Indoles and their derivatives belong to an important substance class which is often found in natural products and in many pharmaceuticals. Great efforts have recently been devoted to transition-metal-catalyzed chemical modifications of indoles, in particular direct C(2)-H or C(3)-H arylations. Palladium has been heavily used in that regard. Herein we present our first results on direct C-H arylations of indoles with arylboronic acids and the 2,2,6,6-tetramethylpiperidine N-oxyl radical (TEMPO) as an external mild oxidant (→ 1a-f) Eq. (1). More importantly, we will show that upon installing a protecting group (PG) on the indole nitrogen atom, the reaction outcome changes and the product formed is that of a highly stereoselective oxidative arylcarboaminox-ylation reaction (→2a-c). To our knowledge metal-catalyzed arylation of indoles by oxidatively intercepting putative cationic intermediates is unknown.
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