Water enables the tunable electrochemical synthesis of heterocyclic 3a-or 5a-bromoindolines

Due to the electron-richness of the C3/C2 positions of the indole moiety, direct nucleophilic bromination at the C5-position of tryptophol and tryptamine derivatives is difficult and remains unexplored. Herein, we report a tunable water-mediated electrooxidative protocol for the dearomative C3/C5-bromocyclization of tryptophol and tryptamine derivatives. This electrosynthetic approach enables selective construction of cyclic 3a- or 5a-bromoindolines simply by the addition of different amounts of water, with no additional electrolytes, external oxidants, or additives being required. Less water delivers the cyclic 3a-bromoindoline, while more water delivers the 5a-bromoindoline. A detailed mechanistic study showed that an unexpected collaborative bromide ionic electrooxidation at the anode and reductive hydrodebromination at the cathode in the presence of more water is key to the tunable chemoselectivity control.

Automated summary

In this study, a water-mediated electrochemical synthesis method was developed for the C3/C5-bromocyclization of tryptophol and tryptamine derivatives. By controlling the amount of water, the selective synthesis of 3a- or 5a-bromoindolines was achieved without the need for additional oxidants or additives.