Abstract

(Photo)Chemical Conversion of Diazonium Salts in Flow Including Online NMR Analysis

Thomas Rehm, Senior Scientist, Fraunhofer IMM

The use of microstructured reactors has become a key technology for (photo)chemical synthesis.  As first example for flow photochemistry we have applied our falling film microreactor (FFMR) technology for the blue light mediated C-H arylation of heteroarenes with diazonium salts using TiO2 as immobilized photocatalyst in the microchannels of the FFMR. Beside catalyst analysis a broad scope of aryldiazonium salts were investigated with furan, thiophene and pyridine as heteroarenes including a long-term run.  As second example we used a capillary photoreactor for the dye-sensitized C-H arylation of furan with a trifluoromethylated diazonium salt.  In this case we adopted a benchtop NMR spectrometer for the online 19F NMR analysis of the reaction progress. On-going research now focuses on the non-catalysed C-C coupling of diazonium salts with arene derivatives in flow reactors connected to our online NMR analytics. All three processes benefit from the mild (photo)chemical reaction conditions and the flow capabilities of microreactors. Additionally, integrated online NMR technology has proved its efficiency as contact-free analytical method for syntheses in flow mode.