Chiral Ammonium Research Articles

Cholesterol-Supported Liquid-Phase Synthesis of Betaines and Organic Cations without Chromatography.

Cholesterol was explored as a support for liquid-phase synthesis. Without the need for chromatography, the cholesterol-supported liquid-phase approach gave access to diverse betaines possessing chiral ammonium, phosphonium, and sulfonium ions. The cholesterol-supported method was further demonstrated by the synthesis of cationic amides and hydrazides.

Chiral Macrocycles for Enantioselective Recognition.

The efficient synthesis of chiral macrocycles with highly enantioselective recognition remains a challenge. We have addressed this issue by synthesizing a pair of chiral macrocycles, namely, R/S-BINOL[2], achieving total isolated yields of up to 62% through a two-step reaction sequence. These macrocycles are readily purified by column chromatography over silica gel without the need for chiral separation, thus streamlining the overall synthesis. R/S-BINOL[2] demonstrated enantioselective recognition toward chiral ammonium salts, with enantioselectivity (KS/KR) values reaching up to 13.2, although less favorable separations were seen for other substrates. R/S-BINOL[2] also displays blue circularly polarized luminescence with a |glum| value of up to 2.2 × 10-3. The R/S-BINOL[2] macrocycles of this study are attractive as chiral hosts in that they both display enantioselective guest recognition and benefit from a concise, high-yielding synthesis. As such, they may have a role to play in chiral separations.

Air-Processed Efficient Cesium-Based Two-Dimensional Perovskite Solar Cells Based on Asymmetric Chiral Ammonium Salts.

Cesium-based two-dimensional (2D) perovskites with attractive phase and environmental stability have broad application prospects in single-junction and tandem perovskite solar cells (PSCs). However, the severe nonradiative recombination and significant energy losses due to disordered phase orientations and phase distributions greatly hinder the carrier transport performance of cesium-based 2D PSCs and severely limit their photovoltaic performance. Here, we employ an asymmetric chiral spacer cation source, (R)-α-phenylethylamine acrylate (R-α-PEAAA), to prepare high-quality 2D cesium-based films with uniform phase distribution and high out-of-plane orientation by air processing, resulting in efficient carrier transport. More importantly, the asymmetric chiral spacer R-α-PEA has a stronger dipole moment than its isomer (PEA), which can regulate the dielectric properties of cesium-based 2D perovskites and promote charge dissociation. In addition, the chiral R-α-PEA can optimize the morphology and out-of-plane orientation of perovskite films, reduce trap density and nonradiative recombination loss, and optimize energy level alignment, thus enhancing carrier transport. As a result, cesium-based 2D PSCs (R-α-PEA2Cs4Pb5I16, n = 5) achieved a record power conversion efficiency of 19.71% and the unencapsulated device maintained over 90% efficiency after 1500 h of continuous light exposure and ambient storage (35 ± 5% relative humidity). This study provides an idea for the development of chiral 2D perovskite with efficient charge carrier transport toward efficient and stable cesium-based 2D PSCs.

Oxidative Dearomative Coupling of Electron-Deficient Arenols Using Hypohalite Catalysis.

We developed performant in situ hypohalite, especially hypobromite, catalysis for the oxidative dearomatization of low-reactivity electron-deficient arenols. The reaction scope encompasses inter- and intramolecular oxidative dearomative C-O, C-N, and C-C coupling reactions. Notably, using a chiral ammonium countercation, we achieved enantioselective hypobromite catalysis for oxidative dearomative coupling reactions. Mechanistic studies revealed that the reaction mechanisms might differ depending on the halide identity.

Confinement‐Driven Bulk Photovoltaic in Lead‐Free Hybrid Perovskite for Self‐Powered X‐Ray Detection

AbstractLead‐free hybrid perovskites (HPs) are considered as environmentally friendly photodetection materials. However, there are few studies on self‐powered photodetection in lead‐free HPs, and most of them are achieved through the introduction of chiral ammonium cations induced bulk photovoltaic effect (BPVE). Notably, enantiomeric pure chiral ammonium cations are difficult to synthesize, the quantity is small and the price is high. Therefore, choosing achiral ammonium cations to induce BPVE in lead‐free HPs is crucial. Herein, achiral cations 1,3‐bis(aminomethyl)cyclohexane (1,3‐BMACH) is exploited to induce bulk photovoltaic through confinement effect in (1,3‐BMACH)BiI5, then self‐powered X‐ray detection is successfully realized. Due to the large space in zero‐dimensional (0 D) (MXD)BiI5 (MXD is meta‐xylylenediammonium), the confinement effect is not obvious, resulting in the disorderly arrangement of MXD2+ cations and a centrosymmetry structure of (MXD)BiI5. Benefiting from the large photovoltage (1.0 V) of (1,3‐BMACH)BiI5 under X‐ray, its single crystal devices exhibit ultra‐high sensitivity of 170 µC Gy−1 cm−2 and extremely low detection limit of 49 nGy s−1 under self‐powered mode, which almost surpassing most low‐dimensional single‐phase lead‐free HPs at 0 V bias. This work has promoted the application of lead‐free HPs in the field of self‐powered X‐ray detection.

Catalytic Asymmetric Construction of Chiral Amines with Three Nonadjacent Stereocenters via Trifunctional Catalysis.

Pharmaceuticals and biologically active natural products usually contain multiple stereocenters. The development of catalytic asymmetric reactions for the direct construction of complex motifs containing three nonadjacent stereocenters is a particularly important and formidable challenge. In this paper, we report an unprecedented method for the direct asymmetric construction of complex chiral amines with 1,3,5- or 1,3,4-stereocenters from readily available achiral and racemic starting materials. The reaction was made possible by the development of highly efficient chiral ammonium catalysts that serve three distinct functions: promoting efficient kinetic resolution by chiral recognition of racemic electrophiles, promoting asymmetric C-C bond forming reactions by recognizing enantiotropic faces of achiral nucleophiles, and mediating a highly stereoselective protonation of carbanions. Using these trifunctional catalysts, the reaction of imines and tulipane derivatives proceeded in a highly regio-, chemo-, and stereoselective manner to produce synthetically useful yields of complex chiral amines. We believe that trifunctional catalysis can be applied in a variety of asymmetric transformations for the streamlined asymmetric synthesis of complex chiral molecules with multiple stereocenters.

Deracemization of Atropisomeric Biaryls Enabled by Copper Catalysis.

Atropisomeric biaryls have found crucial applications in versatile chiral catalysts as well as in ligands for transition metals. Herein, we have developed an efficient crystallization-induced deracemization (CID) method to access chiral biaryls from their racemates with a chiral ammonium salt under copper catalysis including BINOL, NOBIN, and BINAM derivatives. After being significantly accelerated by its bidentate diamine ligand, the copper catalyst exhibits high efficiency and selectivity in racemizing biaryl skeletons, and the cocrystal complex would be enantioselectively formed together with chiral ammonium salt, which on acid-quenching would directly deliver chiral biaryl without further chromatographic purification. This CID process is easily scalable, and the chiral ammonium salt was nicely recoverable. Ligand effect studies showed that bulky alkyl substitution was an indispensable element to ensure efficient racemization, which probably proceeds via a radical-cation intermediate and further allows axial rotation by forming a delocalized radical.

Electron-donating functional groups strengthen ligand-induced chiral imprinting on CsPbBr3 quantum dots

Chiral perovskite nanoparticles and films are promising for integration in emerging spintronic and optoelectronic technologies, yet few design rules exist to guide the development of chiral material properties. The chemical space of potential building blocks for these nanostructures is vast, and the mechanisms through which organic ligands can impart chirality to the inorganic perovskite lattice are not well understood. In this work, we investigate how the properties of chiral ammonium ligands, the most common organic ligand type used with perovskites, affect the circular dichroism of strongly quantum confined CsPbBr3 nanocrystals. We show that aromatic ammonium ligands with stronger electron-donating groups lead to higher-intensity circular dichroism associated with the lowest-energy excitonic transition of the perovskite nanocrystal. We argue that this behavior is best explained by a modulation of the exciton wavefunction overlap between the nanocrystal and the organic ligand, as the functional groups on the ligand can shift electron density toward the organic species-perovskite lattice interface to increase the imprinting.

Shape-Persistent Triptycene-Derived Pillar[6]arenes: Synthesis, Host–Guest Complexation, and Enantioselective Recognitions of Chiral Ammonium Salts

Shape-Persistent Triptycene-Derived Pillar[6]arenes: Synthesis, Host–Guest Complexation, and Enantioselective Recognitions of Chiral Ammonium Salts

Design of Y-Shaped Trimers of Chiral Phase-Transfer Catalysts for the Asymmetric Alkylation of Amino Acid Derivatives

AbstractA series of soluble Y-shaped trimers of chiral phase-transfer catalysts were synthesized and utilized in the asymmetric alkylation of amino acid Schiff bases. The length of the linker between the ammonium cation of the simplified Maruoka catalyst and a phloroglucinol moiety was varied and excellent yields and enantioselectivity were observed when using the Y-shaped trimer of chiral phase-transfer catalyst with an octamethylene linker. A theoretical study of the most efficient Y-shaped trimer of chiral phase-transfer catalyst suggests the chiral ammonium center of the catalyst is more exposed and thus more available in the reaction.

Enantioselective Allylation of Stereogenic Nitrogen Centers.

Most tertiary amines with a stereogenic nitrogen center undergo rapid racemization at room temperature. Consequently, the quaternization of amines under dynamic kinetic resolution seems feasible. N-Methyl tetrahydroisoquinolines are converted into configurationally stable ammonium ions by Pd-catalyzed allylic alkylation. The optimization of conditions and the evaluation of the substrate scope enabled high conversions and an enantiomeric ratio of up to 10:90. We report here the first examples for the enantioselective catalytic synthesis of chiral ammonium ions.

Chiral Calix[3]pyrrole Derivatives: Synthesis, Racemization Kinetics, and Ring Expansion to Calix[9]- and Calix[12]pyrrole Analogues.

Chiral pyrrolic macrocycles continue to attract interest. However, their molecular design remains challenging. Here, we report a calixpyrrole-based chiral macrocyclic system, calix[1]furan[1]pyrrole[1]thiophene (1), synthesized from an oligoketone. Macrocycle 1 adopts a partial cone conformation in the solid state, and undergoes racemization via ring inversion. Molecular dynamics simulations revealed that inversion of the thiophene is the rate determining step. Pyrrole N-methylation suppressed racemization and permitted chiral resolution. Enantioselective N-methylation also occurred in the presence of a chiral ammonium salt, although the stereoselectivity is modest. A unique feature of 1 is that it acts as a useful synthetic precursor to yield several calix[n]furan[n]pyrrole[n]thiophene products (n=2-4), including a calix[12]pyrrole analogue that to our knowledge constitutes the largest calix[n]pyrrole-like species to be structurally characterized.

Chiral Calix[3]pyrrole Derivatives: Synthesis, Racemization Kinetics, and Ring Expansion to Calix[9]‐ and Calix[12]pyrrole Analogues

AbstractChiral pyrrolic macrocycles continue to attract interest. However, their molecular design remains challenging. Here, we report a calixpyrrole‐based chiral macrocyclic system, calix[1]furan[1]pyrrole[1]thiophene (1), synthesized from an oligoketone. Macrocycle 1 adopts a partial cone conformation in the solid state, and undergoes racemization via ring inversion. Molecular dynamics simulations revealed that inversion of the thiophene is the rate determining step. Pyrrole N‐methylation suppressed racemization and permitted chiral resolution. Enantioselective N‐methylation also occurred in the presence of a chiral ammonium salt, although the stereoselectivity is modest. A unique feature of 1 is that it acts as a useful synthetic precursor to yield several calix[n]furan[n]pyrrole[n]thiophene products (n=2–4), including a calix[12]pyrrole analogue that to our knowledge constitutes the largest calix[n]pyrrole‐like species to be structurally characterized.

Inside Cover: Ultra‐fast One‐Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts (Angew. Chem. Int. Ed. 15/2023)

Catalytic ultra-fast helix induction and its static helicity memory in a poly(biphenylylacetylene) were achieved by using chiral ammonium salts, as reported by Katsuhiro Maeda, Eiji Yashima, and co-workers in their Research Article (e202217020). The remarkable acceleration of the helix-induction rate accompanied by strong chiral amplification relies on crown-ether-like specific intermolecular complex formation of the two methoxymethoxy groups of the biphenyl pendants with chiral ammonium species.

Innentitelbild: Ultra‐fast One‐Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts (Angew. Chem. 15/2023)

Innentitelbild: Ultra‐fast One‐Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts (Angew. Chem. 15/2023)

Ultra‐fast One‐Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts

AbstractWe report an ultra‐fast helix induction and subsequent static helicity memory in poly(biphenylylacetylene) (PBPA‐A) assisted by a catalytic amount of nonracemic ammonium salts comprised of non‐coordinating tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (BArF−) as a counter anion. The remarkable acceleration of the helix‐induction rate in PBPA‐A accompanied by the significant amplification of the asymmetry relies on the two methoxymethoxy groups of the biphenyl pendants, which can gain access to enfold the chiral ammoniums in a crown‐ether manner in specific aromatic solvents, leading to ultra‐fast helicity induction, which is completed within 30 s. In aromatic solvents, helicity memory is lost rapidly, but is quite stable in long‐chain hydrocarbons. The best use of specific solvents for helicity induction and static helicity memory, respectively, provides a highly sensitive chirality sensing system toward a small amount of chiral amines and amino acids when complexed with BArF−.

Ultra-fast One-Handed Helix Induction and Its Static Helicity Memory in a Poly(biphenylylacetylene) with a Catalytic Amount of Chiral Ammonium Salts.

We report an ultra-fast helix induction and subsequent static helicity memory in poly(biphenylylacetylene) (PBPA-A) assisted by a catalytic amount of nonracemic ammonium salts comprised of non-coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF- ) as a counter anion. The remarkable acceleration of the helix-induction rate in PBPA-A accompanied by the significant amplification of the asymmetry relies on the two methoxymethoxy groups of the biphenyl pendants, which can gain access to enfold the chiral ammoniums in a crown-ether manner in specific aromatic solvents, leading to ultra-fast helicity induction, which is completed within 30 s. In aromatic solvents, helicity memory is lost rapidly, but is quite stable in long-chain hydrocarbons. The best use of specific solvents for helicity induction and static helicity memory, respectively, provides a highly sensitive chirality sensing system toward a small amount of chiral amines and amino acids when complexed with BArF- .

Catalytic Asymmetric Imine Cross-Coupling Reaction

Catalytic asymmetric cross-coupling of imines constitutes a particularly desirable method for the synthesis of chiral vicinal diamines directly from readily available achiral precursors. The potential of this method lies in the possibility of utilizing a variety of imines as reacting partners. However, the realization of highly stereoselective cross-coupling of two different imines proved to be a formidable challenge. Herein we report an unprecedented catalytic asymmetric cross-coupling reaction that tolerates a variety of ketimines and aldimines as nucleophiles and electrophiles, respectively. The realization of this reaction resulted from the development of a new chiral ammonium catalyst, which was guided by insights from studies of catalyst-substrate interactions. With a 0.5 mol % loading of an organocatalyst, this reaction proceeded in a highly diastereo- and enantioselective manner to afford a diverse range of chiral vicinal diamines as nearly single stereoisomers. This catalytic reaction establishes a new approach for the asymmetric synthesis of chiral vicinal diamines.

3D-printed polymer composite devices based on a ferroelectric chiral ammonium salt for high-performance piezoelectric energy harvesting.

Three-dimensional printing (3DP) is an emerging technology to fabricate complex architectures, necessary to realize state-of-the-art flexible and wearable electronic devices. In this regard, top-performing devices containing organic ferro- and piezoelectric compounds are desired to circumvent significant shortcomings of conventional piezoceramics, e.g. toxicity and high-temperature device processibility. Herein, we report on a 3D-printed composite of a chiral ferroelectric organic salt {[Me3CCH(Me)NH3][BF4]} (1) with a biodegradable polycaprolactone (PCL) polymer that serves as a highly efficient piezoelectric nanogenerator (PENG). The ferroelectric property of 1 originates from its polar tetragonal space group P42, verified by P-E loop measurements. The ferroelectric domain characteristics of 1 were further probed by piezoresponse force microscopy (PFM), which gave characteristic 'butterfly' and hysteresis loops. The PFM amplitude vs. drive voltage measurements gave a relatively high magnitude of the converse piezoelectric coefficient for 1. PCL polymer composites with various weight percentages (wt%) of 1 were prepared and subjected to piezoelectric energy harvesting tests, which gave a maximum open-circuit voltage of 36.2 V and a power density of 48.1 μW cm-2 for the 10 wt% 1-PCL champion device. Furthermore, a gyroid-shaped 3D-printed 10 wt% 1-PCL composite was fabricated to test its practical utility, which gave an excellent output voltage of 41 V and a power density of 56.8 μW cm-2. These studies promise the potential of simple organic compounds for building PENG devices using advanced manufacturing technologies.

Phosphine-catalysed transformations of ortho- and para-quinone methides.

Organocatalytic methodologies for the derivatization of o-QM, p-QM and the analogous aza-QM have been recently developed and involve different catalytic systems such as phosphoric acids, thioureas, squaramides, NHC carbenes or chiral ammonium salts. Besides, phosphines, commonly used as ligands in metal-catalysed reactions, can be also used as organocatalysts. In this case, they are mainly involved as nucleophilic catalysts in reactions such as the Rauhut-Currier (RC) reaction. In this review, an analysis of the recent developments in racemic and enantioselective phosphine-catalysed transformations of o-QM, p-QM and aza-o-QM has been carried out.