Aryl alkynylation versus alkyne homocoupling: unprecedented selectivity switch in Cu, phosphine and solvent-free heterogeneous Pd-catalysed couplings

One-Pot Tandem Coupling Method for the Short-Step Formal Synthesis of Riccardin C

One-pot reactions reduce reagent amounts and circumvent process treatments, such as work-up and purifications in multi-step reactions. In this study, we achieved the formal total synthesis of riccardin C through a one-pot reaction by simultaneously linking four units through two Sonogashira coupling reactions and one Suzuki coupling reaction, followed by reduction and deprotection. Thus, this one-pot method comprised five steps and did not require the purification of intermediate reaction mixtures, which saves resources, such as reagents and solvents, and expedites the work process.

Magnetically Recoverable Nanoparticulate Catalysts for Cross-Coupling Reactions: The Dendritic Support Influences the Catalytic Performance

Carbon-carbon cross-coupling reactions are among the most important synthetic tools for the preparation of pharmaceuticals and bioactive compounds. However, these reactions are normally carried out using copper, phosphines, and/or amines, which are poisonous for pharmaceuticals. The use of nanocomposite catalysts holds promise for facilitating these reactions and making them more environmentally friendly. In the present work, the PEGylated (PEG stands for poly(ethylene glycol) pyridylphenylene dendrons immobilized on silica loaded with magnetic nanoparticles have been successfully employed for the stabilization of Pd2+ complexes and Pd nanoparticles. The catalyst developed showed excellent catalytic activity in copper-free Sonogashira and Heck cross-coupling reactions. The reactions proceeded smoothly in green solvents at low palladium loading, resulting in high yields of cross-coupling products (from 80% to 97%) within short reaction times. The presence of magnetic nanoparticles allows easy magnetic separation for repeated use without a noticeable decrease of catalytic activity due to the strong stabilization of Pd species by rigid and bulky dendritic ligands. The PEG dendron periphery makes the catalyst hydrophilic and better suited for green solvents. The minor drop in activity upon the catalyst reuse is explained by the formation of Pd nanoparticles from the Pd2+ species during the catalytic reaction. The magnetic separation and reuse of the nanocomposite catalyst reduces the cost of target products as well as energy and material consumption and diminishes residual contamination by the catalyst. These factors as well as the absence of copper in the catalyst makeup pave the way for future applications of such catalysts in cross-coupling reactions.

Synergistic Effect of NiLDH@YZ Hybrid and Mechanochemical Agitation on Glaser Homocoupling Reaction

Herein, we report the synthesis of nickel-layered double hydroxide amalgamated Y-zeolite (NiLDH@YZ) hybrids and the evaluation of the synergistic effect of various NiLDH@YZ catalysts and mechanochemical agitation on Glaser homocoupling reactions. Nitrogen adsorption-desorption experiments were carried out to estimate the surface area and porosity of NiLDH@YZ hybrids. The basicity and acidity of these hybrids were determined by CO₂ -TPD and NH₃ -TPD experiments respectively and this portrayed good acid-base bifunctional feature of the catalysts. The NiLDH@YZ-catalyzed mechanochemical Glaser coupling reaction achieved best yield of 83 % for the 0.5NiLDH@0.5YZ hybrid after 60 min of agitation, which revealed the highest acid-base bifunctional feature compared to all the investigated catalysts. The developed catalyst has proven itself as a robust and effective candidate that can successfully be employed up to four catalytic cycles without significant loss in catalytic activity, under optimized reaction conditions. This work demonstrated a new strategy for C-C bond formation enabled by the synergy between mechanochemistry and heterogeneous catalysis.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

N,N-Dimethylformamide-stabilized copper nanoparticles as a catalyst precursor for Sonogashira–Hagihara cross coupling

We found that DMF-stabilized Cu NPs have high catalytic activity in the Sonogashira cross-coupling reaction at low catalyst loadings (0.2 mol%).

Probing Heterogeneity and Bonding at Silica Surfaces through Single-Molecule Investigation of Base-Mediated Linkage Failure

The nature of silica surfaces is relevant to many chemical systems, including heterogeneous catalysis and chromatographies utilizing functionalized-silica stationary phases. Surface linkages must be robust to achieve wide and reliable applicability. However, silyl ether-silica support linkages are known to be susceptible to detachment when exposed to basic conditions. We use single-molecule spectroscopy to examine the rate of surface linkage failure upon exposure to base at a variety of deposition conditions. Kinetic analysis elucidates the role of thermal annealing and addition of blocking layers in increasing stability. Critically, it was found that successful surface modification strategies alter the rate at which base molecules approach the silica surface as opposed to reducing surface linkage reactivity. Our results also demonstrate that the innate structural diversity of the silica surface is likely the cause of observed heterogeneity in surface-linkage disruption kinetics.

A simple, recyclable, and self-assembled palladium(ii)–alkyl Schiff base complex for Suzuki coupling reaction: chain length dependence and heterogeneous catalysis

A simple, recyclable, and self-assembled Pd(ii)–alkyl Schiff base complex for Suzuki coupling reaction: chain length dependence and heterogeneous catalysis.

The mechanism of a self-assembled Pd(ferrocenylimine)–Si compound-catalysed Suzuki coupling reaction

A SAM of Pd(FcL)–Si as a highly active and recyclable heterogeneous catalyst for Suzuki coupling reaction.

Synthesis and characterization of functionalized titania-supported Pd catalyst deriving from new orthopalladated complex of benzophenone imine: catalytic activity in the copper-free Sonogashira cross-coupling reactions at low palladium loadings

Efficient catalytic activity of TiO₂-supported Pd catalyst in the copper, amine and phosphine free Sonogashira cross-coupling reactions at low Pd-loadings.

An organo-NHC catalyzed domino addition approach for the selective synthesis of 5-butynylisoxazoles and subsequent Sonogashira coupling

An organo-NHC catalysed domino addition approach for the selective synthesis of 5-butynylisoxazoles and the subsequent Sonogashira cross-coupling for the selective synthesis of corresponding internal alkynes is described.

Diphosphino-Functionalised MCM-41-Supported Palladium Complex: An Efficient and Recyclable Catalyst for the Formylation of Aryl Halides

The heterogeneous formylation of aryl halides with HCO2Na at atmospheric pressure by carbon monoxide was readily achieved in the presence of the diphosphino-functionalised MCM-41-supported palladium complex in DMF to afford the corresponding aromatic aldehydes in good to excellent yields. This heterogeneous palladium catalyst can be recovered by simple filtration and reused 10 times without any loss of activity.

Practical synthesis of aryl-2-methyl-3-butyn-2-ols from aryl bromides via conventional and decarboxylative copper-free Sonogashira coupling reactions

Two efficient protocols for the palladium-catalyzed synthesis of aryl-2-methyl-3-butyn-2-ols from aryl bromides in the absence of copper were developed. A simple catalytic system consisting of Pd(OAc)₂ and P(p-tol)₃ using DBU as the base and THF as the solvent was found to be highly effective for the coupling reaction of 2-methyl-3-butyn-2-ol (4) with a wide range of aryl bromides in good to excellent yields. Analogously, the synthesis of aryl-2-methyl-3-butyn-2-ols was performed also through the decarboxylative coupling reaction of 4-hydroxy-4-methyl-2-pentynoic acid with aryl bromides, using a catalyst containing Pd(OAc)₂ in combination with SPhos or XPhos in the presence of tetra-n-butylammonium fluoride (TBAF) as the base and THF as the solvent. Therefore, new efficient approaches to the synthesis of terminal acetylenes from widely available aryl bromides rather than expensive iodides and using 4 or propiolic acid rather than TMS-acetylene as inexpensive alkyne sources are described.

Sonogashira cross-coupling under non-basic conditions. Flow chemistry as a new paradigm in reaction control

The Sonogashira reaction under non-basic conditions.

Heterogeneous Pd catalysts supported on silica matrices

Features, advantages and limitations associated with palladium catalysts deposited over various siliceous supports in different types of practically useful organic transformations are reviewed.

Copper- and amine-free Sonogashira–Hagihara coupling reaction catalyzed by Pd(0) nanoparticles supported on modified crosslinked polyacrylamide

Pd(0) nanoparticles supported on modified crosslinked polyacrylamide containing phosphinite ligand was prepared. The complex was a highly efficient catalyst for the Sonogashira– Hagihara coupling reaction in the absence of a copper cocatalyst and an amine base under aerobic condition. This protocol can be applied for different aryl halides including chloroarenes. The catalyst can be reused several times without any considerable decrease in its activity because of low leaching of palladium metal from the support. Transmission electron microscopy (TEM) analysis of the catalyst revealed the formation of stable and polydispersed Pd(0) nanoparticles.