Palladium-Catalyzed Heck Cross-Coupling Reactions in Water: A Comprehensive Review

Ubiquitous Role of Phosphine-Based Water-Soluble Ligand in Promoting Catalytic Reactions in Water

Catalysis has been at the forefront of the developments that has revolutionised synthesis and provided the impetus in the discovery of platform technologies for efficient C-C or C-X bond formation. Current environmental situation however, demands a change in strategy with catalysis being promoted more in solvents that are benign (Water) and for that the development of hydrophilic ligands (especially phosphines) is a necessity which could promote catalytic reactions in water, allow recyclability of the catalytic solutions and make it possible to isolate products using column-free techniques that involve lesser usage of hazardous organic solvents. In this review, we therefore critically analyse such catalytic processes providing examples that do follow the above-mentioned parameter.

Aqueous Micelles as Solvent, Ligand, and Reaction Promoter in Catalysis

Water is considered to be the most sustainable and safest solvent. Micellar catalysis is a significant contributor to the chemistry in water. It promotes pathways involving water-sensitive intermediates and transient catalytic species under micelles' shielding effect while also replacing costly ligands and dipolar-aprotic solvents. However, there is a lack of critical information about micellar catalysis. This includes why it works better than traditional catalysis in organic solvents, why specific rules in micellar catalysis differ from those of conventional catalysis, and how the limitations of micellar catalysis can be addressed in the future. This Perspective aims to highlight the current gaps in our understanding of micellar catalysis and provide an analysis of designer surfactants' origin and essential components. This will also provide a fundamental understanding of micellar catalysis, including how aqueous micelles can simultaneously perform multiple functions such as solvent, ligand, and reaction promoter.

Coupling Photoresponsive Transmembrane Ion Transport with Transition Metal Catalysis

Artificial ion transporters have been explored both as tools for studying fundamental ion transport processes and as potential therapeutics for cancer and channelopathies. Here we demonstrate that synthetic transporters may also be used to regulate the transport of catalytic metal ions across lipid membranes and thus control chemical reactivity inside lipid-bound compartments. We show that acyclic lipophilic pyridyltriazoles enable Pd(II) cations to be transported from the external aqueous phase across the lipid bilayer and into the interior of large unilamellar vesicles. In situ reduction generates Pd(0) species, which catalyze the generation of a fluorescent product. Photocaging the Pd(II) transporter allows for photoactivation of the transport process and hence photocontrol over the internal catalysis process. This work demonstrates that artificial transporters enable control over catalysis inside artificial cell-like systems, which could form the basis of biocompatible nanoreactors for applications such as drug synthesis and delivery or to mediate phototargeted catalyst delivery into cells.

Intermolecular Enamine Mizoroki-Heck Reactions on a Bio-Derived Scaffold

The intramolecular enamine-Mizoroki-Heck reaction allows for the construction of nitrogen-containing heterocycles, although the related intermolecular version is less known. The reactions of enamines derived from Cyrene were investigated under Mizoroki-Heck conditions. An optimization study was used to identify that 1.5 mol % Pd(dba)2 with PCy3 in xylene at reflux temperature gave the highest yield with electron-rich aryl iodides. Arylation occurred predominantly at the C-N center of the enamine, while the diastereoselectivity was dependent on the nitrogen substitution in the enamine.

Sterically demanding Csp2(ortho-substitution)-Csp3(tertiary) bond formation via carboxylate-directed Mizoroki-Heck reaction under extra-ligand-free conditions

Construction of the sterically demanding Csp2(oS)-Csp3(T) bond was achieved by carrying out the Pd-catalyzed carboxylate-directed Mizoroki-Heck reaction under extra-ligand-free aqueous conditions. The cooperative role of the presence of water with the absence of phosphine ligand was proposed to accelerate the migratory insertion process considerably, delivering a broad substrate scope.

Supramolecular Pd@methioine-EDTA-chitosan nanocomposite: an effective and recyclable bio-based and eco-friendly catalyst for the green Heck cross-coupling reaction under mild conditions

Supramolecular palladium(ii) supported on modified chitosan by dl-methionine using an ethylenediaminetetraacetic acid linker (Pd@MET-EDTA-CS) was designed and prepared through a simple procedure. The structure of this novel supramolecular nanocomposite was characterized by different spectroscopic, microscopic and analytical techniques including FTIR, EDX, XRD, FESEM, TGA, DRS, TEM, AA, and BET. The obtained bio-based nanomaterial was successfully investigated, as a highly efficient and green heterogeneous catalyst, in the Heck cross-coupling reaction (HCR) for the synthesis of various valuable biologically active cinnamic acid ester derivatives from the corresponding aryl halides using several acrylates. Indeed, aryl halides containing I or Br survived very well under optimized conditions to afford the corresponding products compared to the substrates with Cl. The prepared Pd@MET-EDTA-CS nanocatalyst promoted the HCR in high to excellent yields and short reaction times with minimum Pd loading (0.0027 mol%) on its structure as well as without any leaching occurring during the process. The recovery of the catalyst was performed by simple filtration and the catalytic activity remained approximately constant after five runs for the model reaction.

Pd-catalyzed intermolecular consecutive double Heck reaction "on water" under air: facile synthesis of substituted indenes

An efficient and environmentally benign method for the preparation of substituted indene derivatives has been developed by using water as the sole solvent. This reaction proceeded under air, tolerated a wide range of functional-groups and was easily scaled up. Bioactive natural products like indriline were synthesized via the developed protocol. Preliminary results demonstrate that the enantioselective variant can also be achieved.

Pd@l-asparagine-EDTA-chitosan: a highly effective and reusable bio-based and biodegradable catalyst for the Heck cross-coupling reaction under mild conditions

In this research, a novel supramolecular Pd(ii) catalyst supported on chitosan grafted by l-asparagine and an EDTA linker, named Pd@ASP-EDTA-CS, was prepared for the first time. The structure of the obtained multifunctional Pd@ASP-EDTA-CS nanocomposite was appropriately characterized by various spectroscopic, microscopic, and analytical techniques, including FTIR, EDX, XRD, FESEM, TGA, DRS, and BET. The Pd@ASP-EDTA-CS nanomaterial was successfully employed, as a heterogeneous catalytic system, in the Heck cross-coupling reaction (HCR) to afford various valuable biologically-active cinnamic acid derivatives in good to excellent yields. Different aryl halides containing I, Br and even Cl were used in HCR with various acrylates for the synthesis of corresponding cinnamic acid ester derivatives. The catalyst shows a variety of advantages including high catalytic activity, excellent thermal stability, easy recovery by simple filtration, more than five cycles of reusability with no significant decrease in its efficacy, biodegradability, and excellent results in the HCR using low-loaded Pd on the support. In addition, no leaching of Pd into the reaction medium and the final products was observed.

Sustainable and practical formation of carbon-carbon and carbon-heteroatom bonds employing organo-alkali metal reagents

Metal-catalysed cross-coupling reactions are amongst the most widely used methods to directly construct new bonds. In this connection, sustainable and practical protocols, especially transition metal-catalysed cross-coupling reactions, have become the focus in many aspects of synthetic chemistry due to their high efficiency and atom economy. This review summarises recent advances from 2012 to 2022 in the formation of carbon-carbon bonds and carbon-heteroatom bonds by employing organo-alkali metal reagents.

Triazine: An Important Building Block of Organic Materials for Solar Cell Application

Since the beginning of the 21st century, triazine-based molecules have been employed to construct different organic materials due to their unique optoelectronic properties. Among their applications, photovoltaics stands out because of the current need to develop efficient, economic, and green alternatives to energy generation based mainly on fossil fuels. Here, we review all the development of triazine-based organic materials for solar cell applications, including organic solar cells, dye-sensitized solar cells, and perovskite solar cells. Firstly, we attempt to illustrate the main synthetic routes to prepare triazine derivatives. Then, we introduce the main aspects associated with solar cells and their performance. Afterward, we discuss different works focused on the preparation, characterization, and evaluation of triazine derivatives in solar cells, distinguishing the type of photovoltaics and the role of the triazine-based material in their performance (e.g., as a donor, acceptor, hole-transporting material, electron-transporting material, among others). Throughout this review, the progress, drawbacks, and main issues of the performance of the mentioned solar cells are exposed and discussed. Finally, some conclusions and perspectives about this research topic are mentioned.

Water as a solvent: transition metal catalyzed dehydrogenation of alcohols going green

The long-established practice of using organic solvents in synthetic chemistry is currently becoming a major focus of environmental alarms as many of the chemical wastes are generated in the form of organic solvents. Recently, various alternative solvents have been recognized by the scientific community, including water, ionic liquids, supercritical fluids, glycerol, polyethylene glycol, etc. Among these alternatives, water is unquestionably an ideal solvent as it is abundant, cheap, non-toxic, and non-flammable. In the last few decades, a breakthrough has been achieved in the field of transition metal-catalyzed dehydrogenation of alcohols and the related chemistry for the sustainable synthesis of a wide range of valuable compounds. Although a large number of reports with new potential are published every year following this alcohol dehydrogenation strategy, the utilization of water as a solvent in alcohol dehydrogenation and related coupling reactions is yet to be highlighted properly. This review summarizes the advances in metal-catalyzed dehydrogenative functionalization of alcohols using water as a solvent.

Synthesis, antitumor activity, 3D-QSAR and molecular docking studies of new iodinated 4-(3H)-quinazolinones 3N-substituted

A novel series of 6-iodo-2-methylquinazolin-4-(3H)-one derivatives, 3a–n, were synthesized and evaluated for their in vitro cytotoxic activity. Compounds 3a, 3b, 3d, 3e, and 3h showed remarkable cytotoxic activity on specific human cancer cell lines when compared to the anti-cancer drug, paclitaxel. Compound 3a was found to be particularly effective on promyelocytic leukaemia HL60 and non-Hodgkin lymphoma U937, with IC₅₀ values of 21 and 30 μM, respectively. Compound 3d showed significant activity against cervical cancer HeLa (IC₅₀ = 10 μM). The compounds 3e and 3h were strongly active against glioblastoma multiform tumour T98G, with IC₅₀ values of 12 and 22 μM, respectively. These five compounds showed an interesting cytotoxic activity on four human cancer cell types of high incidence. The molecular docking results reveal a good correlation between experimental activity and calculated binding affinity on dihydrofolate reductase (DHFR). Docking studies proved 3d as the most potent compound. In addition, the three-dimensional quantitative structure–activity relationship (3D-QSAR) analysis exhibited activities that may indicate the existence of electron-withdrawing and lipophilic groups at the para-position of the phenyl ring and hydrophobic interactions of the quinazolinic ring in the DHFR active site.

New iodinated 4-(3H)-quinazolinones 3N-substituted with antitumor activity and 3D-QSAR and molecular docking studies as dihydrofolate reductase (DHFR) inhibitors.

Pyrene Functionalized Highly Reduced Graphene Oxide-palladium Nanocomposite: A Novel Catalyst for the Mizoroki-Heck Reaction in Water

The formation of a C-C bond through Mizoroki-Heck cross-coupling reactions in water with efficient heterogeneous catalysts is a challenging task. In this current study, a highly reduced graphene oxide (HRG) immobilized palladium (Pd) nanoparticle based catalyst (HRG-Py-Pd) is used to catalyze Mizoroki-Heck cross-coupling reactions in water. During the preparation of the catalyst, amino pyrene is used as a smart functionalizing ligand, which offered chemically specific binding sites for the effective and homogeneous nucleation of Pd NPs on the surface of HRG, which significantly enhanced the physical stability and dispersibility of the resulting catalyst in an aqueous medium. Microscopic analysis of the catalyst revealed a uniform distribution of ultrafine Pd NPs on a solid support. The catalytic properties of HRG-Py-Pd are tested towards the Mizoroki-Heck cross-coupling reactions of various aryl halides with acrylic acid in an aqueous medium. Furthermore, the catalytic efficacy of HRG-Py-Pd is also compared with its non-functionalized counterparts such as HRG-Pd and pristine Pd NPs (Pd-NPs). Using the HRG-Py-Pd nanocatalyst, the highest conversion of 99% is achieved in the coupling reaction of 4-bromoanisol and acrylic acid in an aqueous solution in a relatively short period of time (3 h), with less quantity of catalyst (3 mg). Comparatively, pristine Pd NPs delivered lower conversion (∼92%) for the same reaction required a long reaction time and a large amount of catalyst (5.3 mg). Indeed, the conversion of the reaction further decreased to just 40% when 3 mg of Pd-NPs was used which was sufficient to produce 99% conversion in the case of HRG-Py-Pd. On the other hand, HRG-Pd did not deliver any conversion and was ineffective even after using a high amount of catalyst and a longer reaction time. The inability of the HRG-Pd to promote coupling reactions can be attributed to the agglomeration of Pd NPs which reduced the dispersion quality of the catalyst in water. Therefore, the high aqueous stability of HRG-Py-Pd due to smart functionalization can be utilized to perform other organic transformations in water which was otherwise not possible.

Pd@Py2PZ@MSN as a Novel and Efficient Catalyst for C–C Bond Formation Reactions

In this study, a novel catalyst is introduced based on the immobilization of palladium onto dipyrido (3,2-a:2′,3′-c) phenazine–modified mesoporous silica nanoparticles. The dipyrido (3,2-a:2′,3′-c) phenazine (Py₂PZ) ligand is synthesized in a simple method from the reaction of 1,10-phenanthroline-5,6-dione and 3,4-diaminobenzoic acid as starting materials. The ligand is used to functionalize mesoporous silica nanoparticles (MSNs) and modify their surface chemistry for the immobilization of palladium. The palladium-immobilized dipyrido (3,2-a:2′,3′-c) phenazine–modified mesoporous silica nanoparticles (Pd@Py₂PZ@MSNs) are synthesized and characterized by several characterization techniques, including TEM, SEM, FT-IR, TGA, ICP, XRD, and EDS analyses. After the careful characterization of Pd@Py₂PZ@MSNs, the activity and efficiency of this catalyst is examined in carbon–carbon bond formation reactions. The results are advantageous in water and the products are obtained in high isolated yields. In addition, the catalyst showed very good reusability and did not show significant loss in activity after 10 sequential runs.

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.

Progresses in chitin, chitosan, starch, cellulose, pectin, alginate, gelatin and gum based (nano)catalysts for the Heck coupling reactions: A review

Heck cross-coupling reaction (HCR) is one of the few transition metal catalyzed CC bond-forming reactions, which has been considered as the most effective, direct, and atom economical synthetic method using various catalytic systems. Heck reaction is widely employed in numerous syntheses including preparation of pharmaceutical and biologically active compounds, agrochemicals, natural products, fine chemicals, etc. Commonly, Pd-based catalysts have been used in HCR. In recent decades, the application of biopolymers as natural and effective supports has received attention due to their being cost effective, abundance, and non-toxicity. In fact, recent studies demonstrated that biopolymer-based catalysts had high sorption capacities, chelating activities, versatility, and stability, which make them potentially applicable as green materials (supports) in HCR. These catalytic systems present high stability and recyclability after several cycles of reaction. This review aims at providing an overview of the current progresses made towards the application of various polysaccharide and gelatin-supported metal catalysts in HCR in recent years. Natural polymers such as starch, gum, pectin, chitin, chitosan, cellulose, alginate and gelatin have been used as natural supports for metal-based catalysts in HCR. Diverse aspects of the reactions, different methods of preparation and application of polysaccharide and gelatin-based catalysts and their reusability have been reviewed.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Palladium-Catalyzed Mizoroki-Heck and Copper-Free Sonogashira Coupling Reactions in Water Using Thermoresponsive Polymer Micelles

A few kinds of thermoresponsive diblock copolymers have been synthesized and utilized for palladium-catalyzed coupling reactions in water. Poly(N-isopropylacrylamide) (PNIPAAm) and poly(N,N-diethylacrylamide) (PDEAAm) are employed for thermoresponsive segments and poly(sodium 4-styrenesulfonate) (PSSNa) and poly(sodium 2-acrylamido-methylpropanesulfonate) (PAMPSNa) are employed for hydrophilic segments. Palladium-catalyzed Mizoroki-Heck reactions are performed in water and the efficiency of the extraction process is studied. More efficient extraction was observed for the PDEAAm copolymers when compared with the PNIPAAm copolymers and conventional surfactants. In the study of the Sonogashira coupling reactions in water, aggregative precipitation of the products was observed. Washing the precipitate with water gave the product with satisfactory purity with a good yield.

Chemo-Enzymatic Cascade for the Generation of Fragrance Aldehydes

In this study, we present the synthesis of chiral fragrance aldehydes, which was tackled by a combination of chemo-catalysis and a multi-enzymatic in vivo cascade reaction and the development of a highly versatile high-throughput assay for the enzymatic reduction of carboxylic acids. We investigated a biocompatible metal-catalyzed synthesis for the preparation of α or β substituted cinnamic acid derivatives which were fed directly into the biocatalytic system. Subsequently, the target molecules were synthesized by an enzymatic cascade consisting of a carboxylate reduction, followed by the selective C-C double bond reduction catalyzed by appropriate enoate reductases. We investigated a biocompatible oxidative Heck protocol and combined it with cells expressing a carboxylic acid reductase from Neurospora crassa (NcCAR) and an ene reductase from Saccharomyces pastorianus for the production fragrance aldehydes.

Intracellular Activation of Anticancer Therapeutics Using Polymeric Bioorthogonal Nanocatalysts

Bioorthogonal catalysis provides a promising strategy for imaging and therapeutic applications, providing controlled in situ activation of pro-dyes and prodrugs. In this work, the use of a polymeric scaffold to encapsulate transition metal catalysts (TMCs), generating bioorthogonal "polyzymes," is presented. These polyzymes enhance the stability of TMCs, protecting the catalytic centers from deactivation in biological media. The therapeutic potential of these polyzymes is demonstrated by the transformation of a nontoxic prodrug to an anticancer drug (mitoxantrone), leading to the cancer cell death in vitro.

Water-Sculpting of a Heterogeneous Nanoparticle Precatalyst for Mizoroki-Heck Couplings under Aqueous Micellar Catalysis Conditions

Powdery, spherical nanoparticles (NPs) containing ppm levels of palladium ligated by t-Bu₃P, derived from FeCl₃, upon simple exposure to water undergo a remarkable alteration in their morphology leading to nanorods that catalyze Mizoroki-Heck (MH) couplings. Such NP alteration is general, shown to occur with three unrelated phosphine ligand-containing NPs. Each catalyst has been studied using X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and cryogenic transmission electron microscopy (cryo-TEM) analyses. Couplings that rely specifically on NPs containing t-Bu₃P-ligated Pd occur under aqueous micellar catalysis conditions between room temperature and 45 °C, and show broad substrate scope. Other key features associated with this new technology include low residual Pd in the product, recycling of the aqueous reaction medium, and an associated low E Factor. Synthesis of the precursor to galipinine, a member of the Hancock family of alkaloids, is suggestive of potential industrial applications.