In-plane vinylic SN2 substitution and intramolecular β elimination of β-alkylvinyl(chloro)-λ3-iodanes

The reactions of four (E)-beta-alkylvinyl(phenyl)iodonium salts with chloride ion were examined in acetonitrile and in several other solvents at 25 degrees C. The beta-methyl-, beta-octyl-, and beta-isopropyl-substituted iodonium salts undergo competitive bimolecular nucleophilic substitution to form the corresponding (Z)-1-chloro-1-alkene with inversion of configuration at the vinylic carbon and elimination to form the 1-alkyne. The beta-tert-butyl-substituted iodonium salt affords only the products of the elimination reaction. The UV absorption spectra of the reactants show the rapid coversion of chloride and iodonium ions to an equilibrium mixture of the corresponding chloro-lambda(3)-iodane, with an association constant of 5600-7600 mol(-1) dm(3). A kinetic analysis shows that most of the substitution and elimination products form from reaction of the lambda(3)-iodane. Evidence is presented that the substitution reaction proceeds by a concerted bimolecular S(N)2 mechanism and that the elimination reaction proceeds by a unimolecular reaction mechanism with intramolecular transfer of the beta-proton to the leaving group.