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...
The copper(I) complex [Cu(C18(6)tren)]Br 1 (C18(6)tren = tris(2-dioctadecylaminoethyl)amine) which exhibits a good stability towards aerobic conditions is a versatile, highly reactive and recyclable catalyst for the Huisgen cycloaddition of azides with terminal or internal alkynes and is a useful catalyst for the preparation of "click" dendrimers.
Water-soluble arene-cored "clicked" and non-"clicked" dendrimers terminated by 27, 81, and 243 triethyleneglycol (TEG) tethers (respectively generations G0, G1, and G2) have been synthesized and shown to form dendrimer-encapsulated gold nanoparticles (DEAuNPs) and dendrimer-stabilized gold nanoparticles (DSAuNPs). The dendrimers have been characterized by IR, (1)H NMR, (13)C NMR, size-exclusion chromatography, elemental analysis, MALDI-TOF mass spectroscopy, DOSY NMR, and dynamic light scattering. The AuNPs have been generated and stabilized by these PEGylated dendrimers using a variety of reduction modes, including NaBH(4) in methanol, various single-electron metallocene-type reductants, and even in the absence of additional reductants. The active role of the "clicked" triazole rings, dendrimer generation, stoichiometry of Au precursor, and nature of the reductant and of the solvent are delineated, leading to DSAuNPs with the G0 dendrimer and smaller DEAuNPs with the G1 and G2 dendrimers. Altogether, AuNPs in the size range from 1.8 to 42 nm were formed and characterized by transmission electron microscopy (TEM), high resolution TEM (HRTEM) and UV-vis spectroscopy. Both 1,2,3-triazole and PEGylated Percec-type dendrons are required in the dendrimer structure for the stabilization of AuNPs upon NaBH(4) reduction of HAuCl(4) in methanol. On the other hand, in the absence of other reductant in water, only PEGylated Percec-type dendrons in dendrimers were found to be indispensable, because of their semicavitand shape, for the spontaneous reduction of HAuCl(4) and stabilization of DSAuNPs.
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