Application of the palladium-catalyzed borylation/Suzuki coupling (BSC) reaction to the synthesis of biologically active biaryl lactams

Titania Nanorod-Supported Mercaptoundecanoic Acid-Grafted Palladium Nanoparticles as a Highly Reusable Heterogeneous Catalyst for Substrate-Dependent Ullmann Coupling and Debromination of Aryl Bromides

Herein, by implanting palladium nanoparticles (Pd NPs) onto titanium dioxide (TiO₂) nanorods (NRs) through 11-mercaptoundecanoic acid (MUA), we devised a robust heterogeneous catalyst. The formation of Pd-MUA-TiO₂ nanocomposites (NCs) was authenticated using Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, Brunauer-Emmett-Teller analysis, atomic absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. Pd NPs were synthesized directly onto TiO₂ nanorods without the MUA support for comparative studies. As a means of evaluating the endurance and competency of Pd-MUA-TiO₂ NCs compared to their counterpart (Pd-TiO₂ NCs), both were used as the heterogeneous catalyst for Ullmann coupling of a wide variety of aryl bromides. When Pd-MUA-TiO₂ NCs were used, the reaction produced high yields of homocoupled products (54-88%), whereas the yield was only 76% when Pd-TiO₂ NCs were used. Moreover, Pd-MUA-TiO₂ NCs impressed with their outstanding reusability property, allowing over 14 reaction cycles without losing efficiency. On the flip side, just after seven reaction cycles, the productivity of Pd-TiO₂ NCs dropped around 50%. Presumably, the strong affinity of Pd for the thiol groups of MUA allowed for the substantial control of leaching out of Pd NPs during the reaction. Nonetheless, another crucial feature of the catalyst is that the di-debromination reaction took place with an excellent yield of 68-84% from di-aryl bromides with long alkyl chains instead of macrocyclic or dimerized products. It is worth mentioning that AAS data confirmed that only 0.30 mol % catalyst loading was sufficient to activate a broad substrate scope with large functional group tolerance.

Masuda Borylation–Suzuki Coupling (MBSC) Sequence: A One-Pot Process to Access Complex (hetero)Biaryls

The direct formation of (hetero)biaryls from readily available (hetero)aryl halides under mild reaction conditions can be efficiently achieved through the Masuda borylation–Suzuki coupling (MBSC) sequence. The MBSC sequence catenates Pd-catalyzed Masuda borylation and Suzuki coupling into a one-pot process, giving access to diverse symmetrically and unsymmetrically substituted scaffolds. (Hetero)biaryls are ubiquitous structural motifs that appear in natural products, pharmaceutically relevant scaffolds, functional dyes, and several other structures. This review summarizes the development of the MBSC sequence and its improvements over the past two decades.

Catalytic asymmetric synthesis of medium-sized bridged biaryls

Despite the persistent presence of medium-sized (seven- to nine-membered) scaffolds in natural products and biologically active molecules, their asymmetric syntheses have always been considered a formidable task; therefore, they have remained underdeveloped when compared to the enantioselective synthesis of five- and six-membered ring scaffolds. One important class of such medium-sized ring frameworks includes seven- to nine-membered biaryl bridged carbo- and heterocycles. These medium-ring-sized biaryl frameworks possess more configurational stability than the related smaller ring structures and are common features of valuable natural products, bioactive compounds, chiral catalysts, and molecular motors. Due to these exciting properties and broad applications, over the last few years, the catalytic enantioselective synthesis of medium-sized bridged biaryls has seen an upsurge. This highlight article describes the development of organocatalysed and transition-metal catalysed transformations for procuring seven-, eight-, and nine-membered bridged biaryls bearing a chiral axis/one or more asymmetric carbon centres.

Concise Syntheses of Marine (Bis)indole Alkaloids Meridianin C, D, F, and G and Scalaridine A via One-Pot Masuda Borylation-Suzuki Coupling Sequence

N-Protected 3-iodoindoles were reacted with (di)azine halides in a sequentially Pd-catalyzed one-pot fashion, i.e., by Masuda borylation–Suzuki coupling (MBSC) sequence. This methodology was successfully applied to the concise syntheses of marine indole alkaloids meridianin C, D, F, and G, as well as to the bisindole alkaloid scalaridine A, which were obtained in moderate to excellent yield.

The application of C–H bond functionalization in the total syntheses of indole natural products

The recent advances in total synthesis of indole natural products focusing on the application of C–H bond functionalization are summarized.

A General, Multimetallic Cross-Ullmann Biheteroaryl Synthesis from Heteroaryl Halides and Heteroaryl Triflates

Despite their importance to medicine and materials science, the synthesis of biheteroaryls by cross-coupling remains challenging. We describe here a new, general approach to biheteroaryls: the Ni- and Pd-catalyzed multimetallic cross-Ullmann coupling of heteroaryl halides with triflates. An array of 5-membered, 6-membered, and fused heteroaryl bromides and chlorides, as well as aryl triflates derived from heterocyclic phenols, proved to be viable substrates in this reaction (62 examples, 63 ± 17% average yield). The generality of this approach to biheteroaryls was further demonstrated in 96-well plate format at 10 μmol scale. An array of 96 possible products provided >90% hit rate under a single set of conditions. Further, low-yielding combinations could be rapidly optimized with a single "Toolbox Plate" of ligands, additives, and reductants.

Electrochemical Organic Synthesis of Electron-Rich Biaryl Scaffolds: An Update

Biaryl scaffolds are widely spread in biologically important natural products, in numerous therapeutic agents, but they are also considered a privileged class of ligands and (organo)catalysts; therefore, the development of efficient alternative methodologies to prepare such compounds is always attracting much attention. The present review discusses the organic electrosynthesis of biaryls starting from phenols, anilines, naphthols, and naphthylamines. The most significant examples of the works reported in the last decade are presented and classified according to the single class of molecules: after the introduction, the first three sections relate to the reactions of phenols, naphthols, and anilines, respectively; the other two sections refer to cross-coupling and miscellaneous reactions.

Phosphorus ligand-free Suzuki–Miyaura reactions in the presence of ABTS at room temperature in water

A catalytic system for a phosphorus ligand-free Suzuki–Miyaura reaction in water at room temperature was disclosed. Ammonium 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) was an efficient promoter and acted both as a ligand and as a surfactant for the synthesis of biaryl compounds via the Suzuki–Miyaura reaction in water. The targeted biaryl architectures were achieved under mild conditions with high efficiency and good functional group tolerance.

Palladium-Catalyzed Enantioselective 7-exo-Trig Carbopalladation/Carbonylation: Cascade Reactions To Achieve Atropisomeric Dibenzo[b,d]azepin-6-ones

Enantioselective 7-exo-trig cyclocarbopalladation-initiated carbonylation cascade reactions, leading to seven-membered dibenzo[b,d]azepin-6-ones containing a thermodynamically controlled stereogenic axis, have been realized for the first time. A series of 7-acetate- or 7-acetamide-substituted dibenzo[b,d]azepin-6-ones are obtained under atmospheric pressure of CO in good yields with excellent diastereo- and enantioselectivities. The calculated energy difference between the diastereoisomers generated from the stereogenic biaryl axis and the stereogenic center is approximately 2.8 kcal/mol, which agrees with the excellent diastereoselectivity observed.

Syntheses and medicinal chemistry of azepinoindolones: a look back to leap forward

Nitrogen-containing heterocyclic scaffolds constitute nearly 75% of small molecules which favorably act as drug candidates. For the past few decades, numerous natural and synthetic indole-based scaffolds have been reported for their diverse pharmacological profiles. In particular, indole-fused azepines, termed azepinoindolones, have come under the radar of medicinal chemists owing to their synthetic and pharmacological importance. A plethora of literature reports has been generated thereof, which calls for the need for the compilation of information to understand their current status in drug discovery. Accumulating reports of evidence suggest that compounds containing this privileged scaffold display their cytotoxic effects via inhibition of kinase, topoisomerase I, mitochondrial malate dehydrogenase (mMDH), and tubulin polymerization and as DNA minor groove binding agents. Herein, we endeavor to present a closer look at the advancements of various synthetic and derivatization methods of azepinoindolone-based compounds. We have further extended our efforts to discuss the pharmacological effects of azepinoindolones in the whole range of medicinal chemistry as anti-Alzheimer, anticancer, anti-inflammatory, antidiabetic, antileishmanial, and antipyranosomal agents and as drug delivery vectors. Our analysis of recent advances reveals that azepinoindolones will continue to serve as potential pharmaceutical modalities in the years to come and their substantial pool of synthetic methods will be ever expanding.

Role of the base Cs2CO3 on the palladium-catalyzed intramolecular cyclization of two bromoindole derivatives to yield paullone-type products

Reactions for the palladium-catalyzed intramolecular cyclization of the o-bromoindole and the o-bromo-N-methyl-indole derivatives in the presence and absence of base (Cs₂CO₃) were explored through DFT calculations. For the base-free reactions, the palladium atom firstly interacts with the aromatic rings of the indole molecule to yield a stable adduct. Once this adduct has been formed, reaction proceeds readily to the oxidative addition intermediate that arises from the insertion of the metal atom into the C-Br bond of the organic fragment. Further steps leading to the paullone (or dimethyl paullone) product, mainly those involving the metalation and deprotonation of the inserted intermediate, are not energetically viable for these reactions. When the effect of the base on the metalation-deprotonation steps is modeled by replacing the bromide ion with CO₃^2- in the metal-inserted structure, a feasible pathway connecting the oxidative addition intermediate with the paullone-type product was located for each of the investigated reactions. The results emerging from this study suggest that palladium can insert into the C-Br bond of the indole derivatives to yield the oxidative addition intermediate (without participation of the base). However, the metalation and deprotonation steps that evolve to the paullone-type product take place via a concerted action involving both the metal and the base. Metalation and deprotonation steps that evolve to the paullone-type product take place via a concerted action involving both the metal and the base.

Nature-Inspired (di)Azine-Bridged Bisindole Alkaloids with Potent Antibacterial In Vitro and In Vivo Efficacy against Methicillin-Resistant Staphylococcus aureus

Natural bisindole alkaloids such as Hyrtinadine A and Alocasin A, which are known to exhibit diverse bioactivities, provide promising chemical scaffolds for drug development. By optimizing the Masuda borylation-Suzuki coupling sequence, a library of various natural product-derived and non-natural (di)azine-bridged bisindoles was created. While unsubstituted bisindoles were devoid of antibacterial activity, 5,5'-chloro derivatives were highly active against methicillin-resistant Staphylococcus aureus (MRSA) and further Gram-positive pathogens at minimal inhibitory concentrations ranging from 0.20 to 0.78 μM. These compounds showed strong bactericidal killing effects but only moderate cytotoxicity against human cell lines. Furthermore, the two front-runner compounds 4j and 4n exhibited potent in vivo efficacy against MRSA in a mouse wound infection model. Although structurally related bisindoles were reported to specifically target pyruvate kinase in MRSA, antibacterial activity of 4j and 4n is independent of pyruvate kinase. Rather, these compounds lead to bacterial membrane permeabilization and cellular efflux of low-molecular-weight molecules.

New Nanomagnetic Heterogeneous Cobalt Catalyst for the Synthesis of Aryl Nitriles and Biaryls

Cobalt nanoparticles immobilized on magnetic chitosan (Fe3O4@CS-Co) have been prepared. They were identified using various techniques such as Fourier-transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, thermogravimetric analysis, vibrating sample magnetometry, X-ray photoelectron spectroscopy, and inductively coupled plasma atomic emission spectroscopy analysis and applied efficiently as a cobalt catalyst in the cyanation and fluoride-/palladium-free Hiyama reactions of different types of aryl halides employing K4[Fe(CN)6]·3H2O and triethoxyphenylsilane, respectively. After each reaction, the catalyst was isolated and reused for the second run. The catalytic activity of the catalyst was not lost apparently even after five runs. No considerable changes in its chemical structure and morphology were observed. It is worth to note that in this paper, the cobalt catalyst has been used for the first time for the cyanation of aryl halides.

Selective Insertion of Alkynes into C-C σ Bonds of Indolin-2-ones: Transition-Metal-Free Ring Expansion Reactions to Seven-Membered-Ring Benzolactams or Chromone Derivatives

An unprecedented ring expansion reaction of indolin-2-ones with alkynyl ketones under transition-metal-free conditions has been developed. Base-promoted selective cleavage of a C-C σ bond of the amide is the key in this process, which provides a straightforward and efficient way to synthesize seven-membered-ring benzolactams or chromone derivatives. The significant advantages of this method include readily accessible starting materials, wide scope and functional group tolerance, and high atom economy.

Novel meriolin derivatives as rapid apoptosis inducers

3-(Hetero)aryl substituted 7-azaindoles possessing multikinase inhibitor activity are readily accessed in a one-pot Masuda borylation-Suzuki coupling sequence. Several promising derivatives were identified as apoptosis inducers and, emphasizing the multikinase inhibition potential, as sphingosine kinase 2 inhibitors. Our measurements provide additional insights into the structure-activity relationship of meriolin derivatives, suggesting derivatives bearing a pyridine moiety with amino groups in 2-position as most active anticancer compounds and thus as highly promising candidates for future in vivo studies.

Miyaura borylation/Suzuki-Miyaura coupling (MBSC) sequence of 4-bromo-2,4'-bithiazoles with halides: straightforward access to a heterocylic cluster of d-series of thiopeptide GE2270

Herein, palladium-catalyzed Miyaura borylation of 4-bromo-2,4'-bithiazoles followed by Suzuki-Miyaura cross-coupling reaction (named the MBSC process) with (hetero)aryl- and alkenyl halides is reported. This methodology offers rapid access to various 2',4-disubstituted 2,4'-bithiazole features including naturally-occurring 4-alkenylated and 4-pyridinylated 2,4'-bithiazoles. To prove its application, a concise approach for the synthesis of a heterocyclic cluster of the thiopeptide d-series antibiotic GE2270 is reported through a late-stage MBSC strategy.

Thiazole-based non-symmetric NNC–palladium pincer complexes as catalytic precursors for the Suzuki–Miyaura C–C coupling

New non-symmetric pincer palladacycles, containing [N,N,C] tridentate ligands are synthesized and their catalytic activities in the Suzuki–Miyaura cross coupling reaction are tested using water under aerobic conditions and IR irradiation.

Greener Biogenic Approach for the Synthesis of Palladium Nanoparticles Using Papaya Peel: An Eco-Friendly Catalyst for C-C Coupling Reaction

The development of a green and sustainable synthetic methodology still remains a challenge across the globe. Encouraging the prevailing challenge, herein, we have synthesized Pd nanoparticles (Pd NPs) in a green and environmentally viable route, using the extract of waste papaya peel without the assistance of any reducing agents, high-temperature calcination, and reduction procedures. The biomolecules present in the waste papaya peel extract reduced Pd(II) to nanosize Pd(0) in a one-pot green and sustainable process. As a catalyst, the new Pd NPs offer a simple and efficient methodology in direct Suzuki-Miyaura and Sonogashira coupling with excellent yields under mild reaction conditions.

Unexpected cyclization of 2-(2-aminophenyl)indoles with nitroalkenes to furnish indolo[3,2-c]quinolines

A novel synthetic route to the indoloquinoline core of the alkaloid isocryptolepine involving an unprecedented PPA-mediated reaction of 2-(2-aminophenyl)indenes with nitroalkenes is discovered.

One-step highly selective borylation/Suzuki cross-coupling of two distinct aryl bromides in pure water

A one-step borylation/Suzuki cross-coupling of two distinct aryl bromides by Na₂PdCl₄ catalysis mediated by B₂(OH)₄ was developed in pure water, allowing the synthesis of various biaryls and heterobiaryls.

Pd/Gorlos-Phos-catalyzed cross-coupling between two different aryl chlorides in the presence of B2Pin2 and cytotoxicity studies of the products

With the readily available Gorlos-Phos as a ligand, unsymmetrical biaryls were prepared efficiently from Pd-catalyzed one-pot two-step cross-coupling of two different aryl chlorides in the presence of B₂Pin₂.