"Homeopathic" catalytic activity and atom-leaching mechanism in Miyaura-Suzuki reactions under ambient conditions with precise dendrimer-stabilized Pd nanoparticles
"Homeopathic" catalytic activity and atom-leaching mechanism in Miyaura-Suzuki reactions under ambient conditions with precise dendrimer-stabilized Pd nanoparticles
复制标题
DOI：
10.1002/anie.200703067
复制
发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Astruc, Didier
中科院分区：
文献类型：
--
作者：
Diallo, Abdou Khadri;Ornelas, Catia;Astruc, Didier
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.[3–5] We now report that dendrimer-stabilized Pd nanoparticles synthesized by click chemistry catalyze the Suzuki reaction under ambient conditions with as little as 1ppm 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 CÀC 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 (130–1608C) Heck reaction with simple molecular PdII 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 258C 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: G0 with nine terminal ferrocenyltriazole units, G1 with 36 triazole units (9 interior and 27 terminal), and G2 with 117 triazole units (9+ 27 interior and 81 terminal; Scheme 1).[13c] PdNPs that can be stabilized by these new dendrimers have been shown to catalyze selective olefin hydrogenation. Molecular engineering to introduce triazole ligands into the dendrimers has enabled the precise delineation of the number of Pd atoms in the PdNPs. Indeed, the ferrocenyl groups located at the termini of the G0, G1, and G2 ferrocenyltriazolyl dendrimers make it possible to monitor the complexation of the triazolyl ligands by Pd (OAc) 2 by using cyclic voltammetry (CV). Electrochemical titration on the basis of this CV wave …
