Toward a Continuous-Flow Synthesis of Boscalid®

A two-step continuous-flow protocol for the synthesis of 2-amino-4'-chlorobiphenyl, a key intermediate for the industrial preparation of the fungicide Boscalid (R) is described. Initial tetrakis( triphenylphosphine) palladium-catalyzed high-temperature Suzuki-Miyaura cross-coupling of 1-chloro-2-nitrobenzene with 4-chlorophenylboronic acid in a microtubular flow reactor at 160 degrees C using the tert-butanol/water/potassium tert-butoxide solvent/base system provides 4'-chloro-2-nitrobiphenyl in high yield. After in-line scavenging of palladium metal with the aid of a thiourea-based resin, subsequent heterogeneous catalytic hydrogenation is performed over platinum-on-charcoal in a dedicated continuous-flow hydrogenation device. The overall two-step homogeneous/heterogeneous catalytic process can be performed in a single operation providing the desired 2-amino-4'-chlorobiphenyl in good overall yield and high selectivity.