Mechanically Induced Fe(III) Catalysis at Room Temperature: Solvent-Free Cross-Dehydrogenative Coupling of 3-Benzylic Indoles with Methylenes/Indoles

Mechanochemical Synthesis of α-halo Alkylboronic Esters

α-halo alkylboronic esters, acting as ambiphilic synthons, play a pivotal role as versatile intermediates in fields like pharmaceutical science and organic chemistry. The sequential transformation of carbon-boron and carbon-halogen bonds into a broad range of carbon-X bonds allows for programmable bond formation, facilitating the incorporation of multiple substituents at a single position and streamlining the synthesis of complex molecules. Nevertheless, the synthetic potential of these compounds is constrained by limited reaction patterns. Additionally, the conventional methods often necessitate the use of bulk toxic solvents, exhibit sensitivity to air/moisture, rely on expensive metal catalysts, and involve extended reaction times. In this report, a ball milling technique is introduced that overcomes these limitations, enabling the external catalyst-free multicomponent coupling of aryl diazonium salts, alkenes, and simple metal halides. This approach offers a general and straightforward method for obtaining a diverse array of α-halo alkylboronic esters, thereby paving the way for the extensive utilization of these synthons in the synthesis of fine chemicals.

Divergent synthesis of bis(indolyl)methanes via FeIII-catalysed regioselective dehydrogenative coupling reactions: a biomimetic approach to 6,6'-bis-(debromo)-gelliusine F

The divergent dehydrogenative coupling reactions of tryptamines with the catalysis of nontoxic Fe^III salts in the presence of DDQ as the co-oxidant have been developed. Remarkably, the transformations feature a rapid and regioselective assembly of diverse 2,8'- and N1,8'-bis(indolyl) methane derivatives from readily-available starting materials by simply changing the Fe^III salt and reaction temperature. Besides, the fast reaction rate, mild reaction conditions, low catalyst cost and easy operations make this methodology quite useful. The synthetic utility was further demonstrated in the biomimetic synthesis of 6,6'-bis-(debromo)-gelliusine F.

Preparation of astaxanthin micelles self-assembled by a mechanochemical method from hydroxypropyl β-cyclodextrin and glyceryl monostearate with enhanced antioxidant activity

This research aimed to overcome the current challenges in the application of natural carotenoid antioxidants, such as their complex preparation processes, low bioavailability and poor drug stability. Herein, a mechanochemical method was used to prepare an inclusion complex (IC) that self-assembles into micelles in aqueous solution and achieves solid-phase loading of astaxanthin (AST). The NMR analysis, thermodynamics study, particle size analysis and electron microscopy image results showed that AST, hydroxypropyl β-cyclodextrin (HPβ-CD) and glyceryl monostearate (GMS) formed self-assembled micelles and maintained good stability in aqueous solution. The antioxidant performance experiments showed that the formation of IC increases free radical scavenging activity. The pharmacokinetic studies showed that the bioavailability of the astaxanthin inclusion complex increased 4-fold. The tissue distribution experiments showed that the astaxanthin inclusion complex targets the liver to exert its antioxidant effects. The proposed method uses an innovative preparation technology to produce an efficient drug delivery system without solvents, and it exerts powerful antioxidant activity against astaxanthin.

Inositol hexanicotinate self-micelle solid dispersion is an efficient drug delivery system in the mouse model of non-alcoholic fatty liver disease

Inositol hexanicotinate (IHN) self-micelle solid dispersion (SD) with glycyrrhizic acid (GA) and arabic gum (AG) was prepared by mechanical ball milling process to improve the solubility, stability of amorphous state, and bioavailability of IHN, which enhanced the treatment of IHN on hyperlipidemia and nonalcoholic fatty liver disease (NAFLD). The physicochemical properties of IHN/GA/AG SDs in solid state were characterized by differential scanning calorimetry, X-ray diffraction studies, and scanning electron microscopy. The characteristics of the sample solutions were analyzed by reverse-phase HPLC, particle characterization, critical micelle concentration, and transmission electron microscopy. Further pharmacokinetic study of this SD formulation in rats showed a significant 3.3-fold increase in bioavailability compared to pure IHN. Moreover, biomarkers in serum and liver of NAFLD mice were significantly ameliorated after oral administration of IHN/GA/AG SDs for 15 days. Altogether, these results establish the mechanochemically prepared IHN/GA/AG SDs as an efficacious formulation for the treatment of hyperlipidemia and NAFLD.

Research on Preparation of 5-ASA Colon-Specific Hydrogel Delivery System without Crosslinking Agent by Mechanochemical Method

PURPOSE

This study aims to overcome the challenges of the current oral targeted drug delivery system, such as the complex preparation process, poor biocompatibility, and delayed drug release.

METHODS

Here, a non-covalent polymer hydrogel was prepared using the mechanochemical method, and the solid phase loading of 5-amino salicylic acid (5-ASA) was realized.

RESULTS

The results obtained from the thermodynamics study, particle size analysis, and electron microscopy show that chitosan (CS) and sodium alginate (SA) form a pH-sensitive hydrogel under the mechanochemical force and also maintain good stability in aqueous solution. Fluorescent tracers study showed that the pH-sensitive hydrogel could achieve the targeted drug release in the colon and the retention time was over 12 h. Next, in vivo efficacy studies, change in mice body weight, DAI (disease activity index) score, thymus, and spleen index, and the diseased state of the mice colon revealed that the pH-sensitive hydrogel is an improved drug delivery system over 5-ASA API commercial preparations as observed in the efficacy and toxicological studies.

CONCLUSION

This method uses an innovative preparation technology that without the need of cross-linking agent to produce an efficient colon-targeted drug delivery system for the treatment of ulcerative colitis.

Solubility, Permeability, Anti-Inflammatory Action and In Vivo Pharmacokinetic Properties of Several Mechanochemically Obtained Pharmaceutical Solid Dispersions of Nimesulide

Nimesulide (NIM, N-(4-nitro-2-phenoxyphenyl)methanesulfonamide) is a relatively new nonsteroidal anti-inflammatory analgesic drug. It is practically insoluble in water (<0.02 mg/mL). This very poor aqueous solubility of the drug may lead to low bioavailability. The objective of the present study was to investigate the possibility of improving the solubility and the bioavailability of NIM via complexation with polysaccharide arabinogalactan (AG), disodium salt of glycyrrhizic acid (Na₂GA), hydroxypropyl-β-cyclodextrin (HP-β-CD) and MgCO₃. Solid dispersions (SD) have been prepared using a mechanochemical technique. The physical properties of nimesulide SD in solid state were characterized by differential scanning calorimetry and X-ray diffraction studies. The characteristics of the water solutions which form from the obtained solid dispersions were analyzed by reverse phase and gel permeation HPLC. It was shown that solubility increases for all complexes under investigation. These phenomena are obliged by complexation with auxiliary substances, which was shown by ¹H-NMR relaxation methods. The parallel artificial membrane permeability assay (PAMPA) was used for predicting passive intestinal absorption. Results showed that mechanochemically obtained complexes with polysaccharide AG, Na₂GA, and HP-β-CD enhanced permeation of NIM across an artificial membrane compared to that of the pure NIM. The complexes were examined for anti-inflammatory activity on a model of histamine edema. The substances were administered per os to CD-1 mice. As a result, it was found that all investigated complexes dose-dependently reduce the degree of inflammation. The best results were obtained for the complexes of NIM with Na₂GA and HP-β-CD. In noted case the inflammation can be diminished up to 2-fold at equal doses of NIM.

Mechanically induced solvent-free esterification method at room temperature

Herein, we describe two novel strategies for the synthesis of esters, as achieved under high-speed ball-milling (HSBM) conditions at room temperature. In the presence of I2 and KH2PO2, the reactions afford the desired esterification derivatives in 45% to 91% yields within 20 min of grinding. Meanwhile, using KI and P(OEt)3, esterification products can be obtained in 24% to 85% yields after 60 min of grinding. In addition, the I2/KH2PO2 protocol was successfully extended to the late-stage diversification of natural products showing the robustness of this useful approach. Further application of this method in the synthesis of inositol nicotinate was also discussed.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Palladium-Catalyzed Q-Tube-Assisted Protocol for Synthesizing Diaza-dibenzo[a,e]azulene and Diaza-benzo[a]fluorene Derivatives via O2 Acid-Promoted Cross-Dehydrogenative Coupling

An appropriate and efficient Q-tube-assisted palladium-catalyzed strategy for the synthesis of novel, unparalleled diaza-dibenzo[a,e]azulene and diaza-benzo[a]fluorene derivatives has been sophisticated, which includes oxygen and AcOH-induced oxidative C(sp³)-C(sp²) cross-dehydrogenative coupling reactions of 1-amino-2-imino-4-arylpyridine-3-carbonitriles with benzocyclic ketones such as benzosuberone, tetralone, thiochromone, and chromone, respectively. This Q-tube gas purging kit assisted-protocol features safe due to easy pressing and sealing, a wide substrate scope, easy workup and purifying phases, and the use of O₂ as a benign oxidant, in addition to being scalable and having a high atom economy. The suggested mechanistic pathway includes a formal dehydrative step followed by palladium AcOH-induced CH(sp³)-CH(sp²) oxidative cross-coupling. In this study, X-ray crystallographic analysis has been used to authenticate the targeted products.

Selectivity Control in Gold-Catalyzed Hydroarylation of Alkynes with Indoles: Application to Unsymmetrical Bis(indolyl)methanes

Gold-catalyzed hydroarylation of unactivated alkynes with indoles have previously been reported to proceed with double indole addition to produce symmetrical bis(indolyl)methanes (BIMs). We demonstrate for the first time that the selectivity of the gold-catalyzed reaction can be fully switched to allow for isolation of the vinylindole products instead. Furthermore, this selective reaction can be utilized to synthesize the more difficult to access unsymmetrical BIMs from readily available starting materials.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.

Cross-dehydrogenative coupling involving benzylic and allylic C–H bonds

Benzylic and allylic C–H bonds are coupled with C(sp)–H, C(sp²)–H and C(sp³)–H bonds in a straightforward and high atom-economic manner.

Synthetic Strategy for Pyrazolo[1,5-a]pyridine and Pyrido[1,2-b]indazole Derivatives through AcOH and O2-Promoted Cross-dehydrogenative Coupling Reactions between 1,3-Dicarbonyl Compounds and N-Amino-2-iminopyridines

An efficient method has been developed for the synthesis of uniquely substituted pyrazolo[1,5-a]pyridine and pyrido[1,2-b]indazole derivatives, which involves acetic acid and molecular oxygen promoted cross-dehydrogenative coupling reactions of respective β-ketoesters and β-diketones (like ethyl acetoacetate, ethyl benzoylacetate, methyl propionylacetate, acetylacetone, dimedone, 1,3-cyclohexanedione, and 1,3-cyclopentanedione) with N-amino-2-iminopyridines. The proposed tentative mechanism involves formal acetic acid-promoted oxidative C(sp3)-C(sp2) dehydrogenative coupling followed by dehydrative cyclization under a catalyst-free condition within high atom economy processes.

Sustainable Synthetic Approach for (Pyrazol-4-ylidene)pyridines By Metal Catalyst-Free Aerobic C(sp2)-C(sp3) Coupling Reactions between 1-Amino-2-imino-pyridines and 1-Aryl-5-pyrazolones

A novel, metal catalyst-free, and efficient method has been developed for the synthesis of (pyrazol-4-ylidene)pyridine derivatives. The process involves dehydrogenative coupling of 1-amino-2-imino-pyridines with 1-aryl-5-pyrazolone derivatives utilizing O2 as the sole oxidant. The new method benefits from a high atom economy, efficiency, and substrate scope, as well as the simplicity of reaction and product purification procedures.

Olefin-accelerated solid-state C-N cross-coupling reactions using mechanochemistry

Palladium-catalyzed cross-coupling reactions are one of the most powerful and versatile methods to synthesize a wide range of complex functionalized molecules. However, the development of solid-state cross-coupling reactions remains extremely limited. Here, we report a rational strategy that provides a general entry to palladium-catalyzed Buchwald-Hartwig cross-coupling reactions in the solid state. The key finding of this study is that olefin additives can act as efficient molecular dispersants for the palladium-based catalyst in solid-state media to facilitate the challenging solid-state cross-coupling. Beyond the immediate utility of this protocol, our strategy could inspire the development of industrially attractive solvent-free palladium-catalyzed cross-coupling processes for other valuable synthetic targets.

Cross-coupling reactions have been achieved in solution, yet tend to be inefficient in a solid state. Here, the authors report a solid-state palladium-catalyzed Buchwald-Hartwig cross-coupling using olefins as molecular dispersants, enabling reduction of solvent waste.

Decarboxylative acylation of N-free indoles enabled by a catalytic amount of copper catalyst and liquid-assisted grinding

A mechanochemically Cu(ii)-catalyzed decarboxylative acylation of N-free indoles with O₂ as a terminal oxidant was developed for the mild synthesis of 3-acylindoles.

Preparation of curcumin self-micelle solid dispersion with enhanced bioavailability and cytotoxic activity by mechanochemistry

An amorphous solid dispersion (SD) of curcumin (Cur) with disodium glycyrrhizin (Na2GA) was prepared by mechanical ball milling. Curcumin loaded micelles were self-formed by Na2GA when SD dissolved in water. The physical properties of Cur SD in solid state were characterized by differential scanning calorimetry, X-ray diffraction studies, and scanning electron microscope. The characteristics of the sample solutions were analyzed by reverse phase HPLC, UV-visible spectroscopy, 1H NMR spectroscopy, gel permeation LC, and transmission electron microscopy. In vitro cytotoxic tests demonstrated that Cur SD induced higher cytotoxicity against glioblastoma U-87 MG cells than free Cur. Besides, an improvement of membrane permeability of Cur SD was confirmed by parallel artificial membrane permeability assay. Further pharmacokinetic study of this SD formulation in rat showed a significant ∼19-fold increase of bioavailability as comparing to free Cur. Thus, Cur SD provide a more potent and efficacious formulation for Cur oral delivery.

Bromide-assisted chemoselective Heck reaction of 3-bromoindazoles under high-speed ball-milling conditions: synthesis of axitinib

A mechanically-activated chemoselective Heck coupling for the synthesis of 3-vinylindazoles has been developed with the aid of catalytic amounts of TBAB and NaBr as both dehalogenation restrainer and grinding auxiliary. After tuning of the chemical conditions and mechanical parameters, a series of non-activated 3-bromoindazoles and a broad scope of olefins worked well to give the corresponding coupling products in good to excellent yields. A further application of this protocol was performed in a two-step mechanochemical Heck/Migita cross coupling, which provided a highly efficient route for the synthesis of axitinib.

Mechanochemistry as an emerging tool for molecular synthesis: what can it offer?

Mechanochemistry is becoming more widespread as a technique for molecular synthesis with new mechanochemical reactions being discovered at increasing frequency. Whilst mechanochemical methods are solvent free and can therefore lead to improved sustainability metrics, it is more likely that the significant differences between reaction outcomes, reaction selectivities and reduced reaction times will make it a technique of interest to synthetic chemists. Herein, we provide an overview of mechanochemistry reaction examples, with 'direct' comparators to solvent based reactions, which collectively seemingly show that solid state grinding can lead to reduced reaction times, different reaction outcomes in product selectivity and in some instances different reaction products, including products not accessible in solution.

Mechanochemical synthesis of small organic molecules

With the growing interest in renewable energy and global warming, it is important to minimize the usage of hazardous chemicals in both academic and industrial research, elimination of waste, and possibly recycle them to obtain better results in greener fashion. The studies under the area of mechanochemistry which cover the grinding chemistry to ball milling, sonication, etc. are certainly of interest to the researchers working on the development of green methodologies. In this review, a collection of examples on recent developments in organic bond formation reactions like carbon–carbon (C–C), carbon–nitrogen (C–N), carbon–oxygen (C–O), carbon–halogen (C–X), etc. is documented. Mechanochemical syntheses of heterocyclic rings, multicomponent reactions and organometallic molecules including their catalytic applications are also highlighted.