Onium Research Articles

Recent Advances in Catalytic Asymmetric Syntheses of Functionalized Heterocycles via Halogenation/Chalcogenation of Carbon‐Carbon Unsaturated Bonds

AbstractHalogen (fluorine, chlorine, bromine, iodine) or chalcogen (sulfur, selenium)‐containing heterocycles are widely recognized as key building blocks in many natural products and bioactive targets. Catalytic asymmetric halogenation/chalcogenation of carbon‐carbon unsaturated bonds via onium ion transformations are efficient methods to obtain diverse chiral heterocyclic backbones. In the past few years, catalytic enantioselective versions of halo/thio/seleno‐functionalizations with various halogen and chalcogen electrophiles have experienced constant development. This review highlights those advances in preparing functionalized heterocycles promoted by chiral organocatalysts or metal‐based catalysts.magnified image

(Digital Presentation) Phosphonium Salts As Potential Guest Substances for Ionic Semiclathrate Hydrates

Semiclathrate hydrates (SCHs) are ionic crystalline compounds consisting of an organic guest cation in several cages formed by hydrogen bonding of water1. SCHs have recently been considered for application to air conditioning systems and refrigeration systems since desirable equilibrium temperatures and the considerable decomposition enthalpies are observed in the solid-liquid equillibria2. Ionic guest substances of SCHs include quaternary onium compounds such as tetrabutylammonium2 and tetrabutylphosphonium based salts3. We report here the preparation and thermodynamic properties of SCHs based on quaternary phosphonium bromides containing an unsaturated carbon-carbon double bond, as shown in Fig. 1, in comparison with the conventional saturated phosphonium salt, such as tetrabutylphosphonium bromide (P4444-Br).The unsaturated phosphonium bromides (P444(1All)-Br and P444(2All)-Br) were synthesized by quaternarization reaction of tributylphosphine with alkenyl bromides in a nitrogen atmosphere. The aqueous solutions were prepared from 5 to 50 w/w %. After thorough stirring, the aqueous solutions were cooled in a freezer to form SCHs. The SCHs were immersed into a thermostatic bath at 268 K for at least 1 day. The temperature in the thermostatic bath was raised from 268 K at a rate of 0.1 K/step (1 step: more than 5 h). The equilibrium temperatures were defined as the temperature at which SCHs were completely dissolved. The ionic crystal structure was analyzed by a powder X-ray diffraction (PXRD) technique, the dissociation enthalpy was measured by a micro differential scanning calorimeter (μDSC).Fig. 2 depicts the solid-liquid phase equilibrium diagram representing the phase equilibrium temperatures of SCHs. The equilibrium temperature of P444(1All)-Br SCH was higher than P4444-Br SCH. These results might be due to the steric effect of alkenyl group. As shown in Fig. 3, the ionic crystal structure of both P444(1All)-Br and P444(All)-Br SCHs were orthorhombic. The dissociation enthalpy of P444(1All)-Br SCHs and P444(2All)-Br SCHs were 188 J/g and 199 J/g, respectively. The unsaturated phosphonium bromides would be regarded as potential guest substances used for latent heat storage materials in air conditioning systems.References M. Oshima, et al, J. Chem. Thermodynamics, 90, 277 (2015).H. Oyama, et al, Fluid Phase Equilibria, 234, 131 (2005).J. Shimada, et al, Chem. Eng. Sci., 236, 116514 (2021). Figure 1

Intermolecular Interaction of Tetrabutylammonium and Tetrabutylphosphonium Salt Hydrates by Low-Frequency Raman Observation.

Semi-clathrate hydrates are attractive heat storage materials because the equilibrium temperatures, located above 0 °C in most cases, can be changed by selecting guest cations and anions. The equilibrium temperatures are influenced by the size and hydrophilicity of guest ions, hydration number, crystal structure, and so on. This indicates that intermolecular and/or interionic interaction in the semi-clathrate hydrates may be related to the variation of the equilibrium temperatures. Therefore, intermolecular and/or interionic interaction in semi-clathrate hydrates with quaternary onium salts was directly observed using low-frequency Raman spectroscopy, a type of terahertz spectroscopy. The results show that Raman peak positions were mostly correlated with the equilibrium temperatures: in the semi-clathrate hydrates with higher equilibrium temperatures, Raman peaks around 65 cm−1 appeared at a higher wavenumber and the other Raman peaks at around 200 cm−1 appeared at a lower wavenumber. Low-frequency Raman observation is a valuable tool with which to study the equilibrium temperatures in semi-clathrate hydrates.

Biodegradable Copolymers from CO2, Epoxides, and Anhydrides Catalyzed by Organoborane/Tertiary Amine Pairs: High Selectivity and Productivity

Copolymerization of epoxides and carbon dioxide (CO2) with cyclic anhydrides has been developed to be a facile route to obtain biodegradable poly(ester-co-carbonate)s with diversified structures. Metal-free Lewis acid–base pairs emerging in recent years have offered great opportunities for preparing polymers without metal residuals. Herein, the scope of Lewis bases is extended to the tertiary amines with different pKa values and steric hindrances. Especially, the simplest Lewis pair composed of triethylborane (TEB) and triethylamine (TEA) exhibits high catalytic activity even at very low catalyst loadings in the copolymerization of propylene oxide (PO), CO2, and phthalic anhydride (PA). With a PO/PA/TEB/TEA feeding ratio of 16,000/2000/4/1, a low cyclic carbonate content not exceeding 5.0 wt % and a high productivity up to 1.2 kg polymer/g catalyst have been realized at 65 °C. Moreover, the molecular weight of the resultant copolymer climbs to 131 kg/mol with a relatively narrow polymer dispersity index (PDI = 1.31), which is the highest record among the results catalyzed by the homogenous binary catalyst systems. Furthermore, the TEB/TEA (1/1) pair is applicable to provide random copolymers from CO2 with various epoxides and anhydrides, while quasi-block copolymers are yielded with inferior selectivity by replacing TEA with onium salt. Interestingly, the low activity in cyclohexene oxide (CHO) involving copolymerization with PA and CO2 can be dramatically boosted upon adding a small amount of PO (1 mol % of CHO), which can be attributed to the stronger ring-opening ability of PO during the initiation period. The glass transition temperatures of the resultant polymers are adjustable in the range of 34–131 °C by applying different epoxides and anhydrides.

Cooperative Tertiary Phosphine/Palladium Catalyzed Nucleophilic Allylation between Morita‐Baylis‐Hillman Carbonates and Alkenes

AbstractIn this work, we report an unusual nucleophilic allylation reaction between isatin‐derived Morita–Baylis–Hillman carbonates and simple alkenes synergistically catalyzed by tertiary phosphines and palladium complexes, through chemoselective assemblies of in situ formed onium salts and η1‐allylpalladium species, respectively. A range of 3,3‐disubstituted oxindoles were easily constructed in moderate to good yields with exclusive linear regioselectivity. Moreover, a number of control and deuterium experiments have been conducted to elucidate the catalytic mechanism.

Synthesis of Unsaturated (Co)polyesters from Ring-Opening Copolymerization by Aluminum Bipyridine Bisphenolate Complexes with Improved Protonic Impurities Tolerance

Synthesis of unsaturated polyesters (UPES) via ring-opening copolymerization (ROCOP) of maleic anhydride (MAH) and epoxides still remains a great challenge, because the reported catalytic systems generally showed low catalytic activity or complete deactivation at low catalyst concentration and exhibit poor tolerance to protonic impurities. In this work, we synthesized a series of bipyridine bisphenolate aluminum (BpyBph-Al) complexes and used these complexes to catalyze the MAH/epoxide copolymerization for synthesis of UPES. UPES with perfectly alternating structure could be synthesized using BpyBph-Al complexes without onium salt as cocatalyst, and the catalytic activities seem to be dependent on the steric and electronic effects of the ligands as well as the leaving ability of the axial group. The catalytic activities were further increased in the binary catalysts consisting of a BpyBph-Al as Lewis acid and an onium salt as a Lewis base. Compared with salen-Al/onium salt and typical Lewis pairs, the BpyBph-Al/onium salt binary system exhibited a completely maintained catalytic activity even when the [epoxide]/[MAH]/[catalyst pair] feed ratio was high up to 25 000:5000:1 ([catalyst pair] = 0.004 mol %). In addition, BpyBph-Al/onium salt exhibited better tolerance to protonic impurity than salen-Al/onium salt and Lewis pairs. The catalytic activity was slightly decreased (about 15%) in the presence of 100 equiv (relative to catalyst) of external-added maleic acid, while a substantial decrease in activity or complete deactivation was observed in the ROCOPs catalyzed by salen-Al/onium salt and Lewis pairs. These features of BpyBph-Al/onium salt further allowed UPES synthesis by using commercial MAH without any purification as a comonomer. The maleic acid residuals in the commercial MAH could serve as a chain transfer agent (CTA), and UPES polyols with uniform −OH chain ends were obtained via reversible-deactivation chain transfer reaction. Chain extension could be initiated by the active alkoxide end when adding the second cyclic anhydride after the full conversion of MAH, finally producing ABA-type triblock copolymers.

Utilization of CO2-Available Organocatalysts for Reactions with Industrially Important Epoxides

Recent knowledge in chemistry has enabled the material utilization of greenhouse gas (CO2) for the production of organic carbonates using mild reaction conditions. Organic carbonates, especially cyclic carbonates, are applicable as green solvents, electrolytes in batteries, feedstock for fine chemicals and monomers for polycarbonate production. This review summarizes new developments in the ring opening of epoxides with subsequent CO2-based formation of cyclic carbonates. The review highlights recent and major developments for sustainable CO2 conversion from 2000 to the end of 2021 abstracted by Web of Science. The syntheses of epoxides, especially from bio-based raw materials, will be summarized, such as the types of raw material (vegetable oils or their esters) and the reaction conditions. The aim of this review is also to summarize and to compare the types of homogeneous non-metallic catalysts. The three reaction mechanisms for cyclic carbonate formation are presented, namely activation of the epoxide ring, CO2 activation and dual activation. Usually most effective catalysts described in the literature consist of powerful sources of nucleophile such as onium salt, of hydrogen bond donors and of tertiary amines used to combine epoxide activation for facile epoxide ring opening and CO2 activation for the subsequent smooth addition reaction and ring closure. The most active catalytic systems are capable of activating even internal epoxides such as epoxidized unsaturated fatty acid derivatives for the cycloaddition of CO2 under relatively mild conditions. In case of terminal epoxides such as epichlorohydrin, the effective utilization of diluted sources of CO2 such as flue gas is possible using the most active organocatalysts even at ambient pressure.

Asymmetric Tandem Conjugate Addition and Reaction with Carbocations on Unsaturated Heterocycles

AbstractHeterocycles possessing multiple substituents and stereogenic centers are important structural motifs in many compounds of interest. We present here stereoselective tandem transformation based on Cu‐catalyzed conjugate addition of Grignard reagents to heterocyclic Michael acceptors, which is followed by one‐pot trapping of in situ formed enolates with stabilized carbocations or their equivalents. The transformations are highly enantio‐ and diastereoselective with Josiphos‐type ferrocene ligand. The reaction of chiral metal enolates with onium compounds allows the installation of structurally attractive substituents on chromenone or piperidinone core. Moreover, cycloheptatrienyl and benzodithiolyl substituents can be further modified, thus expanding synthetic possibilities of this methodology.magnified image

Cationic Curing of Epoxy–Aromatic Matrices for Advanced Composites: The Assets of Radiation Processing

Cross-linking polymerization of multifunctional aromatic monomers initiated by exposure to high energy radiation continues to be explored as a promising alternative to thermal curing for the production of high-performance composite materials. High-energy radiation processing offers several advantages over thermosetting technology by allowing for fast and out-of-autoclave curing operations and for its adaptability in the manufacturing of large and complex structures at reduced energy costs. The present article covers the basic aspects of radiation curing by cationic polymerization of epoxy resins, providing a status report on recent investigations conducted in our group to improve the properties of epoxy matrices and gain better control over the process for producing composites. A selection of results based on blends prepared with different composition of epoxy aromatics, transfer agents, thermoplastic toughening agents and onium salt initiators exemplifies the importance of the composition on polymerization kinetics and on the properties of resulting materials. The superiority of radiation-triggered polymerization-induced phase separation of thermoplastic additives is emphasized by the obtained morphology of toughened materials. The low initial temperature and fast curing of the reactive blends limits the expansion of phase-separated thermoplastic domains, resulting in an enhancement of the toughness.

Development of near-infrared mitochondrial polarity fluorescent probe for evaluating mitophagy in mice heart and potential cancer diagnosis

Mitophagy is a process that selectively removes excess or damaged mitochondria, and regulates the number of mitochondria in the cell to maintain the normal mitochondrial function. According to reports, mitophagy can cause significant changes to the intracellular microenvironment, such as affecting cell polarity. Therefore, detecting changes in polarity in the intracellular microenvironment is an effective means to monitor the autophagy process. Herein, we synthesized a polarity-sensitive fluorescent probe NIR-BT-P with a strong ICT mechanism and 900 nm near infrared region. The probe was constructed by an onium salt as the electron-deficient parent fluorophore and triphenylamine as a strong electron donating group. Studies indicated that the fluorescence intensity of NIR-BT-P increases with decreasing environmental polarity, accompanied by a redshift (812 nm-853 nm) of the maximum emission wavelength. In addition, NIR-BT-P did not interfere by viscosity, pH, or common active substances. Notably, the probe NIR-BT-P can monitor starvation and drug-induced mitophagy in real time, and successfully distinguished normal cells from cancer cells. More importantly, by visualizing the cardiomyocytes of the heart of the starvation mice group, it was found that mitophagy occurred during heart starvation. Consequently, we anticipate that NIR-BT-P could not only provide insight into the relationship between mitophagy and heart disease, but may also emerge as a potential tool for the clinical diagnosis of cancer.

Energy Capabilties of 5,5'-Azotetrazol-1,1'-Diol and Its Onium Salts as Components of Solid Composite Propellants

Energy Capabilties of 5,5'-Azotetrazol-1,1'-Diol and Its Onium Salts as Components of Solid Composite Propellants

Asymmetric Hydroboration of Ketones by Cooperative Lewis Acid–Onium Salt Catalysis: A Quantum Chemical and Microkinetic Study to Combine Theory and Experiment

We apply microkinetic modeling in homogeneous catalysis and show how it can be used to reveal important details of a complex mechanism and how this can lead to a direct comparison between theory and experiment. While regularly used in heterogeneous catalysis, its applications to organic chemistry or homogeneous catalysis are still comparatively scarce. This approach is exemplarily applied to the mechanism of the asymmetric hydroboration of acetophenone with a highly active cooperative Lewis acid–ammonium salt catalyst. In combination with density functional theory, it is a gateway to shed light into important mechanistic details. In our study, it reveals that the counterion of the ammonium salt of the catalyst facilitates the hydride transfer step of the cycle. Chloride replacing iodide speeds up the main reaction but simultaneously has the same effect on a side reaction that consumes the product. This observation is confirmed by experimental measurements of both the main catalytic cycle and the side reaction. A sensitivity analysis showed that the transition from the product complex to the hydride transfer is rate-limiting and that it determines the enantioselectivity. Based on this insight, an enantioselective kinetic model was applied, from which the difference of the Gibbs free energy barriers of the two pathways forming the two enantiomers can be extracted. The barriers are in fairly good agreement with the ones calculated by DFT, which reveal that the asymmetric backbone interacts with the reactant sterically to favor asymmetric product formation.

Halonium, chalconium, and pnictonium salts as noncovalent organocatalysts: a computational study on relative catalytic activity.

This theoretical study sheds light on the relative catalytic activity of pnictonium, chalconium, and halonium salts in reactions involving elimination of chloride and electrophilic activation of a carbonyl group. DFT calculations indicate that for cationic aromatic onium salts, values of the electrostatic potential on heteroatom σ-holes gradually increase from pnictogen- to halogen-containing species. The higher values of the potential on the halogen atoms of halonium salts result in the overall higher catalytic activity of these species, but in the case of pnictonium and chalconium cations, weak interactions from the side groups provide an additional stabilization effect on the reaction transition states. Based upon quantum-chemical calculations, the catalytic activity of phosphonium(V) and arsenonium(V) salts is expected to be too low to obtain effective noncovalent organocatalytic compounds, whereas stibonium(V), telluronium(IV) and iodonium(III) salts exhibit higher potential in application as noncovalent organocatalysts.

Ring-opening polymerization of ε-caprolactone mediated by a di-zinc complex bearing a macrocyclic thioether-phenolate [OSSO]-type ligand

A unique example of zinc bromide complexes bearing a macrocyclic [OSSO]-type thioetherphenolate ligand has been successfully explored toward ring-opening polymerization of ε-caprolactone in the presence of epoxides and onium salts.

CO2 fixation into cyclic carbonates catalyzed by single-site aprotic organocatalysts

The catalytic activity of a series of onium salts for the synthesis of cyclic carbonates have been investigated experimentally and theoretically.

Evaluation of suitable onium tetrafluoroborates for cationic polymerization of epoxides

AbstractEpoxides are frequently used in coatings due to their great adhesion to most materials, high resistance towards chemicals and well‐defined material properties. Onium salts such as iodonium and sulfonium salts are one of the most widespread classes of initiators used in light‐induced cationic polymerization of such epoxides. We successfully synthesized onium salts based on group 14 to 16 elements in the periodic table. They were then characterized using cyclic voltammetry and UV–visible spectroscopy before determining their reactivity in epoxy‐based resins. Using commercial bisphenol‐A‐based resins, thiopyrylium, bismuithonium and pyrylium salts show good reactivity and epoxy group conversions above 60% in simultaneous thermal analysis. Photochemically interesting new cationic initiators were identified to be oxonium, thiopyrylium and selenonium salts. Additionally, bismuthonium and stibonium salts show great potential towards sensitization with anthracene. © 2021 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Recent Advances in the Synthesis of Diverse Libraries of Small-Molecule Building Blocks in Ionic Liquids (ILs)

AbstractThe Account describes recent advances, from the authors’ laboratories, in the synthesis of diverse libraries of small-molecule building blocks employing ionic liquids (ILs). The ability of ILs to act as catalysts/promoters/solvents for electrophilic and onium ion chemistry, as well as in metal-mediated cross-coupling reactions, and the potential to sequence/hyphenate these methods, have opened up new opportunities for facile assembly of functional small molecules with increased complexity from readily available precursors. While Brønsted acidic IL/IL solvent mixtures are suitable media for carbocation and onium ion chemistry, piperidine-appended IL/IL solvent mixtures can successfully catalyze a variety of base-catalyzed reactions. Several widely practiced transformations including ‘name reactions’ were adapted and performed efficiently in ILs.1 Introduction2 Aryldiazonium Salts and Aryltriazenes as Coupling Partners in Metal-Mediated C–C Cross-Coupling Reactions in ILs3 Expanding the Scope of Metal-Mediated Cross-Coupling Reactions in ILs4 Application of ILs in Synthesis and Functionalization of Hetero­cycles5 Expanding the Scope of Amide Synthesis in ILs6 Generation and Chemistry of ‘Tamed’ Propargylic Cations in ILs7 Newer Nitration Methods for Arenes and Heteroarenes in ILs8 Halofunctionalization in ILs9 ‘Name Reactions’and Other Widely Practiced Synthetic Transformations in ILs9.1 The Biginelli Reaction9.2 Nitrile Synthesis by the Schmidt Reaction9.3 Rupe Rearrangement9.4 Synthesis of 1,3-Dioxanes via Prins Reaction in [BMIM(SO3H)][OTf]9.5 Synthesis of Cyclopropanes and Oxiranes by the Corey–Chaykovsky (CC) Reaction10 Conclusions and Closing Remarks

High-performance LED induces cationic photopolymerization using novel 1,3,5-triaryl-2-pyrazoline as photosensitizer

A series of 1,3,5-triaryl-2-pyrazolines was designed and synthesized as photosensitizers for cationic photopolymerization (CP). The light absorption within the ultraviolet-visible (UV–vis) range (λmax = 344–376 nm; εmax = 17,300–20,000 M−1 cm−1), which was strongly influenced by the substituents on the C3 atom of pyrazoline ring, matched the emission of light-emitting diodes (LEDs) within 365–425 nm. The pyrazolines, together with iodonium salt (Iod) or sulfonium salts, can generate acid upon the irradiation with LEDs, and the quantum yields of the systems were high enough (0.11–0.17 for Iod and 0.04–0.12 for sulfonium salts) for high-performance LED-induced CP at 365–425 nm irradiation. UV–vis spectra, theoretical calculation, electrochemistry, electron spin resonance–spin trapping, and fluorescence quenching were investigated to determine the photochemical mechanism. The introduction of electron-pushing group and heterocycle rings can efficiently enhance the sensitization capabilities of pyrazolines to onium salts. These pyrazoline-based photoinitiating systems have promising applications in LED-induced photopolymerization.

2-3-3 テトラヒドロフランに第四級オニウム塩を混合したクラスレートハイドレートの熱力学的安定性

2-3-3 テトラヒドロフランに第四級オニウム塩を混合したクラスレートハイドレートの熱力学的安定性

Surfactant-Emulating Amphiphilic Polycarbonates and Other Functional Polycarbonates through Metal-Free Copolymerization of CO2 with Ethylene Oxide

Three types of ethylene oxide-based and carbonate-containing copolymers were prepared through copolymerization of ethylene oxide (EO) with CO2 under metal-free conditions in the presence of triethylborane (TEB), using onium salts (OS) as initiator. Hydrophobic poly[(ethylene carbonate)x-co-(ethylene oxide)y] (PECEO) samples with carbonate contents above 90% (x ≫ y) were first prepared under a CO2 pressure of 10–30 bar with a ratio of [TEB] to [OS] of 1–1.2 equiv in tetrahydrofuran (THF) or in hexane. The above PECEO (carbonate > 91%) then served as a macroinitiator to grow two external poly(ethylene oxide) (PEO) blocks and generate in one-pot amphiphilic PEO-b-PECEO-b-PEO triblocks. Lastly, the copolymerization of EO under a low pressure of CO2 (1–2 bar) with a ratio of [TEB] to [OS] of 1.2–2.0 equiv afforded hydrophilic poly[(ethylene oxide)y-co-(ethylene carbonate)x] (PEOEC) random copolymers with carbonate contents below 10% (y ≫ x); allyl glycidyl ether (AGE) was also terpolymerized with EO and CO2 under the same conditions to introduce functional groups along the backbone of PEO chains. Critical micelle concentrations (CMC) and size of micelles were measured for amphiphilic PEO-b-PECEO-b-PEO samples and compared with the values of other nonionic surfactants. The properties of “PEO-like” hydrophilic PEOECs were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and wettability test; their degradation behavior was further investigated under different conditions.