Vinylic SN2 Reaction of 1-Decenyliodonium Salt with Halide Ions

Article abstract of DOI:10.1246/bcsj.71.243

Reactions of (E)-1-decenyl(phenyl)iodonium salt with halide ions are examined under various conditions. The products are those of substitution and elimination, usually (Z)-1-halo-1-decene and 1-decyne, as well as iodobenzene, except for the reaction with fluoride which leads exclusively to elimination. The labeling experiments show that the elimination induced by fluoride occurs via an α mode, while that caused by the other halides is a syn β elimination. Kinetic results show that the substitution and β elimination occur mainly from the halo-λ³ -iodane intermediate; the substitution occurs as a bimolecular reaction with the external halide ion while the elimination is a unimolecular (intramolecular) reaction. The imermediacy of the hypervalent λ³-iodane as well as of the iodate are also confirmed by UV spectroscopy. The secondary kinetic isotope effects, leaving group substituent effects, and pressure effects on the rate are compatible with the in-plane vinylic S_N2 mechanism for the substitution with inversion. Some retained substitution product, (E)-1-halo-1-decene, is formed in polar protic solvents like trifluoroacetic acid, formic acid, and 1,1,1,3,3,3-hexafluoro-2-propanol. The reaction is very slow in these solvents and no sign of formation of ionic intermediates is found. This slow substitution with retention is considered to occur via ligand coupling within the halo-λ³-iodane intermediate.