TBAI-assisted direct C–H activation of indoles with β-E-styrene sulfonyl hydrazides: a stereoselective access to 3-styryl thioindoles

The current work describes the challenging introduction of a vinyl sulfide group by simple C–H activation on a variety of substrates. The direct C–H activation of indoles with β-(E)-styrene sulfonyl hydrazides under the sulfenylation conditions, assisted by the iodic catalyst tert-butyl ammonium iodide (TBAI), afforded a series of (E)-styrylthioindoles. Accordingly, β-(E)-styrene sulfonyl hydrazides undergo radical cross-coupling reactions with a variety of substituted indoles to afford structurally diverse indole vinyl thioethers in moderate to high yields with E-stereoselectivity. This method is metal-catalyst-free and is valuable not only because of its novelty, but also for providing a convenient synthetic pathway to a variety of (E)-styrylthioindoles with retention of the configuration. The current study paves the way for the use of β-(E)-styrene sulfonyl hydrazides as a unique styryl mercaptan source in chemical synthesis.