A Review on Solvent-free Methods in Organic Synthesis

Chalcone Synthesis by Green Claisen-Schmidt Reaction in Cationic and Nonionic Micellar Media

In this paper, micellar-mediated synthesis of chalcones was explored. After optimization of the reaction conditions, the cationic surfactant CTAB and the nonionic one, Tween 80, were taken into consideration. Both surfactants were used to study the scope of Claisen-Schmidt reactants, and a wide scope on both aromatic aldehydes and methyl ketones was explored, obtaining from good to very good yields in most cases and thus demonstrating that the chalcones can be proficiently synthesized in micellar solutions with a wide functional group tolerability. Often, when one surfactant did not perform well, the other surfactant performed better, demonstrating that the use of different surfactants can constitute a good alternative to overcome reactivity problems. Besides, Tween 80 can be proposed as a good and greener alternative to CTAB in most cases. Some reactions gave low yields, showing that some specific improvements would be needed to address the low reactivity. The micellar medium was studied by NMR to search for information about the association of the Claisen-Schmidt reactants with the micelles and their locations within them. Diffusion Ordered Spectroscopy (DOSY) was applied to assess the interaction and the percentage of incorporation of reactants into the micelles.

Synthesis of Core-Functionalised Naphthalenediimides from Naphthalenetetracarboxylic Dianhydride using a Vibratory Ball Mill: Bromination, Imidization and Heck-Type Reactions

The synthesis of 9 core-functionalised naphthalene diimide (c-NDI) residues is reported via a 3-step synthesis from naphthalenetetracarboxylic dianhydride using only mechanochemical activation. Selective dibromination and subsequent diimidization were achieved for the first time in a vibratory ball mill, resulting in the key structural intermediate, 2,6-dibromonaphthalenediimide (DBND), which is the basis for elaboration into a multitude of organic electronic materials. Our new synthesis of DBND is achieved in just 5 hours reaction time over two steps compared to typical solution state times of 24 hours. Subsequent Heck-type cross coupling reactions, with a range of styrene residues, produced a series of c-NDIs in good yields. The Heck-type reactions are rapid (1.5 hours), require no additional heating or solvent and are tolerant of atmospheric moisture and air.

Chemically engineered essential oils prepared through thiocyanation under solvent-free conditions: chemical and bioactivity alteration

The generation of chemically engineered essential oils (CEEOs) prepared from bi-heteroatomic reactions using ammonium thiocyanate as a source of bioactive compounds is described. The impact of the reaction on the chemical composition of the mixtures was qualitatively demonstrated through GC-MS, utilizing univariate and multivariate analysis. The reaction transformed most of the components in the natural mixtures, thereby expanding the chemical diversity of the mixtures. Changes in inhibition properties between natural and CEEOs were demonstrated through acetylcholinesterase TLC autography, resulting in a threefold increase in the number of positive events due to the modification process. The chemically engineered Origanum vulgare L. essential oil was subjected to bioguided fractionation, leading to the discovery of four new active compounds with similar or higher potency than eserine against the enzyme. The results suggest that the directed chemical transformation of essential oils can be a valuable strategy for discovering new acetylcholinesterase (AChE) inhibitors.

A eutectogels-catalyzed one-pot multi-component reaction: access to pyridine and chromene derivatives

The demand for a wide array of functional chemicals and materials has experienced a significant surge in tandem with the advancement of civilization. Regrettably, a number of perilous solvents are employed in chemical laboratories and industrial settings, posing significant risks to the well-being of researchers and contributing to environmental degradation through pollution. Eutectogels, which are based on the eutectic concept, may be synthesized by self-assembling or self-polymerization of various components when put under UV irradiation (254 nm). A novel copolymeric deep eutectic solvent (DES) was successfully synthesized, comprising choline chloride (HBA) as the hydrogen bond acceptor, acetamide (HBD) as the hydrogen bond donor, tetraethyl orthosilicate (TEOS), and formic acid. In this study, we present the preparation of four-component ETGs for synthesizing pyridine and chromene derivatives as a reusable catalyst through a multi-component pathway without solvents. The procedure of synthesizing these heterocyclic compounds is free of using toxic solvents and it could be categorized as a green method.

Unraveling the access to the regioselective synthesis of highly functionalized pyranopyrazoles using an ionic liquid catalyst

An efficient and green strategy for the regioselective synthesis of highly functionalized pyranopyrazole via one-pot condensation of 3-methyl-1-phenyl-5-pyrazolone or EAA and hydrazine hydrate, substituted aromatic aldehydes with NMSM [(E)-N-Methyl-1-(methylthio)-2-nitro-ethenamine] in the existence of IL [(EMIM)Ac] as catalyst with solvent-free condition (SFC) is described. This domino protocol produces biologically substantial heterocycles through Knoevenagel condensation proceeded by Michael addition and O-cyclization with an eradication of methanethiol group, which create the one stereo-center and creation of "C-C, C-N, C-O, C=C, C=N, bonds." The final product is produced by exceptionally easy filtering after the reaction mass was triturated with ethanol. The strategy's noteworthy features include the use of biodegradable IL catalyst, excellent to exceptional yield with rapid reaction times, applicability to a wide range of substrate, clear reaction profile, and straightforward workup process.

Green multicomponent synthesis of pyrano[2,3-c]pyrazole derivatives: current insights and future directions

The past decade has witnessed significant progress in synthesizing structurally diverse and biologically relevant pyrano[2,3-c]pyrazole derivatives through the integration of green methodologies. This review summarizes the recent advances in the green multicomponent synthesis of pyrano[2,3-c]pyrazole and spiro-pyrano[2,3-c]pyrazole derivatives. These include the application of energy-efficient techniques such as microwave and ultrasound-assisted synthesis, benign catalysts and biodegradable composites, solvent selection with a focus on water as a renewable and non-toxic medium, and solvent-free conditions. The review consolidates the current knowledge and future research directions, providing a valuable resource for researchers dedicated to advancing green chemistry practices.

A Lamellar Zn-Based Coordination Polymer Showing Increasing Photoluminescence upon Dehydration

The present study reports on a 2D lamellar coordination polymer (CP) of {[Zn(µ3-pmdc)(H2O)]·H2O}n formula (pmdc = pyrimidine-4,6-dicarboxylate). This CP is synthesized under an appropriate acid-base reaction between the gently mortared reagents in the solid state through a solvent-free procedure that avoids the presence of concomitant byproducts. The X-ray crystal structure reveals the occurrence of Zn2 entities connected through carboxylate groups of pmdc, which behave as triconnected nodes, giving rise to six-membered ring-based layers that are piled up through hydrogen bonding interactions. In addition to a routine physico-chemical characterization, the thermal evolution of the compound has been studied by combining thermogravimetric and thermodiffractometric data. The photoluminescence properties are characterized in the solid state and the processes governing the spectra are described using time-dependent density-functional theory (TD-DFT) with two different approaches employing different program packages. The emissive capacity of the material is further analyzed according to the dehydration and decreasing temperature of the polycrystalline sample.

Direct formation of amide/peptide bonds from carboxylic acids: no traditional coupling reagents, 1-pot, and green

Technology for generating especially important amide and peptide bonds from carboxylic acids and amines that avoids traditional coupling reagents is described. The 1-pot processes developed rely on thioester formation, neat, using a simple dithiocarbamate, and are safe and green, and rely on Nature-inspired thioesters that are then converted to the targeted functionality.

An Efficient and Sustainable Synthesis of the Antimalarial Drug Tafenoquine

An 11-step, 8-pot synthesis of the antimalarial drug tafenoquine succinate was achieved in 42% overall yield using commercially available starting materials. Compared to the previous manufacturing processes that utilize environmentally egregious organic solvents and toxic reagents, the current route features a far greener (as measured by Sheldon's E Factors) and likely more economically attractive sequence, potentially expanding the availability of this important drug worldwide.

Recent Advances in the Enzymatic Synthesis of Polyester

Polyester is a kind of polymer composed of ester bond-linked polybasic acids and polyol. This type of polymer has a wide range of applications in various industries, such as automotive, furniture, coatings, packaging, and biomedical. The traditional process of synthesizing polyester mainly uses metal catalyst polymerization under high-temperature. This condition may have problems with metal residue and undesired side reactions. As an alternative, enzyme-catalyzed polymerization is evolving rapidly due to the metal-free residue, satisfactory biocompatibility, and mild reaction conditions. This article presented the reaction modes of enzyme-catalyzed ring-opening polymerization and enzyme-catalyzed polycondensation and their combinations, respectively. In addition, the article also summarized how lipase-catalyzed the polymerization of polyester, which includes (i) the distinctive features of lipase, (ii) the lipase-catalyzed polymerization and its mechanism, and (iii) the lipase stability under organic solvent and high-temperature conditions. In addition, this article also focused on the advantages and disadvantages of enzyme-catalyzed polyester synthesis under different solvent systems, including organic solvent systems, solvent-free systems, and green solvent systems. The challenges of enzyme optimization and process equipment innovation for further industrialization of enzyme-catalyzed polyester synthesis were also discussed in this article.

Organocatalytic Enantioselective α-Nitrogenation of α,α-Disubstituted Aldehydes in the Absence of a Solvent

A highly efficient enantioselective α-nitrogenation method of α,α-disubstituted aldehydes with azodicarboxylates promoted by a chiral carbamate-monoprotected cyclohexa-1,2-diamine as organocatalyst has been developed. The process was carried out without any solvent, and the corresponding α,α-disubstituted α-nitrogenated aldehydes were obtained with excellent yields and enantioselectivities up to 99% ee. The sustainability of the procedure was established through the calculation of green metrics, such as EcoScale and E-factor. In addition, theoretical calculations have been used to justify the obtained enantioselectivity sense.

The green chemistry of chalcones: Valuable sources of privileged core structures for drug discovery

The sustainable use of resources is essential in all production areas, including pharmaceuticals. However, the aspect of sustainability needs to be taken into consideration not only in the production phase, but during the whole medicinal chemistry drug discovery trajectory. The continuous progress in the fields of green chemistry and the use of artificial intelligence are contributing to the speed and effectiveness of a more sustainable drug discovery pipeline. In this light, here we review the most recent sustainable and green synthetic approaches used for the preparation and derivatization of chalcones, an important class of privileged structures and building blocks used for the preparation of new biologically active compounds with a broad spectrum of potential therapeutic applications. The literature here reported has been retrieved from the SciFinder database using the term "chalcone" as a keyword and filtering the results applying the concept: "green chemistry", and from the Reaxys database using the keywords "chalcone" and "green". For both databases the time-frame was 2017-2022. References were manually selected based on relevance.

Solvent‐Free Synthesis of Core‐Functionalised Naphthalene Diimides by Using a Vibratory Ball Mill: Suzuki, Sonogashira and Buchwald–Hartwig Reactions

Solvent‐free synthesis by using a vibratory ball mill (VBM) offers the chance to access new chemical reactivity, whilst reducing solvent waste and minimising reaction times. Herein, we report the core functionalisation of N,N’‐bis(2‐ethylhexyl)‐2,6‐dibromo‐1,4,5,8‐naphthalenetetracarboxylic acid (Br₂‐NDI) by using Suzuki, Sonogashira and Buchwald–Hartwig coupling reactions. The products of these reactions are important building blocks in many areas of organic electronics including organic light‐emitting diodes (OLEDs), organic field‐effect transistors (OFETs) and organic photovoltaic cells (OPVCs). The reactions proceed in as little as 1 h, use commercially available palladium sources (frequently Pd(OAc)₂) and are tolerant to air and atmospheric moisture. Furthermore, the real‐world potential of this green VBM protocol is demonstrated by the double Suzuki coupling of a monobromo(NDI) residue to a bis(thiophene) pinacol ester. The resulting dimeric NDI species has been demonstrated to behave as an electron acceptor in functioning OPVCs.

Metal-bio functionalized bismuthmagnetite [Fe3-x Bi x O4/SiO2@l-ArgEt3 +I-/Zn(ii)]: a novel bionanocomposite for the synthesis of 1,2,4,5-tetrahydro-2,4-dioxobenzo[b][1,4]diazepine malononitriles and malonamides at room temperature and under sonication

In this work, a new magnetized composite of bismuth (Fe3-x Bi x O4) was prepared and functionalized stepwise with silica, triethylargininium iodide ionic liquid, and Zn(ii) to prepare a multi-layered core-shell bio-nanostructure, [Fe3-x Bi x O4/SiO2@l-ArgEt3 +I-/Zn(ii)]. The modified bismuth magnetic amino acid-containing nanocomposite was characterized using several techniques including Fourier-transform infrared (FT-IR), X-ray fluorescence (XRF), vibrating sample magnetometer (VSM), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDAX), thermogravimetric/differential scanning calorimetric (TGA/DSC) analysis, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and inductively coupled plasma-optical emission spectrometry (ICP-OES). The magnetized bionanocomposite exhibited high catalytic activity for the synthesis of 1,2,4,5-tetrahydro-2,4-dioxobenzo[b][1,4]diazepine malononitriles via five-component reactions between 1,2-phenylenediamines, Meldrum's acid, malononitrile, aldehydes, and isocyanides at room temperature in ethanol. The efficacy of this protocol was also examined to obtain malonamide derivatives via pseudo six-component reactions of 1,4-phenylenediamine, Meldrum's acid, malononitrile, aldehydes, and isocyanides. When the above-mentioned MCRs were repeated under the same conditions with the application of sonication, a notable decrease in the reaction time was observed. The recovery and reusability of the metal-bio functionalized bismuthmagnetite were examined successfully in 3 runs. Furthermore, the characteristics of the recovered Fe3-x Bi x O4/SiO2@l-ArgEt3 +I-/Zn(ii) were investigated though FESEM and EDAX analysis.

Solvent-Free Method for Nanoparticles Synthesis by Solid-State Combustion Using Tetra(Imidazole)Copper(II) Nitrate

The development of solvent-free techniques for nanoparticles synthesis is one of the challenges of Green chemistry. In this work, the principled opportunity to obtain copper-containing nanosized particles without use of any solvents was shown. The copper complexes were prepared as precursors by the melting-assisted solvent-free synthesis. The formation of tetra(imidazole)copper(II) nitrate complex was confirmed by XRD, elemental analysis, FTIR spectroscopy, and thermal analysis. It was noted that their thermal decomposition occurs in two stages: (I) the low-temperature step may be related to redox interaction between organic ligands and nitrate-anions; (II) the high-temperature step may be related to the oxidation of the products of incomplete imidazole decomposition. TEM and XRD studies of solid products of complex combustion have shown that they are oxides with particle size less than 40 nm. Thus, the combustion of [Cu(Im)4](NO3)2 complex under air can be considered as a new approach to prepare nanosized particles of copper oxides without the use of solvents.

The liquid phase of 4,4'-trimethylenedipiperidine: a safe and greener dual-task agent for clean and facile synthesis of coumarin derivatives

A practical and facile synthesis of various coumarin derivatives was conducted using a liquid phase of 4,4'-trimethylenedipiperidine as a safe and greener dual-task reagent under catalyst-free and solvent-free conditions. This reagent is a commercially available solid and can be handled easily, having a liquid phase over a vast temperature range, high thermal stability, low toxicity, and good solubility in green solvents such as water and ethanol. It is worth mentioning that 4,4'-trimethylenedipiperidine could be completely recovered and regenerated after a simple process. The current method has other merits, including (a) minimizing the use of high-risk and toxic reagents and solvents; (b) the use of a secure and recoverable medium-organocatalyst instead of metal-based catalysts, (c) avoid tedious processes, harsh conditions, and a multi-step process for the preparation of catalysts, (d) transform phenol and salicyladehyde derivatives into the corresponding coumarin derivatives in good to high yields, (e) minimize hazardous waste generation. TMDP could be easily recovered and reused several times with no change in its activity. Furthermore, the current work demonstrated that the liquid phase of 4,4'-trimethylenedipiperidine can be a promising medium in organic reaction at higher temperatures due to its broad liquid range temperature, thermal stability, acceptor/donor hydrogen bond property, and other unique merits. New methodology for the synthesis of coumarines using liquid phase of TMDP under mild conditions.

Zn(OAc)2•2H2O-Catalyzed Betti Base Synthesis under Solvent-free Conditions

:

Zn(OAc)2•2H2O-catalyzed one-pot multicomponent reaction of Betti bases using β- naphthol, aldehydes, and amines, under neat conditions in moderate to excellent yields (68- 96%) is reported in this study. This synthetic protocol offers several advantages, such as operational simplicity, shorter reaction period, high yields, and easy work-up procedures.

High-component reactions (HCRs): An overview of MCRs containing seven or more components as versatile tools in organic synthesis

Recently, multi-component reactions (MCRs) have gained special attention due to their versatility for the synthesis of polycyclic heterocycles. Moreover, their applicability can become more widespread as they can be combined together as a union of MCRs. In this overview, the authors have tried to collect the MCRs containing more than seven components that can lead to effectual heterocycles in organic and/or pharmaceutical chemistry. The review contains papers published up to the end of 2020. The subject is classified based on the number of substrates, such as seven-, eight-, nine-, ten-, and more components. The authors expect their report to be helpful for researchers to clarify their route to significant MCRs.

Regioselective Mercury(I)/Palladium(II)-Catalyzed Single-Step Approach for the Synthesis of Imines and 2-Substituted Indoles

An efficient synthesis of ketimines was achieved through a regioselective Hg(I)-catalyzed hydroamination of terminal acetylenes in the presence of anilines. The Pd(II)-catalyzed cyclization of these imines into the 2-substituted indoles was satisfactorily carried out by a C-H activation. In a single-step approach, a variety of 2-substituted indoles were also generated via a Hg(I)/Pd(II)-catalyzed, one-pot, two-step process, starting from anilines and terminal acetylenes. The arylacetylenes proved to be more effective than the alkyl derivatives.

Reactivity of substrates with multiple competitive reactive sites toward NBS under neat reaction conditions promoted by visible light

Regioselectivity of visible-light-induced transformations of a range of (hetero)aryl alkyl-substituted ketones bearing several competitive reactive sites (α-carbonyl, benzyl and aromatic ring) with N-bromosuccinimide (NBS) was studied under solvent-free reaction conditions (SFRC) and in the absence of inert-gas atmosphere, radical initiators and catalysts. An 8-W energy-saving household lamp was used for irradiation. Heterogeneous reaction conditions were dealt with throughout the study. All substrates were mono- or dibrominated at the α-carbonyl position, and additionally, some benzylic or aromatic bromination was observed in substrates with benzylic carbon atoms or electron-donating methoxy groups, respectively. Surprisingly, ipso-substitution of the acyl group with a bromine atom took place with (4-methoxynaphthyl) alkyl ketones. While the addition of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy) decreased the extent of α- and ring bromination, it completely suppressed the benzylic bromination and α,α-dibromination with NBS under SFRC.

Mechanochemical Aza-Vinylogous Povarov Reactions for the Synthesis of Highly Functionalized 1,2,3,4-Tetrahydroquinolines and 1,2,3,4-Tetrahydro-1,5-Naphthyridines

The aza-vinylogous Povarov reaction between aromatic amines, α-ketoaldehydes or α-formylesters and α,β-unsaturated dimethylhydrazones was carried out in a sequential three-component fashion under mechanochemical conditions. Following extensive optimization work, the reaction was performed on a vibratory ball mill operating at 20 Hz and using zirconium oxide balls and milling jar, and afforded 1,2,3,4-tetrahydroquinolines and 1,2,3,4-tetrahydro- 1,5-naphthyridines functionalized at C-2, C-4 and also at C-6, in the latter case. This protocol generally afforded the target compounds in good to excellent yields and diastereoselectivities. A comparison of representative examples with the results obtained under conventional conditions revealed that the mechanochemical protocol affords faster Povarov reactions in comparable yields using a solvent-less environment.

DBU Catalyzed Phospho-Aldol-Brook Rearrangement for Rapid Preparation of α-Phosphates Amide in Solvent-Free Conditions

The 1,8-diazabicyclo [5.4.0] undec-7-ene DBU-catalyzed Phospho-Aldol-Brook Rearrangement reaction of α-ketoamide and dialkyl phosphites was developed under solvent-free at room temperature. The novel α-Phosphate Amide derivatives could be obtained with good yield (86–96%), which also exhibited good tolerance of various dialkyl phosphites and α-ketoamide, including isatins. In addition, the reaction was conducted in both gram-scale and mol-scale, and the title compounds could also be obtained in excellent yield (more than 91%) within 5 min.

Novel Methodologies for Chemical Activation in Organic Synthesis under Solvent-Free Reaction Conditions

One central challenge for XXI century chemists is the development of sustainable processes that do not represent a risk either to humanity or to the environment. In this regard, the search for more efficient and clean alternatives to achieve the chemical activation of molecules involved in chemical transformations has played a prominent role in recent years. The use of microwave or UV-Vis light irradiation, and mechanochemical activation is already widespread in many laboratories. Nevertheless, an additional condition to achieve "green" processes comes from the point of view of so-called atom economy. The removal of solvents from chemical reactions generally leads to cleaner, more efficient and more economical processes. This review presents several illustrative applications of the use of sustainable protocols in the synthesis of organic compounds under solvent-free reaction conditions.

Synthesis of silyl formates, formamides, and aldehydes via solvent-free organocatalytic hydrosilylation of CO2

Carbon dioxide (CO2) was used as a C1 source to prepare silyl formates, formamides, and aldehydes. Tetrabutylammonium acetate (TBAA) catalyzed the solvent-free N-formylation of amines with CO2 and hydrosilane to give formamides including Weinreb formamide, Me(MeO)NCHO, which was successively converted into aldehydes by one-pot reactions with Grignard reagents.