Weak Nucleophiles Research Articles

Enhancing the Reactivity of an Aromatic cyclo-P5 Ligand via Electrophilic Activation.

Electrophilic activation of the aromatic cyclo-P5 ligand in [Cp*Fe(h5-P5)] is demonstrated to drastically enhance its reactivity towards weak nucleophiles. Unprecedented functionalized, contracted as well as complexly aggregated polyphosphorus compounds are accessed utilizing [Cp*Fe(h5-P5Me)][OTf] (A), highlighting the great potential of this underexplored mode of reactivity. Addition of carbenes to A affords novel 1,2- or 1,1-difunctionalized cyclo-P5 complexes [Cp*Fe(h4-P5(1-L)(2-Me)][OTf] (L=IDipp (1), EtCAAC (2), IiPr (3b)) and [Cp*Fe(h4-P5(1-IiPr)(1-Me)][OTf] (3a). For the first time, the much smaller IMe4 leads to the contraction of the cyclo-P5 ligand and formation of [Cp*Fe(h4-P4(1-IMe)(4-Me)] (4). DFT calculations shed light on the delicate mechanism of this type of reaction, which is reinforced by the experimental identification of key intermediates. Even the comparably weak nucleophile IDippCH2 reacts with A to form [Cp*Fe(h4-P5(1-IDippCH2)(1/2-Me)][OTf] (6a/b), highlighting its explicitly more reactive nature. Moreover, exposure of A to IDippEH (E = N, P) leads to a unique aggregation reaction affording [{Cp*Fe}2{m2,h4:3:1‑P10Me2(IDippN)}][OTf] (8) and [{Cp*Fe}2{m2,h4:1:1:1‑P11Me2(IDipp)}][OTf] (9), respectively.

Синтез 5-амино-3-арил-1H-пиразол-4-карбонитрилов на основе гидразинов или бензгидразидов в условиях ультразвуковой активации

Pyrazoles containing amino and carbonitrile groups have a wide range of biological activities, including antimicrobial, antiinflammatory, antitumor, antioxidant, and are used to create pesticides and dyes. Also, these compounds are synthons for the preparation of various polyheterocyclic compounds. New potentially biologically active 5-amino-3-aryl-1H-pyrazole-4-carbonitriles containing pharmacophoric substituents have been obtained via three-component condensation reactions of malonic dinitrile with substituted aromatic aldehydes and benzhydrazides or hydrazines. This work considers the limits of applicability of hydrazines and hydrazides as weak nucleophiles in similar processes. The described target compounds have been synthesized using the «green chemistry» approach under ultrasonic activation in water or a mixture of water and isopropyl alcohol in the presence of triethylamine as base catalyst. The applicability of this environmentally friendly, economical and efficient approach for the synthesis of nitrogen-unsubstituted (7, 8) and N-aryl- (4, 6) or N-aroyl-substituted (1–3) pyrazole-4-carbonitriles has been demonstrated. The target pyrazole-4-carbonitrile (2) is formed and Schiff base (2’) is released as a by-product in the reaction of the weak binucleophile 3-nitrobenzhydrazide with 4-benzyloxy-3-methoxybenzaldehyde and malonic dinitrile. Step-by-step syntheses have shown that the initial reaction of a three-component interaction can be either croton condensation or the formation of a Schiff base. In any case, during subsequent heterocyclization, unstable substituted pyrazolines are formed, which aromatize to the target pyrazoles in the presence of atmospheric oxygen under synthesis conditions. The composition and structure of the products have been confirmed by elemental analysis, NMR 1H, 13C spectroscopy, HSQC, and HMBC heteronuclear correlations.

Fluorinated Dialkyl Chloronium Salts: Synthesis and Reactivity for Fluoroalkylation and Hydride Abstraction.

A new concept for the synthesis of dialkyl chloronium cations [R-Cl-R]+ is described (R=CH3, CH2CF3), that allows the formation of fluorinated derivatives. By utilizing the xenonium salt [XeOTeF5][M(OTeF5)n] (M=Sb, n=6; M=Al, n=4) chlorine atoms of chloroalkanes or the deactivated chlorofluoroalkane CH2ClCF3 are oxidized and removed as ClOTeF5 leading to the isolation of the corresponding chloronium salt. Since the resulting highly electrophilic cation [Cl(CH2CF3)2]+ is able to alkylate weak nucleophiles, this compound can be utilized for the introduction of a fluorinated alkyl group to those. In addition, the fluorinated alkyl chloronium cation displays a high hydride ion affinity, enabling the activation of linear hydrocarbons by hydride abstraction even at low temperatures ultimately leading to the formation of branched carbocations.

Fluorierte Dialkyl Chloronium Salze: Synthese und Reaktivität für CH2CF3‐Übertragungs‐ und Hydridabstraktions‐Reaktionen

AbstractEs wird ein neuer Ansatz für die Synthese von Dialkylchloronium‐Kationen [R−Cl−R]+ beschrieben (R=CH3, CH2CF3), der die Herstellung von fluorierten Derivaten ermöglicht. Unter Verwendung des Xenonium‐Salzes [XeOTeF5][M(OTeF5)n] (M=Sb, n=6; M=Al, n=4) werden Chloratome von Chloralkanen oder dem deaktivierten Chlorfluoralkan CH2ClCF3 oxidiert und als ClOTeF5 abgespalten, wodurch die Isolierung des entsprechenden Chloronium‐Salzes möglich ist. Da das resultierende stark elektrophile Kation [Cl(CH2CF3)2]+ in der Lage ist, schwache Nukleophile zu alkylieren, kann diese Verbindung für die Einführung einer fluorierten Alkylgruppe verwendet werden. Darüber hinaus weist das fluorierte Alkylchloronium‐Kation eine hohe Hydrid‐Ionen‐Affinität auf, sodass lineare Kohlenwasserstoffe auch bei niedrigen Temperaturen durch Hydridabstraktion aktiviert werden und so verzweigte Carbokationen bilden.

Base‐Promoted Regioselective Synthesis of α‐Amino BODIPYs through Cross‐Dehydrogenative Coupling with Anilines

AbstractThe construction of C−N bond at the α‐position of BODIPYs (boron dipyrromethene) not only enables flexible reactivity but also leads to strong electron‐donating properties. In this study, a base‐promoted oxidative cross‐dehydrogenative coupling was developed to synthesize α‐amino BODIPYs with air as a green oxidant. By this strategy, a series of weak nitrogen nucleophiles, such as heteroaryl and aryl anilines, were installed to the α‐position. Remarkably, pyrrole, pyrazole and indole were also employed to realize the synthesis of α‐heteroaryl BODIPYs through direct functionalization. Further aggregation‐induced emission enhancement (AIE) properties and lipid droplet‐targeting bioimaging experiments of representative dyes demonstrated their potential applications as organic probes.

Indoor organic photovoltaic module with 30.6 % efficiency for efficient wireless power transfer

Organic photovoltaic (OPV) cells possess substantial advantages for indoor applications. However, the underdeveloped cathode interlayer hinders the industrialization of indoor OPV cells. Here, we introduce a phenanthroline derivative with weak nucleophilicity, dicarbolong-phenanthroline (DCP), as the cathode interlayer and comprehensively examine its utilization in indoor OPV cells. DCP exhibits thin depletion region width, which is beneficial for the charge extraction at the interface, thereby contributing to improved power conversion efficiency (PCE). Moreover, DCP presents high adaptability for coating process due to its thickness insensitivity. A DCP-based OPV module is fabricated by blade-coating method, reaching a high maximum output voltage of 4.00 V and a remarkable PCE of 30.6 % at 1000 lux. The OPV module can be successfully applied for efficient wireless power transfer. Furthermore, the DCP-based cell demonstrates outstanding device stability with a lifetime of 32000 hours under indoor lighting. These results demonstrate the promising potential of DCP-based OPV cells for indoor applications.

Superelectrophilic Activation of Phosphacoumarins towards Weak Nucleophiles via Brønsted Acid Assisted Brønsted Acid Catalysis.

The electrophilic activation of various substrates via double or even triple protonation in superacidic media enables reactions with extremely weak nucleophiles. Despite the significant progress in this area, the utility of organophosphorus compounds as superelectrophiles still remains limited. Additionally, the most common superacids require a special care due to their high toxicity, exceptional corrosiveness and moisture sensitivity. Herein, we report the first successful application of the "Brønsted acid assisted Brønsted acid" concept for the superelectrophilic activation of 2-hydroxybenzo[e][1,2]oxaphosphinine 2-oxides (phosphacoumarins). The pivotal role is attributed to the tendency of the phosphoryl moiety to form hydrogen-bonded complexes, which enables the formation of dicationic species and increases the electrophilicity of the phosphacoumarin. This unmasks the reactivity of phosphacoumarins towards non-activated aromatics, while requiring only relatively non-benign trifluoroacetic acid as the reaction medium.

Activatable NIR Fluorescence Probe for Epinephrine Detection and Bioimaging Based on Anionic Heptamethine Cyanine.

Epinephrine (EP) is an essential catecholamine in the human body. Currently, most EP detection methods are not suitable for in vivo detection due to material limitations. An organic small molecule fluorescent probe based on a chemical cascade reaction for the detection of EP was designed. Anionic heptamethine cyanine dye was selected as a fluorescent dye because of its NIR fluorescence emission with excellent biocompatibility. The secondary amine of EP nucleophilically attacks the carbonate of the probe with its stronger nucleophilicity and further undergoes intramolecular nucleophilic cyclization to release the fluorophore. Other substances containing only primary amines or no β-OH lack reaction competitiveness due to their weaker nucleophilicity or inability to undergo further cyclization. The fluorescence recovery of the probe was linearly related to the EP concentration of 2-75 μmol/L. The detection limit was 0.4 μmol/L. The recovery rate was 94.78-111.32%. Finally, we successfully achieved bioimaging of EP in living cells and EP analogue in nematodes.

Gold-catalyzed intermolecular amination of allyl azides with ynamides: efficient construction of 3-azabicyclo[3.1.0] scaffold

Gold-catalyzed amination reactions based on azides via α-imino gold carbene intermediates have attracted extensive attention in the past decades because this methodology leads to the facile and efficient construction of synthetically useful N-containing molecules, especially valuable N-heterocycles. However, successful examples of intermolecular generation of α-imino gold carbenes by using azides as amination reagents are rarely explored probably due to the weak nucleophilicity of azides. Herein, we disclose an efficient gold-catalyzed intermolecular aminative cyclopropanation of ynamides with the allyl azides, enabling flexible synthesis of a wide range of valuable 3-azabicyclo[3.1.0]hex-2-ene derivatives in good to excellent yields with excellent diastereoselectivities. Importantly, this protocol represents the first use of allyl azide as an efficient amination reagent in gold-catalyzed alkyne amination reactions.

Catalyst-free electrochemical SNAr of electron-rich fluoroarenes using carboxylic acids

Herein, an electrochemically driven catalyst-free nucleophilic aromatic substitution (SNAr) of electron-rich fluoroarenes with carboxylic acids as weak nucleophiles under mild conditions was reported. A series of highly valuable ester derivatives were obtained in a direct and rapid way. This transformation features commercially available reagents and an exceptionally broad substrate scope with good functional group tolerance, using cheap and abundant electrodes and completed within a short reaction time. Gram-scale synthesis and complex biorelevant compounds ligation further highlighted the potential utility of the methodology. The mechanistic investigations and density functional theory (DFT) calculations verified the feasibility of the proposed pathway of this transformation.

Backside versus Frontside SN2 Reactions of Alkyl Triflates and Alcohols.

Nucleophilic substitution reactions are elementary reactions in organic chemistry that are used in many synthetic routes. By quantum chemical methods, we have investigated the intrinsic competition between the backside SN2 (SN2-b) and frontside SN2 (SN2-f) pathways using a set of simple alkyl triflates as the electrophile in combination with a systematic series of phenols and partially fluorinated ethanol nucleophiles. It is revealed how and why the well-established mechanistic preference for the SN2-b pathway slowly erodes and can even be overruled by the unusual SN2-f substitution mechanism going from strong to weak alcohol nucleophiles. Activation strain analyses disclose that the SN2-b pathway is favored for strong alcohol nucleophiles because of the well-known intrinsically more efficient approach to the electrophile resulting in a more stabilizing nucleophile-electrophile interaction. In contrast, the preference of weaker alcohol nucleophiles shifts to the SN2-f pathway, benefiting from a stabilizing hydrogen bond interaction between the incoming alcohol and the leaving group. This hydrogen bond interaction is strengthened by the increased acidity of the weaker alcohol nucleophiles, thereby steering the mechanistic preference toward the frontside SN2 pathway.

A-series agent A-234: initial in vitro and in vivo characterization

A-series agent A-234 belongs to a new generation of nerve agents. The poisoning of a former Russian spy Sergei Skripal and his daughter in Salisbury, England, in March 2018 led to the inclusion of A-234 and other A-series agents into the Chemical Weapons Convention. Even though five years have already passed, there is still very little information on its chemical properties, biological activities, and treatment options with established antidotes. In this article, we first assessed A-234 stability in neutral pH for subsequent experiments. Then, we determined its inhibitory potential towards human recombinant acetylcholinesterase (HssAChE; EC 3.1.1.7) and butyrylcholinesterase (HssBChE; EC 3.1.1.8), the ability of HI-6, obidoxime, pralidoxime, methoxime, and trimedoxime to reactivate inhibited cholinesterases (ChEs), its toxicity in rats and therapeutic effects of different antidotal approaches. Finally, we utilized molecular dynamics to explain our findings. The results of spontaneous A-234 hydrolysis showed a slow process with a reaction rate displaying a triphasic course during the first 72 h (the residual concentration 86.2%). A-234 was found to be a potent inhibitor of both human ChEs (HssAChE IC50 = 0.101 ± 0.003 µM and HssBChE IC50 = 0.036 ± 0.002 µM), whereas the five marketed oximes have negligible reactivation ability toward A-234-inhibited HssAChE and HssBChE. The acute toxicity of A-234 is comparable to that of VX and in the context of therapy, atropine and diazepam effectively mitigate A-234 lethality. Even though oxime administration may induce minor improvements, selected oximes (HI-6 and methoxime) do not reactivate ChEs in vivo. Molecular dynamics implies that all marketed oximes are weak nucleophiles, which may explain the failure to reactivate the A-234 phosphorus-serine oxygen bond characterized by low partial charge, in particular, HI-6 and trimedoxime oxime oxygen may not be able to effectively approach the A-234 phosphorus, while pralidoxime displayed low interaction energy. This study is the first to provide essential experimental preclinical data on the A-234 compound.

The influence of acceptor acidity on hydrogen bond mediated aglycone delivery (HAD) through the picoloyl protecting group

AbstractThe outcome of glycosylation reactions heavily relies on the specific protecting group patterns employed on both the donor and acceptor molecules. The picoloyl (Pico) protecting group stands out as it can steer the stereoselectivity in a glycosylation reaction through hydrogen bond‐mediated aglycone delivery (HAD). This provides syn‐stereoselectivity, with respect to the stereochemistry of the Pico group, by forming a hydrogen bond between the incoming acceptor and the picoloyl ring nitrogen. We here probe how acceptor acidity influences the stereodirecting effect of the picoloyl protecting group. A set of 3‐O‐functionalized glucosyl and mannosyl donors, each bearing different protecting groups (picolinate, nicotinate, isonicotinate, and benzoate), were synthesized for systematic evaluation. For the 3‐O‐picoloyl‐glucose series, the picoloyl group exhibited minimal influence on stereoselectivity, with only weak nucleophiles showing a modest shift in selectivity for the 3‐O‐Pico protected glucosyl donor in comparison to the other C‐3‐acyl glucosides. In contrast, in the 3‐O‐picoloyl‐mannose series a much stronger β‐directing effect was observed, wherein more acidic acceptors led to increased β‐selectivity. The results provide insights into the complex interplay of acceptor acidity and glycosylation stereoselectivity mediated by the picoloyl protecting group.

Capturing the generation and structural transformations of molecular ions.

Molecular ions are ubiquitous and play pivotal roles1-3 in many reactions, particularly in the context of atmospheric and interstellar chemistry4-6. However, their structures and conformational transitions7,8, particularly in the gas phase, are less explored than those of neutral molecules owing to experimental difficulties. A case in point is the halonium ions9-11, whose highly reactive nature and ring strain make them short-lived intermediates that are readily attacked even by weak nucleophiles and thus challenging to isolate or capture before they undergo further reaction. Here we show that mega-electronvolt ultrafast electron diffraction (MeV-UED)12-14, used in conjunction with resonance-enhanced multiphoton ionization, can monitor the formation of 1,3-dibromopropane (DBP) cations and their subsequent structural dynamics forming a halonium ion. We find that the DBP+ cation remains for a substantial duration of 3.6 ps in aptly named 'dark states' that are structurally indistinguishable from the DBP electronic ground state. The structural data, supported by surface-hopping simulations15 and ab initio calculations16, reveal that the cation subsequently decays to iso-DBP+, an unusual intermediate with a four-membered ring containing a loosely bound17,18 bromine atom, and eventually loses the bromine atom and forms a bromonium ion with a three-membered-ring structure19. We anticipate that the approach used here can also be applied to examine the structural dynamics of other molecular ions and thereby deepen our understanding of ion chemistry.

Efficient one-step amide formation using amino porphyrins.

Amide bonds are one of the most prevalent phenomena in nature and are utilized frequently in drug and material design. However, forming amide bonds is not always efficient or high yielding, particularly when the amine used to conjugate to a carboxylic acid is a weak nucleophile. This limitation precludes many useful amino compounds from participating in conjugation reactions to form amides. A particularly valuable amino compound, which is also a very weak nucleophile, is the amino porphyrin, valued for its role as a photosensitizer, fluorescent agent, catalyst, or, upon metalation, even a very efficient contrast agent for magnetic resonance imaging (MRI). In this work, we propose fast and high-yield coupling of an unreactive amine - the amino porphyrin - to carboxylic acid via isothiocyanate conjugation. Reactions can be achieved in one step at room temperature in one hour, achieving quantitative conversion and near perfect selectivity. Both metalated and unmetalated porphyrin, as well as fluorescein isothiocyanate (FITC), demonstrated efficient conjugation. To illustrate the value of the proposed method, we created a new blood-pool MRI contrast agent that reversibly binds to serum albumin. This new blood-pool agent, known as MITC-Deox (MRI isothiocyanate that links with deoxycholic acid), substantially reduced T1 relaxation times in blood vessels in mice, remained stable for 1 hour, cleared from blood by 24 hours, and was eliminated from the body after 4 days. The proposed method for efficient amide formation is a superior alternative to existing coupling methods, opening a door to novel synthesis of MRI contrast agents and beyond.

Double strain-release enables formal C-O/C-F and C-N/C-F ring-opening metathesis.

Metathesis reactions have been established as a powerful tool in organic synthesis. While great advances were achieved in double-bond metathesis, like olefin metathesis and carbonyl metathesis, single-bond metathesis has received less attention in the past decade. Herein, we describe the first C(sp3)-O/C(sp3)-F bond formal cross metathesis reaction between gem-difluorinated cyclopropanes (gem-DFCPs) and epoxides under rhodium catalysis. The reaction involves the formation of a highly electrophilic fluoroallyl rhodium intermediate, which is capable of reacting with the oxygen atom in epoxides as weak nucleophiles followed by C-F bond reconstruction. The use of two strained ring substrates is the key to the success of the formal cross metathesis, in which the double strain release accounts for the driving force of the transformation. Additionally, azetidine also proves to be a suitable substrate for this transformation. The reaction offers a novel approach for the metathesis of C(sp3)-O and C(sp3)-N bonds, presenting new opportunities for single-bond metathesis.

Palladium-catalyzed allylation and carbonylation: access to allylhydrazones and allyl acylhydrazones.

A palladium-catalyzed allylation of hydrazines with allyl alcohols and aldehydes was developed, enabling the syntheses of a series of allylhydrazones in good to excellent yields with high regioselectivity. Furthermore, the four-component tandem allylation carbonylation of hydrazines with allyl alcohols and aldehydes was established using the catalytic system, producing various allyl acylhydrazones. Additionally, the functionalized allyl acylhydrazones could be smoothly constructed with the catalytic system employing allylhydrazones as a partner. The catalytic system exhibited good functional tolerance with excellent regioselectivities and scaled-up capability, overcoming the limitations of chemoselectivity of the multicomponent transformation and poor conversion of the weak nucleophile.

Раскрытие экзоцикла форбинов слабыми О-нуклеофилами. Изучение и оптимизация путей синтеза свободного хлорина е6

The article considers issues concerning the reaction of exocycle opening in phorbin molecules (on the example of methylpheophorbide a) under the O nucleophiles - water and alcohols action. According to the author, under certain conditions relatively weak nucleophiles as water and alcohols can cause the opening of the exocycle in the molecule of methylpheophorbide a with the formation of chlorine derivatives e6. The study of this reaction, in addition, reveals new synthetic methods to obtain chlorine e6 esters of different degrees of substitution, and eventually - free chlorine e6 in the form of triacids. Free chlorine is a valuable photosensitiser for photodynamic therapy (PDT), and a starting compound for the preparation of other photosensitizers. The article also discusses possible synthetic approaches to obtain free chlorine e6.

Opening of forbinexocycle by weak O-nucleophiles. Study and optimisation of free chlorine e6 synthesis methods

The article considers issues concerning the reaction of exocycle opening in phorbin molecules (on the example of methylpheophorbide a) under the O nucleophiles - water and alcohols action. According to the author, under certain conditions relatively weak nucleophiles as water and alcohols can cause the opening of the exocycle in the molecule of methylpheophorbide a with the formation of chlorine derivatives e6. The study of this reaction, in addition, reveals new synthetic methods to obtain chlorine e6 esters of different degrees of substitution, and eventually - free chlorine e6 in the form of triacids. Free chlorine is a valuable photosensitiser for photodynamic therapy (PDT), and a starting compound for the preparation of other photosensitizers. The article also discusses possible synthetic approaches to obtain free chlorine e6.

Binding Properties of Small Electrophilic Anions [B6 X5 ]- and [B10 X9 ]- (X=Cl, Br, I): Activation of Small Molecules Based on π-Backbonding.

Superelectrophilic anions constitute a special class of molecular anions that show strong binding of weak nucleophiles despite their negative charge. In this study, the binding characteristics of smaller gaseous electrophilic anions of the types [B6 X5 ]- and [B10 X9 ]- (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B12 X11 ]- . The positive charge of vacant boron increases from [B6 X5 ]- via [B10 X9 ]- to [B12 X11 ]- , as evidenced by increasing attachment enthalpies towards typical σ-donor molecules (noble gases, H2 O). However, this behavior is reversed for σ-donor-π-acceptor molecules. [B6 Cl5 ]- binds most strongly to N2 and CO, even more strongly than to H2 O. Energy decomposition analysis confirms that the orbital interaction is responsible for this opposite trend. The extended transition state natural orbitals for chemical valence method shows that the π-backdonation order is [B6 X5 ]- >[B10 X9 ]- >[B12 X11 ]- . This predicted order explains the experimentally observed red shifts of the CO and N2 stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B6 Cl5 N2 ]- : 222 cm-1 . Therefore, strong activation of unreactive σ-donor-π-acceptor molecules (commonly observed for cationic transition metal complexes) is achieved with metal-free molecular anions.