Halogen Cations Research Articles

"Cation Pool" generated from DMSO and 1,2-dihaloethanes and their application in organic synthesis.

Conventionally, carbenium and onium ions are prepared in the presence of nucleophiles due to their instability and transient nature. The nucleophiles that are unstable or inert to the reaction media cannot be used for reaction with the cationic species to access the desired compounds. To overcome these limitations, developing methods for generating organic cations irreversibly in the absence of nucleophiles is essential. The "cation pool" method developed by Yoshida and co-workers stands out as a reliable strategy to generate and accumulate the reactive cations in solution in the absence of nucleophiles. The cation pool method involves the electrolysis of the substrate in the absence of nucleophiles, usually at low temperature. Moreover, the generation of halogen and chalcogen cations through electrolysis needs extra care because of their low stability. This review covers our effort in generating and accumulating halogen cations as "cation pools", most importantly by simply heating a mixture of dimethyl sulfoxide (DMSO) and 1,2-dihaloethane (DXE, X = Cl, Br, I), and their use in the halogenation reactions. Furthermore, condition-dependent Pummerer-type fragmentations of DMSO-stabilized halogen cations to methyl(methylene)sulfonium ions and chlorodimethylsulfonium ions for synthetic applications are described.

Evaluate the Limits of F Covalent Bonding with Transition Metals at High Valent States in Li2MO2f ( M = Mn, Co, Ni ) Oxyfluoride

The capacity of transition metal oxides as Li-ion battery cathodes are limited by instabilities that arise when high states of charge are achieved1. Oxyfluorides with a disordered rock-salt structure have emerged as attractive alternatives2, but the role of F in their electrochemical function, particularly when metals reach high formal oxidation states, remains to be ascertained so far. In our recent study3, using X-ray Absorption Spectroscopy (XAS) measurements of Mn, O and F, we revealed the existence of Mn-F covalent interactions in Li2MnO2F. The results challenged the assumption of F as largely a spectator ion, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggested the existence of unique knobs of design of battery cathodes in these chemical spaces, by manipulating the covalent interactions between transition metals and two different anions. To further expand the understanding of F participation in the covalent bonding and its electrochemical effect on oxyfluoride materials, we synthesized Li2CoO2F and Li2NiO2F4. Solid fluorides of ions like Co(IV) and Ni(IV) are known to be aggressive oxidizers5. Analysis of the transition metal fluoride literature reveals that oxidation states of IV and higher could lead to unstable phases for late transition metals, particularly Co or Ni5. Unlike oxides, rather than reductively releasing F2, they have a strong propensity to act as highly oxidizing F- donors6,7, being able to oxidize even other halogen cations to their VII state. Yet the oxidation of Ni(II) features prominently in oxyfluoride cathodes that were recently discovered. This puzzle and the exact role of F in modulating the formal redox chemistry of a late metal like Ni and Co in the presence of O remains to be elucidated. To address this question and determine a periodic trend on the role of a mixture of anions in improving the energy density in such cathode materials, we conducted a deep dive into Li2CoO2F and Li2NiO2F. We interrogated the covalent interaction between the oxygen 2p states, fluorine 2p states, and the transition metal 3d orbitals, and their respective contribution to the charge compensation mechanism using XAS. These two oxyfluorides data were compared with the previous Li2MnO2F data to understand the effect of varying the transition metal and how that affects the overall electrochemistry of these materials. We also have estimated the M-O and M-F hybridization and provided a periodic trend. This study allowed us to define a rule to manipulate each element in a oxyfluoride that determines the electrochemical properties of these cathode materials.

Stability performance enhancement and mechanistic research of CH3NH3PbBr3 perovskite quantum dots

Organic-inorganic lead halide perovskites quantum dots (OIPeQDs) have shown excellent performance and great promise for commercial applications. However, their instability is a major challenge for their commercial viability. To address this issue, this study optimized the precursor PbBr2 to DMAPbBr3 (DMA+ = (CH3)2NH2+) and synthesis the CH3NH3PbBr3 quantum dots (QDs) by ligand-assisted reprecipitation (LARP) method at room temperature. This process aims to provide a halogen-rich environment to reduce halide vacancy defects, as well as incorporating symmetrical A-site ions, which enhances hydrogen bonding between the halogen and organic cations. The QDs-DMAPbBr3 obtained through this optimized precursor showed preferable time, water and light stability compared to those synthesized through conventional PbBr2. The result demonstrates that the QDs-DMAPbBr3 maintained 85.60% of the initial photoluminescence (PL) value even after one month. To test the stability of the QDs further, the QDs were mixed with deionized (DI) water at a 20:1 ratio, and the QDs-DMAPbBr3 retained 408.33% of the initial PL value after 3 h. In addition, after continuous irradiation under a 365 nm ultraviolet lamp for 20 h, the PL value of the QDs-DMAPbBr3 remained stable, at 65.48% of the initial value. These findings indicate that QDs-DMAPbBr3 possess remarkable stability, which suggests their suitability for a wide range of applications.

A computational study of the electronic and thermoelectric properties of metal halide cubic perovskites CsBX3 (B = Ge, Sn, Pb and X = Cl, Br, I)

Metal halide perovskites have received a tremendous interest recently in new applications. Photovoltaic, diverse photonic and optoelectronic applications of these materials are in full expansion, but thermoelectricity also instigates a great interest. In this work, we will focus on the thermoelectric properties of a particular material family of metal halide cubic perovskites CsBX3 (B = Ge, Sn, Pb and X = Cl, Br, I). The structural and electronic properties of CsBX3 are computed using first-principles calculations based on the density functional theory which allows calculating equilibrium lattice parameters, band structures, the nature (direct/indirect) and value of the band gap. These studied compounds are semiconductors with direct band gap energy. We have also detected the effect of replacement of halogen and metal cation atoms with other halogen and metal cation atoms on electronic and thermoelectric properties. Boltzmann transport calculations are carried out to explore their thermoelectric properties like the Seebeck coefficient, electrical conductivity and power factor. Large values of the Seebeck coefficient and the power factor obtained for these compounds indicate that these compounds can be used for thermoelectric devices. Our theoretical analysis of the electronic and thermoelectric properties of these compounds suggests that CsSnBr3 and CsGeBr3 are the best Pb-free inorganic metal halide semiconductor for a high thermoelectric performance.

Halogen cation-promoted and solvent-regulated electrophilic cyclization for the regioselective synthesis of 3-haloquinolines and 3-halospirocyclohexadienones.

A novel approach for the production of halogen cations through the reaction of halogens with silver ions is described in this paper. On this basis, the regioselective synthesis of 3-haloquinolines and 3-halospirocyclohexadienones is realized through solvent regulation. The gram-scale reaction and the compatibility of complex substrates demonstrate the synthetic potential of this protocol, which will be an appealing strategy in organic synthesis.

Simultaneously tuning interlayer spacing and termination of MXenes by Lewis-basic halides

The surface and interface chemistry are of significance on controlling the properties of two-dimensional transition metal carbides and nitrides (MXenes). Numerous efforts have been devoted to the regulation of Ti3C2Tx MXene, however, tuning interlayer spacing and surface halogen termination of other MXenes (besides Ti3C2Tx) is rarely reported while demanded. Here we propose a Lewis-basic halides treatment, which is capable of simultaneously engineering the interlayer spacing and surface termination of various MXenes. Benefited from the abundant desolvated halogen anions and cations in molten state Lewis-basic halides, the -F termination was substituted by nucleophilic reaction and the interlayer spacing was enlarged. Ti3C2Tx MXene treated by this method showed a high specific capacity of 229 mAh g−1 for Li+ storage, which is almost 2 times higher than pristine one. Considering the universality, our method provides an approach to regulating the properties of MXenes, which may expand their potential applications in energy storage, optoelectronics and beyond.

Generation of Dimethyl Sulfoxide Coordinated Thermally Stable Halogen Cation Pools for C−H Halogenation

AbstractA method to generate halogen cation pools from the reaction of 1,2‐dihaloethanes (hal=Br, I) and dimethyl sulfoxide (DMSO) for C−H halogenation of arenes and heteroarenes was reported. The initial reaction of DMSO and 1,2‐dihaloethane generates the sulfur ylide, which undergoes pyrolytic elimination of ethylene by affording halonium ions. These ions were accumulated and stabilized by DMSO through coordination by forming halogen cation pools for the halogenation reaction. This protocol was selective for electrophilic monohalogenation of arenes at room temperature; however, polyhalogenated products were formed by raising the reaction temperature. Late‐stage halogenation of heteroarenes and some commonly marketed drugs signifies the synthetic utility of this protocol in pharmaceutical chemistry. Unlike the classical methods, the in‐situ generated electrophilic bromonium ion was further exploited for the direct synthesis of α‐diketones from the alkenes under base‐free conditions.magnified image

Molecular Bond Engineering and Feature Learning for the Design of Hybrid Organic–Inorganic Perovskite Solar Cells with Strong Noncovalent Halogen–Cation Interactions

Hybrid organic-inorganic perovskites are exceedingly interesting candidates for new solar energy technologies, for both ground-based and space applications. However, their large-scale production is hampered by the lack of long-term stability, mostly associated to ion migration. The specific role of non-covalent bonds in contributing to the stability remains elusive, and in certain cases controversial. Here, we perform an investigation on a large perovskite chemical space via a combination of first-principles calculations for the bond strengths and the recently developed Sure Independent Screening and Sparsifying Operator (SISSO) algorithm. The latter is used to formulate mathematical descriptors that, by highlighting the importance of specific non-covalent molecular bonds, can guide the design of perovskites with suppressed ion migration. The results unveil the distinct nature of different non-covalent interactions, with remarkable differences compared to previous arguments and interpretations in the literature on the basis of smaller chemical spaces. In particular, we clarify the origin of the higher stability offered by FA compared to MA, which shows to be different from previous arguments in the literature, the reasons of the improved stability given by the halogen F, and explain the exceptional case of overall stronger bonds for Guanidiunium. The found descriptors reveal the criteria that, within the stability boundaries given by the Goldschmidt factor, give an all-in-one picture of non-covalent interactions which provide the more stable configurations, also including interactions other than H-bonds. Such descriptors are more informative than previously used quantities and can be used as universal input to better inform new machine learning studies.

Magnetodielectric effect in an organic-inorganic hybrid layered perovskite (CH3NH3)2MnCl4 with canted antiferromagnetism

Herein, we report the magnetodielectric effect in a single crystal of (CH3NH3)2MnCl4, an organic–inorganic hybrid layered perovskite having a composition of A2MX4 (A = organic anion, M = metal divalent ion, X = halogen cation). Magnetic property measurements reveal that a magnetic phase transition to antiferromagnetism appears at 47 K and a spin-flop transition occurs at 3.5 T in the magnetization (M) curve. We observed a considerable maximum magnetodielectric (MD) effect of 0.4% at 8 K. Furthermore, it was found that the MD effect is proportional to M2 in low magnetic fields and the MD effect starts to saturate at the spin-flop transition field. We proposed a magnetic structure model to explain the mechanism of the peculiar step-like MD effect induced by the spin reorientation.

Halogen-mediated electrochemical organic synthesis.

In general, halogenide anions are anodically oxidized into active species, which can be elemental halogen, halogen cations, or halogen radicals. These species subsequently react with substrates, such as olefins, ketones, or amines, to generate halogenated products. We review the mechanisms of these reactions.

Anodic Oxidation for the Stereoselective Synthesis of Heterocycles.

Stereodefined aliphatic heterocycles are one of the fundamental structural motifs observed in natural products and biologically active compounds. Various strategies for the synthesis of these building blocks based on transition metal catalysis, organocatalysis, and noncatalytic conditions have been developed. Although electrosynthesis has also been utilized for the functionalization of aliphatic heterocycles, stereoselective transformations under electrochemical conditions are still a challenging field in electroorganic chemistry. This Account consists of four main topics related to our recent efforts on the diastereo- and/or enantioselective synthesis of aliphatic heterocycles, especially N-heterocycles, using anodic oxidations as key steps. The first topic is the development of stereoselective synthetic methods for multisubstituted piperidines and pyrrolidines from anodically prepared α-methoxy cyclic amines. Our strategies were based primarily on N-acyliminium ion chemistry, and the key electrochemical transformations were diastereoselective anodic methoxylation, diastereoselective arylation, and anodic deallylative methoxylation. Furthermore, we found a unique property of the N-cyano protecting group that enabled the electrochemical α-methoxylation of α-substituted cyclic amines. The second topic of investigation is memory of chirality in electrochemical decarboxylative methoxylation. We observed that the electrochemical decarboxylative methoxylation of oxazolidine and thiazolidine derivatives with the appropriate N-protecting group occurred in a stereospecific manner even though the reaction proceeded through an sp2 planar carbon center. Our findings demonstrated the first example of memory of chirality in N-acyliminium ion chemistry. The third topic is the synthesis of chiral azabicyclo-N-oxyls and their application to chiral organocatalysis in the electrochemical oxidative kinetic resolution of secondary alcohols. The final topic is stereoselective transformations utilizing anodically generated halogen cations. We investigated the oxidative kinetic resolution of amino alcohol derivatives using anodically generated bromo cations. We also developed an intramolecular C-C bond formation of keto amides, a diastereoselective bromoiminolactonization of α-allyl malonamides, and an oxidative ring expansion reaction of allyl alcohols. It is noteworthy that most of the electrochemical reactions were performed in undivided cells under constant-current conditions, which avoided a complicated reaction setup and was beneficial for a large-scale reaction. In addition, we developed some enantioselective electrochemical transformations that are still challenges in electroorganic chemistry. We hope that our research will contribute to the further development of diastereo- and/or enantioselective transformations and the construction of valuable heterocyclic compounds using an electrochemical approach.

Direct α-amination of nitrones achieved by DBU-activated N-haloimides

Direct α-amination of nitrones has been developed by using N-haloimide-DBU protocol. Reaction of nitrones with N-haloimide takes place readily in the presence of DBU in DCM, affording directly α-aminonitrones in moderate to excellent yields. N-Haloimide activated by DBU provides both a halogen cation and a nitrogen anion source. E- and Z-isomerization of α-aminonitrones was examined and found to be highly sensitive to acidic substances like silica gel and Brønsted acid. In addition to nitrones, quinoline N-oxides are also suitable substrates. Direct amination achieved by the protocol demonstrates new reactivity of nitrones and quinoline N-oxides.

Electrochemical Generation of Stabilized Halogen Cation Pools

Halogen cations such as I+ and Br+ are highly unstable species, although some of them are identified in the gas phase. The electrochemical oxidation provides a powerful method for generating and accumulating highly reactive cationic species in the solution, whereas the chemical methods are not effective because of reversibility of cation generation. We have already reported that some carbocations and onium ions can be generated and accumulated as cation pools at low temperatures (the “cation pool” method). However, halogen cations are very difficult to accumulate in the solution as “cation pools” because they are too unstable. We envisioned that the use of a stabilizing agent that coordinates the cation being generated would solve the problem. Coordination of a neutral molecule gives a positively charged species and coordination of an anionic species gives a neutral species, although the halogen is the +1 oxidation state in both cases. Some stabilizing agents have already been reported. For example, Miller and co-workers reported the generation and accumulation of “I+” cation in acetonitrile (CH3CN), and later we reported the detection of CH3CN−I+ and (CH3CN)2I+ by cold-spray-ionization mass spectroscopy. Shono and co-workers reported that “I+” could be accumulated in trimethyl orthoformate (TMOF). In this paper we report that dimethyl sulfoxide (DMSO) serves as a neutral stabilizing agent and that TfO− anion serves as a negatively charged stabilizing agent in the electrochemical generation and accumulation of halogen cations such as I+ and Br+. We also report their reactions with organic molecules to give the corresponding halogen-containing products, which serve as useful transformations in organic synthesis.

Alkene Difunctionalization Via b-Haloalkoxysulfonium Ions

Alkene difunctionalization via three-membered ring halonium ion intermediates is an important transformation in organic synthesis. The most straightforward method to generate the halonium intermediates is to use halogen cations, which are usually too unstable to accumulate in solution. Recently, we reported that halogen cations stabilized by dimethyl sulfoxide (DMSO) were successfully generated and accumulated in solution by low-temperature electrochemical oxidation. Reactions of DMSO-stabilized halogen cations such as Br+ and I+ with alkenes gave b-haloalkoxysulfonium ions. Subsequent treatment of the b-haloalkoxysulfonium ions with triethylamine gave α-halocarbonyl compounds via Swern−Moffatt type oxidation. Here, we report that treatment of the b-haloalkoxysulfonium ions with different bases gives different products, i.e. the use of sodium hydroxide and sodium methoxide gives halohydrins and epoxides, respectively.

ChemInform Abstract: Halogen and Chalcogen Cation Pools Stabilized by DMSO. Versatile Reagents for Alkene Difunctionalization.

AbstractBy modification of the procedure, also the corresponding alcohols are accessible in a diastereoselective one‐pot reaction [viz.

Halogen and Chalcogen Cation Pools Stabilized by DMSO. Versatile Reagents for Alkene Difunctionalization

Halogen and chalcogen cations (X(+) = Br(+), I(+), ArS(+), and ArSe(+)) were generated by low-temperature electrochemical oxidation in the presence of dimethyl sulfoxide (DMSO) and were accumulated in the solution. DFT calculations indicated that DMSO stabilizes these cations by coordination. The complexes of I(+) with one and two DMSO molecules were observed by cold-spray-ionization MS analyses. The stability of the resulting cation pools of X(+) increased in the order of Br(+) < I(+) < ArS(+) < ArSe(+), which could be explained in terms of the electronegativity of X. The cation pools served as versatile reagents for organic synthesis; the reactions with alkenes gave β-X-substituted alkoxysulfonium ions, which were converted to the corresponding carbonyl compounds by the treatment with triethylamine, whereas the treatment with methanol gave the corresponding alcohols. The reactions with aminoalkenes and 1,6-dienes gave the cyclized products.

Mutual Influence between Halogen Bonds and Cation–π Interactions: A Theoretical Study

The interplay between halogen bonds and cation-π interactions is investigated by ab initio calculations at the MP2 level of theory. Different energetic effects are observed in the studied complexes in which halogen bonds and cation-π interactions coexist, which can be ascribed to the direction of charge transfer for the two interactions. These effects are analyzed in detail in terms of the structural, energetic, and charge-transfer properties of the complexes. In addition, the quantum theory of atoms in molecules is employed to characterize the interactions and to examine their enhancement and attenuation in terms of the variations in electron density at the bond and cage critical points. Finally, experimental evidence for a combination of the two interactions is obtained from the Cambridge Structural Database.

ChemInform Abstract: Three‐Component Coupling of Arynes and Organic Bromides.

AbstractA novel three‐component reaction of benzyne derivatives and alkynyl halides (III), (V) with isocyanides (II) or (XII) is presented, where in situ generated dipoles can be captured with halogen cations and alkynyl anions to furnish alkynyl aryl imines (IV) and (VI).

An Overview of Lewis Basicity and Affinity Scales

The impossibility of establishing a universal scale of Lewis basicity does not prevent the determination of the quantitative behavior of Lewis bases, thanks to scales constructed against particular Lewis acids: BF3, 4-FC6H4OH, I2, Li+, Na+, K+, Al+, Mn+, CpNi+, and CH3NH3+. These scales encompass important types of bonds formed in a Lewis acid–base adduct: the dative bond, the conventional and ionic hydrogen bonds, the halogen bond and cation–molecule bonds for metal cations of groups 1, 7, 10, 11, and 13. Moreover, although these scales are generally not interrelated, there exist family-dependent relationships that permit ranking, in a rather general order, of bases belonging to a given chemical family, for example, the family of oxygen bases. Therefore, the skepticism about the quantitative usefulness of the Lewis concept of acids and bases is no longer founded.

Stereoselective Construction of Quaternary Carbon Stereocenters via a Semipinacol Rearrangement Strategy

Quaternary carbon stereocenters are found in a broad range of organic compounds, including important bioactive natural products and medicinal agents. Given their ubiquity and the significant synthetic challenges they present, quaternary carbon stereocenters have long attracted great interest from synthetic organic chemists. Numerous efforts have been devoted to their construction, leading to a spectrum of strategies for creating stereogenic quaternary carbon centers. In this context, the semipinacol rearrangement has proven successful. In this extension of the pinacol rearrangement, the 1,2-carbon-to-carbon migration in a 1,2-diol has been expanded to include leaving groups other than the hydroxyl group. Over the past decade, our laboratory has explored the semipinacol rearrangement strategy for the stereoselective construction of quaternary carbon stereocenters. We have investigated various substrates, including 2,3-epoxy alcohols (also termed α-hydroxy epoxides), 2,3-aziridino alcohols, and allylic alcohols. Several promoters that effect the semipinacol rearrangement have been identified, including Lewis acids based on Al, Sm, B, Zn, and Ti for the rearrangement of α-hydroxy epoxides and 2,3-aziridino alcohols; cationic halogen species for the rearrangement of allylic alcohols; and cinchona alkaloids and chiral phosphoric acid for the asymmetric semipinacol rearrangement. Our research efforts have led to a series of valuable synthetic methods, including (1) a tandem semipinacol rearrangement and Meerwein-Ponndorf-Verley reduction, (2) a tandem semipinacol rearrangement and Tishchenko reaction, (3) a tandem semipinacol rearrangement with either an allylation or a propargylation, (4) a tandem semipinacol rearrangement and Schmidt reaction, (5) a semipinacol rearrangement of 2,3-aziridino alcohols, (6) a semipinacol rearrangement of allylic alcohols induced by halogen cation, (7) a tandem aziridination and semipinacol rearrangement of allylic alcohols, and (8) asymmetric semipinacol rearrangements with chiral organic catalysts. One hallmark of these reactions is the creation of stereogenic quaternary carbon centers with high levels of stereocontrol. In this Account, we describe the development of these synthetically useful methodologies and their successful application to the total syntheses of natural products. Our results demonstrate that the semipinacol rearrangement of carefully designed substrates constitutes an efficient approach to the stereoselective construction of quaternary carbon centers. These reactions have produced a broad array of useful compounds that lend themselves to further elaboration. Furthermore, the total synthesis of a series of alkaloids, with significant bioactivity and intriguing molecular architecture, was achieved through these semipinacol rearrangement strategies, highlighting their synthetic value.