Chlorosulfonation of 2-acylthiophenes: an examination on the reaction regiochemistry

Chlorosulfonation of thenoyltrifluoroacetone with neat chlorosulfuric acid was found to give a 25:75 mixture of 3- and 2-chlorosulfonated thenoyltrifluoroacetone isomers in 33% overall yield. The use of dichloromethane as solvent for the chlorosulfonation reaction gave only the 2-chlorosulfonated isomer in 45% yield. On the contrary, the reaction of 2-acetylthiophene with neat chlorosulfuric acid gave only the 3-chlorosulfonated-5-acetylthiophene in 35% yield. The identity of these chlorosulfonated compounds was unambiguously established by NMR techniques and confirmed by the crystal structure determination of the 2-chlorosulfonated isomer.