Kinetics for Scale-Up of a One-Pot Pathway to 5-(3-Fluorophenyl)-2,4-dihydro-2,4-dimethyl-3H-1,2,4-triazole-3-thione Using a Hybrid Model of Parallel and Consecutive Reactions

Kinetic results are reported for the one-pot reaction of 2,4-dimethylthiosemicarbazide (1) and 3-fluorobenzoyl chloride (2) to the unisolated benzamide intermediate (3), and on to the ring-closed product, 5-(3-fluorophenyl)-2,4-dihydro-2,4-dimethyl-3H-1,2,4-triazole-3-thione (4). The sensitivity of the synthetic route to competing side reactions was examined by studying the kinetics of each reaction step. A first-order kinetic Model of the hybrid consecutive and parallel reaction scheme was developed and fit to experimental data. The Model was extended to address potential mixing concerns resulting from the formation of the insoluble intermediate 3 upon scale-up where a more concentrated reaction would be used. Scale-up of the product drug substance from laboratory to 50-gal scale was successfully completed using this recently developed one-pot pathway.