Visible-light-induced oxidant and metal-free dehydrogenative cascade trifluoromethylation and oxidation of 1,6-enynes with water

Abstract

Generally, oxy-trifluoromethylation in olefins is achieved using oxidants and transition metal catalysts. However, labile olefins remain unexplored due to their incompatibility with harsh reaction conditions. Here, unprecedented light-induced oxidant and metal-free tandem radical cyclization–trifluoromethylation and dehydrogenative oxygenation of 1,6-enynes have been achieved using a photoredox catalyst, CF₃SO₂Na, and phenanthrene-9,10-dione (PQ), Langlois’ reagent (CF₃SO₂Na) and water as the oxygen source. This benign protocol allows for access to various CF₃-containing C₃-aryloyl/acylated benzofurans, benzothiophenes, and indoles. Moreover, the oxidized undesired products, which are inherently formed by the cleavage of the vinylic carbon and heteroatom bond, have been circumvented under oxidant free conditions. The mechanistic investigations by UV-visible and ESR spectroscopy, electrochemical studies, isotope labelling and density functional theory (DFT) suggest that light induced PQ produced a CF₃ radical from CF₃SO₂Na. The generated CF₃ radical adds to the alkene, followed by cyclization, to provide a vinylic radical that transfers an electron to PQ and generates a vinylic cation. Alternatively, electron transfer may occur from the CF₃-added alkene moiety, forming a carbocation, which would undergo cationic cyclization to generate a vinylic carbocation. The subsequent addition of water to the vinylic cation, followed by the elimination of hydrogen gas, led to the formation of trifluoromethylated C₃-aryloyl/acylated heterocycles.