Aryl Cation Research Articles

Distinguishing the XUV-induced Coulomb explosion dynamics of iodobenzene using covariance analysis

Abstract The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I+ with phenyl or phenyl-like cations (CnHx + or CnHx 2+, with n = 1-6 and x = 1-5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of C6Hx 2+ intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps.

Gas‐phase Smiles rearrangement of ortho‐nitro‐substituted diaryliodonium cations and their mechanistic insights

RationaleDiaryliodonium salts are useful electrophilic reagents in organic chemistry, finding extensive applications in arylations and photo‐induced polymerizations. However, comprehensive mechanistic investigations, particularly concerning the mass spectrometric behaviors of diaryliodonium salts, are relatively scarce in the literature.MethodsDiaryliodonium salts could be readily ionized using electrospray ionization mass spectrometry (ESI‐MS) to give [Ar1‐I+‐Ar2], and high‐resolution ESI‐tandem mass spectrometry (MS/MS) experiments were conducted to investigate their gas‐phase chemical reactions.ResultsInvestigations on ESI‐MS/MS of [Ar1‐I+‐Ar2] revealed two major fragmentation patterns: (1) reductive elimination resulting in the diaryl coupling product ion [Ar1‐Ar2]+• by the loss of I; (2) generation of aryl cations [Ar1]+ or [Ar2]+ through cleavage of the C–I bonds. We revealed that the introduction of NO2 into Ar2 of [Ar1‐I+‐Ar2] could lead to an unexpected fragmentation ion [Ar1O]+ in MS/MS, arising from an O‐atom transfer process from NO2 to Ar1. Particularly, when NO2 was ortho‐positioned to the iodine in Ar2, the [Ar1O]+ sometimes exhibited dominant behavior.ConclusionsComprehensive ESI‐MS/MS studies and theoretical calculations provided strong support for the O‐atom transfer mechanistic pathway: [Ar1‐I+‐(o‐NO2‐Ar2)] initially underwent a Smiles rearrangement to the intermediate [Ar1‐O‐(o‐NO‐Ar2I)]+, which subsequently dissociated to [Ar1O]+ or [o‐NO‐Ar2I]+•. Herein, we proposed an unexpected “ortho effect” in the gas‐phase fragmentation reaction of [Ar1‐I+‐(o‐NO2‐Ar2)], in which the crucial determinant factor for the aryl migration was identified as the Smiles rearrangement reaction.

Aurated Aryl Cations as Halogen Abstractors – Easy Access to 1-Halogenated Naphthalenes

Aurated Aryl Cations as Halogen Abstractors – Easy Access to 1-Halogenated Naphthalenes

Chiral-Achiral Cations Intercalation Induced Lead-Free Chiral-Polar Hybrid Perovskites Enable Self-Powered X-Ray and Ultraviolet-Visible-Near-Infrared Photo Detection.

Lead halide hybrid perovskites have made great progress in direct X-ray detection and broadband photodetection, but the existence of toxic Pb and the demand for external operating voltage have severely limited their further applications and operational stability improvements. Therefore, exploring "green" lead-free hybrid perovskite that can both achieve X-ray detection and broadband photodetection without external voltage is of great importance, but remains severely challenging. Herein, using centrosymmetric (BZA)3BiI6 (1, BZA = benzylamine) as a template, a pair of chiral-polar lead-free perovskites, (BZA)2(R/S-PPA)BiI6 (2-R/S, R/S-PPA = (R/S)-1-Phenylpropylamine) are successfully obtained by introducing chiral aryl cations of (R/S)-1-Phenylpropylamine. Compared to 1, chiral-polar 2-R presents a significant irradiation-responsive bulk photovoltaic effect (BPVE) with an open circuit photovoltage of 0.4V, which enables it with self-powered X-ray, UV-vis-NIR broadband photodetection. Specifically, 2-R device exhibits an ultralow detection limit of 18.5nGys-1 and excellent operational stability. Furthermore, 2-R as the first lead-free perovskite achieves significant broad-spectrum (377-940nm) photodetection via light-induced pyroelectric effect. This work sheds light on the rational crystal reconstruction engineering and design of "green" hybrid perovskite toward high-demanded self-powered radiation detection and broadband photodetection.

Visible-Light-Promoted Aryl Cation Formation: Aromatic SN1 Reactions of Areneazo-2-(2-nitro)propanes.

The visible light excitation of areneazo-2-(2-nitro)propane·HCl salts generated the singlet aryl cation that readily underwent aromatic SN1 reactions with a variety of nucleophiles. The in situ generated singlet aryl cation was stabilized by a counter nitronate anion that prevented other intersystem crossing and single electron transfer processes. With the improved safety features of neutral areneazo-2-(2-nitro)propane derivatives, the current visible-light-promoted aromatic SN1 reactions provide an alternative aryl Csp2-X bond forming strategy.

Divergent Mechanistic Scenarios for the C−sp2 and C−sp3 C−H Insertion of Aurated Aryl Cations

AbstractBy employing the sterically highly demanding steering ligand IPr* at a gold(I) center, it is possible to induce a 6‐endo‐dig cyclization pathway in 1,5‐diynes bearing terminal alkyne moieties. The resulting aurated naphthyl cations are utilized for the intermolecular formation of C−C bonds via C−H insertion of aromatic and aliphatic C−H bonds. The mechanisms of these transformations were studied by quantum chemical calculations.

Catalytic Strategies and Mechanism Analysis Orbiting the Center of Critical Intermediates in Lignin Depolymerization

Lignin, as a precious resource given to mankind by nature with abundant functional aromatic structures, has drawn much attention in the recent decade from academia to industry worldwide, aiming at harvesting aromatic compounds from this abundant and renewable natural polymer resource. How to efficiently depolymerize lignin to easy-to-handle aromatic monomers is the precondition of lignin utilization. Many strategies/methods have been developed to effectively degrade lignin into monomers, such as the traditional methods of pyrolysis, gasification, liquid-phase reforming, solvolysis, chemical oxidation, hydrogenation, reduction, acidolysis, alkaline hydrolysis, alcoholysis, as well as the newly developed redox-neutral process, biocatalysis, and combinatorial strategies. Therefore, there is a strong demand to systemically summarize these developed strategies and methods and reveal the internal transformation principles of the lignin. Focusing on the topic of lignin depolymerization to aromatic chemicals, this review reorganizes and categorizes the strategies/methods according to their mechanisms, orbiting the center of critical intermediates during the lignin linkage transformation, which includes the critical anionic intermediates, cationic intermediates, organometallic intermediates, organic molecular intermediates, aryl cation radical intermediates, and neutral radical intermediates. The corresponding introduction involves the generation and the transformation chemistry of the critical intermediates via the corresponding C-H/O-H/C-C/C-O chemical bond transformations, leading to the cleavage of the C-C/C-O linkage bonds. Accompanying the brief introduction of lignin chemistry and the final concluding remarks and perspectives on lignin depolymerization, this review aims to provide a current research process of lignin depolymerization, which may provide useful suggestions for this vigorous research field.

Aryl-aryl cross-coupling reactions without reagents or catalysts: photocyclization of ortho-iodoaryl ethers and related compounds via triplet aryl cation intermediates.

Cyclisations of benzyl ortho-iodoaryl ethers to benzo[c]chromenes can be effected without reagents or catalysts by irradiation with UVC under flow. Reactions proceed via triplet aryl cation generation, 5-exo and 3-exo-cyclisations, and rearomatisation. They have wide scope, are easy to effect and extend to a myriad of related ring systems.

Design, development and applications of copper-catalyzed regioselective (4 + 2) annulations between diaryliodonium salts and alkynes

Diaryliodonium salts have been extensively applied in organic synthesis as aryl cation equivalents. However, in the electrophilic reactions with alkenes or alkynes, only the electrophilic carbon of the diaryliodonium salts was involved while the other part of the aryl ring was not utilized. Herein, a reaction pattern of diaryliodonium was reported as oxa-1,4-dipoles to undergo (4 + 2) cycloaddition reactions with alkynes. Broad spectrum of the two reaction partners could be utilized in this protocol, enabling an operationally simple, high yielding, and regioselective synthetic approach to isocoumarins. Particularly, good to excellent regioselectivities were achieved for the sterically unbiased unsymmetrical diaryl acetylenes, which was challenging for other transition metal-catalyzed processes. The reaction could be scaled up with the ideal 1:1 stoichiometry and the isocoumarin type natural products Oospolactone and Thunberginol A could be obtained in one or three steps through this methodology.

Computational study on the reduction and solvolysis of triplet chlorobenzenes

AbstractIn this work we explored the excited state reactivity of triplet chlorobenzenes with density functional theory and a cluster‐continuum approach. We modeled two competing reactions: a direct abstraction of hydrogen from a solvent molecule and solvolysis via photo‐SN2Ar. Electron donating (–OMe, –CH2SiMe3, –SiMe3) and withdrawing (–CN) substituents not only have distinct effects on the triplet geometries, inducing structural distortions due to the relief of excited state antiaromaticity, but also affect the reactivity of the system. Therefore, electron‐rich chlorobenzenes favor the radical reduction, while electron deficiency opens the possibility for solvolysis. Both reactions are energetically comparable to or more favorable then the dissociation of the C‐Cl bond to form triplet or singlet aryl cations, the intermediates considered responsible for these reactivities. Our findings can be correlated with experimental results on similar systems available from the literature, deeming the proposed pathways as viable alternatives to established mechanisms involving aryl cations.

Diradicals Photogeneration from Chloroaryl-Substituted Carboxylic Acids.

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double‐strand DNA cleavage, the photochemistry of a set of chloroaryl‐substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron‐transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO2 loss in a structure‐dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.

Effects of Electrolytes on the Dediazoniation of Aryldiazonium Ions in Acidic MeOH/H2O Mixtures

Aryldiazonium, ArN2+, ions decompose spontaneously through the formation of highly reactive aryl cations that undergo preferential solvation by water, showing a low selectivity towards the nucleophiles present in their solvation shell. In this work, we investigate the effects of electrolytes (NaCl, LiCl, and LiClO4) on the dediazoniation of 2-, 3-, and 4-methylbenzenediazonium ions in acidic MeOH/H2O mixtures. In the absence of electrolytes, the rates of dediazoniation, kobs, increase modestly upon increasing the MeOH content of the reaction mixture. At any solvent composition, the rate of ArN2+ loss is the same as that for product formation. The main dediazoniation products are cresols (ArOH) and methyl phenyl ethers (ArOMe). Only small amounts (less than 5%) of the reduction product toluene (ArH), which are detected at high percentages of MeOH. Quantitative yields of are obtained at any solvent composition. The addition of LiCl or NaCl ([MCl] = 0–1.5 M) to the reaction mixtures has a negligible effect on kobs but leads to the formation, in low yields (<10%), of the ArCl derivative. The addition of LiClO4 (0–1.5 M) to 20% MeOH/H2O mixtures has a negligible effect on both kobs and on the product distribution. However, at 99.5% MeOH, the addition of the same amounts of LiClO4 leads to a modest decrease in kobs but to a significant decrease in the yields of ArOMe. Results are interpreted in terms of the preferential solvation of perchlorate ions by the aryl cations, removing MeOH molecules from the solvation shell.

Alkyl-Aryl Cation Mixing in Chiral 2D Perovskites.

We report 2D hybrid perovskites comprising a blend of chiral arylammonium and achiral alkylammonium spacer cations (1:1 mole ratio). These new perovskites feature an unprecedented combination of chirality and alkyl-aryl functionality alongside noncovalent intermolecular interactions (e.g., CH···π interactions), determined by their crystal structures. The mixed-cation perovskites exhibit a circular dichroism that is markedly different from the purely chiral cation analogues, offering new avenues to tune the chiroptical properties of known chiral perovskites, instead of solely relying on otherwise complex chemical syntheses of new useable chiral cations. Further, the ability to dilute the density of chiral cations by mixing with achiral cations may offer a potential way to tailor the spin-based properties in 2D hybrid perovskites, such as Rashba-Dresselhaus spin splitting and chirality-induced spin selectivity and magnetization effects.

Excitation of aryl cations as the key to catalyst-free radical arylations

While Sandmeyer-type brominations/iodinations and Meerwein-type arylations are useful methodologies, they are not without their drawbacks. Here, we report an alternative catalyst- and additive-free access to Sandmeyer-type brominations/iodinations and to a diverse set of Meerwein-type arylations. The key for the highly selective transformations is the excitation of slowly and thermally formed aryldiazonium-derived aryl cations by blue LEDs, conditions that differ from the known photodissociation pathway. An excited singlet state (diradical character) of the cation is shown to be the key intermediate, which interacts with the solvent MeOH to eventually form the sp2-centered chain-initiating radical. Our results demonstrate a facile, mild, atom-economic, sustainable, and economically attractive route for a broad spectrum of aryl radical arylations, with methanol as the solvent and blue LED irradiation being the only restrictions. In addition, this methodology can be directly pursued from the abundant feedstock, the anilines.

Effective N2 capture by aryl cations at ambient temperature and pressure.

Here, we show that molecular N2 was efficiently captured by organic arylium cations in a well-defined manner at ambient pressure and temperature, which was monitored by on-line mass spectrometry analysis. A kinetic picture was proposed to disclose the principle of the ion-molecule reaction behavior for exclusive aryldiazonium production. The observation has an implication for direct nitrogen fixation into an organic framework via the intermediacy of such cationic species.

Leaving Groups in Metal‐Free Arylations: Make Your Choice!

Cleavage of an Aryl–Leaving Group bond (Ar–LG) is the key step in most arylation processes for the synthesis of highly functionalized (hetero)arenes. In metal‐free arylations, depending on the nature of the starting substrates, different thermal and photochemical approaches can be adopted: Single Electron Transfer (SET) and homolysis/heterolysis of an Ar–LG bond to afford an aryl radical (Ar·) or an aryl cation (Ar+), respectively. Accordingly, the nature of LG plays a key‐role in determining the mechanism and efficiency of the process.

Biaryl Cross‐Coupling Enabled by Photo‐Induced Electron Transfer

AbstractWe report a protocol for aryl cross‐coupling of electron‐deficient aryl halides with electron‐rich (hetero)arenes that is driven solely by violet light. This process takes advantage of formation of photo‐excited state of electron‐deficient aryl halides, that are reduced by electron‐rich (hetero)arenes to form a pair of aryl anion and cation radicals. The resulting aryl anion radicals of aryl halides undergo mesolysis of the carbon‐halogen bond to generate aryl radicals, that are coupled most likely with aryl cation radicals to afford functionalized biaryls.magnified image

Chemical tuning for potential antitumor fluoroquinolones

Phototoxic effects of 6,8 dihalogenated quinolones confers to this type of molecules a potential property as photochemotherapeutic agents. Two photodehalogenation processes seem to be involved in the remarkable photoinduced cellular damage. In this context, a new 6,8 dihalogenated quinolone 1 (1-methyl-6,8-difluoro-4-oxo-7-aminodimethyl-1,4-dihydroquinoline-3-carboxylic acid) was synthetized looking for improving the phototoxic properties of fluoroquinolones (FQ) and to determine the role of the photodegradation pathways in the FQ phototoxicity. With this purpose, fluorescence emissions, laser flash photolysis experiments and photodegradation studies were performed with compound 1 using 1-ethyl-6,8-difluoro-4-oxo-7-aminodimethyl-1,4-dihidroquinoline-3-carboxylic acid (2) and lomefloxacin (LFX) as reference compounds. The shortening of alkyl chain of the N(1) of the quinolone ring revealed a lifetime increase of the reactive aryl cation generated from photolysis of the three FQ and a significant reduction of the FQ photodegradation quantum yield. The fact that these differences were smaller when the same study was done using a hydrogen donor solvent (ethanol-aqueous buffer, 50/50 v/v) evidenced the highest ability of the reactive intermediate arising from 1 to produce intermolecular alkylations. These results were correlated with in vitro 3T3 NRU phototoxicity test. Thus, when Photo-Irritation-Factor (PIF) was determined for 1, 2 and LFX using cytotoxicity profiles of BALB/c 3T3 fibroblasts treated with each compound in the presence and absence of UVA light, a PIF more higher than 30 was obtained for 1 while the values for 2 and LFX were only higher than 8 and 10, respectively. Thereby, the present study illustrates an approach to modulate the photosensitizing properties of FQ with the purpose to improve the chemotherapeutic properties of antitumor quinolones. Moreover, the results obtained in this study also evidence that the key pathway responsible for the phototoxic properties associated with dihalogenated quinolones is the aryl cation generation.

DFT B3LYP/6-311+G* calculation study of a new nonclassical mechanism of electrophilic functionalization of aromatic C–H bond via aryl cation formation

The DFT B3LYP-/6-311+G* calculations of the reactions beween 1,3,5-Ad3C6H3 (Ad = 1-adamantyl) and CBr3+Al2Br7− with or without CO reveal that only sp3 C–H route occurs.

Diaryl-λ3-chloranes: Versatile Synthesis and Unique Reactivity as Aryl Cation Equivalent.

We have developed a versatile, high-yield synthesis of diarylchloroniums/λ3-chloranes through the reaction of various chloroarenes with readily prepared mesityldiazonium tetrakis(pentafluorophenyl)borate under mild conditions. The scope of the reaction is broad, including ArCl, ArBr, and ArI. The diarylchloroniums/λ3-chloranes prepared here show unique reactivity in various respects, enabling intermolecular electrophilic arylation reaction of weak nucleophiles, and chlorane-halogane exchange reaction.