Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substitutedcis- andtrans-Stilbenes in Protic Solvents

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 mY Br relationships (linear for nucleophilic solvents only, with m = 0.8), the deviations of the two nonnucleophilic solvents from the mY Br plots (Δ AcOH and Δ TFE positive, negative, or negligible), the kinetic solvent isotope effects ( k MeOH / k MeOD = 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.