“Homeopathic” Catalytic Activity and Atom‐Leaching Mechanism in Miyaura–Suzuki Reactions under Ambient Conditions with Precise Dendrimer‐Stabilized Pd Nanoparticles

Abstract

Low catalyst loading is essential for the development of "green" palladium-catalyzed CÀC bond-forming processes, [1] such as Heck and Suzuki reactions. [2] In this regard, transitionmetal nanoparticles (NPs) have attracted much attention as a major alternative to molecular homogeneous catalysts. [3][4][5] We now report that dendrimer-stabilized Pd nanoparticles synthesized by click chemistry catalyze the Suzuki reaction under ambient conditions with as little as 1 ppm of the PdNP catalyst: Our studies indicate that an atom-leaching mechanism is operating. We also show that the extraction of these PdNPs with thiols provides catalytically active PdNPs of complementary mechanistic interest.Polymer-stabilized [4] and dendrimer-stabilized NPs [5] have proved to be efficient catalysts for a variety of reactions. The advantage of dendrimers, besides their loose binding of the NP surface, is that a precise and predictable number of Pd atoms exist in the PdNP catalyst or precatalyst. Thus, it is possible to study the influence of NP size on the catalytic parameters (efficiency and stability) and mechanism. The use of dendrimers to form dendrimer-encapsulated NPs (DENs) [6] and dendrimer-stabilized NPs (DSNs), [7] in which dendrimers surround the NPs, was pioneered with PAMAM dendrimers. DENs have been studied largely in catalytic olefin-hydrogenation reactions [6a, c, 8] and in several cases in C À C coupling reactions, including the Suzuki reaction. [9] A leaching mechanism upon Pd catalysis has been proposed by de Vries and co-workers for the high-temperature (130-160 8C) Heck reaction with simple molecular Pd II catalysts, [10] and these catalysts indeed decompose to give PdNPs at high temperature. They propose that Pd atoms are removed from the PdNP surface subsequent to oxidative addition of the aryl halide, and that efficient Pd catalysis proceeds thereafter in solution before these catalytically active Pd atoms are trapped by a PdNP.[10] The mechanism of the Suzuki reaction, however, is controversial. One research group proposed a mechanism that operates only at the NP surface, [11] whereas another research group proposed that under microwave irradiation Pd-atom leaching occurs subsequent to oxidative addition of the aryl halide, as in the Heck reaction.[12] All of these studies were carried out at high temperature, under which conditions Oswald ripening, including atom loss from PdNPs, occurs. We wondered how the reaction would proceed under mild, ambient conditions with a PdNP catalyst. Herein, we show that the use of precise DSNs and DENs of various sizes for comparative styrenehydrogenation and Suzuki reactions at 25 8C provides a unique means to reach decisive conclusions concerning the leaching mechanism of the Suzuki reaction under ambient conditions.We recently described dendrimers synthesized by click chemistry [13] with 9, 27, and 81 ferrocenyl termini and 1,2,3-triazole ligands on the dendritic tethers: G 0 with nine terminal ferrocenyltriazole units, G 1 with 36 triazole units (9 interior and 27 termi...

Palladium‐Based Nanomaterials: A Platform to Produce Reactive Oxygen Species for Catalyzing Oxidation Reactions

“…Astruc et al have identifi ed that step 3 in Figure 8 b limits the reaction effi ciency of Suzuki-Miyaura couplings. [ 110 ] Thus it can be assumed that the presence of oxygen species (e.g., 1 O 2 analogue) not only promotes the formation of R-Pd-X, but also restrains the unfavorable step 3.…”

A reação "click" na síntese de 1,2,3-triazóis: aspectos químicos e aplicações

“…Outros exemplos de catálise utilizando ligantes sintetizados a partir da reação "click" são relatados como, por exemplo, catalisadores em reações de acoplamento do tipo Suzuki-Miyaura; 145 catalisadores que promovem reação de alquilação alílica regiosseletiva; 146 catalisadores que promovem benzoilação enantiosseletiva de dióis 147 e catalisadores que promovem hidrogenação assimétrica.…”

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

“…The same problems appear when relationships between the catalyst amount and the activity applied for distinguishing homogeneous catalysis from heterogeneous are analyzed. 3,28,35 The results might differ essentially depending on both the manner and the time chosen for the measurement of the catalyst activity (initial, overall or maximal activity over a certain period of time). In this work we have attempted to interpret obtained experimental data taking into account all problems described above.…”

“…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.…”