Reaction of Phosphorus Pentachloride with 2-Acetylthiophene and Acetophenone

Article abstract of DOI:10.1021/jo00153a016

The treatment of 2-acetylthiophene with PCl5, followed by dehydrochlorination, is known to be a poor method for synthesizing 2-ethynylthiophene (1a).Reinvestigation of the reaction showed the major products to be the E- and Z- isomers of 1,2-dichloro-1-(2-thienyl)ethylene (7a), with minor amounts of 1,1-dichloro-1-(2-thienyl)ethane (3a), 1-chloro-1-(2-thienyl)ethylene (4a), 2-(chloroacetyl)thiophene, and 2-(dichloroacetyl)thiophene.The treatment of acetophenone with PCl5 yielded similar products, and the mechanism of these reactions is discussed.The major product 7a could be converted into 1a by reaction with magnesium.The yield of 4a was increased when pyridine was also used, when only 1 equiv of PCl5 was added by portions to the ketone, or when catecholphosphorus trichloride was used instead of PCl5.The best method for converting 2-acetylthiophene into 1a goes through the enol phosphonate of 2-(bromoacetyl)thiophene, which is treated with sodium amide.