Electrophilic Activation Research Articles

Bio‐Inspired Thiolate‐FeII‐Hydrazine Adduct Towards Iron‐Mediated C‐N Bond Formation

The bio‐inspired dinuclear iron(II) complex [FeII(L)]₂ (L2‐ = 2,2’‐(2,2’‐bipyridine‐6,6’‐diyl)bis(1,1’‐diphenylethanethiolate)), featuring alkyl thiolate ligands, was found to react with hydrazine (N₂H₄) to form an unprecedented alkyl thiolate‐supported FeII‐N₂H₄ adduct, [LFeII(N₂H₄)]. This complex was characterized structurally by X‐ray crystallography, spectroscopically (1H NMR, UV‐vis, IR), and electrochemically. Unexpectedly, in CH3CN solution, [LFeII(N₂H₄)] gradually evolved to yield an FeII‐acetohydrazonohydrazide complex, [LFeII(H₂N₂C(CH₃)N₂H₃)], the C‐N bond formation resulting from the reaction between N₂H₄ and CH₃CN mediated by the FeII ion. Its structure, confirmed by X‐ray diffraction on single crystals, reveals a η¹‐coordination of the acetohydrazonohydrazide ligand. The proposed mechanism is based on the electrophilic activation of a CH3CN molecule by FeII, followed by the nucleophilic attack of a free hydrazine. This work expands the reactivity landscape of iron‐hydrazine complexes and provides new insights into potential pathways for N‐N bond functionalization in nitrogen fixation processes.

Late-stage (radio)fluorination of alkyl phosphonates via electrophilic activation

Constructing organic fluorophosphines, vital drug skeletons, through the direct fluorination of readily available alkyl phosphonates has been impeded due to the intrinsic low electrophilicity of PV and the high bond energy of P═O bond. Here, alkyl phosphonates are electrophilically activated with triflic anhydride and N-heteroaromatic bases, enabling nucleophilic fluorination at room temperature to form fluorophosphines via reactive phosphine intermediates. This approach facilitates the late-stage (radio)fluorination of broad dialkyl and monoalkyl phosphonates. Monoalkyl phosphonates derived from targeted drugs, including cyclophosphamide, vortioxetine, and dihydrocholesterol, are effectively fluorinated, achieving notable yields of 47−71%. Radiofluorination of medically significant 18F-tracers and synthons are completed in radiochemical conversions (radio-TLC) of 51−88% and molar activities up to 251 ± 12 GBq/μmol (initial activity 11.2 GBq) within 10 min at room temperature. Utilizing a phosphonamidic fluoride building block (BFPA), [18F]BFPA-Flurpiridaz and [18F]BFPA-E[c(RGDyK)]2 demonstrate high-contrast target imaging, excellent pharmacokinetics, and negligible defluorination.

N-H Bond Activation of Ammonia by a Redox-Active Carboranyl Diphosphine.

In this work, we report the room-temperature N-H bond activation of ammonia by the carboranyl diphosphine 1-PtBu2-2-PiPr2-closo-C2B10H10 (1) resulting in the formation of zwitterionic 7-P(NH2)tBu2-10-P(H)iPr2-nido-C2B10H10 (2). Unlike the other phosphorus-based ambiphiles that require geometric constraints to enhance electrophilicity, the new mode of bond activation in this main-group system is based on the cooperation between electron-rich trigonal phosphine centers and the electron-accepting carborane cluster. As an exception among many other metal-based and metal-free systems, the N-H bond activation of gaseous ammonia or aqueous ammonium hydroxide by carboranyl diphosphine 1 proceeds with tolerance of air and water. Mechanistic details of ammonia activation were explored computationally by DFT methods, demonstrating an electrophilic activation of ammonia by the phosphine center. This process is driven by the reduction of the boron cluster followed by an ammonia-assisted deprotonation and proton transfer. A subsequent reaction of 2 and TEMPO results in the cleavage of all N-H and P-H bonds with the formation of a cyclic phosphazenium cation supported by an anionic cluster N(7-PtBu2-8-PiPr2)-nido-C2B9H10 (3). Transformations reported herein represent the first example of ammonia oxidation via triple hydrogen atom abstraction facilitated by a metal-free system.

Nickel-Catalyzed Photoredox Allenylation of Alkyl Halides.

We report a dual Ni/photoredox-catalyzed cross-coupling method for propargyl carbonates and nonactivated alkyl bromides, facilitating the synthesis of a variety of substituted allenes under mild and practical conditions. Mechanistically, the reaction integrates Ni-catalyzed activation of the propargyl electrophile via SN2' oxidative addition at Ni(I) with silyl radical-induced activation of the alkyl halide through halogen-atom transfer. This methodology provides a gentle approach for introducing allenyl groups into complex halogenated aliphatic molecules, offering further opportunities for derivatization.

Non-electrophilic NRF2 activators promote wound healing in human keratinocytes and diabetic mice and demonstrate selective downstream gene targeting

The transcription factor NRF2 plays an important role in many biological processes and is a promising therapeutic target for many disease states. NRF2 is highly expressed in the skin and is known to play a critical role in diabetic wound healing, a serious disease process for which treatment options are limited. However, many existing NRF2 activators display off-target effects due to their electrophilic mechanism, underscoring the need for alternative approaches. In this work, we investigated two recently described non-electrophilic NRF2 activators, ADJ-310 and PRL-295, and demonstrated their efficacy in vitro and in vivo in human keratinocytes and Leprdb/db diabetic mice. We also compared the downstream targets of PRL-295 to those of the widely used electrophilic NRF2 activator CDDO-Me by RNA sequencing. Both ADJ-310 and PRL-295 maintained human keratinocyte cell viability at increasing concentrations and maintained or improved cell proliferation over time. Both compounds also increased cell migration, improving in vitro wound closure. ADJ-310 and PRL-295 enhanced the oxidative stress response in vitro, and RNA-sequencing data showed that PRL-295 activated NRF2 with a narrower transcriptomic effect than CDDO-Me. In vivo, both ADJ-310 and PRL-295 improved wound healing in Leprdb/db diabetic mice and upregulated known downstream NRF2 target genes in treated tissue. These results highlight the non-electrophilic compounds ADJ-310 and PRL-295 as effective, innovative tools for investigating the function of NRF2. These compounds directly address the need for alternative NRF2 activators and offer a new approach to studying the role of NRF2 in human disease and its potential as a therapeutic across multiple disease states.

Multi-defense Pathways against Electrophiles through Adduct Formation by Low Molecular Weight Substances with Sulfur Atoms.

There is a variety of electrophiles in the environment. In addition, there are precursor chemicals that undergo metabolic activation by enzymes and conversion to electrophiles in the body. Although electrophiles covalently bind to protein nucleophiles, they also form adducts associated with adaptive or toxic responses. Low molecular weight compounds containing sulfur are capable of blocking such adduct formation by capturing the electrophiles. In this review, we present out findings on the capture and inactivation of electrophiles by 1) intracellular glutathione, 2) reactive sulfur species and 3) extracellular cysteine (formed during the production of sulfur adducts). These actions not only substantially suppress electrophilic activity but also regulate protein adduct formation.

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

Electrophilic activation of the aromatic cyclo-P5 ligand in [Cp*Fe(η5-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(η5-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(η4-P5(1-L)(2-Me)][OTf] (L=IDipp (1), EtCAAC (2), IiPr (3 b)) and [Cp*Fe(η4-P5(1-IiPr)(1-Me)][OTf] (3 a). For the first time, the much smaller IMe4 leads to the contraction of the cyclo-P5 ligand and formation of [Cp*Fe(η4-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(η4-P5(1-IDippCH2)(1/2-Me)][OTf] (6 a/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{μ2,η4:3:1-P10Me2(IDippN)}][OTf] (8) and [{Cp*Fe}2{μ2,η4:1:1:1-P11Me2(IDipp)}][OTf] (9), respectively.

Enhancing the Electrocatalytic Hydrogenation of Furfural via Anion-Induced Molecular Activation and Adsorption.

The electrocatalytic hydrogenation (ECH) of furfural (FF) to furfuryl alcohol, which does not require additional hydrogen or high pressure, is a green and promising production route. In this study, we explore the effects of anions on FF ECH in two buffer electrolytes (KHCO3 and phosphate-buffered saline [PBS]). Anions influence the yield of furfuryl alcohol through molecular activation and adsorption. Molecular dynamics simulations show that bicarbonate is present in the first shell layer of the FF molecule and induces strong hydrogen bonding interactions. In contrast, hydrogen phosphate is present only in the second shell layer, resulting in weak hydrogen bonding interactions. Owing to the interfacial anions and hydrogen bonding, FF molecules exhibit strong flat adsorption on the electrode surface in the KHCO3 solution, while weak adsorption is observed in the PBS solution, as confirmed by operando synchrotron-radiation Fourier-transform infrared spectroscopy and in situ Raman spectroscopy. Density-functional theory calculations reveal that the overall anionic hydrogen bonding network promotes the activation of the carbonyl group in the FF molecule in KHCO3, whereas electrophilic activity is inhibited in PBS. Consequently, FF ECH demonstrates much faster kinetics in KHCO3, while it exhibits sluggish ECH kinetics and a severe hydrogen evolution reaction in PBS. This work introduces a new strategy to optimize the catalytic process through the modulation of the microenvironment.

Difluoroenoxysilane: Expanding Allenamide Hydrodifluoroalkylation for Diverse Carbon Frameworks.

This study presents an effective route to access functionalizable fluorinated enamides characterized by their high regiospecificity around the allenamide. Synthetic applications of the resulting difluorocarbonyl-bearing enamide products were pursued through straightforward synthetic transformations to prepare unknown functionalized valuable halogenated O-heterocycles and C5 skeletons. Experimental mechanistic studies showed that hydrodifluoroalkylation occurs via a hidden Brønsted acid activation, thereby establishing a new electrophilic activation mode for allenamide through a conjugated iminium intermediate.

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.

Electron Dynamics in Alkane C-H Activation Mediated by Transition Metal Complexes.

Alkanes, ideal raw materials for industrial chemical production, typically exhibit limited reactivity due to their robust and weakly polarized C-H bonds. The challenge lies in selectively activating these C-H bonds under mild conditions. To address this challenge, various C-H activation mechanisms have been developed. Yet, classifying these mechanisms depends on the overall stoichiometry, which can be ambiguous and sometimes problematic. In this study, we utilized density functional theory calculations combined with intrinsic bond orbital (IBO) analysis to examine electron flow in the four primary alkane C-H activation mechanisms: oxidative addition, σ-bond metathesis, 1,2-addition, and electrophilic activation. Methane was selected as the representative alkane molecule to undergo C-H heterolytic cleavage in these reactions. Across all mechanisms studied, we find that the CH3 moiety in methane consistently uses an electron pair from the cleaved C-H bond to form a σ-bond with the metal. Yet, the electron pair that accepts the proton differs with each mechanism: in oxidative addition, it is derived from the d-orbitals; in σ-bond metathesis, it resulted from the metal-ligand σ-bonds; in 1,2-addition, it arose from the π-orbital of the metal-ligand multiple bonds; and in electrophilic activation, it came from the lone pairs on ligands. This detailed analysis not only provides a clear visual understanding of these reactions but also showcases the ability of the IBO method to differentiate between mechanisms. The electron flow discerned from IBO analysis is further corroborated by results from absolutely localized molecular orbital energy decomposition analysis, which also helps to quantify the two predominant interactions in each process. Our findings offer profound insights into the electron dynamics at play in alkane C-H activation, enhancing our understanding of these critical reactions.

Activation/Cyclization of 2H‐Azirines and 3‐Amino‐2‐fluoropyridines Towards 2‐Aryl‐Pyrido[2,3‐b]pyrazines

AbstractThe electrophilic activation of 2H‐azirines with triflic anhydride (Tf2O) for an addition/cyclization reaction producing 2‐aryl‐pyrido[2,3‐b]pyrazines regioselectively is described. This reaction proceeds via nucleophilic addition of 3‐amino‐2‐fluoropyridines onto 1‐trifloyl‐aziridin‐2‐yl triflates and subsequent cyclization via SNAr. Desulfonylation and oxidation provides a single regioisomer of the aza‐bicyclic heterocycle after treatment with Et3N. Optimization of the reaction conditions lead to a range of 2‐aryl‐pyrido[2,3‐b]pyrazines in isolated yields of 20 % to 60 %.

Transformation mechanisms of antidepressants in biological wastewater treatment: Removal kinetic, transformation products and pathways

The extensive detection of antidepressants in wastewater has raised increasing concerns for human beings. During biological wastewater treatment processes, antidepressants cannot be completely removed and mineralized, which caused the formation of transformation products (TPs). To date, knowledges on TPs that participated in the transformation processes of antidepressants were scarce. To fill this gap, molecular networking nontarget screening based on lab-scale batch assays were employed to explore the removal and transformation of five antidepressants (amitriptyline (AMI), clomipramine (CLO), duloxetine (DUL), nortriptyline (NOR) and paroxetine (PAR)). The removal rate constants per unit of biomass (kbio) of the five antidepressants were found to range from 0.29 to 0.56 L/(gMLSS*d). Comparatively, clomipramine was removed more slowly than other antidepressants, which attributed to its lower electrophilic properties. Subsequently, 22 TPs were tentatively identified, among which 19 ones were newly found in the present study. The proposed transformation pathways indicated that demethylation, N-formylation, N-acetylation, N-succinylation, N-hydroxylation, dealkylation, hydrolysis and nitrosation participated in the transformation of antidepressants in wastewater. Particularly, N-acylation and N-hydroxylation were found as characteristics reactions for antidepressants in wastewater and these reactions consistently occurred at the site of amino functional group, where high electrophilic activity was the dominant causes for the observed characteristics reactions. In silico results revealed that TPs formed during biological wastewater treatment generally had lower toxicity, lower bioaccumulation and higher bioavailability than their parent compounds. Collectively, the present study provides insights into the transformation processes of antidepressants in wastewater, indicating that characteristic transformation reactions occur across different antidepressants, which is theoretically useful to effectively remove pharmaceuticals from wastewater.

Mesoporous Electrocatalysts with p-n Heterojunctions for Efficient Electroreduction of CO2 and N2 to Urea.

The electrocatalytic synthesis of high-value-added urea by activating N2 and CO2 is a green synthesis technology that has achieved carbon neutrality. However, the chemical adsorption and C-N coupling ability of N2 and CO2 on the surface of the catalyst are generally poor, greatly limiting the improvement of electrocatalytic activity and selectivity in electrocatalytic urea synthesis. Herein, novel hierarchical mesoporous CeO2/Co3O4 heterostructures are fabricated, and at an ultralow applied voltage of -0.2 V, the urea yield rate reaches 5.81 mmol g-1 h-1, with a corresponding Faraday efficiency of 30.05%. The hierarchical mesoporous material effectively reduces the mass transfer resistance of reactants and intermediates, making it easier for them to access active centers. The emerging space-charge regions at the heterointerface generate local electrophilic and nucleophilic regions, facilitating CO2 targeted adsorption in the electrophilic region and activation to produce *CO intermediates and N2 targeted adsorption in the nucleophilic region and activation to generate *N ═ N* intermediates. Then, the electrons in the σ orbitals of *N ═ N* intermediates can be easily accepted by the empty eg orbitals of Co3+ in CeO2/Co3O4, which presents a low-spin state (LS: t2g6eg0). Subsequently, *CO couples with *N ═ N* to produce the key intermediate *NCON*. Interestingly, it was discovered through in situ Raman spectroscopy that the CeO2/Co3O4 catalyst has a reversible spinel structure before and after the electrocatalytic reaction, which is due to the surface reconstruction of the catalyst during the electrocatalytic reaction process, producing amorphous active cobalt oxides, which is beneficial for improving electrocatalytic activity.

On the Mechanisms of Hypohalous Acid Formation and Electrophilic Halogenation by Non‐Native Halogenases

AbstractEnzymatic electrophilic halogenation is a mild tool for functionalization of diverse organic compounds. Only a few groups of native halogenases are capable of catalyzing such a reaction. In this study, we used a mechanism‐guided strategy to discover the electrophilic halogenation activity catalyzed by non‐native halogenases. As the ability to form a hypohalous acid (HOX) is key for halogenation, flavin‐dependent monooxygenases/oxidases capable of forming C4a‐hydroperoxyflavin (FlC4a‐OOH), such as dehalogenase, hydroxylases, luciferase and pyranose‐2‐oxidase (P2O), and flavin reductase capable of forming H2O2 were explored for their abilities to generate HOX in situ. Transient kinetic analyses using stopped‐flow spectrophotometry/fluorometry and product analysis indicate that FlC4a‐OOH in dehalogenases, selected hydroxylases and luciferases, but not in P2O can form HOX; however, the HOX generated from FlC4a‐OOH cannot halogenate their substrates. Remarkably, in situ H2O2 generated by P2O can form HOI and also iodinate various compounds. Because not all enzymes capable of forming FlC4a‐OOH can react with halides to form HOX, QM/MM calculations, site‐directed mutagenesis and structural analysis were carried out to elucidate the mechanism underlying HOX formation and characterize the active site environment. Our findings shed light on identifying new halogenase scaffolds besides the currently known enzymes and have invoked a new mode of chemoenzymatic halogenation.

On the Mechanisms of Hypohalous Acid Formation and Electrophilic Halogenation by Non-Native Halogenases.

Enzymatic electrophilic halogenation is a mild tool for functionalization of diverse organic compounds. Only a few groups of native halogenases are capable of catalyzing such a reaction. In this study, we used a mechanism-guided strategy to discover the electrophilic halogenation activity catalyzed by non-native halogenases. As the ability to form a hypohalous acid (HOX) is key for halogenation, flavin-dependent monooxygenases/oxidases capable of forming C4a-hydroperoxyflavin (FlC4a-OOH), such as dehalogenase, hydroxylases, luciferase and pyranose-2-oxidase (P2O), and flavin reductase capable of forming H2O2 were explored for their abilities to generate HOX in situ. Transient kinetic analyses using stopped-flow spectrophotometry/fluorometry and product analysis indicate that FlC4a-OOH in dehalogenases, selected hydroxylases and luciferases, but not in P2O can form HOX; however, the HOX generated from FlC4a-OOH cannot halogenate their substrates. Remarkably, in situ H2O2 generated by P2O can form HOI and also iodinate various compounds. Because not all enzymes capable of forming FlC4a-OOH can react with halides to form HOX, QM/MM calculations, site-directed mutagenesis and structural analysis were carried out to elucidate the mechanism underlying HOX formation and characterize the active site environment. Our findings shed light on identifying new halogenase scaffolds besides the currently known enzymes and have invoked a new mode of chemoenzymatic halogenation.

Stereospecific syn-dihalogenations and regiodivergent syn-interhalogenation of alkenes via vicinal double electrophilic activation strategy

Whereas the conventional anti-dihalogenation of alkenes is a valuable synthetic tool with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, we report a conceptually distinctive strategy for the simultaneous double electrophilic activation of the two alkene carbons from the same side. Then, the resulting vicinal leaving groups can be displaced iteratively by nucleophilic halides to complete the syn-dihalogenation. For this purpose, thianthrenium dication is employed, and all possible combinations of chlorine and bromine are added onto internal alkenes successfully, particularly resulting in the syn-dibromination and the regiodivergent syn-bromochlorination.

Acetate Assistance in Regioselective Hydroamination of Allenamides: A Combined Experimental and Density Functional Theory Study.

A key factor in the development of selective nucleophilic addition to allenamides is controlling the reactivity of electrophilic intermediates, which is generally achieved using an electrophilic activator via conjugated iminium intermediates. In this combined experimental and computational study, we show that a general and highly chemoselective hydroamination of allenamides can be accomplished using a combination of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and NaOAc. Experimental mechanistic studies revealed that HFIP mediates proton transfer to activate the allenamide, while the acetate additive significantly contributes to N-selective interception. This strategy enables a general hydroamination of allenamides without the use of metals. We demonstrated that various functionalized 1,3-diamines could be readily synthesized and diversified into value-added structural motifs. Detailed mechanistic investigations using the density functional theory revealed the role of NaOAc in the formation of reactive electrophilic intermediates, which ultimately governed the selective formation of 1,3-diamine products. Critically, calculations of the potential energy surface around the proton-transfer transition state revealed that two different reactive electrophilic intermediates were formed when NaOAc was added.

A Lewis Acid–Lewis Base Hybridized Electrocatalyst for Roundtrip Sulfur Conversion in Lithium–Sulfur Batteries

AbstractElectrocatalysts can optimize the sulfur/sulfide reaction kinetics in Li–S batteries to compete with the loss of lithium polysulfides (LiPSs) caused by the shuttling effect. However, the design rationale of electrocatalysts to drive roundtrip sulfur/sulfide conversion is lacking. Here, pairing Lewis acidic and Lewis basic active sites to reach collective adsorption of LiPSs and simultaneous activation of electrophiles and nucleophiles in LiPSs is proposed. This concept is validated by doping polyaniline with protonated metatungstate anions, which enables reduced activation energies for both sulfur reduction reaction and sulfide oxidation reaction and results in significantly improved kinetics. Such electrocatalysts enable a Li–S battery to reach a low capacity‐decay rate of 0.029% per cycle for 1000 cycles. This work would offer insights into battery technologies where sulfur electrocatalysis will play pivotal roles.

Elektrophile Aktivierung von S−Si‐Reagenzien mit Silyliumionen für deren regio‐ und diastereoselektive Addition an C−C‐Mehrfachbindungen

AbstractEs wird eine regioselektive silyliumionvermittelte Thiosilylierung von internen C−C‐Dreifachbindungen mit Kontrolle der Doppelbindungsgeometrie beschrieben. Im Zuge dieser Zweikomponentenreaktion eines Alkins und eines Thiosilans werden sowohl eine C(sp2)−S‐ als auch eine C(sp2)−Si‐Bindung mit einer trans‐Beziehung gebildet. Die hierdurch entstandene orthogonal funktionalisierte C−C‐Doppelbindung lässt sich chemoselektiv defunktionalisieren oder unter Erhalt der Alkenkonfiguration in Kreuzkupplungsreaktionen weiter umsetzen. Die Reaktionsvorschrift ist ebenfalls auf die regio‐ und diastereoselektive Thiosilylierung von terminalen Allenen anwendbar, was zu allylischen Thioethern, die eine Vinylsilaneinheit enthalten, führt. Diese Reaktionen beinhalten die elektrophile Aktivierung des S−Si‐Reagenzes, ein silyliertes Thiophenol und sogar Alkylthiolderivat, durch eine in situ erzeugte Carbokationzwischenstufe. Der Katalysecyclus wird durch ein bissilyliertes Aryl‐ oder Alkylsulfoniumion als ein Shuttle für das kationische Siliciumelektrophil aufrechterhalten. Dessen unabhängige Darstellung und strukturelle Charakterisierung durch Röntgenbeugung werden ebenfalls berichtet.