Extraction of monodisperse palladium nanoparticles from dendrimer templates

We describe a strategy for extracting nearly size-monodisperse, 1.7-nm-diameter Pd nanoparticles from within dendrimer hosts. The approach involves two steps. First, an aqueous solution of dendrimer-encapsulated Pd nanoparticles is prepared, followed by the addition of a toluene solution containing an n-alkanethiol. When the two-phase system is stirred, n-alkanethiols presumably self-assemble onto the surface of the Pd nanoparticles, extract them from the dendrimer, and transport them to toluene phase. UV-vis spectroscopy indicates that the Pd nanoparticles, in the form of monolayer-protected clusters, are transported into the toluene phase. FT-IR spectroscopy shows that the dendrimers remain in the aqueous phase. High-resolution transmission electron microscopy shows that the average size and size distribution of the particles do not change during the extraction process.     

Effect of Pd nanoparticle size on the catalytic hydrogenation of allyl alcohol

We report a particle size dependence for the rate of hydrogenation of allyl alcohol using 1.3-1.9 nm Pd dendrimer-encapsulated nanoparticle (DEN) catalysts. For particles with diameters of <1.5 nm and containing <147 Pd atoms, the modulation in catalytic activity is due to the electronic properties of the particle. For the larger particles, 1.5-1.9 nm in diameter and containing an average of 147-250 Pd atoms, the size effect is a result of geometrical constraints. Specifically, the hydrogenation reaction is shown to occur preferentially on the face atoms of the larger nanoparticles.     

Enhanced stability of charged dendrimer-encapsulated Pd nanoparticles in ionic liquids

Highly stable dendrimer-encapsulated Pd nanoparticles in ionic liquids were prepared for the first time by using charged PAMAM dendrimers as templates, which could maintain hydrogenation efficiency for up to at least 12 recycles.     

Size controlled synthesis of Pd nanoparticles in water and their catalytic application in C-C coupling reactions

Catalytically active Pd nanoparticles have been synthesized in water by a novel reduction of Pd(II) with a Fischer carbene complex where polyethylene glycol (PEG) was used as stabilizer. PEG molecules wrap around the nanoparticles to impart stability and prevent agglomeration, yet leave enough surface area available on the nanoparticle for catalytic activity. Our method is superior to others in terms of rapid generation and stabilization of Pd nanoparticles in water with a cheap, readily available PEG stabilizer. The size of the nanoparticles generated can be controlled by the concentration of PEG in water medium. The size decreased with the increase in the PEG: Pd ratio. This aqueous nano-sized Pd is a highly efficient catalyst for Suzuki, Heck, Sonogashira, and Stille reaction. Water is used as the only solvent for the coupling reactions.     

The mechanism of the Stille reaction investigated by electrospray ionization mass spectrometry

On-line monitoring of Stille reactions was performed via direct infusion electrospray ionization mass spectrometry (ESI-MS) and its tandem version (ESI-MS/MS). When operated in the positive ion mode, ESI(+)-MS was able to transfer, directly from solution to the gas phase, the species involved in all main steps of a Stille reaction, that is, the catalytically active palladium species Pd(PPh3)2, in its molecular ion form as well as the key cationic Pd(II) intermediates, including cyclic IPd-(CH2CH)Sn species. When searching for anionic species, ESI(-)-MS monitoring showed I- as the only anion detectable in the reaction medium. A detailed catalytic cycle for a Stille reaction was elaborated in which reaction intermediates and the previously elusive catalytically active Pd(0) species are shown in association with the respective ionic species intercepted by ESI-MS and further characterized by ESI-MS/MS.     

Palladium-mediated functionalization of heteroaromatic cations: comparative study on quinolizinium cations

An efficient palladium-catalyzed cross-coupling reaction on heteroaromatic cations is described. A comparative study of the Stille and Suzuki reactions shows that only the Stille reaction is able to produce an efficient C-C bond formation between any of the four isomeric bromoquinolizinium bromides and a variety of stannanes. In the presence of the catalysts Pd(PPh3)4 or Pd2(dba)3P(o-Tol)3, vinyl, ethynyl, aryl, and heteroaryl groups are successfully incorporated into the quinolizinium system in satisfactory yields under mild reaction conditions. This procedure represents a marked improvement on the functionalization of this class of heteroaromatic cation.     

Palladium nanoparticles as efficient green homogeneous and heterogeneous carbon-carbon coupling precatalysts: a unifying view

Pd catalysis of C-C bond formations is briefly reviewed from the angle of nanoparticles (NPs) whether they are homogeneous or heterogeneous precatalysts and whether they are intentionally preformed or generated from a Pd derivative such as Pd(OAc)2. The most studied reaction is the Heck coupling of halogenoarenes with olefins that usually proceeds at high temperature (120-160 degrees C). Under such conditions, the PdII precursor is reduced to Pd0, forming PdNPs from which Pd atom leaching, subsequent to oxidative addition of the aryl halide onto the PdNP surface, is the source of very active molecular catalysts. Other C-C coupling reactions (Suzuki, Sonogashira, Stille, Negishi, Hiyama, Corriu-Kumada, Ullmann, and Tsuji-Trost) can also be catalyzed by species produced from preformed PdNPs. For catalysis of these reactions, leaching of active Pd atoms from the PdNPs may also provide a viable molecular mechanistic scheme. Thus, the term "PdNP catalysis of C-C coupling" used in this review refers to this function of PdNPs as precursors of catalytically active Pd species (i.e., the PdNPs are precatalysts of C-C coupling reactions).     

Efficient Stille cross-coupling reaction catalyzed by the Pd(OAc)2/Dabco catalytic system

[reaction: see text] An efficient Pd(OAc)2/Dabco-catalyzed Stille cross-coupling reaction procedure has been developed. In the presence of Pd(OAc)2 and Dabco (triethylenediamine), various aryl halides including aryl iodides, aryl bromides, and activated aryl chlorides were coupled efficiently with organotin compounds to afford the corresponding biaryls, alkene, and alkynes in good to excellent yields. Furthermore, high TONs [turnover numbers, up to 980,000 TONs for the coupling reaction of 1-bromo-4-nitrobenzene and furan-2-yltributyltin] for the Stille cross-coupling reaction were observed.     

Supramolecular catalysts by encapsulating palladium complexes within dendrimers

Dendrimers are well-defined and highly branched macromolecules. By utilizing their capsular architectures, dendrimers encapsulating various catalytically active species can be prepared, which often bring about unique catalysis. Treatment of the alkylated PPI dendrimer with 4-diphenylphosphinobenzoic acid and [PdCl(C3H5)]2 afforded the dendrimer-encapsulated Pd complex using ionic interactions. The dendrimers encapsulating Pd complexes acted as unique supramolecular catalysts for the Heck reaction and allylic amination. The specific nanoenvironment created by the dense amino groups inside the dendrimers can provide high catalytic activity and stability for the Pd complexes. Facile recovery of the dendritic catalysts could be achieved by thermomorphic systems.     

Palladium nanoparticles stabilized by an ionic polymer and ionic liquid: a versatile system for C-C cross-coupling reactions

Highly stable palladium nanoparticles (Pd NPs), protected by an imidazolium-based ionic polymer (IP) in a functionalized ionic liquid (IL), have been prepared and characterized by transmission electron microscopy (TEM). These Pd NPs are excellent precatalysts for Suzuki, Heck, and Stille coupling reactions and can be stored without undergoing degradation for at least 2 years. The NP-IP-IL system may therefore be considered as an alternative to the traditional palladium on carbon (Pd/C) precatalyst employed in many C-C coupling reactions, also allowing reactions to be conducted under "solvent-free" conditions.     

Dendrimer-encapsulated nanoparticle precursors to supported platinum catalysts

In this contribution, we report the successful preparation of supported metal catalysts using dendrimer-encapsulated Pt nanoparticles as metal precursors. Polyamidoamine (PAMAM) dendrimers were first used to template and stabilize Pt nanoparticles prepared in solution. These dendrimer-encapsulated nanoparticles were then deposited onto a commercial high surface area silica support and thermally activated to remove the organic dendrimer. The resulting materials are active oxidation and hydrogenation catalysts. The effects of catalyst preparation and activation on activity for toluene hydrogenation and CO oxidation catalysis are discussed.     

PEG-400 promoted Pd(OAc)2/DABCO-catalyzed cross-coupling reactions in aqueous media

PEG-400 [poly(ethylene glycol-400)] was found to improve the Pd(OAc)₂/DABCO-catalyzed aqueous Suzuki-Miyaura and Stille cross-coupling reactions. In the presence of Pd(OAc)₂, DABCO, and PEG-400, a variety of aryl halides were coupled with arylboronic acids or organotin compounds efficiently to afford the corresponding cross-coupled products in moderate to excellent yields. The turnover numbers was up to 900,000 for the Suzuki-Miyaura reaction and up to 9800 for the Stille reaction. The catalyst system was also effective for Heck and Sonogashira cross-coupling reactions to some extent.     

Synthesis, characterization, and surface immobilization of platinum and palladium nanoparticles encapsulated within amine-terminated poly(amidoamine) dendrimers

Platinum and palladium dendrimer-encapsulated nanoparticles (DENs) were prepared within commercially available, fourth-generation, amine-terminated, poly(amidoamine) dendrimers (G4-NH2). The synthesis is carried out by selectively encapsulating metal complexes within the dendrimer and then reducing the resulting composite. Intradendrimer complexation requires control over the solution pH to prevent attachment of the metal complexes to primary amine groups on the dendrimer periphery. That is, the surface primary amines of the dendrimer must be selectively protonated in the presence of the interior tertiary amines. The metal-ion encapsulation and reduction processes were characterized by UV-vis spectroscopy. Forty-atom Pt and Pd DENs were examined by high-resolution transmission electron microscopy, which showed that the mean particle sizes were 1.4 and 1.5 nm, respectively, and that both were nearly monodisperse (standard deviation = 0.3 nm). The free amine groups on the dendrimer surface were used to link Pd DENs to monolithic Au surfaces via an intermediate self-assembled monolayer adhesion layer.     

Cine-substitution in the Stille coupling: evidence for the carbenoid reactivity of sp3-gem-organodimetallic iodopalladio-trialkylstannylalkane intermediates

Two complementary routes to sp3-gem-organodimetallic iodopalladio-trialkylstannanylalkanes are presented. Such intermediates have been proposed as Pd-carbenoid precursors in the Busacca-Farina cine-substitution mechanism in the Stille coupling. The decomposition of iodomethyltrialkylstannanes by Pd(0) catalysts was monitored by 1H, 2D, and 119Sn NMR. The formation of ethylene, trace formaldehyde, and iodotrialkylstannanes was detected. When the reaction was carried out in the presence of norbornene, the corresponding cyclopropane was produced in good yield. These observations are consistent with the intermediacy of a Pd-carbenoid species. sp3-gem-Organodimetallic iodopalladio-trialkylstannanylalkane complexes were also prepared under stoichiometric conditions via transmetalation from tin to Pd(II). Me3SnCH2Sn(CH2CH2CH2)3 reacted with [(D-t-BPF)PdI]+I-, yielding the (D-t-BPF)Pd(II)ICH2SnMe3 complex that dimerized to form ethylene and cyclopropanated norbornene. The carbenoid reactivity of iodopalladio-trialkylstannanylalkanes complexes validates the Busacca-Farina mechanism of the cine-substitution in the Stille coupling.     

Silica-dendrimer core-shell microspheres with encapsulated ultrasmall palladium nanoparticles: efficient and easily recyclable heterogeneous nanocatalysts

We report the synthesis, characterization, and catalytic properties of novel monodisperse SiO(2)@Pd-PAMAM core-shell microspheres containing SiO(2) microsphere cores and PAMAM dendrimer-encapsulated Pd nanoparticle (Pd-PAMAM) shells. First, SiO(2) microspheres, which were prepared by the St?ber method, were functionalized with vinyl groups by grafting their surfaces with vinyltriethoxysilane (VTS). The vinyl groups were then converted into epoxides by using m-chloroperoxybenzoic acid. Upon treatment with amine-terminated G4 poly(amidoamine) (PAMAM) dendrimers, the SiO(2)-supported epoxides underwent ring-opening and gave SiO(2)@PAMAM core-shell microspheres. Pd nanoparticles within the cores of the SiO(2)-supported PAMAM dendrimers were synthesized by letting Pd(II) ions complex with the amine groups in the cores of the dendrimers and then reducing them into Pd(0) with NaBH(4). This produced the SiO(2)@Pd-PAMAM core-shell microspheres. The presence of the different functional groups on the materials was monitored by following the changes in FTIR spectra, elemental analyses, and weight losses on thermogravimetric traces. Transmission electron microscopy (TEM) images showed the presence of Pd nanoparticles with average size of 1.56 ± 0.67 nm on the surface of the monodisperse SiO(2)@Pd-PAMAM core-shell microspheres. The SiO(2)@Pd-PAMAM core-shell microspheres were successfully used as an easily recyclable catalyst for hydrogenation of various olefins, alkynes, keto, and nitro groups, giving ~100% conversion and high turnover numbers (TONs) under 10 bar H(2) pressure, at room temperature and in times ranging from 10 min to 3 h. In addition, the SiO(2)@Pd-PAMAM core-shell microspheres were proven to be recyclable catalysts up to five times with barely any leaching of palladium into the reaction mixture.     

Biaryl formation via Suzuki and Stille coupling reactions using palladium nanoparticle/polymeric N‐heterocyclic carbene grafted silica as recyclable and efficient catalyst

Palladium nanoparticles supported on polymeric N‐heterocyclic carbene grafted silica as an efficient organic–inorganic hybrid catalyst is introduced. Pd⁰ nanoparticle formation, which is stabilized by the polymeric N‐heterocyclic carbene ligands and ionic liquid units, was confirmed using X‐ray photoelectron spectroscopy. Scanning electron microscopy images showed microparticles of modified silica while transmission electron microscopy images displayed a fine distribution of Pd nanoparticles. The modified structure was applied successfully in biaryl formation via Suzuki and Stille coupling reactions. Various biaryls were generated through the reaction of phenylboronic acid or tetraphenyltin with a variety of haloarenes via cross‐coupling reactions. This catalyst showed promising activity after being recycled several times. Copyright © 2016 John Wiley & Sons, Ltd.     

Stille reactions catalytic in tin: a "Sn-F" route for intermolecular and intramolecular couplings

Polymethylhydrosiloxane (PMHS) made hypercoordinate by KF(aq) allows Me(3)SnH to be recycled during a Pd(0)-catalyzed hydrostannation/Stille cascade. Starting with a variety of alkynes, in situ vinyltin formation is followed by Stille reaction with aryl, styryl, benzyl, or vinyl electrophiles present in the reaction mixture. Both inter- and intramolecular versions of the process are possible with tin loads of approximately 6 mol %. Regeneration of the organotin hydride is believed to proceed through a Me(3)SnF intermediate. Given the aggregated nature of organotin fluorides and the ability to use these organotins in substoichiometric quantities, the hazards and purification problems associated with the removal of organotin wastes from reaction mixtures are minimized.     

An efficient Stille cross-coupling reaction catalyzed by Pd(OAc)2/DAB-Cy catalytic system

An efficient palladium-catalyzed Stille cross-coupling reaction has been developed. In the presence of 3 mol% of Pd(dba)₂ and 6 mol% of DAB-Cy (1,4-dicyclohexyl-diazabutadiene), various aryl halides (iodides and bromides) were coupled with organotin compounds to afford the corresponding biaryls and alkyne in good to excellent yields. Furthermore, high TONs [turnover numbers, TONs up to 950,000 for the reaction of 1-iodo-4-nitrobenzene and tributyl(phenyl)stannane] for the Stille cross-coupling reaction were observed.     

Computational characterization of a complete palladium-catalyzed cross-coupling process: the associative transmetalation in the Stille reaction

[reaction: see text] The species presumably involved in the associative ligand substitution mechanism for the Stille cross coupling of vinyl bromide and trimethylvinyl stannane with Pd(PMe(3))(2)/PMe(3) as catalysts in DMF (as ligand and solvent) have been structurally and energetically characterized. The cyclic four-coordinate transition state for the rate-determining transmetalation step explains the retention of configuration in the Stille coupling of chiral nonracemic alkyl stannanes.     

Palladium supported on modified magnetic nanoparticles: a phosphine‐free and heterogeneous catalyst for Suzuki and Stille reactions

An efficient magnetic nanoparticle‐supported palladium (Fe₃O₄/SiO₂‐PAP‐Pd) catalyst is reported for the Suzuki cross‐coupling and Stille reactions. This method provides a novel and much improved modification of the Suzuki and Stille coupling reactions in terms of phosphine‐free catalyst, short reaction time, clean reaction and small quantity of catalyst. Another important feature of this method is that the catalyst can be easily recovered from the reaction mixture and reused with no loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.