Ethylammonium Nitrate (EAN)/Tf2O and EAN/TFAA: Ionic Liquid Based Systems for Aromatic Nitration

Regioselective synthesis of 3-nitroindoles under non-acidic and non-metallic conditions

An electrophilic substitution reaction, without acid and metal, of indole with ammonium tetramethylnitrate for accessing 3-nitroindole has been developed. In this protocol, trifluoroacetyl nitrate (CF3COONO2) was produced by metathesis of ammonium tetramethyl nitrate and trifluoroacetic anhydride at sub-room temperature. Trifluoroacetyl nitrate (CF3COONO2) is an electrophilic nitrating agent for a variety of indoles, aromatic and heterocyclic aromaticity. Meanwhile, this strategy could be applied to construct the skeleton structure of many kinds of bioactive molecules. Interestingly, 3-nitroindole can be further derivatived as a pyrrolo[2,3-b]indole.

Microwave Assisted Esterification of Aryl/Alkyl Acids Catalyzed by N-Fluorobenzenesulfonimide

The susceptibility of the carbonyl group towards nucleophilic attack affords the construction of various organic compounds. Thus, investigations of carbonyl activation applying greener methodologies are highly important. In the present work, among the investigated N-halo compounds, N-fluorobenzenesulfonimide (NFSi) has been found as an efficient and selective catalyst in the reaction of direct esterification of aryl and alkyl carboxylic acids supported by microwave (MW) irradiation. The comprehensive esterification of different benzoic acids and mono-, di- and tri-carboxy alkyl derivatives was performed, whereby significant reaction time reductions were achieved. The presented method used NFSi as an easily manipulatable, non-metal, water- and air-tolerant catalyst, allowing simple synthetic and isolation procedures and energy saving, compared to conventional methodologies. Importantly, in contrast to esterification under thermal conditions, where N-halo compounds behave as pre-catalysts, in the MW-supported protocol, a distinct reaction mechanism has been proposed that assumes NFSi as a sustainable catalyst. Moreover, a scale-up of the industrially important derivative was performed.

Organic Nitrating Reagents

Nitro compounds are vital raw chemicals that are widely used in academic laboratories and industries for the preparation of various drugs, agrochemicals, and materials. Thus, nitrating reactions are of great importance for chemists and are even taught in schools as one of the fundamental transformations in organic synthesis. Since the discovery of the first nitrating reactions in the 19th century, progress in this field has been constant. Yet, for many years the classical electrophilic nitration approach using a mixture of strong mineral acids dominated the field. However, in recent decades, the attention of researchers has focused on new reactivity and new reagents that can provide access to nitro compounds in a practical and straightforward way under mild reaction conditions. Organic nitrating reagents have played a special role in this field since they have enhanced reactivity. They also allow nitration to be carried out in an ecofriendly and sustainable manner. This review examines the development and application of organic nitrating reagents.

1 Introduction

2 Organic Nitrating Reagents

2.1 Alkyl Nitrites

2.2 Nitroalkanes

2.3 Alkyl Nitrates

2.4 N-Nitroamides

2.5 N-Nitropyrazole

2.6 N-Nitropyridinium Salts

3 Organic Nitrating Reagents Generated In Situ

3.1 Acyl Nitrates

3.2 Trimethylsilyl Nitrate

3.3 Nitro Onium Salts

4 Organic Nitronium Salts

5 Organic Nitrates and Nitrites

5.1 Ammonium Nitrates

5.2 Heteroarylium Nitrates

5.3 Other Organic Nitrates

5.4 Organic Nitrites

6 Conclusion and Outlook

Recent Advances in the Synthesis of Diverse Libraries of Small-Molecule Building Blocks in Ionic Liquids (ILs)

The Account describes recent advances, from the authors’ laboratories, in the synthesis of diverse libraries of small-molecule building blocks employing ionic liquids (ILs). The ability of ILs to act as catalysts/promoters/solvents for electrophilic and onium ion chemistry, as well as in metal-mediated cross-coupling reactions, and the potential to sequence/hyphenate these methods, have opened up new opportunities for facile assembly of functional small molecules with increased complexity from readily available precursors. While Brønsted acidic IL/IL solvent mixtures are suitable media for carbocation and onium ion chemistry, piperidine-appended IL/IL solvent mixtures can successfully catalyze a variety of base-catalyzed reactions. Several widely practiced transformations including ‘name reactions’ were adapted and performed efficiently in ILs.

1 Introduction

2 Aryldiazonium Salts and Aryltriazenes as Coupling Partners in Metal-Mediated C–C Cross-Coupling Reactions in ILs

3 Expanding the Scope of Metal-Mediated Cross-Coupling Reactions in ILs

4 Application of ILs in Synthesis and Functionalization of Hetero­cycles

5 Expanding the Scope of Amide Synthesis in ILs

6 Generation and Chemistry of ‘Tamed’ Propargylic Cations in ILs

7 Newer Nitration Methods for Arenes and Heteroarenes in ILs

8 Halofunctionalization in ILs

9 ‘Name Reactions’and Other Widely Practiced Synthetic Transformations in ILs

9.1 The Biginelli Reaction

9.2 Nitrile Synthesis by the Schmidt Reaction

9.3 Rupe Rearrangement

9.4 Synthesis of 1,3-Dioxanes via Prins Reaction in [BMIM(SO3H)][OTf]

9.5 Synthesis of Cyclopropanes and Oxiranes by the Corey–Chaykovsky (CC) Reaction

10 Conclusions and Closing Remarks

Ipso Nitration of Aryl Boronic Acids Using Fuming Nitric Acid

The ipso nitration of aryl boronic acid derivatives has been developed using fuming nitric acid as the nitrating agent. This facile procedure provides efficient and chemoselective access to a variety of aromatic nitro compounds. While several activating agents and nitro sources have been reported in the literature for this synthetically useful transformation, this report demonstrates that these processes likely generate a common active reagent, anhydrous HNO₃. Kinetic and mechanistic studies have revealed that the reaction order in HNO₃ is >2 and indicate that the ^•NO₂ radical is the active species.

A Useful Method for the Conversion of Olefins to Nitro Olefins

Triflyl nitrate is easily generated from tetra-n-butylammonium nitrate in CH₂Cl₂ solution and serves as an effective nitrating agent for a wide range of unsaturated substrates to form nitro olefins.

Etherification of phenols by amines via transient diazonium intermediates

In this paper, the synthesis of alkyl aryl ethers from electron poor phenols and amines, using 1,3-propanedinitrite, is described. Due to the mild conditions, functionalized primary, secondary, and tertiary alkyl groups were successfully introduced, denoting a highly tolerant process that allows for unprotected alcohols and acetals. The reaction is thought to proceed through the formation of a diazonium intermediate that undergoes subsequent SN2 or SN1 reactions.

Coupling reactions between reduced intermediates of insensitive munitions compound analog 4-nitroanisole

Explosives, pesticides, and pharmaceuticals contain toxic nitroaromatic compounds that may form even more toxic azo compounds if they encounter reducing conditions in the environment. We investigated the mechanism by which 4,4'-dimethoxyazobenzene forms in anaerobic sludge incubations of 4-nitroanisole, an analog for the insensitive munitions compound 2,4-dinitroanisole (DNAN). Because studies have reported the mechanism to involve the coupling of reduced nitroaromatic intermediates, specifically aromatic amines and nitrosoaromatics, by nucleophilic processes, we abiotically paired 10 mM 4-aminoanisole with 2 mM 4-nitrosoanisole in nitrogen-flushed microcosms. However, only 7 μM of 4,4'-dimethoxyazobenzene had formed after 24 h. We identified the major product to be 4-methoxy-4'-nitrosodiphenylamine. Repeating this experiment in phosphate buffer at pH 5.1, 7.1, and 8.6 demonstrated that the formation of this unexpected product is acid catalyzed. We found that 4-methoxy-4'-nitrosodiphenylamine is more toxic than 4,4'-dimethoxyazobenzene to the bioluminescent bacterium Aliivibrio fischeri, with IC₅₀ values of 0.1 μM and 0.5 μM, respectively. Both products are several orders of magnitude more toxic than reduced 4-nitroanisole intermediates 4-aminoanisole and 4-nitrosoanisole, as well as DNAN and its aromatic amine metabolites. Six-fold more 4,4'-dimethoxyazobenzene formed when we incubated 4-nitrosoanisole with ascorbic acid, a reducing agent, than when we incubated 4-nitrosoanisole with 4-aminoanisole in the absence of ascorbic acid. We therefore suspect that 4-hydroxylaminoanisole, the first reduction product of 4-nitrosoanisole, is a better nucleophile than 4-aminoanisole and couples more readily with 4-nitrosoanisole. Slightly basic and reducing conditions can prevent the formation and persistence of toxic coupling products on sites contaminated with nitroaromatics, i.e. DNAN-contaminated firing ranges.

Unprecedented Behavior of (9 R)-9-Hydroxystearic Acid-Loaded Keratin Nanoparticles on Cancer Cell Cycle

Histone deacetylases, HDACs, have been demonstrated to play a critical role in epigenetic signaling and were found to be overexpressed in several type of cancers; therefore, they represent valuable targets for anticancer therapy. 9-Hydroxystearic acid has been shown to bind the catalytic site of HDAC1, inducing G0/G1 phase cell cycle arrest and activation of the p21^WAF1 gene, thus promoting cell growth inhibition and differentiation in many cancer cells. Despite the ( R) enantiomer of 9-hydroxystearic acid (9R) displaying a promising in vitro growth-inhibitory effect on the HT29 cell line, its scarce water solubility and micromolar activity require novel solutions for improving its efficacy and bioavailability. In this work, we describe the synthesis and in vitro biological profiling of 9R keratin nanoparticles (9R@ker) obtained through an in-water drug-induced aggregation process. The anticancer activity of 9R@ker was investigated in the HT29 cell line; the results indicate an increased fluidity of cell membrane and a higher intracellular ROS formation, resulting in an unexpected S phase cell cycle arrest (25% increase as compared to the control) induced by 9R@ker with respect to free 9R and an induction of cell death.

Prospective Symbiosis of Green Chemistry and Energetic Materials

A global increase in environmental pollution demands the development of new "cleaner" chemical processes. Among urgent improvements, the replacement of traditional hydrocarbon-derived toxic organic solvents with neoteric solvents less harmful for the environment is one of the most vital issues. As a result of the favorable combination of their unique properties, ionic liquids (ILs), dense gases, and supercritical fluids (SCFs) have gained considerable attention as suitable green chemistry media for the preparation and modification of important chemical compounds and materials. In particular, they have a significant potential in a specific and very important area of research associated with the manufacture and processing of high-energy materials (HEMs). These large-scale manufacturing processes, in which hazardous chemicals and extreme conditions are used, produce a huge amount of hard-to-dispose-of waste. Furthermore, they are risky to staff, and any improvements that would reduce the fire and explosion risks of the corresponding processes are highly desirable. In this Review, useful applications of almost nonflammable ILs, dense gases, and SCFs (first of all, CO₂ ) for nitration and other reactions used for manufacturing HEMs are considered. Recent advances in the field of energetic (oxygen-balanced and hypergolic) ILs are summarized. Significant attention is paid to the SCF-based micronization techniques, which improve the energetic performance of HEMs through an efficient control of the morphology and particle size distribution of the HEM fine particles, and to useful applications of SCFs in HEM processing that makes them less hazardous.

syn-Selective crotylation of aldehydes using bismuth-crotyl bromide-(1-butyl-3-methylimidazolium bromide) combination: some synthetic applications

The Bi-[bmim][Br] combination has been found to offer high syn-selectivity in the crotylation of aldehydes with crotyl bromide using practically stoichiometric amounts of the reagents. The room temperature ionic liquid (RTIL), [bmim][Br], activated Bi metal in the presence of oxygen to produce crotylbismuthdibromide, which reacted with the aldehydes at room temperature. The major anti-syn diastereomeric product obtained from the crotylation of (R)-cyclohexylideneglyceraldehyde was utilized for the synthesis of dictyostatin and cryptophycin segments, and (+)-cis-aerangis lactone, using standard synthetic protocols.

Engineered P450 biocatalysts show improved activity and regio-promiscuity in aromatic nitration

Nitroaromatics are among the most important and commonly used chemicals but their production often suffers from multiple unsolved challenges. We have previously described the development of biocatalytic nitration processes driven by an engineered P450 TxtE fusion construct. Herein we report the creation of improved nitration biocatalysts through constructing and characterizing fusion proteins of TxtE with the reductase domain of CYP102A1 (P450BM3, BM3R). The majority of constructs contained variable linker length while one was rationally designed for optimizing protein-protein interactions. Detailed biochemical characterization identified multiple active chimeras that showed improved nitration activity, increased coupling efficiency and higher total turnover numbers compared with TxtE. Substrate promiscuity of the most active chimera was further assessed with a substrate library. Finally, a biocatalytic nitration process was developed to nitrate 4-Me-dl-Trp. The production of both 4-Me-5-NO₂-l-Trp and 4-Me-7-NO₂-l-Trp uncovered remarkable regio-promiscuity of nitration biocatalysts.

Using weak interactions to control C–H mono-nitration of indolines

By utilising simultaneous cooperative multiple weak interactions (soft forces), mild and selective C₅–H or C₇–H mono-nitration of indoline was demonstrated.

Heterogenization of an imidazolium ionic liquid based on magnetic carbon nanotubes as a novel organocatalyst for the synthesis of 2-amino-chromenes via a microwave-assisted multicomponent strategy

The microwave-assisted synthesis of the first ionic liquid stabilized CNTs–Fe₃O₄as a novel nanocatalyst for the synthesis of 2-amino-chromenesviaa multicomponent reaction is reported.

Gold-Catalyzed Proto- and Deuterodeboronation

A mild gold-catalyzed protodeboronation reaction, which does not require acid or base additives and can be carried out in "green" solvents, is described. As a result, the reaction is very functional-group-tolerant, even to acid- and base-sensitive functional groups, and should allow for the boronic acid group to be used as an effective traceless directing or blocking group. The reaction has also been extended to deuterodeboronations for regiospecific ipso-deuterations of aryls and heteroaryls from the corresponding organoboronic acid. Based on density functional theory calculations, a mechanism is proposed that involves nucleophilic attack of water at boron followed by rate-limiting B-C bond cleavage and facile protonolysis of a Au-σ-phenyl intermediate.

Highly efficient synthesis of amides from ketoximes using trifluoromethanesulphonic anhydride

Trifluoromethanesulphonic anhydride (triflic anhydride: TA) has been successfully used as a reagent for Beckmann rearrangement in the conversion of a variety of ketoximes into amides without any additive or base.

Operative conversions of 3-carboxy-4-quinolones into 3-nitro-4-quinolones via ipso-nitration: potential antifilarial agents as inhibitors of Brugia malayi thymidylate kinase

An efficient, cost effective and green methodology for ipso nitration in the synthesis of the 3-nitro derivative of 3-carboxy 4-quinolones has been developed by the quantitative use of copper acetate and silver nitrate in water.

Synthesis of 13C and 15N labeled 2,4-dinitroanisole

Syntheses of [(13)C6]-2,4-dinitroanisole (ring-(13)C6) from [(13)C6]-anisole (ring-(13)C6) and [(15)N2]-2,4-dinitroanisole from anisole using in situ generated acetyl nitrate and [(15)N]-acetyl nitrate, respectively, are described. Treatment of [(13)C6]-anisole (ring-(13)C6) with acetyl nitrate generated in 100% HNO3 gave [(13)C6]-2,4-dinitroanisole (ring-(13)C6) in 83% yield. Treatment of anisole with [(15)N]-acetyl nitrate generated in 10 N [(15)N]-HNO3 gave [(15)N2 ]-2,4-dinitroanisole in 44% yield after two cycles of nitration. Byproducts in the latter reaction included [(15)N]-2-nitroanisole and [(15)N]-4-nitroanisole.