Photosensitizers Enable the Formation of Biphenyls with UV-LEDs and Sunlight

The regioselective synthesis of biphenyls, which are economically important pharmaceuticals, agrochemicals, and liquid crystals, is a challenging task. Current methods rely on metal-dependent cross-coupling reactions, which unfortunately require the use of harmful halogenated aryls and heavy metal catalysts that are toxic and difficult to remove from the final products. Recently, we have circumvented these problems by developing a metal-free and broadly applicable photochemical method for biphenyl synthesis using UV-C light, called photosplicing. Here we present an improved method using photosensitizers in combination with UV-B, UV-A light, or sunlight. Using a high-precision flow reactor with deep-UV LEDs, we investigated the ability of commonly available organic photosensitizers to enhance the photosplicing reaction and identified a number of suitable photosensitizers with the required triplet energy. This method allows for easy batch synthesis of biaryls in borosilicate glassware and paves the way for their large-scale production without the need for flow reactors.