Onium Research Articles

The use of a diammonium salt in the synthesis of organic carbonates from epoxides and CO2: promoting effect of support

Onium salt, N1, N1, N1, N5, N5, N5-hexaethylpentane-1,5-diammonium dibromide, was used as a catalyst for the addition of CO2 to epoxides. An influence of insoluble supports on the rate of the process as well as possibility of recycling of the catalytic system are described. In addition, the effects of temperature and CO2 pressure on the yields of products are discussed.

Green Application of Phase-Transfer Catalysis in Oxidation: A Comprehensive Review

Oxidation reactions have emerged as one of the most versatile tools in organic chemistry. Various onium salts such as ammonium, phosphonium, arsonium, bismuthonium, tellurium have been used as phase transfer catalysts in many oxidation reactions. Certainly, considerable catalysts have been widely used in Phase-Transfer Catalysis (PTC). This review focuses on the application of PTC in various oxidation reaction. Furthermore, PTC also conforms to the concept of “Green Chemistry”. <p></p> • Oxidation has become one of the most widely used tools in organic chemistry and phase transfer catalysts has been widely used in oxidation. <p></p> • The application of phase transfer catalysts in oxidation reaction will be summarized. <p></p> • Phase transfer catalysts have important application in various oxidation reaction.

Double Role of Diphenylpyridine Derivatives as Fluorescent Sensors for Monitoring Photopolymerization and the Determination of the Efficiencies of the Generation of Superacids by Cationic Photoinitiators.

Novel fluorescent sensors with electron-donating or electron-withdrawing substituents incorporated into a chromophore group based on 2,6-diphenylpyridine were designed and synthesised. The spectroscopic properties of these compounds were studied. Moreover, the positive solvatochromism of 2,6-bis-(4-methylsulphanylphenyl)pyridine (PT-SCH3) in selected solvents was studied by measurement of the absorption and emission spectra and analysed using the Dimroth–Reichardt solvent parameter set. After that, the performance of a series of 2,6-diphenylpyridine derivatives as fluorescent molecular sensors for monitoring free-radical and cationic photopolymerization processes by the Fluorescence Probe Technique (FPT) was studied. As a consequence of this stage of research, the effect of substituents on the sensitivity of the 2,6-diphenylpyridine derivatives as sensors during photopolymerization has been evaluated and discussed. It has been found that compounds containing strong electron-donating substituent (PT-SCH3) slightly shift their fluorescence spectrum during the free-radical polymerization of monomer, which enables the monitoring of the polymerization progress using the fluorescence intensity ratio measured at two different wavelengths as the progress indicator. The position of the fluorescence spectrum of 2,6-diphenylpyridine derivatives with electron-withdrawing substituents is practically insensitive to changes occurring in their environment. Hence, it is recommended to use these compounds with different indicators of the progress of the photopolymerization process based on normalised intensity of fluorescence (Imax/I0). Among the compounds studied, 2,6-bis(4-methylsulphanylphenyl)pyridine (PT-SCH3) turned out to be the best fluorescent sensor for the purpose of monitoring free-radical polymerization by FPT. Consequently, the dual application of the selected 2,6-diphenylpyridine derivatives is proposed: (a) as fluorescent sensors for monitoring the free-radical photopolymerization progress, and (b) as spectroscopic sensors for the determination of efficiencies of the generation of superacids by cationic photoinitiators during the cationic photopolymerization process. Finally, a new method for determining the relative efficiency of the photogeneration of superacids during the photo cleavage of onium salt has been devised and applied for the evaluation of the performance of 2,6-diphenylpyridine derivatives.

UV-Induced Cationic Ring-Opening Polymerization of 2-Oxazolines for Hot Lithography.

The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility. Up to now, poly(2-oxazoline)s were obtained by classical thermal polymerization methods not aiming for application in bulk curing of structural polymers. We introduce the cationic photopolymerization of 2-oxazolines at elevated temperatures for the direct UV-induced curing of materials with exclusive chemical and structural particularities. After efficient photoinitiation via onium salt photoacid generators (PAGs), the immanent low-rate propagation is crucially promoted by thermal energy input to the ring-opening reaction. In simultaneous thermal analysis (STA), photo-DSC, and (thermo)mechanical analyses we investigated the UV-induced CROP of 2-oxazolines in a temperature range of 100-140 °C and show the exceptional potential of the introduced photopolymers. Furthermore, we applied the photopolymerizable system in Hot Lithography, a stereolithography-based 3D printing technology at elevated temperatures.

Photochemical Study of a New Bimolecular Photoinitiating System for Vat Photopolymerization 3D Printing Techniques under Visible Light

In this work, we presented a new bimolecular photoinitiating system based on 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives as visible photosensitizers of diphenyliodonium salt. Real-time FTIR and photo-DSC photopolymerization experiments with a cycloaliphatic epoxide and vinyl monomers showed surprisingly good reactivity of the bimolecular photoinitiating systems under UV-A, as well as under visible light sources. Steady-state photolysis, fluorescence experiments, theoretical calculations of molecular orbitals, and electrochemical analysis demonstrated photo-redox behavior as well as the ability to form initiating species via photo-reduction or photo-oxidation pathways, respectively. Therefore, the 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives were also investigated as a type II free-radical photoinitiator with amine. It was confirmed that the 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives, in combination with different types of additives, e.g., amine as a co-initiator or the presence of onium salt, can act as bimolecular photoinitiating systems for cationic, free-radical, and thiol-ene photopolymerization processes by hydrogen abstraction and/or electron transfer reactions stimulated by either near-UV or visible light irradiation. Finally, the 2-amino-4,6-diphenylpyridine-3-carbonitrile derivatives were selected for 3D printing rapid prototyping experiments. Test objects were successfully printed using purely cationic photosensitive resin, created on a 3D printer with a visible LED light source.

Hydrogen-Bonding Catalysis of Alkyl-Onium Salts.

The synthetic utility of alkyl-onium salt compounds is widely recognized in the field of organic chemistry. Among the wide variety of onium salts, quaternary ammonium, phosphonium, and tertiary sulfonium salts have been the most useful compounds in organic syntheses. These compounds have been very useful reagents in the construction of organic building blocks. In addition, onium salts are known as reliable catalysts, which are used to promote important organic transformations by serving as phase-transfer and ion-pair catalysts through the activation of nucleophiles. Although phase-transfer catalysis is a major direction for onium salt catalysis, hydrogen-bonding catalysis of alkyl-onium salts, which is promoted via the activation of electrophiles, has recently become a relevant topic in the field of onium salt chemistry. This Minireview introduces new possibilities and future directions for alkyl-onium salt chemistry based on its use in hydrogen-bonding catalysis and on its overall utility.

Theoretical evaluation of hexazinane as a basic component of nitrogen-rich energetic onium salts

In the present paper, we report a comprehensive theoretical evaluation of a hypothetical compound, hexazinane (cyclo-H6N6), and its 10 onium salts as high-energy density materials.

Onium salts improve the kinetics of photopolymerization of acrylate activated with visible light

The aim was study the influence of onium salts on the kinetics of photopolymerization in the visible light region. Trimethylolpropane triacrylate TMPTA was selected as a monomer, and activated by 1,3-bis(phenylamino)squaraine (SQ) used as a photosensitizer in addition to tetramethylammonium n-butyltriphenylborate (B2). The iodonium salt [A–I–B]+X− functioned as a second radical initiator, bearing a different substitution pattern for the cation. The ternary system was formulated with different concentrations of both borate and diphenyliodonium salts. Differential scanning calorimetry was used to investigate the polymerization reaction over the photoactivation time carried out at 300 nm < λ < 500 nm irradiation. When the squaraine dye/borate salt was used as photoinitiator, a slow polymerization reaction was observed and a lower monomer conversion. The addition of a third component (onium salt) increased the polymerization rate and conversion. Ternary photoinitiator systems showed improvement in the polymerization rate of triacrylate leading to high conversion in a short photoactivation time. The photoinitiating ability of bi- and tri-component photoinitiators acting in the UV-Vis region for initiation polymerization of triacrylate was compared with those of some commercially used photoinitiating systems. It was also found, that, the parallel electron transfer from an excited state of the sensitizer to [A–I–B]+X−, and an electron transfer from a ground state of R(Ph)3B−N(CH3)4+ to an excited state of the sensitizer results in two types of initiating radical.

Visible-light-mediated arylation of ortho-hydroxyarylenaminones: direct access to isoflavones.

In this work we highlight two new methods for the synthesis of isoflavones through consecutive domino arylation of ortho-hydroxyarylenaminones with in situ photogenerated aryl radicals. As precursors for aryl radicals we used aryl onium reagents such as diazonium and diaryliodonium salts. Notably, the photo-Meerwein arylation by aryl diazonium tetrafluoroborates demonstrated high efficiency in terms of yields and can be considered as a method of choice for the straightforward assembly of 3-aryl-substituted chromones. Ultimately, 26 compounds were prepared in good to excellent yields using the developed synthetic protocols.

Quest for a Symmetric [C-F-C]+ Fluoronium Ion in Solution: A Winding Path to Ultimate Success.

In this Account, we chronicle our tortuous but ultimately fruitful quest to synthesize a [C-F-C]+ fluoronium ion in solution, thus providing the last piece of the organic halonium ion puzzle. Inspiration for the project can be traced all the way back to the graduate career of the corresponding author, wherein the analogy between a [C-H-C]+ "hydrido" bridge and a hypothetical [C-F-C]+ bridge was first noted. The earliest attempt to construct a bicyclo[5.3.3]tridecane-based fluoronium ion (based on the analogous hydrido bridged cation) proved to be synthetically difficult. A subsequent attempt involving a 1,8-substituted naphthalene ring was theoretically naïve in retrospect, and it resulted in a classical benzylic carbocation instead. A biphenyl-based substrate, although computationally sound, proved to be kinetically untenable. At last, after some tweaking (including a dead-end detour into a fluoraadamantane skeleton), we finally achieved success with a highly rigid, semicage precursor based on the decahydro-1,4:5,8-dimethanonaphthalene system. This strained substrate possessed a triflate leaving group to enhance its solvolytic reactivity. Detailed isotopic labeling and kinetic studies supported the generation of a symmetrical [C-F-C]+ bridge; interesting solution behavior allowed the manipulation of the rate-determining step for solvolysis depending on solvent nucleophilicity. After initial generation as a transient intermediate, the fluoronium ion was later produced as a stable species in solution and was fully characterized by 19F, 1H, and 13C NMR, with the resultant species displaying evident Cssymmetry through coordination of a molecule of SbF5. This remarkable ion proved stable to -30 °C. We also address a disagreement surrounding the nomenclature of fluoronium ions in particular and its potential impact upon the naming of onium ions in general. We strove to highlight the dangers of confusing the arbitrary concept of calculated partial charge with IUPAC nomenclature. Finally, we discuss future directions, for example, the synthesis of a fluoronium ion in which fluorine resides within an aromatic ring.

Fine tuning of the Fermi level of single-walled carbon nanotubes with onium salts and application for thermoelectric materials

Fermi level is one of the most important parameters for control of semiconductor polarity and improvement of thermoelectric performances. Although single-walled carbon nanotubes (SWCNTs) are promising candidates for thermoelectric materials, control of the Fermi level of SWCNTs with chemical dopants is still challenging. Reported here is the fine-tuning of the Fermi level of SWCNTs with onium salts. The Fermi level of SWCNTs was very sensitive to substituent size on the cation and nucleophilicity of the anion, and the Seebeck coefficients markedly changed between −50 μV K−1 and 68 μV K−1. The optimum Fermi level of p-type SWCNTs that exhibits the maximum power factor was realized by ammonium chloride or pyridine hydrochloride, and the power factor (PF) was improved by about 7 times as compared with non-doped SWCNTs. SWCNTs doped with onium salts were stable in both p-type and n-type states, and thermoelectric performances did not change more than 100 days in the air.

Cationic Organic Catalysts or Ligands in Concert with Metal Catalysts.

Cooperative dual catalysis and bifunctional catalysis have emerged as reliable strategies for the development of hitherto difficult asymmetric transformations because they could deliver new reactivity and selectivity, and allow for the employment of substrates not amenable to reaction systems relying on a single, monofunctional catalysts. Furthermore, these modes of catalysis often improve yields and stereoselectivities via the precise recognition and simultaneous activation of nucleophiles and electrophiles. Efforts towards utilizing chiral cationic organic catalysts for asymmetric cooperative catalysis with metal complexes have provided a unique platform to address the challenging issues associated with reaction development. Chiral onium ions, such as tetraalkylammonium, guanidinium, and azolium ions, are employed mainly to control the reactivity and stereochemistry of anionic intermediates through electrostatic and hydrogen-bonding interactions. Metal complexes complement the synergy of the catalysis by activating the substrates via the formation of electrophilic π-allyl complexes, Lewis acid-base adducts, nucleophilic ate complexes, etc. The electrostatic interactions between cations and anions also offer a means to construct complex molecular assemblies, and, thus, onium ions are useful not only for controlling pairing with anionic species, but also for the design of supramolecular catalysts. The combination of onium ions and metal complexes leads to the introduction of novel concepts and powerful strategies for the development of catalysts and chemical transformations.

Transformations of Modified Morita-Baylis-Hillman Adducts from Isatins Catalyzed by Lewis Bases.

The development of synthetic protocols to access architectures with broad structural and functional diversity from readily available starting materials is very attractive in both organic and medicinal chemistry fields. Toward this objective, the multifunctional isatin-derived Morita-Baylis-Hillman (MBH) adducts provide opportunities to construct a variety of complex scaffolds containing a "privileged" oxindole motif through several catalytic pathways. By forming the ammonium or phosphonium salts with Lewis bases, isatin-derived MBH adducts can undergo allylic substitutions with a range of nucleophiles, usually in a SN 2'-SN 2' pattern. Besides, assisted by Brønsted bases, the corresponding onium salts can be converted into the allylic ylide intermediates, which can undergo various annulation reactions or even 1,3-difunctionalizations. Moreover, recent cooperative catalysis of Lewis bases and transition metal complexes further puts forward the application of isatin-derived MBH adducts. This tutorial review covers the significant transformations of isatin-derived MBH adducts, mostly in an asymmetric version, catalyzed by various Lewis bases over the past decade.

Development of New High-Performance Biphenyl and Terphenyl Derivatives as Versatile Photoredox Photoinitiating Systems and Their Applications in 3D Printing Photopolymerization Processes

Novel 2-amino-4-methyl-6-phenyl-benzene-1,3-dicarbonitrile derivatives were proposed as photosensitizes of iodonium salt for a highly effective bimolecular photoinitiating system upon soft irradiation conditions under long-wave ultraviolet (UV-A) and visible light. Remarkably, these structures are highly versatile, allowing access to photoinitiating systems for the free-radical polymerization of acrylates, the cationic photopolymerization of epoxides, glycidyl, and vinyl ethers, the synthesis of interpenetrated polymer networks (IPNs) and the thiol-ene photopolymerization processes. Excellent polymerization profiles for all of the monomers, along with the high final conversions, were obtained. The initiation mechanisms of these bimolecular systems based on the 2-amino-4-methyl-6-phenyl-benzene-1,3-dicarbonitrile derivatives were investigated using the real-time FT-IR technique, steady-state photolysis, fluorescence experiments, theoretical calculations of molecular orbitals, and electrochemical analysis. Moreover, the 2-amino-4-methyl-6-phenyl-benzene-1,3-dicarbonitrile derivatives were investigated as a type II free-radical photoinitiator with amine. It was confirmed that the 2-amino-4-methyl-6-phenyl-benzene-1,3-dicarbonitrile derivatives, in combination with different types of additives, e.g., amine as co-initiator or in the presence of onium salt, can act as a bimolecular photoinitiating system via the photo-reduction or photo-oxidation pathways, respectively.

Near‐IR and UV‐LED Sensitized Photopolymerization with Onium Salts Comprising Anions of Different Nucleophilicities

AbstractIodonium‐ and sulfonium‐type onium salts with counter anions of different nucleophilicities, namely [Al(O‐t‐C(CF3)4)4]−, [(CF3SO2)2N]−, SCN−, CF3‐SO3−, PF6−, and 3,5‐bis(methoxycarbonyl)benzenesulfonate, were investigated in both NIR and UV‐LED sensitized photoinitiated polymerizations at 790 nm and 395 nm wavelength exposures, respectively. The reactivity of the neutral sensitizer Sens1, and the cationic sensitizer Sens2 were examined in combination with the onium salts. The reaction rate was compared with isopropylthioxanthone (Sens3) as sensitizer. The reactivity of the initiating system is proposed to be associated with the conductivity of the salts in acrylate monomers such as tri(propylene glycol) (TPGDA), trimethylol propane triacrylate (TMPTA), and 1,6‐hexane diol diacrylate (HDDA). Salts carrying the [Al(O‐t‐C(CF3)4)4]− anion require new consideration of previously introduced models because the reactivity of the salts does not directly correlate with their conductivities. Rhodamine B lactone quantitatively probed formation of acidic cations between considering the reaction between [(t‐C4H9‐Ph)2I]+[Al(O‐t‐C(CF3)4)4]− and photoexcited Sens2 comprising either BF4−, PF6− or [Al(O‐t‐C(CF3)4)4]−. No significant differences were observed in terms of the reactivity.

Clean transesterification process for biodiesel production using heterogeneous polymer-heteropoly acid nanocatalyst

In the present study, new hetrogeneous catalysts, in which phosphotungstic acid is chemically bonded to hydrophobic highly crosslinked poly(4-divinylbenzene-co-vinylbenzyl chloride) via a soft linker, ethylenediamine (Cat-2C) or 1,8-diaminooctane (Cat-8C). The synthesized polymer was identified by infrared, Raman spectroscopy, scanning and transmittance electron microscopy, and dispersibility test. The prepared catalysts possessed uniform core-shell structure and high acidic sites. The presence of vinyl benzyl chloride in the polymer enables more phosphotungstic acid on the surface of the polymer with excellent dispersibility. Utilizing the polymer structure, synergies between acidic and hydrophilic sites (onium ion) sites lead to good results than methyl and ethyl oleate (99.8%, 84.3%) using a low ratio of alcohol and oleic acid and a short time compared to other catalysts. The catalysts showed superior catalytic performance originated from favoring the smooth transportation of reactant and product. The durability of catalysts was unaltered up to 7 cycles with no loss of phosphotungstic acid. Cat-8C was tested in the transesterification of castor oil process. Transesterification of castor oil was conducted at the determined optimal condition, and the characterization of the produced biodiesel is reported. • Successful design of PTA-functionalized cross-linked polymers. • PTA is chemically bonded to porous polymers through a soft linker. • Methyl, and ethyl oleate (99.8%, 84.3%) yielded at low molar ratio & short time. • The durability of catalysts was almost retained up to 7th cycles. • Transesterification of castor oil for biodiesel production was reported.

Near‐visible stereolithography of a low shrinkage cationic/free‐radical photopolymer blend and its nanocomposite

ABSTRACTIn this work, a newly prepared cationic/free‐radical photopolymer, which consists of two epoxies and a tetrafunctional acrylate, is presented for the first time for a visible light stereolithography (SL), showing the advantages of both cationic and free‐radical resins. An onium salt, commonly used as a cationic UV initiator, and a photosensitizer make the blend suitable for a near‐visible (405 nm) SL. An increase in the polymerization rate and a drop in the induction period are observed for the newly prepared cationic/free‐radical blend, compared with either only cationic systems or free‐radical resins. This suggests that the combination of cationic and free‐radical polymerizations in a single resin has a positive synergistic effect. The addition of silica nanoparticles to the blend provides a reinforcing and toughening effect. Indeed, the resin loaded with silica shows a 31% increase in the elastic modulus, compared with the unfilled resin. Regarding the values of tensile strength and elongation at break, they, respectively, grow by 47 and 15%, when the nanocomposite resin is compared with the neat resin. A very low volumetric shrinkage of 0.7% and a remarkable printing quality of objects obtained with this new photopolymer will enable the 3D printing of microrobots, bioengineering microdevices, and sensors. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48333.

Mass‐Spectrometric Observation of Counter Anion Production in SU‐8 Exposed to UV Light and its Use for Dill C Parameter Determination

ABSTRACTPhotopolymerization is a phenomenon that is the basis of much of today's microfabrication technology and intense research is conducted to improve its control and the characteristics of end products for a variety of applications. The design of microscopic structures often relies on the accurate knowledge and modeling of photopolymer's behavior upon exposure, i.e. the Dill parameters, for each radiation species of interest and therefore the development of flexible characterization techniques is of great importance. SU‐8 is a popular compound that is representative of a whole class that relies on cationic polymerization, where an acid is obtained via photolysis of an onium salt during exposure. Here we report on the observation of SbF6− via laser desorption mass spectrometry on SU‐8 exposed to UV light at the wavelength of 365 nm and demonstrate that the yield of this counter‐anion as a function of exposure is consistent with the Dill C parameter value available in the literature. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 967–972

Onium salts-derived B and P dual-doped carbon microspheres as anode material for high-performance sodium-ion batteries

Boron (B) and phosphorus (P) dual-doped carbon microspheres (BPC) were successfully synthesized from onium salts by a facile one-step pyrolysis method. When employed as the anode material for sodium (Na)-ion batteries (NIBs), the resulting BPC delivers a reversible capacity of 221.9 mAh g−1 at 50 mA g−1. Impressively, the BPC can still deliver a capacity of 102 mAh g−1 after 1500 cycles at 1 A g−1 with the capacity retention up to 78%. The high performance of the BPC can be attributed to the significantly increased defects and dilated interlayer distance induced by synergistically B and P co-doping.

Facile method to prepared photoinitiated cationic curable plant oil and its enhancement on the impact strength of cycloaliphatic diepoxide

In this study, a modified soybean oil (SBO) with urethane branched chain terminated by epoxy group (SBO-URE) was prepared by highly efficient thiol-ene photo-click chemistry and isocyanate polyurethane reaction. The structure of modified plant oil was characterized by FT-IR and 1H-NMR. Then, the obtained bio-resin was blended with a petroleum-based cycloaliphatic epoxy 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (ECC) with different composition ratios. The photoinitiated cationic process and properties of curing formulation were investigated. The epoxy conversion was improved by adding chlorothioxanthone (CTX) through photosensitization process of onium salts. The DMA, TGA, and a series of tensile tests indicated that the addition of SBO-URE can increase the impact resistance with a modest decrease of tensile properties without sacrificing thermal stability. The formulation system exhibited a better performance than the formulation with its commercial counterpart epoxidized soybean oil (ESO), thereby revealing that the introduction of urethane branched structure terminated by epoxy group of soybean oil can react with epoxy matrix during curing process, and offered a certain improvement of impact strength without serious depletion of tensile properties.