Catalytic Activity of Triphenylphosphine for Electrophilic Aromatic Bromination Using <i>N</i>-Bromosuccinimide and Process Safety Evaluation

Electrophilic aromatic bromination using N-bromosuccinimide (NBS) is the most widely used reaction to synthesize highly functionalized aromatic compounds. We encountered catalytic activity of triphenylphosphine for aromatic bromination. This catalytic activity was successfully applied to a wide range of organic solvents and enabled the addition of NBS below the flash point of various organic solvents. Toward the industrial implementation of this bromination, we evaluated the process safety including the reaction heat and thermal decomposition. The analysis revealed that the characteristic behavior of the reaction heat made it difficult to suppress the increase of the internal temperature. However, precise evaluation of the reaction heat suggested the sequential addition of NBS. This procedure suppressed the increase of the internal temperature below 5 °C, which made the industrial implementation of this bromination feasible with process safety.