Reactions of 2,2-dialkylvinyl iodonium salt with halide ions

Reactions of (E)- and (Z)-2-methyl-5-phenyl-1-pentenyl(phenyl) (1) salts with halide ions were examined in various solvents at 50 or 60 degreesC. The main products are those of substitution, 1-halo-2-methyl-5-phenyl-1-pentenes, mainly of inversion but involving some retained products. Varying amounts of rearranged products are also formed. Reaction follows pseudo-first-order kinetics at [1] << [halide ion]. The observed rate constants show curved dependence on halide concentration typical of reactions involving a pre-equilibrium formation of the adduct, lambda (3)-haloiodane, and first-, second-, and third-order reaction pathways. The first-order rate constant for the E isomer, (E)-1, is greater than that for (Z)-1, while the opposite is the case for the second- and third-order terms. The main reaction is considered to proceed via a vinylic in-plane S(N)2 mechanism to lead to inversion while some retention routes via out-of-plane S(N)2 and/or ligand coupling mechanism are also possible. Competing rearrangement reactions occur by the beta -alkyl participation and no evidence for formation of the primary vinylic cation was obtained.