Kinetics and mechanism of heterogeneous palladium-catalyzed coupling reactions of chloroaryls in water

Abstract

Coupling reactions of substituted chlorobenzenes to biphenyls catalyzed by palladium on carbon are performed in water using sodium hydroxide and sodium formate in the presence of a surface active agent. Thus, chlorobenzene, p-chloro-o-xylene, p-chloro-1,1,1-trifluorotoluene, p-chloroanisole, and p-chlorotoluene are coupled under moderate conditions to the respective biaryls. A competing reduction process occurs (e.g. chlorobenzene is reduced to benzene), which can be minimized by altering conditions. The relationship of product selectivity to reaction temperature, formate concentration, base concentration, and surfactant type is examined. The roles of formate, Pd catalyst, and surfactant are discussed. It is proposed that the reduction is dependent on the participation of palladium hydride [Pd²⁺(H^–)₂], while the coupling occurs via single electron-transfer from Pd⁰ to the substrate, with subsequent decomposition of the chloroaryl radical anions to obtain aryl radicals and chloride anions. This mechanism is supported by experiments with stoichiometric and sub-stoichiometric amounts of palladium which indicate that selective coupling can occur also in the absence of hydrogen (providing that reduced palladium Pd⁰, is present in sufficient amount), and by kinetic investigations which indicate that the coupling is actually a first-order reaction, for which the rate-determining step may be the dissociation of the chloroaryl radical anion.