Professor Steven Ley and his team at Cambridge University have demonstrated how visible light can be used to perform complex photoredox cross-couplings in continuous flow.
Using a Vapourtec E-series equipped with the UV-150 photochemical reactor for blue LEDs irradiation, the Ley group have used the enhanced light irradiation properties of continuous flow to add to the field of carbon radical chemistry. An iridium based photocatalyst was used to generate a C(sp3) radical from commercially available boronic pinacol esters using visible light, which could then be coupled with aryl bromides and heteroaromatic nitriles. The approach tolerated a range of functionality on the aryl bromides, including aldehyde and alkyne groups, and the group were also able to demonstrate that the method could be applied to a range of N-heterocycles, with particular success for those built around a 4-cyanopyridine scaffold.
The Ley group’s expertise with continuous flow is renowned, and it is remarked in the text that “more efficient irradiation with microchannel devices and shorter residence times often result in faster and cleaner photo-reactions in flow”. Using continuous flow, Ley and his team have avoided common limitations with batch photochemical reactions resulting from poor penetration depths through the reaction stream.
Dr Ryan Skilton, Research Scientist at Vapourtec comments “Professor Ley and his team have demonstrated an exciting, novel method for performing quite a complex coupling. It’s very pleasing to see the Vapourtec E-series and UV-150 playing a part in this research. Flow chemistry allows a more efficient light absorption, which has helped the Ley group to significantly reduce the reaction times compared to the equivalent batch process. The advantage is that the throughput of this novel reaction can be increased, which is an important consideration for any future scale up”.
As always, Professor Ley and his group have demonstrated how continuous flow can be used as a tool for new synthesis and improved reaction efficiency.