A new and efficient synthetic method for the herbicide carfentrazone-ethyl based on the Heck reaction

The herbicide carfentrazone-ethyl (1) was prepared by a new and improved synthetic method. The common and inexpensive reagent ethyl acrylate was employed to replace commercially unavailable ethyl 3-hydroxy-2-methylenebutanoate, which was used in the synthetic route reported previously. Starting from iodination of 1-(4-chloro-2-fluorophenyl)-4-difluoromethyl-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazole (2), an intermediate 1-(4-chloro-2-fluoro-5-iodophenyl)-4-difluoromethyl-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazole (3) was afforded in an excellent yield. Then, an intermediate ethyl 3-[2-chloro-5-(4-difluoromethyl-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl)-4-fluorophenyl]-2-propenoate (4) was synthesized by Heck coupling of 3 with ethyl acrylate. Next, oxidative addition-elimination of 4 with OXONE®/HCl-Et₃N in one pot produced ethyl 2-chloro-3-[2-chloro-5-(4-difluoromethyl-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl)-4-fluorophenyl]-2-propenoate (5). Finally, the target product 1 was obtained via reduction of 5 by H₂. This new synthetic method exhibits the advantages of mild conditions, atom economy, low-cost, and efficiency.