Photochemical and photoredox reactions in continuous microreactors : application to cycloaddition, controlled polymerization and radical chemistry

In order to mimic nature’s highly energy efficient photosynthesis reaction, this work focuses on photochemical reactions using UV/visible light, metal based recyclable catalysts and metal free catalysts in flow to synthesize organic material that have pharmaceutical and industrial applications. The utilized microfluidic systems have small path lengths (500 μm) resulting in improved illumination. Using chemical actinometry, it was shown that ≈ 98% of the light supplied reached the reaction mixture inside the widely used Mikroglas® Dwell device. [2+2] cycloaddition, used in total synthesis, was tested in flow using a sensitizer under UV. The optimized reaction was quantitative after 2 h vs. 47% after 10 h in literature’s batch system. Metal free ATRP was assessed using the commercial Eosin Y in flow with green LEDs. Only 6 h of irradiation were enough to give narrow dispersed polymers that have wide applications (plastics, latex…). Metal free catalysts are of critical importance as they are more ecofriendly. Forming new C-C and C-O bonds is the heart of organic synthesis. Using UV LEDs and a photoredox catalyst, adducts of trifluoroborate salts with TEMPO and with Michael acceptors were obtained (>99%) after only 2.5 min of irradiation in flow compared to 8-24 h in batch. Our results highlight the impact of miniaturization on accelerating photochemical reactions. Less time and energy usage, improved yields and strictly linear kinetic graphs are main features of flow technology. In addition, miniaturization requires less safety precautions rendering it favorable for large scale industry. This work supports considering the microfluidic technology for greener industrial systems.