Cobaltaphotocatalysed intramolecular hydroarylation of alkenes for the construction of tetrahydroquinolines.

1,2,3,4-Tetrahydroquinolines are a class of heterocycles that commonly feature in both biologically active natural products and pharmaceuticals. However, their synthesis is often dominated by either high-pressure hydrogenation reactions or acid-mediated Friedel-Crafts-like hydroarylations. Recently, cobalt-salen catalysed cycloisomerisation reactions have been reported as a promising method to construct heterocycles, but there are limited examples of tetrahydroquinolines. Moreover, these protocols often rely on stoichiometric additives and proceed through radical pathways in which catalyst control is difficult to exert. Herein, we report a photochemical cobalt-salen catalysed intramolecular hydroarylation for the synthesis of C4-functionalised tetrahydroquinolines. The mild reaction conditions tolerate a wide array of functional groups, enabling the preparation of a diverse catalogue of tetrahydroquinoline derivatives. This protocol was readily adapted for scale-up by employing a photoflow system, a first for a cobalt-salen metal-hydride hydrogen atom transfer process. Collectively, experimental observations are consistent with a radical-polar crossover pathway involving formation of a free carbocation or a putative alkylcobalt(iv) carbocation surrogate.