Supported Palladium Catalysts for Suzuki Reactions: Structure‐Property Relationships, Optimized Reaction Protocol and Control of Palladium Leaching

Abstract

Palladium on metal oxides and on activated carbon with particular properties (high palladium dispersion, low degree of reduction, water content) are shown to be highly active (tunrover number, TON = 20,000; turnover frequency, TOF = 16,600), selective and robust catalysts for Suzuki cross-couplings of aryl bromides and activated aryl chlorides. Catalysts and reaction protocol offer combined advantages of high catalytic efficiency under ambient conditions (air and moisture), easy separation and reuse and quantitative recovery of palladium. The palladium concentration in solution during the reaction correlates clearly with the progress of the reaction and indicates that dissolved molecular palladium is in fact the catalytically active species. Dissolved palladium is redeposited onto the support at the end of the reaction. Additional minimization of the palladium content in solution (down to 0.1 ppm) could be achieved by simple procedures which meet the requirements of pharmaceutical industry.

Suzuki reaction: mechanistic multiplicity versus exclusive homogeneous or exclusive heterogeneous catalysis

“…For instance, Pd leaching under aryl halide action found earlier for the Heck reaction should theoretically (based on scheme 3) occur in the Suzuki reaction. Numerous confirmations of this were, indeed, found, 4,7,10,31,32 moreover the correlation between the activity and the amount of However, from the previous discussion of the Heck chemistry, it can be concluded that distinguishing between true homogeneous catalysis from heterogeneous catalysis in the Suzuki reaction is undoubtedly a very complicated task. Unfortunately, the results of the conventional tests on reaction homogeneity-heterogeneity 33 are ambiguous due to the interconversions of different catalyst forms (Scheme 3).…”

“…Now it is quite clear that there is no consensus on the nature of the catalysis of this reaction (see review 2 published in 2006 and references cited therein). Analysis of the literature for the last two years indicates that some researchers advance the hypothesis of a homogeneous catalysis mechanism [3][4][5][6][7][8][9][10][11] while the others suggest a heterogeneous mechanism. [12][13][14][15][16][17][18] It is worth mentioning that the most convincing evidence is for the homogeneous mechanism of the Suzuki reaction.…”

“…[12][13][14][15][16][17][18] It is worth mentioning that the most convincing evidence is for the homogeneous mechanism of the Suzuki reaction. [3][4][5]10,11 But mechanistic studies of the reactions never provide absolute unequivocal evidence for any mechanism and it makes judging only possible about the relative probability; a set of different experimental approaches (as wide as possible) to perform the mechanistic research. In this work a new approach to the Suzuki reaction was applied for the first time: i.e.…”

Graphitized nanocarbon-supported metal catalysts: synthesis, properties, and applications in heterogeneous catalysis

“…Ligand-free and recyclable heterogeneous Pd catalysts represent promising options to address these issues [174,175]. The Pd NPs supported on some traditional carbon materials such as mesoporous carbon [176], AC [177], and CNTs [8] to induce coupling reactions have been developed into a distinct research area. Recently, a number of graphenebased hybrids were synthesized and employed for coupling reactions.…”

C-C Coupling Reactions in Water Catalyzed by Palladium

“…To address this issue, heterogeneous catalysts have provided a desirable alternative regarding their recyclability, less contamination to the products, cost benefits and environmental acceptability. Palladium species including palladium nanoparticles and palladium(II) ion could be heterogenized on the surface of the support materials such as ionic liquids, [17] zeolites and molecular sieves, [18] activated carbon, [19] metal oxides, [20] graphene oxide, [21] clays,433 polymers. [24] However, the reaction performance of the heterogeneous catalysts was affected signally by the intrinsic properties of the support materials as well as the links between the active components and the support.…”