Reactions of Unsaturated Compounds with Iodine and Bromine on γ-Alumina

Article abstract of DOI:10.1021/jo00254a011

Iodine reacts with a diverse group of unsaturated substrates in the presence of dehydrated alumina.Aromatic substrates, for example, are iodinated at room temperature by electrophilic aromatic substitution.The yields of the iodinated products can be improved dramatically by running the reaction at 110 deg C.Aniline, even at 100 deg C, is unreactive, presumably because its nitrogen lone pair is complexed to surface aluminum cation or protons.Alkenes react readily with HI, formed in the reaction of surface hydroxyls with I2, to form alkyl iodides by an ionic mechanism involving carbocations.This is an excellent method for the synthesis of alkyl iodides from alkenes.Alkynes, on the other hand, add I2 (stereospecific, anti) under very mild conditions to form vinyl diiodides in excellent yield.The behavior of Br2 with cyclohexene on alumina is considerably different than that of I2 withcyclohexene.Two products are formed: trans-1,2-dibromocyclohexane (1), which is formed in solution during workup, and trans-1-bromo-2-chlorocyclohexane (2), which is formed on the surface.The chlorine in 2 arise from intrinsic chloride already on the alumina and CCl4, the solvent used to adsorb Br2 onto alumina.Further experiments demonstrate that cyclohexene undergoes intergranular hopping, but that Br2 does not.