Cu(I) Complexes Catalyzed the Dehydrogenation of N-Heterocycles

A copper-catalyzed method for the dehydrogenation of various nitrogen-containing heterocycles to furnish quinolines and indoles has been developed. A range of 1,2,3,4-tetrahydroquinolines underwent dehydrogenation by employing 2 mol % of copper complex Cat 3 as a catalyst and using O₂ as an oxidant at 120 °C in 1,2-dichlorobenzene to afford the desired quinolines. The method enables the dehydrogenation of a variety of indolines in the presence of 2 mol % of copper complex Cat 2, using 10 mol % of TEMPO as an additive and O₂ as an oxidant under room temperature in tetrahydrofuran to furnish indoles in high yields. Mechanistic studies suggested that the dehydrogenative activity is ascribed to the formation of a copper(II) active species from copper(I) complexes oxidized by O₂, which was proved by high-resolution mass spectrometry (HRMS). The copper-catalyzed dehydrogenation reaction proceeds via a superoxide radical anion (·O₂^-) as proved by electron paramagnetic resonance (EPR) spectrometry. In situ infrared spectroscopy revealed that the dihydroquinoline intermediate was formed in the dehydrogenation of 1,2,3,4-tetrahydroquinolines.