Photocatalytic Synthesis of 3,4-Dihydroquinolone from Tetrahydroquinolines by a High-Throughput Microfluidic System and Insights into the Role of Organic Bases

3,4-dihydroquinolone and its derivatives are structural motifs found in diverse pharmacologically active compounds. Direct oxidation of tetrahydroquinolines represents the most efficient synthetic route to 3,4-dihydroquinolone. However, the reaction conditions reported in previous studies were either relatively harsh or complex. We also attempted previously reported photocatalytic oxidation methods for the α-carbonylation of amines, but these approaches failed to efficiently produce 3,4-dihydroquinolone. Herein, we present an efficient photocatalytic oxidation methodology facilitated by our in-house high-throughput microfluidic system, which can be carried out under mild conditions with a short reaction time. Moreover, a new reaction mechanism, in which the organic base DBU serves a dual role as both an electron donor and a hydrogen atom transfer (HAT) mediator, is proposed and supported by DFT calculations.