Novel Access to Indazoles Based on Palladium-Catalyzed Amination Chemistry

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Two efficient methods to construct the indazole nucleus have been developed, both of which utilize palladium-catalyzed intramolecular carbon-nitrogen bond formation. One is based on intramolecular Buchwald-Hartwig amination reaction of 2-halobenzophenone tosylhydrazones. The catalyst system we developed for this reaction allows the cyclization to proceed under very mild conditions and thus could be applied to a wide range of substrates with acid- or base-sensitive functional groups. Furthermore, this methodology could be applied for the construction of benzoisoxazole ring system. In addition, catalytic C-H activation with palladium followed by intramolecular amination of benzophenone tosylhydrazones was also accomplished with the aid of the catalyst system such as Pd(OAc)(2)/Cu(OAc)(2)/AgOCOCF(3), which gave another route to indazoles. Using this combination, indazoles with various functional groups could be obtained in good to high yields, especially in the case of substrates having electron donating group such as methoxy group on benzene ring. Interesting chemo- and regioselectivity were also observed in this reaction.