An associative electron upconversion is proposed as a key step determining the selectivity of thiol-yne coupling. The developed synthetic approach provided an efficient tool to access a comprehensive range of products - four types of vinyl sulfides were prepared in high yields and selectivity. We report practically important transition-metal-free regioselective thiol-yne addition and formation of the demanding Markovnikov-type product by a radical photoredox process. The photochemical process was directly monitored by mass-spectrometry in a specially designed ESI-MS device with green laser excitation in the spray chamber. The proposed reaction mechanism is supported by experiments and DFT calculations.
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