Heck reactions of alkenes with aryl iodides and aryl bromides: Rate-determining steps deduced from a comparative kinetic study of competing and noncompeting reactions

Joint kinetic analysis of competition and noncompetition experiments can provide important information and sometimes a direct answer concerning the rate-determining step of a catalytic reaction. Applying this approach to the Heck reaction has demonstrated that alkenes; reactive aryl iodides; and, contrary to the conventional opinion, unactivated aryl bromides participate in rapid steps of the Heck reaction. For aryl iodides, the rate-determining step includes the dissociation of a C-H bond. At the same time, kinetic data for the dissolution of palladium reacting with aryl bromides suggest that the rate of the catalytic reaction is determined by the slow dissolution of palladium aggregates.