New photocatalyzed C-3 alkylation of unprotected indoles with Michael acceptors: Push-pull TPA derivative as emerging photosensitizer

Finding relatively simple, robust, and affordable photosensitizers is highly valuable for the development of photocatalytic methods. Herein, we report a UV-light-promoted Friedel–Crafts type alkylation of 1 H -indole with α,β-unsaturated ketones, utilizing 4-nitro- N , N -diphenylaniline as a photosensitizer and silver(I) triflate as a co-catalyst. The reaction proceeds at room temperature in an open flask. Remarkably, it does not require N -protection of the indole, affording 3-substituted indoles in modest to good yields across a range of chalcones. Control experiments indicate that UV light, Ag(I), and the photosensitizer are all necessary; TEMPO inhibits the reaction, supporting the proposal of a radical mechanism of the reaction. TD-DFT calculations were performed to rationalize the performance of the nitro-substituted triphenylamine (TPA) in terms of frontier orbitals and photophysical properties. To the best of our knowledge, this is the first report of the use of a nitro-TPA derivative as a photosensitizer for C C bond formation in the Friedel–Crafts type alkylation of indole. The simplicity and accessibility of this system make TPA derivatives a suitable addition to the photocatalysis toolbox.