Polar addition of olefins. Part I. Stereochemistry of the halogenation of cis- and trans-2-t-butylstyrene. Rotational isomerism of the products

Abstract

Halogenation of cis- and trans-2-t-butylstyrene in carbon tetrachloride occurs by a non-stereospecific addition. The stereochemistry of dibromide formation depends strongly on the starting olefin, but the same ratio (50%) of dichloro-compounds is obtained from either cis- or trans-olefin. The results provide further support for an unsymmetrical bridged bromonium ion intermediate in the bromination reaction, but indicate that an open carbonium ion is involved in the chlorination reaction. The coupling constants obtained for the dihalogeno-compounds show that conformers with gauche t-butyl and phenyl groups are substantially populated in both the threo- and the erythro-compounds. This observation is shown to have analogies with the rotamer populations in halogenoethanes.