Abstract

In the past few years it has been shown that microreactor/continuous flow technology can overcome the problems associated with large scale photochemical transformations. The relatively low reactor volume and in particular the high surface to volume ratio of capillary flow reactors assures an intense and uniform irradiation of the reaction mixture for a specific time period very precisely controlled by the pump flow rate, thus avoiding over-irradiation of the solution.

Herein we present two case studies of photochemical reactions successfully developed under a continuous flow regime in our group. The first is a protocol for the continuous bromination of benzylic compounds with N-bromosuccinimide (NBS). Using a photochemical reactor consisting of readily available materials, FEP tubing, and an inexpensive household CFL lamp, the reaction proceeds to completion within minutes in acetonitrile, thus avoiding the use of radical initiators or hazardous CCl4 as solvent.

An efficient procedure for the two-step a-trifluoromethylation of ketones will also be presented. During the first step the ketones were converted in flow into their silyl enol ethers, which were then subjected to a visible light induced trifluoromethylation. In this context, a novel system has been introduced for the a-trifluoromethylation of carbonyl compounds, which combines CF3SO2Cl as a source of CF3 radicals and Eosin Y as an inexpensive, non-toxic, and readily available photoredox catalyst. The two-step process requires only 20 min for completion, and has been successfully tested for a variety of substrates.