Greener Building Block For Fungicides

Scientists at specialty chemical company Solvay have developed a cost-effective, environmentally friendlier synthesis of a difluoromethylpyrazole intermediate that provides a key structural unit in the fast-growing family of succinate dehydrogenase inhibitor (SDHI) fungicides, which includes Syngenta’s Sedaxane, BASF’s Fluxapyroxad, and Bayer’s Bixafen (Org. Process Res. Dev. 2014, DOI: 10.1021/op500128p). The current method starts with tetrafluoroethylene and proceeds through ethyldifluoroacetate, which is converted to the pyrazole intermediate by a Claisen condensation with ethyl acetate followed by a cyclization step with methylhydrazine. This approach requires excess reagents and generates a large amount of waste salts and aqueous waste contaminated with halogenated organics. The Solvay team of Janis Jaunzems and Max Braun created a procedure that largely avoids those limitations by starting with trichlorodifluoroethane (a waste product from another process), using a photooxidation step to make an acyl chloride intermediate, and then concluding with a ketene condensation before the methylhydrazine cyclization. The new, fully scalable process uses nearly equimolar amounts of reagents, has better yield, and reduces total waste from about 10 kg per kg of difluoromethylpyrazole made to 2.4 kg per kg.