Pd Nanoparticle-Catalyzed Stereospecific Mizoroki-Heck Arylation of cis-1,2-Disilylarylethylenes

In the presence of catalytic amounts of Pd nanoparticles, generated from Pd2dba3/Ag(I), cis-1,2-ditrimethylsilylarylethylenes undergo with aryl iodides a stereospecific Mizoroki-Heck arylation leading to trans-ditrimethylsilyldiarylethylenes. This chemoselectivity is in contrast to that of their trimethylgermyl analogues, which are arylated at the position of the C-Ge bonds. trans-1,2-Ditrimethylsilylarylethylenes are completely unreactive under the standard reaction conditions. The reaction tolerates the presence of boryl, silyl, or bromine substituents on the aryl iodides. From a mechanistic point of view, the process involves syn-arylpalladation followed by syn-dehydropalladation.

Mechanically Robust Hybrid Gel Beads Loaded with "Naked" Palladium Nanoparticles as Efficient, Reusable, and Sustainable Catalysts for the Suzuki-Miyaura Reaction

The increase in demand for Pd and its low abundance pose a significant threat to its future availability, rendering research into more sustainable Pd-based technologies essential. Herein, we report Pd scavenging mechanically robust hybrid gel beads composed of agarose, a polymer gelator (PG), and an active low-molecular-weight gelator (LMWG) based on 1,3:2,4-dibenzylidenesorbitol (DBS), DBS-CONHNH ₂ . The robustness of the PG and the ability of the LMWG to reduce Pd(II) in situ to generate naked Pd(0) nanoparticles (PdNPs) combine within these gel beads to give them potential as practical catalysts for Suzuki-Miyaura cross-coupling reactions. The optimized gel beads demonstrate good reusability, green metrics, and most importantly the ability to sustain stirring, improving reaction times and energy consumption compared to previous examples. In contrast to previous reports, the leaching of palladium from these next-generation beads is almost completely eliminated. Additionally, for the first time, a detailed investigation of these Pd-loaded gel beads explains precisely how the nanoparticles are formed in situ without a stabilizing ligand. Further, detailed catalytic investigations demonstrate that catalysis occurs within the gel beads. Hence, these beads can essentially be considered as robust "nonligated" heterogeneous PdNP catalysts. Given the challenges in developing ligand-free, naked Pd nanoparticles as stable catalysts, these gel beads may have future potential for the development of easily used systems to perform chemical reactions in "kit" form.

Room-temperature coalescence of Pd nanoparticles with sacrificial templates and sintering agents, and their catalytic activities in the Suzuki coupling reaction

Porous metal structures are very useful for heterogeneous catalysts in organic syntheses. This study reports a novel method to fabricate porous Pd structures by room-temperature (RT) coalescence of Pd nanoparticles (Pd NPs). First, oleylamine-capped Pd NPs were synthesized, and then Pd NP pastes were fabricated by mixing with tri-n-octylphosphine oxide as a sacrificial template. Finally, the Pd NP paste was dipped into methanol containing a sintering agent. When KOH was used as the sintering agent, porous Pd structures could be successfully obtained at RT. The catalytic activities of porous Pd structures were investigated in the Suzuki coupling reaction and they increased with the increase of the KOH concentration in the sintering process. These results indicate that pre-activation of porous Pd structures by KOH increased the catalytic activities.

Synthesis of Mono- and Bis-Pyrazoles Bearing Flexible p-Tolyl Ether and Rigid Xanthene Backbones, and Their Potential as Ligands in the Pd-Catalysed Suzuki–Miyaura Cross-Coupling Reaction

The present work describes the synthesis of mono- and bis-pyrazole compounds bearing flexible p-tolyl ether and rigid 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene backbones as pyrazolyl analogues of DPEphos and Xantphos ligands, respectively. The synthesis of new pyrazolyl analogues was accomplished following an Ullmann coupling protocol, and the resulting products were isolated in overall good yields. In addition, a hybrid imidazolyl–pyrazolyl analogue bearing a xanthene backbone was synthesized using the same protocol, whereas a hybrid selanyl–pyrazolyl analogue with a xanthene backbone was synthesized in a good yield employing a second C–H activation step. The symmetrical bis-pyrazolyl and the hybrid imidazolyl–pyrazolyl analogues were found to be the most active among the new ligands evaluated in the Pd-catalysed Suzuki-Miyaura cross-coupling of aryl halides with aryl boronic acids. A simple catalytic system based on Pd(OAc)2/2a was developed, which efficiently catalyses the Suzuki–Miyaura reaction of aryl halides and aryl boronic acids and provides moderate to excellent yields of the corresponding cross-coupling products.

Electrochemical instability of highly fluorinated tetraphenyl borates and syntheses of their respective biphenyls

Highly fluorinated tetraphenyl borate anions are of importance as weakly coordinating anions in metalorganic reactions. However, at high positive potentials their electrochemical stability in organic solvents is not sufficient. This was investigated by a comprehensive cyclic voltammetry study and can be used synthetically to generate highly fluorinated biphenyls.

An effective Pd nanocatalyst in aqueous media: stilbene synthesis by Mizoroki-Heck coupling reaction under microwave irradiation

Aqueous Mizoroki–Heck coupling reactions under microwave irradiation (MW) were carried out with a colloidal Pd nanocatalyst stabilized with poly(N-vinylpyrrolidone) (PVP). Many stilbenes and novel heterostilbenes were achieved in good to excellent yields starting from aryl bromides and different olefins. The reaction was carried out in a short reaction time and with low catalyst loading, leading to high turnover frequency (TOFs of the order of 100 h⁻¹). The advantages like operational simplicity, high robustness, efficiency and turnover frequency, the utilization of aqueous media and simple product work-up make this protocol a great option for stilbene syntheses by Mizoroki–Heck reaction.

Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand

Molecular and colloidal palladium systems with a new NHC ligand show different catalytic reactivities.

Palladium nanoparticles in the catalysis of coupling reactions

Palladium catalyzed coupling reactions have emerged as a versatile, convenient, selective and mild protocol that can usually be adapted in any synthetic scheme for important target molecules with various degrees of structural complexity.

Aerobic oxidation at benzylic positions catalyzed by a simple Pd(OAc)2/bis-triazole system

An extremely active palladium catalyst system for the aerobic oxidation of benzyl alcohols and benzylic C–H oxidation is described.