Azobisisobutyronitrile → 4-Bromomethyl-1-(2-chloro-phenyl)-5-(4-chloro-phenyl)-1H-pyrazole-3-carboxylic acid ethyl ester

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A mixture of 1-(2-chloro-phenyl)-5-(4-chloro-phenyl)-4-methyl-1H-pyrazole-3-carboxylic acid ethyl ester I-4a (2.8 g, 7.46 mmol), N-bromosuccinimide (1.6 g, 8.95 mmol), AIBN (245 mg, 1.49 mmol) in CCl4 (60 ml) was heated under reflux for 17 hours. The reaction was cooled to room temperature, filtered to remove any solids, and concentrated under vacuum. The crude residue was purified via silica gel chromatography (Flash 40) using a solvent gradient of 10% EtOAc/hexanes to 20% EtOAc/hexanes to give the desired product (I-4b) as an amorphous solid (2.2 g, 64%): +APCI MS (M+1) 455.0.

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