Electrophilic Additions of Positive Iodine to Alkynes through an Iodonium Mechanism

Alkynes react with bis(pyridine)iodonium(I) tetrafluoroborate (1) and nucleophiles (CH3COOH, HCOOH, Cl-, pyridine, Br-, I-) to give 1,2-iodofunctionalized alkenes. The regiochemistry of the processes is in accordance with the polar effects of the triple bond substituents. With regard to stereochemistry, terminal alkynes yield anti addition products, but internal ones produce anti addition with weaker nucleophiles (CH3COOH, HCOOH, Cl-, pyridine), syn with stronger nucleophiles (I-), and a mixture of anti and syn with borderline nucleophiles (Br-) or when the internal alkyne bears a bulky group. An ionic mechanism through a vinyleneiodonium ion is proposed to explain the obtained results. © 1990 American Chemical Society