Kinetic Studies of Heck Coupling Reactions Using Palladacycle Catalysts: Experimental and Kinetic Modeling of the Role of Dimer Species

Abstract

Experimental kinetic studies of the coupling of p-bromobenzaldehyde (1) with butyl acrylate (2) using the dimeric palladacycles complex (4) with chelating nitrogen ligands were carried out together with kinetic modeling using a reaction rate expression based on the mechanism shown in Scheme 2. The oxidative addition product of 1 was found to be the resting state within the catalytic cycle. The formation of dimeric Pd species external to the catalytic cycle helped to rationalize a non-first-order rate dependence on catalyst concentration. Theoretical modeling showed how the relative concentrations of the different intermediate species within the catalystic cycle can influence the observed rate dependence on Pd concentration. It was shown how conventional kinetic studies may give reaction orders in substrates which differ from those which would be observed under practical synthetic conditions. Comparison between phosphine- and nonphosphine-based palladacycles suggests that they follow the same reaction mechanism. The role of water in accelerating the initial formation of the active catalyst species is noted.

Mechanisms of the Mizoroki–Heck Reaction

“…However, Pd(OAc) 2 must be reduced in situ to a Pd(0) species which initiates the catalytic cycle by an oxidative addition to the aryl iodide. Some reagents of Mizoroki-Heck reactions may play the role of reducing agents, such as alkenes proposed by Heck [3b], according to the mechanism depicted in Scheme 1.9: intramolecular nucleophilic attack of acetate onto the alkene coordinated to Pd(OAc) 2 , followed by a β-hydride elimination leading to HPdOAc and subsequent formation of Pd (0) in the presence of a base [3b, 11, 12].…”

“…Indeed, a β-hydride elimination may take place in the amine coordinated to the Pd(II) centre, leading to HPdOAc and then to Pd (0) in the presence of the amine (Scheme 1.10) [13]. Some additives, such as ammonium salts R 4 N + X − (X = Br, Cl, R = n-Bu), greatly improve Mizoroki-Heck reactions performed in the absence of phosphine ligands (Jeffery process [1h]).…”

“…Such a mechanism written by Heck as successive reactions [1a c] is presented as the catalytic cycle in Scheme 1.8. After formation of a Pd(0) catalyst from the precursor Pd(OAc) 2 by a vaguely defined reduction process, the following steps of the catalytic cycle were proposed: (i) The first step of the catalytic cycle is an oxidative addition of the aryl halide to a Pd (0) complex. Such a step is supported by oxidative additions of aryl halides to Pd 0 (PPh 3 ) 4 reported by Fitton and Rick [6c] in 1971 with the reactivity order: ArI > ArBr ArCl and p-EWG-ArX > p-EDG-ArX (EDG = electron-donating group).…”

“…The mechanisms of Mizoroki-Heck reactions performed from aryl derivatives are presented herein by highlighting how the catalytic precursors, the bases and the ligands may affect the structure and reactivity of intermediate palladium (0) or palladium(II) complexes in one or more steps of the catalytic cycle and, consequently, how they may affect the efficiency and regioselectivity of the catalytic reactions.…”

Graphene Oxide-Supported Oxime Palladacycles as Efficient Catalysts for the Suzuki–Miyaura Cross-Coupling Reaction of Aryl Bromides at Room Temperature under Aqueous Conditions

“…The presence of water in the reaction medium, as demonstrated by Blackmond [79], would also play a key role in accelerating the initial formation of the monomeric active catalytic species. As commented above, palladium nanoparticles on graphene nanoplatelets (PdNPs-G, average nanoparticle size: 4.50 nm) and reduced graphene oxide (PdNPs-rGO, average nanoparticle size: 10.9 nm) have shown lower activity in the model Suzuki reaction under the optimized reaction conditions, probably due to the agglomeration of the nanoparticles under the reaction conditions.…”