A Detailed Study of Irradiation Requirements Towards an Efficient Photochemical Wohl-Ziegler Procedure in Flow

A platform has been developed to enable standardization of light sources, allowing consistent scale-up from high-throughput screening in batch to flow, using the same pseudo-monochromatic light source. The impact of wavelength and light intensity on a photochemical reaction was evaluated within this platform using the Wohl-Ziegler benzylic bromination of 4-methyl-3-(trifluoromethyl)benzonitrile with N-bromosuccinimide as a model system. It was found that only 40% of the maximum light intensity was required while still maintaining reaction rate, allowing more reliable temperature control and lower energy consumption. The optimized reaction conditions were subsequently applied to a range of synthetically relevant (hetero)aromatic compounds under continuous conditions, exploring the scope of the process within a mild and scalable procedure.