Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substituted<i>cis</i>- and<i>trans</i>-Stilbenes in Protic Solvents

Abstract

Rates and products of electrophilic bromination of ring-substituted cis- and trans-stilbenes have been investigated in acetic acid, trifluoroethanol, ethanol, methanol, and water−methanol mixtures. The mYBr relationships (linear for nucleophilic solvents only, with m = 0.8), the deviations of the two nonnucleophilic solvents from the mYBr plots (ΔAcOH and ΔTFE positive, negative, or negligible), the kinetic solvent isotope effects (kMeOH/kMeOD = 1.1−1.6), the chemoselectivity (predominant dibromide, DB, or solvent-incorporated adducts, MA), and the high dependence of the stereochemistry on the solvent and the substituents (from stereoconvergency to stereospecificity) are discussed and interpreted in terms of a mechanistic scheme, analogous to the Jencks scheme for aliphatic nucleophilic substitutions, in which preassociation, free-ion, and ion-pair pathways compete. In particular, the stereochemical outcome of these reactions is consistent with a marked change in the nucleophilic partners of the product-forming ionic intermediate arising from different ionization routes. Return, i.e. change in the rate-limiting step from ionization to product formation, is shown to be related to substituent-dependent, but not solvent-dependent, bromine bridging.