Visible light–promoted reactions offer industrial chemists simple yet unique routes to valuable pharmaceutical compounds. But the chemists have struggled to run these powerful transformations on a large scale because it is hard to get light where it needs to go in a reactor. For example, lamps and light-emitting diodes can penetrate only a few millimeters into a solution.
To address this issue, chemists have turned to tubular flow, pumping their reaction solutions through spindly tubes for maximum exposure to light. Now, researchers at AbbVie report a new solution to the light-penetration problem: high-powered lasers (ACS Central Science 2019, DOI: 10.1021/acscentsci.8b00728).
The team hooked up a laser to a beam expander and fiber optic cables to irradiate a continuous stirred-tank reactor (CSTR)—a standard reactor used in industry. This set-up avoids heating the reaction, which is an issue for large-scale tubular flow because chemists must point an array of lights at the reaction. The tank reactor also allowed chemists to use insoluble reagents that aren’t tolerated in typical flow systems because they clog the lines.
But the main advantage, the authors say, is the increased penetration of light into the reaction vessel. They found that the rate of reaction, and thus reaction yields and throughput, increased as the intensity of the light source increased. The team achieved reaction rates of kilograms of product per day in a 100 mL reactor.
Ian W. Davies, director of Princeton University’s Catalysis Initiative, calls the set-up a creative adaptation of CSTRs and a valuable addition to photoredox flow technology.