Mechanistic insights for the photoredox organocatalytic fluorination of aliphatic carbons by anthraquinone using time-resolved and DFT studies

Abstract

Chemoselective photoredox fluorination is an appealing approach to access fluorinated fine chemicals such as active pharmaceutical ingredients, but most of the known procedures currently lack time-resolved mechanistic insights. We use nanosecond transient absorption spectroscopy and density functional theory (DFT) calculations to elucidate the elementary steps after irradiation in a photocatalytic fluorination procedure that we reported previously. Time-resolved optical spectroscopy suggests that direct reaction only occurs between the photoexcited anthraquinone (AQN) and Selectfluor®. We have observed spectroscopic evidence of a novel transient AQN–Selectfluor® species for the first time. Further studies by DFT calculations suggest that the AQN–Selectfluor® triplet exciplex formed by photoirradiation is responsible for initiating and sustaining the fluorination reaction.