"On water": unique reactivity of organic compounds in aqueous suspension

Accelerated Reaction Rates within Self-Assembled Polymer Nanoreactors with Tunable Hydrophobic Microenvironments

Performing reactions in the presence of self-assembled hierarchical structures of amphiphilic macromolecules can accelerate reactions while using water as the bulk solvent due to the hydrophobic effect. We leveraged non-covalent interactions to self-assemble filled-polymer micelle nanoreactors (NR) incorporating gold nanoparticle catalysts into various amphiphilic polymer nanostructures with comparable hydrodynamic nanoreactor size and gold concentration in the nanoreactor dispersion. We systematically studied the effect of the hydrophobic co-precipitant on self-assembly and catalytic performance. We observed that co-precipitants that interact with gold are beneficial for improving incorporation efficiency of the gold nanoparticles into the nanocomposite nanoreactor during self-assembly but decrease catalytic performance. Hierarchical assemblies with co-precipitants that leverage noncovalent interactions could enhance catalytic performance. For the co-precipitants that do not interact strongly with gold, the catalytic performance was strongly affected by the hydrophobic microenvironment of the co-precipitant. Specifically, the apparent reaction rate per surface area using castor oil (CO) was over 8-fold greater than polystyrene (750 g/mol, PS 750); the turnover frequency was higher than previously reported self-assembled polymer systems. The increase in apparent catalytic performance could be attributed to differences in reactant solubility rather than differences in mass transfer or intrinsic kinetics; higher reactant solubility enhances apparent reaction rates. Full conversion of 4-nitrophenol was achieved within three minutes for at least 10 sequential reactions demonstrating that the nanoreactors could be used for multiple reactions.

Metal-Free and Regioselective Synthesis of Substituted and Fused Chromenopyrrole Scaffolds via the Divergent Reactivity of α-Azido Ketones in Water

A green and simple approach was developed for the regioselective synthesis of structurally diverse chromenopyrrole frameworks from 3-formylchromones, active methylenes, and α-azido ketones using piperidine as a catalyst in the aqueous medium through a tandem one-pot multicomponent reaction. Further, the synthesized pyrrole framework was successfully converted into biologically significant 6-azaindole derivatives in a simple synthetic transformation. An exciting feature of this synthetic protocol is that the reaction mechanism and formation of the products depend on the nature of the active methylene used. This approach has several advantages such as a transition-metal-free catalyst, a short reaction time, easy separation, an excellent yield, practically simple execution, high regioselectivity, very good atom economy, low E-factor, and no requirement of toxic solvents and chromatographic purification.

A kinetic description of how interfaces accelerate reactions in micro-compartments

A kinetic expression is derived to explain how interfaces alter bulk chemical equilibria and accelerate reactions in micro-compartments. This description, aided by the development of a stochastic model, quantitatively predicts previous experimental observations of accelerated imine synthesis in micron-sized emulsions. The expression accounts for how reactant concentration and compartment size together lead to accelerated reaction rates under micro-confinement. These rates do not depend solely on concentration, but rather the fraction of total molecules in the compartment that are at the interface. Although there are ∼10⁷ to 10¹³ solute molecules in a typical micro-compartment, a kind of "stochasticity" appears when compartment size and reagent concentration yield nearly equal numbers of bulk and interfacial molecules. Although this is distinct from the stochasticity produced by nano-confinement, these results show how interfaces can govern chemical transformations in larger atmospheric, geologic and biological compartments.

Molecular reactions at aqueous interfaces

This Review aims to critically analyse the emerging field of chemical reactivity at aqueous interfaces. The subject has evolved rapidly since the discovery of the so-called 'on-water catalysis', alluding to the dramatic acceleration of reactions at the surface of water or at its interface with hydrophobic media. We review critical experimental studies in the fields of atmospheric and synthetic organic chemistry, as well as related research exploring the origins of life, to showcase the importance of this phenomenon. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

"On Water" Palladium Catalyzed Direct Arylation of 1H-Indazole and 1H-7-Azaindazole

The C3 direct arylation of 1H-indazole and 1H-7-azaindazole has been a significant challenge due to the lack of the reactivity at this position. In this paper, we describe a mild and an efficient synthesis of new series of C3-aryled 1H-indazoles and C3-aryled 1H-7-azaindazoles via a C3 direct arylation using water as solvent. On water, PPh₃ was effective as a ligand along with a lower charge of the catalyst Pd(OAc)₂ (5 mol%) at 100 °C, leading to C3-aryled 1H-indazoles or C3-aryled 1H-7-azaindazoles in moderate to good yields.

Iodine Promoted Efficient Synthesis of 2-Arylimidazo[1,2-a]pyridines in Aqueous Media: A Comparative Study between Micellar Catalysis and an "On-Water" Platform

In a new and environmentally sustainable approach, a series of 2-arylimidazo[1,2-a]pyridine derivatives were synthesized in aqueous media in the presence of iodine as a catalyst. The reaction proceeded by condensation of various aryl methyl ketones with 2-aminopyridines to afford 2-arylimidazo[1,2-a]pyridines in good overall yields. Although several of the reactions were efficiently performed "on water", the addition of a surfactant, namely, sodium dodecyl sulphate , was found effective in terms of substrate scope and yield enhancement. Both methods were successfully used for the gram-scale synthesis of a marketed drug, zolimidine. The simple experimental setup, water as "green" media, and inexpensive catalyst are some of the merits of this protocol.

A highly green approach towards aromatic nitro group substitutions: catalyst free reactions of nitroimidazoles with carbon nucleophiles in water

We have successfully developed a flexible green aqueous approach for the formation of a carbon–carbon bond by the reaction of highly-enolizable carbanions (mostly derived from 1,3-dicarbonyl compounds) with an aromatic carbon bearing a nitro group. The key step involves a nucleophilic displacement reaction. All newly synthesized compounds were unambiguously characterized via various spectroscopic techniques including NMR, mass spectrometry and single-crystal X-ray diffraction as applicable. We believe that our study will be useful in providing new insights into catalyst-free water-mediated nucleophilic substitution reactions.

We have successfully developed a flexible green aqueous approach for the formation of a carbon–carbon bond by the reaction of highly-enolizable carbanions (mostly derived from 1,3-dicarbonyl compounds) with an aromatic carbon bearing a nitro group.

The mRNA-Binding Protein HuR Is a Kinetically-Privileged Electrophile Sensor

The key mRNA-binding proteins HuR and AUF1 are reported stress sensors in mammals. Intrigued by recent reports of sensitivity of these proteins to the electrophilic lipid prostaglandin A2 and other redox signals, we here examined their sensing abilities to a prototypical redox-linked lipid-derived electrophile, 4-hydroxynonenal (HNE). Leveraging our T-REX electrophile delivery platform, we found that only HuR, and not AUF1, is a kinetically-privileged sensor of HNE in HEK293T cells, and sensing functions through a specific cysteine, C13. Cells depleted of HuR, upon treatment with HNE, manifest unique alterations in cell viability and Nrf2-transcription-factor-driven antioxidant response (AR), which our recent work shows is regulated by HuR at the Nrf2-mRNA level. Mutagenesis studies showed that C13-specific sensing alone is not sufficient to explain HuR-dependent stress responsivities, further highlighting a complex context-dependent layer of Nrf2/AR regulation through HuR.

Acoustic levitation and infrared thermography: a sound approach to studying droplet evaporation

Herein we report a new technique combining acoustic levitation and infrared thermography to directly monitor droplet surface temperatures. Using it, temperature profiles were recorded during the evaporation of deionized water, methanol, n-propanol, and isopropanol. Results support the viability of this inexpensive and easily-accessed technique for studying chemical and physical changes in droplets.

Substrate-controlled selectivity switch in a three-component reaction: sequential synthesis of spiro-oxazolidinedione-cyclopentenones and hydroxy enaminobarbiturates in water

An efficient eco-friendly catalyst-free three-component domino multicyclization for the synthesis of new spirobicyclic oxazolidinedione containing cyclopentenone moieties has been established by mixing amines, β-dicarbonyl compounds and N,N'-dimethylalloxan in water at room temperature. This domino process involves multiple reactions such as enamination/aldol-like reaction/Stork enamine annulation/intramolecular cyclization under mild conditions.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

DESign of Sustainable One-Pot Chemoenzymatic Organic Transformations in Deep Eutectic Solvents for the Synthesis of 1,2-Disubstituted Aromatic Olefins

The self-assembly of styrene-type olefins into the corresponding stilbenes was conveniently performed in the Deep Eutectic Solvent (DES) mixture 1ChCl/2Gly under air and in the absence of hazardous organic co-solvents using a one-pot chemo-biocatalytic route. Here, an enzymatic decarboxylation of p-hydroxycinnamic acids sequentially followed by a ruthenium-catalyzed metathesis of olefins has been investigated in DES. Moreover, and to extend the design of chemoenzymatic processes in DESs, we also coupled the aforementioned enzymatic decarboxylation reaction to now concomitant Pd-catalyzed Heck-type C-C coupling to produce biaryl derivatives under environmentally friendly reaction conditions.

Cucurbit[7]uril as a catalytic nanoreactor for one-pot synthesis of isoxazolidines in water

The main objective of supramolecular chemistry is to mimic the macrosystems present in nature, a goal that fits perfectly with the green chemistry guidelines. The aim of our work is to use the hydrophobic cavity of cucurbit[7]uril (CB[7]) to mimic nature for performing different dehydration and cycloaddition reactions in water. The hydrophobic cavity of CB[7] made it possible to synthesize nitrones and isoxazolidines in a one-pot fashion using water as a reaction solvent. Substituted isoxazolidines were obtained from the cycloaddition of nitrones with various styrenes and cinnamates, under microwave irradiation, with a catalytic amount of CB[7], and a moderate increase in the formation of the trans adduct was observed, compared to the reaction being carried out in toluene. The mechanism of the reaction and the inclusion of reagents and products in the CB[7] cavity have been studied and rationalized by NMR spectroscopy, ESI-MS experiments, and molecular modeling calculations.

Sum frequency generation, calculation of absolute intensities, comparison with experiments, and two-field relaxation-based derivation

The experimental sum frequency generation (SFG) spectrum is the response to an infrared pulse and a visible pulse and is a highly surface-sensitive technique. We treat the surface dangling OH bonds at the air/water interface and focus on the absolute SFG intensities for the resonant terms, a focus that permits insight into the consequences of some approximations. For the polarization combinations, the calculated linewidths for the water interface dangling OH SFG band at 3,700 [Formula: see text] are, as usual, too large, because of the customary neglect of motional narrowing. The integrated spectrum is used to circumvent this problem and justified here using a Kubo-like formalism and theoretical integrated band intensities rather than peak intensities. Only relative SFG intensities are usually reported. The absolute integrated SFG intensities for three polarization combinations for sum frequency, visible, and infrared beams are computed. We use molecular dynamics and the dipole and the polarizability matrix elements obtained from infrared and Raman studies of [Formula: see text]O vapor. The theoretical expressions for two of the absolute susceptibilities contain only a single term and agree with experiment to about a factor of 1.3, with no adjustable parameters. The Fresnel factors are included in that comparison. One of the susceptibilities contains instead four positive and negative terms and agrees less well. The expression for the SFG correlation function is normally derived from a statistical mechanical formulation using a time-evolving density matrix. We show how a derivation based on a two-field relaxation leads to the same final result.

Regioselective, one-pot, multi-component, green synthesis of substituted benzo[c]pyrazolo[2,7]naphthyridines

An efficient and environmentally benign synthetic protocol has been developed for the synthesis of benzo[c]pyrazolo[2,7]naphthyridine derivatives through regioselective multi-component "on-water" reaction of isatin, malononitrile and 3-aminopyrazole. The Knoevenagel condensation of isatin with malononitrile resulted in the formation of arylidene, which subsequently underwent Michael addition with 3-aminopyrazole followed by basic hydrolysis, cyclization, decarboxylation and aromatization to give the target naphthyridines in good to excellent yields. The one-pot multi-component protocol was also employed to obtain the said naphthyridines in a lower yield (10-15%) than obtained by basic hydrolysis of spiro-intermediates. The present study shows attractive features such as the use of water as a green solvent, short reaction time, reduced waste products and transition metal free C-C and C-N bond formation. The structures of the synthesized derivatives were established through FTIR, 1H-NMR, 13C-NMR spectroscopy and ESI-mass spectrometry.

TBAB-Catalyzed 1,6-Conjugate Sulfonylation of para-Quinone Methides: A Highly Efficient Approach to Unsymmetrical gem-Diarylmethyl Sulfones in Water

A highly efficient sulfonylation of para-quinone methides with sulfonyl hydrazines in water has been developed on the basis of the mode involving a tetrabutyl ammonium bromide (TBAB)-promoted sulfa-1,6-conjugated addition pathway. This reaction provides a green and sustainable method to synthesize various unsymmetrical diarylmethyl sulfones, showing good functional group tolerance, scalability, and regioselectivity. Further transformation of the resulting diarylmethyl sulfones provides an efficient route to some functionalized molecules.

A facile and efficient method for the synthesis of crystalline tetrahydro-β-carbolines via the Pictet-Spengler reaction in water

A facile and efficient synthesis of tetrahydro-β-carbolines (tryptolines) in one step from tryptamine and aldehydes, in an environmentally friendly water solvent, has been investigated. This convenient and clean synthesis of various tryptolines was facilitated by l-tartaric acid, a natural compound, to obtain the desired products as clear crystals. Among the four crystalline products, the most substituted tryptoline 2 showed the best inhibitory activity against EJ cells and the least cytotoxicity, with an LC₅₀ value of 1.49 mg/mL, against brine shrimp larvae.

Biomimetic total syntheses of baefrutones A-D, baeckenon B, and frutescones A, D-F

Biomimetic total syntheses of baefrutones A-D (1-4), baeckenon B (5), and frutescones A, D-F (6-9), isolated from the leaves of Baeckea frutescens, were achieved in 9, 8, and 5 steps, respectively, in moderate to good yields (72-83%). The synthetic routes feature the Michael addition, oxidative [4 + 2] cycloaddition, and water-promoted Diels-Alder click reactions as the key steps. This study helped gain thorough mechanistic insights into the biosynthetic origins and provided a facile approach for the construction of a library of natural tasmanone-based meroterpenoid analogues. Moreover, compounds 1-9 show potent inhibitory effects against S. paratyphi and/or C. albicans with MIC values of 3.125-25 μg mL^-1, and they could be promising lead molecules for the design of new antibiotic agents.

Diels-Alder Reactions in Water Are Determined by Microsolvation

Nanoconfined aqueous environments and the recent advent of accelerated chemistry in microdroplets are increasingly being investigated for catalysis. The mechanisms underlying the enhanced reactivity in alternate solvent environments and whether the enhanced reactivity due to nanoconfinement is a universal phenomenon are not fully understood. Here, we use ab initio molecular dynamics simulations to characterize the free energy of a retro-Diels-Alder reaction in bulk water at very different densities and in water nanoconfined by parallel graphene sheets. We find that the broadly different global solvation environments accelerate the reactions to a similar degree with respect to the gas-phase reaction, with activation free energies that do not differ by more than k_bT from each other. The reason for the same acceleration factor in the extremely different solvation environments is that it is the microsolvation of the dienophile's carbonyl group that governs the transition-state stabilization and mechanism, which is not significantly disrupted by either the lower density in bulk water or the strong nanoconfinement conditions used here. Our results also suggest that significant acceleration of Diels-Alder reactions in microdroplets or on-water conditions cannot arise from local microsolvation when water is present but instead must come from highly altered reaction environments that drastically change the reaction mechanisms.

Interfacial Water Mediates Oligomerization Pathways of Monoterpene Carbocations

The air-water interface plays central roles in "on-droplet" synthesis, living systems, and the atmosphere; however, what makes reactions at the interface specific is largely unknown. Here, we examined carbocationic reactions of monoterpene (C₁₀H₁₆ isomer) on an acidic water microjet by using spray ionization mass spectrometry. Gaseous monoterpenes are trapped in the uppermost layers of a water surface via proton transfer and then undergo a chain-propagation reaction. The oligomerization pathway of β-pinene (β-P), which showed prompt chain-propagation, is examined by simultaneous exposure to camphene (CMP). (CMP)H⁺ is the most stable isomer formed via rearrangement of (β-P)H⁺ in the gas phase; however, no co-oligomerization was observed. This indicates that the oligomerization of (β-P)H⁺ proceeded via ring-opening isomerization. Quantum chemical calculations for [carbocation-(H₂O)_n=1,2] complexes revealed that the ring-opened isomer is stabilized by hydrogen-π bonds. We propose that partial hydration is a key factor that makes the interfacial reaction unique.

Diels–Alder reactions between cyclopentadiene analogs and benzoquinone in water and their application in the synthesis of polycyclic cage compounds

Diels–Alder reactions between cyclopentadiene analogs and p-benzoquinone were explored in water and yielded 83–97% product, higher than the results reported in water with a catalyst or cetrimonium bromide (CTAB) micelles. The novel adduct 10 was synthesized and further used to synthesize the bi-cage hydrocarbon 4,4′-spirobi[pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane], which has a high density (1.2663 g cm⁻³) and a high volumetric heat of combustion (53.353 MJ L⁻¹). Four novel bi-cage hydrocarbon compounds were synthesized in water using this method starting from 2,2′-bi(p-benzoquinone) and cyclopentadiene analogs.

Diels–Alder reactions between cyclopentadiene analogs and p-benzoquinone were explored in water and yielded 83–97% product, higher than the results reported in water with a catalyst or cetrimonium bromide (CTAB) micelles.

Recent designer surfactants for catalysis in water

Recent development of new designer surfactants further spurs the development of micellar catalysis in water for chemical transformations and catalysis, providing reliable alternatives to the employment of organic solvents.

Nitro compounds as the core structures of promising energetic materials and versatile reagents for organic synthesis

This review addresses some promising areas of chemistry of nitro compounds extensively developed in recent years in Russia (particularly at the N.D.Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences) and worldwide. The most important results in the synthesis of novel energetic N-, C- and O-nitro compounds are summarized. New environmentally friendly approaches to the preparation of known compounds of this series, used as components of energetic compositions, are considered. Methods for selective transformations of various nitro compounds to valuable products of organic synthesis, primarily biologically active products and their precursors, are systematically analyzed.

The bibliography includes 446 references.

The unique catalytic role of water in aromatic C–H activation at neutral pH: a combined NMR and DFT study of polyphenolic compounds

NMR and DFT studies demonstrate the unique catalytic role of H₂O in aromatic C–H activation which results in a reduction of ΔG^# values by a factor of 20–30 kcal mol⁻¹.

Electrochemical sulfonylation of alkenes with sulfonyl hydrazides: a metal- and oxidant-free protocol for the synthesis of (E)-vinyl sulfones in water

An electrochemical sulfonylation of alkenes with sulfonyl hydrazides for the synthesis of (E)-vinyl sulfones in water is reported.

Recent progress in the synthesis of limonoids and limonoid-like natural products

Recent progress in syntheses of limonoids and limonoid-like natural products is reviewed. The current “state-of-art” advance on novel synthetic strategy are summarized and future outlook will be presented.

Recent Developments on 1,3-Dipolar Cycloaddition Reactions by Catalysis in Green Solvents

The use of eco-compatible synthetic procedures in organic reactions and, in particular, in 1,3-dipolar cycloaddition reactions, has recently received a great deal of attention and considerable progress has been achieved in this area in the last years. This review summarizes the approaches currently employed to synthesize heterocyclic compounds by catalyzed 1,3-dipolar cycloadditions in green solvents in the last six years. Our choice to do a selection of the literature from 2014 to 2019 was made considering the absence of a recent review about this period, to our knowledge. Several examples to construct heterocycles by 1,3-dipolar cycloadditions will be discussed in this work subdivided in function of the most important class of non-conventional and green solvents, i.e., ionic liquids (ILs), deep eutectic solvents (DES), and water.

The base-free van Leusen reaction of cyclic imines on water: synthesis of N-fused imidazo 6,11-dihydro β-carboline derivatives

Construction of imidazoles has been demonstrated on water under base-free conditions. The reaction of dihydro β-carboline imines and p-toluenesulfonylmethyl isocyanides furnished the corresponding substituted N-fused imidazo 6,11-dihydro β-carboline derivatives in very good yields under ambient conditions. The use of deuterium oxide (D2O) as a solvent enabled the incorporation of deuterium isotopes in the imidazole ring.