Tropylium Research Articles

Pd(II)‐Catalyzed Conversion Of N‐Troponyl‐Amino Acid Derivatives Into Cationic Troponyl‐Oxazolidines

AbstractBenzoxazole and benzoxazolidine are constituents of various bioactive natural/synthetic molecules comprising benzenoid aromatic residue. However, non‐benzenoid aromatic scaffolds such as tropone and tropolone are also found in natural products. This report describes the synthesis of unusual cationic troponyl‐oxazolidine derivatives from N‐troponyl‐amino acids/peptides by Pd(II)‐catalyzed transformation. X‐ray studies confirm the formation of the first tropylium cation annelated oxazolidine structures and support the formation of unique supramolecular self‐assembly structures owing to the H‐bonding. The CV studies of new tropylium cations reveal their reduction potential ca −1.2 V. Hence, cationic troponyl‐oxazolidinyl amino acids/peptides could be promising analogs of benzoxazolidine for the development of therapeutic drugs.

Synthesis of Dibenzoazulene Derivatives via Pd‐Catalyzed (3+2) Alkyne Annulation of Dibenzosuberenone

AbstractNon‐benzenoid PAHs with pentagon/heptagon pairs have unique electronic structures and promising opto‐electronic properties. However, the construction of pentagon/heptagon pair units in these PAHs is challenging. Herein, we developed a Pd‐catalyzed (3+2) alkyne annulation reaction of heptagons, and constructed heptagon/pentagon pairs. The substrate scope is wide and the yield is good to moderate. A series of dibenzoauzlene derivatives were synthesized and their opto‐electronic properties were systematically studied by UV‐vis absorption and cyclic voltammetry. In addition, the dibenzoazulene derivative was further transformed to tropylium cation and tropyl radical, which showed bathochromic shifted absorption and switched aromaticity.

Linear Non-benzenoid Isomer of Acene Fusing Chrysene with Azulene Units.

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received considerable attention owing to their distinctive optical and electrical properties. Nevertheless, the synthesis and optoelectronic application of non-benzenoid PAHs remain challenging. Herein, we present a facile synthesis of linear non-benzenoid PAH with an armchair edge, diACh, by fusing chrysene with two azulene units. We systematically investigated the optical and electrical properties, which were also compared to its isomers, including benzenoid and non-benzenoid zigzag edge isomers. diACh exhibits global aromaticity, good planarity, and suitable highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels. The protonation of diACh in solution successively forms a stable tropylium cation and dication. Moreover, the neutral, cationic, and dicationic states of diACh can be transformed with remarkable reversibility during the protonation-deprotonation process, as confirmed by ultraviolet-visible absorptions, fluorescence spectra, 1H nuclear magnetic resonance, and theoretical calculations. Additionally, we fabricate p-type organic field-effect transistor (OFET) devices based on diACh with hole mobility up to 0.026 cm2 V-1 s-1, and we further develop OFET-based acid vapor sensors with good sensitivity, recyclability, and selectivity. These findings underscore the unique properties of linear non-benzenoid PAHs with an armchair edge engendered by the fusion of azulene with the acene backbone, showcasing prospective applications in organic optoelectronics.

Electrospray ionization tandem mass spectrometry of 4-aryl-3,4-dihydrocoumarins.

We have investigated the gas-phase fragmentation reactions of 11 synthetic 4-aryl-3,4-dihydrocoumarins by electrospray ionization tandem mass spectrometry (ESI-MS/MS) on a quadrupole-time-of flight (Q-TOF) hybrid mass spectrometer. We have also estimated thermochemical data for the protonated coumarins (precursor ion A) and product ion structures by computational chemistry at a B3LYP level of theory to establish the ion structures and to rationalize the fragmentation pathways. The most abundant ions in the product ion spectra of coumarins 1-11 resulted from C8H8O2, CO2, C4H4O3, C8H10O3, C8H8O2, and CH3OH eliminations through retro-Diels-Alder (RDA) reactions, remote hydrogen rearrangements (β-eliminations), and β-lactone ring contraction. Although the investigated coumarins shared most of the fragmentation pathways, formation of a benzylic product ion and its corresponding tropylium ion was diagnostic of the substituents at ring C. The thermochemical data revealed that the nature and position of the substituents at ring C played a key role in the formation of this product ion and determined its relative intensity in the product ion spectrum. The results of this study contribute to knowledge of the gas-phase ion chemistry of this important class of organic compounds.

Light-Driven Ring Slippage in [Re(η7-C7H7)(η5-C7H9)]+ and the Inertness of Its Technetium Homologue.

Here, we present the light-driven reactions of [Re(η7-C7H7)(η5-C7H9)]+ (1+) with nitriles, phosphines, and isocyanides, which are added to 1+ via a ring slippage of the tropylium cation from η7 to η3, forming [Re(η3-C7H7)(η5-C7H9)(L)2]+ (L= acetonitrile 2+; 2-phenylacetonitrile 3+; 1,3,5-triaza-5-phosphoadamantane (PTA) 4+; tert-butyl isocyanide 6+; benzyl isocyanide 7+) and [Re(η3-C7H7)(η5-C7H9)(L)]+ with L = (ethane-1,2-diyl)bis(diphenylphosphane) (dppe) 5+. To compare the reactivities of rhenium and technetium, we also investigated the synthesis of [99Tc(η6-C10H8)2]+, its substitution of naphthalene with cyclohepta-1,3,5-triene to obtain [99Tc(η7-C7H7)(η5-C7H9)]+, and its reactivity (or lack thereof) with light.

Charge-transfer inclusion complex formation of the tropylium cation with prism[6]arenes.

Herein, we report that prism[6]arenes (PrS[6]R) can form charge-transfer (CT) inclusion complexes with tropylium tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate (G) in chloroform solution with an obvious CT band at 560 nm. Moreover, the CT complex PrS[6]Et⊃G showed Cl-/Ag+ responsiveness which can be easily monitored by the naked eye.

Efficient amide bond formation via tropylium ion organocatalysis

An efficient method for amide bond formation is reported using tropylium ion organocatalysis. High yields are achieved under mild conditions with a broad substrate scope. DFT calculations were performed to assess substituent effects on reaction energies, tautomerism, and orbital splitting.

Computational Studies on the Tropylium‐Catalyzed Carboxylic Acid O−H Insertion with Diazoesters: The Role of Homoaromaticity**

AbstractA tropylium‐catalyzed carboxylic acid O−H insertion with diazoesters was reported to provide α‐hydroxy esters through an activated carbene intermediate. This study presents a revised mechanism based on density functional theory (DFT) calculations involving a unique homoaromatic intermediate formed between the tropylium ion and the diazoester. The calculated energies of the intermediates and transition states support this revised mechanism. Our computational model explains the tropylium ion‘s binding to the diazoester, resulting in the homoaromatic intermediate. By comparing the reaction profiles of four different pathways, our findings contribute to a better understanding of the reaction mechanism and have potential implications for designing more efficient catalytic systems.

A Negatively Curved π‐Expanded Pyracylene Comprising a Tropylium Cation

AbstractWe disclose π‐expanded pyracylenes and their cationic species comprising 7‐membered rings. The compounds were synthesized by stepwise oxidative cyclodehydrogenation to monitor the effect of successive cyclization on the structural and optoelectronic properties. As shown by X‐ray crystallography, the complete cyclization leads to a boat‐shaped scaffold featuring negative curvature provided by the 7‐membered ring. The embedded tropone unit enabled the convenient generation of a stabilized tropylium cation, showing bathochromically shifted absorption bands reaching into the near‐infrared region beyond 1000 nm. The altered structural features, supported by theoretical calculations, point towards the positively charged 7‐membered ring having aromatic character.

Tropylium Ion, an Intriguing Moiety in Organic Chemistry.

The tropylium ion is a non-benzenoid aromatic species that works as a catalyst. This chemical entity brings about a large number of organic transformations, such as hydroboration reactions, ring contraction, the trapping of enolates, oxidative functionalization, metathesis, insertion, acetalization, and trans-acetalization reactions. The tropylium ion also functions as a coupling reagent in synthetic reactions. This cation's versatility can be seen in its role in the synthesis of macrocyclic compounds and cage structures. Bearing a charge, the tropylium ion is more prone to nucleophilic/electrophilic reactions than neutral benzenoid equivalents. This ability enables it to assist in a variety of chemical reactions. The primary purpose of using tropylium ions in organic reactions is to replace transition metals in catalysis chemistry. It outperforms transition-metal catalysts in terms of its yield, moderate conditions, non-toxic byproducts, functional group tolerance, selectivity, and ease of handling. Furthermore, the tropylium ion is simple to synthesize in the laboratory. The current review incorporates the literature reported from 1950 to 2021; however, the last two decades have witnessed a phenomenal upsurge in the utilization of the tropylium ion in the facilitation of organic conversions. The importance of the tropylium ion as an environmentally safe catalyst in synthesis and a comprehensive summary of some important reactions catalyzed via tropylium cations are described.

Pagoda[n]arenes (n = 4, 5): Extremely Strong Binders for the Tropylium Cation

Herein, host-guest complexation between pagoda[n]arenes (n = 4, P4; n = 5, P5) and tropylium cation (G) was investigated in detail. It was found that both P4 and P5 showed surprisingly strong binding affinities toward the tropylium cation with association constants of more than 107 M-1 for the 1:1 host-guest complexes. The theoretical calculations showed different host-guest complexion ways for complexes G@P4 and G@P5 and the strong π···π interactions and multiple C-H···π interactions play a very important role in the formation of these stable complexes, respectively. Moreover, the switchable processes of guest binding and release in the complexes can be effectively controlled by redox stimuli, and they can be also visible by the color and fluorescence changes.

Thermally Activated Aromatic Ionic Dopants (TA‐AIDs) Enabling Stable Doping, Orthogonal Processing and Direct Patterning

AbstractDoping plays a critical role in organic electronics, and dopant design has been central in the development of functional and stable doping. In this study, there is departure from conventional molecular dopants and a new class of dopants are reported – aromatic ionic dopants (AIDs). AIDs consist of a pair of aromatic cation and anion that are responsible for molecular doping reaction and charge balancing separately. It is shown that the first AID made from cycloheptatrienyl (tropylium) cation and pentacyanocyclopentadienide anion (PCCp), abbreviated as T‐PCCp, can function as an effectivep‐type dopant to dope polydioxythiophenes. Here, tropylium cation induces the doping reaction while the PCCp anion stabilizes the generated polarons and bipolarons. With T‐PCCp, a highly doped (≈120 S/cm) and stable system is achieved up to 150 °C, an orthogonal (sequential)solution processing resulting from the immiscibility of the dopant and the polymer host, and a high‐resolution direct micropatterning with laser writing resulted from a thermally activated doping process.

Synthesis, Structures, and Properties of Tropone-Fused Coumarin Dyes

Abstract A nonbenzenoid aromatic ring, tropone-fused 7-methoxycoumarin dye (1a) and its chlorinated product (1b) were designed and successfully synthesized via transannulation of a coumarin derivative and dehydrobromination of the dibromo- and tribromoprecursors. X-ray crystallography revealed that 1a and 1b are distorted at the annulated tropone skeleton with bond alternation, unlike typical 3,4-annulated π-extended coumarin dyes. These tropone-fused coumarin dyes exhibited redshifted absorption and emission bands compared with that of planar 3,4-benzo-7-methoxycoumarin. These compounds demonstrated relatively efficient fluorescence emission (ΦF = 2–10%) despite their curved skeletons, and solvent dependency owing to the intramolecular charge transfer in the excited state. A computational study clarified that compound 1 changes to a flat structure including the tropylium unit in the excited state. Furthermore, in the fluorescence spectra of 1a with trifluoroacetic acid, the emission intensity increased dramatically to ΦF = 19% owing to the formation of a charge-separated structure with the tropylium cation.

In the quest of Hückel–Hückel and Hückel–Baird double aromatic tropylium (tri)cation and anion derivatives

AbstractBesides the most common form of aromaticity involving a π‐ring, hexaiodobenzene and hexakis(phenylselenyl)benzene dications also present σ‐aromaticity in the outer ring formed by the main group substituents. These two compounds are considered σ‐ and π‐double aromatic, and their characterization is of special interest to the fields of organic and structural chemistry. In this work, we decided to explore the double aromaticity in substituted tropylium cations for three reasons: (i) the seven neutral halogen substituents of the tropylium cations will, without oxidation, lead to 14 σ‐electrons (a 4n + 2 Hückel number); (ii) tropylium cations are highly stable and can be easily generated experimentally; and (iii) whereas in substituted benzenes the distances between substituents in the optimized structures or X‐ray crystals are too large to allow strong σ‐aromaticity, these distances are expected to be shorter in substituted tropylium cations. Yet, instead of the expected σ‐aromaticity, we found that the most stable geometries are highly puckered, meaning that delocalization in both π‐ and σ‐systems is lost. Our results, which include also the tropylium anion and trication in the singlet and triplet state, show that there is a need to open a lone pair hole by oxidation to generate σ‐aromaticity. Among the systems studied, only triplet C7Br7+3 with an internal Hückel aromatic tropylium ring and an external incipient Baird aromatic Br7 ring shows double π‐ and σ‐aromaticity. This result, however, is functional‐dependent and reveals that 3C7Br73+ is at the borderline for onset of double aromaticity.

Direct C(sp3)-N Bond Formation between Toluene and Amine in Water Microdroplets.

Unlike the inertness of bulk water, water microdroplets exhibit some remarkable reactivities. We report that water microdroplets can directly produce stable C7H7+ cations (a combination of benzylic and tropylium cations) from toluene and other substrates at room temperature with a positive voltage (+4 kV) applied to the droplet spray source. The C7H7+ cation and the benzyl radical (C6H5CH2·) are both generated via hydroxyl radicals at the water-gas interface of the microdroplets. The C7H7+ signal is observed directly by mass spectrometry. Dissolved amines (primary, secondary, and tertiary) in the microdroplets can react with both C7H7+ and C6H5CH2· to form the corresponding alkyl C(sp3)-N coupling products in one step, which cannot be achieved in bulk water or other solvents. The products were identified using tandem mass spectrometry (MS2) and 1H NMR spectroscopy. Notably, the direct C(sp3)-N bond formation products were obtained in the absence of a catalyst. In the presence of a radical scavenger, the mass spectra of the C(sp3)-N coupling products are strongly suppressed, which supports the hypothesis that this reaction is driven by hydroxyl radicals generated in the water microdroplets. Taken together, these results show that water microdroplets provide a new method for direct one-step C(sp3)-N bond formation without the need for a metal catalyst.

A New Family of Tropylium Ions: Tropone–Triarylborane Complexes

AbstractAir‐ and thermally stable tropone–triarylborane 1 : 1 complexes were synthesized in excellent yields by treating tropones, including benzo‐fused derivatives, with an equimolar amount of B(C6F5)3. NMR and X‐ray analyses and DFT calculations including nuclear independent chemical shift (NICS) and anisotropy of the induced current density (ACID) studies revealed that the tropone–triarylborane complexes have betaine structures consisting of tropylium and borate moieties and that their aromaticity depends on the number of fused benzene rings.

Recent advance in carbocation-catalyzed reactions

Carbocations such as tropylium and trityl cation, can be stable enough to be isolated and used without inert conditions. They can act as Lewis acids to lower the LUMO of electrophile, thus promoting reactions with nucleophiles. Additionally, the interaction between carbocations and alcohols can form Brønsted acids with enhanced acidity. Furthermore, electrophoto activation of TAC+ (trisaminocyclopropenium ion) delivers the excited radical dication TAC•2+*, which is a strong oxidant and capable of oxidizing a range of challenging substrates. Moreover, nPr-DMQA+ is disclosed as a versatile photoredox catalyst as its excited state can be quenched through both oxidation and reduction. This review summarizes recent advance in carbocation-catalyzed reactions. These developed methods provide an environmentally friendly pathway for the synthesis of valuable compounds and will inspire chemists to discover more interesting transformations promoted by carbocations.

Structural phase transition in a charge-transfer compound: tropylium hexafluoridoantimonate(V)-1,4-dimethylnaphthalene (1/1).

Molecular motion in crystals has attracted much attention for the development of stimuli-responsive materials. The most studied are molecules with few atoms or highly symmetrical molecules. To develop molecules with new motion characteristics, we synthesized a charge-transfer compound, namely, tropylium hexafluoridoantimonate(V)-1,4-dimethylnaphthalene (1/1), (C7H7)[SbF6]·C12H12, and studied its structural phase transition. In this compound, the tropylium cation and the 1,4-dimethylnaphthalene molecule have planar geometry, but the latter has low symmetry. They are stacked as a one-dimensional chain structure through π-π charge-transfer interactions. Weak intermolecular interactions and planar molecular geometry result in a large degree of freedom of in-plane motion. Upon heating, due to the in-plane rotation of the molecules, the compound undergoes an order-disorder structural phase transition (phase-transition temperature = 334 K). The space group of the room-temperature phase is P21/m and the space group of the high-temperature phase is P4/mmm. This phase transition is accompanied by significant dielectric anomalies. The current investigation shows that the structural features of the title compound can be used to construct functional materials with phase transitions, such as molecular ferroelectrics.

Ring Contraction of Tropylium Ions into Benzenoid Derivatives.

We report a method to convert substituted tropylium ions into benzenoid derivatives.

Untargeted Identification of Alkyne-Containing Natural Products Using Ruthenium-Catalyzed Azide Alkyne Cycloaddition Reactions Coupled to LC-MS/MS.

Alkyne-containing natural products have been identified from plants, insects, algae, fungi, and bacteria. This class of natural products has been characterized as having a variety of biological activities. Polyynes are a subclass of acetylenic natural products that contain conjugated alkynes and are underrepresented in natural product databases due to the fact that they decompose during purification. Here we report a workflow that utilizes alkyne azide cycloaddition (AAC) reactions followed by LC-MS/MS analysis to identify acetylenic natural products. In this report, we demonstrate that alkyne azide cycloaddition reactions with p-bromobenzyl azide result in p-bromobenzyl-substituted triazole products that fragment to a common brominated tropylium ion. We were able to identify a synthetic alkyne spiked into the extract of Anabaena sp. PCC 7120 at a concentration of 10 μg/mL after optimization of MS/MS conditions. We then successfully identified the known natural product fischerellin A in the extract of Fischerella muscicola PCC 9339. Lastly, we identified the recently identified natural products protegenins A and C from Pseudomonas protegens Pf-5 through a combination of genome mining and RuAAC reactions. This is the first report of RuAAC reactions to detect acetylenic natural products. We also compare CuAAC and RuAAC reactions and find that CuAAC reactions produce fewer byproducts compared to RuAAC but is limited to terminal-alkyne-containing compounds. In contrast, RuAAC is capable of identification of both terminal and internal acetylenic natural products, but byproducts need to be eliminated from analysis by creation of an exclusion list. We believe that both CuAAC and RuAAC reactions coupled to LC-MS/MS represent a method for the untargeted identification of acetylenic natural products, but each method has strengths and weaknesses.