oxidative homocoupling of diarylamines to 2,2 '-bis(arylamino)-1,1 '-biaryls followed by a twofold palladium(II)-catalyzed oxidative cyclization.** image