Direct synthesis of 2-arylpyrole from calcium carbide

Abstract

2-Arylpyrrole is an important building block found in pharmaceuticals and organic material compounds. A conventional way to make such compounds involves Trofimov reaction, requiring acetylene gas as a starting material. This gas however, has many disadvantages such as high flammability, toxicity and difficulty to control the amount of reagent. In this work, we therefore develop a new method to make 2-arylpyrrole directly from primary chemical feedstock calcium carbide which has low cost and less toxicity. In the optimization study, treatment of aryloximes, KOH and calcium carbide at 100℃ for 15 h in wet DMSO as a solvent, resulted in the formation of 2-phenylpyrrole in 73% yield along with small amount of 2-pheny-N-vinylpyrrole in 8% yield. Applying such condition to different oximes carrying various substituents afforded 2-aryl-N-vinylpyroles in 0-52% yield. Furthermore, we also developed a one-pot synthesis of 2-phenylpyrrole from acetophenone to give 56% yield of the desired product without purification of the oxime intermediate. For the application of this methodology, we are able to convert 2-phenylpyrrole into 4,4-difluoro-4bora-3a,4a-diaza-s-indacene (BODIPY) in 3 steps in 54% yield which is more convenient than the conventional method. Due to the high conjugation of 2-phenylpyrrole, the prepared BODIPY showed the strong emission in the red region (600 nm).