The Use of Conductivity Measurements for the Characterization of Cationic Palladium(II) Complexes and for the Determination of Kinetic and Thermodynamic Data in Palladium‐Catalyzed Reactions

Abstract

AbstractThis review illustrates how conductivity measurements are used to characterize the cationic character of aryl‐, vinyl‐, hydrido‐, and (η3‐allyl)palladium(II) complexes generated by oxidative addition of aryl and vinyl triflates, acetic/formic acid, and allylic carboxylates/carbonates, respectively, with palladium(0) complexes in DMF. These cationic palladium(II) complexes have been postulated as key intermediates in Stille, Heck, Tsuji−Trost, and Trost reactions, but most of them have never been characterized in the context of real oxidative additions, i.e., with the counteranion delivered by the reagent (triflates, carboxylates, and carbonates). The role of chloride ions has been examined and the mechanistic consequences are reported. Conductivity measurements are also used to obtain thermodynamic and kinetic data on the formation of cationic palladium(II) complexes and, consequently, to gain a deeper insight into the mechanism of palladium‐catalyzed reactions. The reversibility of oxidative additions of acetic/formic acid and allylic carboxylates/carbonates with Pd0 complexes has been established and equilibrium constants have been determined. The isomerization of cyclic trans/cis cationic (η3‐allyl)palladium(II) complexes is evidenced by a rate constant that depends on the Pd0 precursor: kinetic evidence for an SN2 mechanism that is responsible for the loss of stereoselectivity observed in catalytic reactions. Under conditions in which the oxidative additions are irreversible, the kinetics of the formation of cationic (η3‐allyl)palladium(II) complexes has been monitored by conductivity measurements for the determination of the rate constants. By comparing the rate of disappearance of Pd0, it has been shown that oxidative additions of allylic carboxylates proceed in two steps with the detection of intermediate neutral, kinetic Pd0 complexes ligated to the allylic carboxylate unit. As an example of its application, the mechanism of a Stille reaction has been investigated by conductivity measurements of an ionic by‐product in DMF. © Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003