Helical Enantiomers Research Articles

Conformationally Locked Cyclo[2]Dipyrrins Linked with Anthracene Subunits: Synthesis and Chiroptical Properties.

Herein, we report the synthesis of anthracene-containing twisted cyclo[2]dipyrrin 1 by utilizing a non-planar building block, 1,5-dipyrrylanthracene (1,5-DPA). The non-planar nature of the macrocycle enhanced the solubility and helped in structural characterization. Macrocycle 1 adopts a twisted 'figure of eight' conformation stabilized by strong intramolecular H-bonding interactions and exists as a pair of helical enantiomers, as revealed by X-ray crystallographic analysis. More importantly, the sterically locked structure enabled facile optical resolution using chiral HPLC. The (P,P) and (M,M) enantiomers show moderate chiroptical properties, such as absorption dissymmetry factors |gabs | in the order of 10-3 , and luminescence dissymmetry factors |glum | of 3.8×10-3 and 2.9×10-3 at 702 nm, respectively.

Conformationally Locked Cyclo[2]Dipyrrins Linked with Anthracene Subunits: Synthesis and Chiroptical Properties

AbstractHerein, we report the synthesis of anthracene‐containing twisted cyclo[2]dipyrrin 1 by utilizing a non‐planar building block, 1,5‐dipyrrylanthracene (1,5‐DPA). The non‐planar nature of the macrocycle enhanced the solubility and helped in structural characterization. Macrocycle 1 adopts a twisted ‘figure of eight’ conformation stabilized by strong intramolecular H‐bonding interactions and exists as a pair of helical enantiomers, as revealed by X‐ray crystallographic analysis. More importantly, the sterically locked structure enabled facile optical resolution using chiral HPLC. The (P,P) and (M,M) enantiomers show moderate chiroptical properties, such as absorption dissymmetry factors |gabs| in the order of 10−3, and luminescence dissymmetry factors |glum| of 3.8×10−3 and 2.9×10−3 at 702 nm, respectively.

Nitrile grafted thia[5]Helicenes through a photocyclization pathway of styrylnaphtho[2,1-b]thiophenes: Synthesis and photophysical properties

Six nitrile grafted pentacyclic π-conjugated systems have been designed and synthesized, in 44–68% overall yields, through a photochemical route under mild conditions and using a suitable phenanthrene-like derivative as a basic molecular skeleton. Their structural characterization has been performed using various spectroscopic techniques among them X-ray diffraction, which indicated the presence of pair of P and M helical enantiomers in the crystal structure. These helical compounds were found readily soluble in a variety of organic solvents and exhibit a good thermal stability. The UV–vis absorption and photoluminescence properties of these π-conjugated helical scaffolds have been experimentally examined indicating an absorption maxima around 281–284 nm and a remarkable dual emissions in the blue region as their characteristic. All compounds exhibited irreversible redox-active characteristics. HOMO and LUMO energy level have been evaluated experimentally by cyclic voltammetry showing an electrochemical band gap (Eg-el) ranging from 2.87 eV to 3.01 eV.

The origin of conformational solvatochromism in phenylmethylidene-bis(pyrrolo[2,3-b]quinoxaline) derivative

Phenylmethylidene-bis(pyrrolo[2,3-b]quinoxaline) derivative (1) was synthesized by the reaction between the 3-thiobenzoylpyrrolo[2,3-b]quinoxaline (SPQ) and a secondary amine or NaOH in DMF solution. The obtained compound was characterized by single crystal X-ray diffraction, which indicated the presence of pair of P and M helical enantiomers in the crystal structure. Molecules of 1 showed positive solvatochromism, with about 40 nm bathochromic shift. The theoretical calculations showed that the solvatochromism of the investigated compound depends on the conformational changes induced by the formation of the hydrogen bond with a strong acceptor. These conformational changes described by the angle between chromophore planes correlate with the solvent polarity.

Correction: How the magnetic field impacts the chiroptical activities of helical copper enantiomers

Correction for ‘How the magnetic field impacts the chiroptical activities of helical copper enantiomers’ by Jialu Wu et al., New J. Chem., 2021, DOI: 10.1039/D1NJ03803F.

How the magnetic field impacts the chiroptical activities of helical copper enantiomers

A pair of enantiomers {[Cu(l-pro)(l-tyr)]·2H2O}n (L-1) and {[Cu(d-pro)(d-tyr)]·2H2O}n (D-1) based on the chiral ligands l/d-proline and l/d-tyrosine were synthesized and investigated using single-crystal X-ray structure analysis, IR, thermogravimetric analysis, solid-state UV-Vis spectroscopy, circular dichroism, magnetic studies, and DFT calculations.

Oligophenyls with Multiple Disulfide Bridges as Higher Homologues of Dibenzo[c,e][1,2]dithiin: Synthesis and Application in Lithium-Ion Batteries.

Higher homologues of dibenzo[c,e][1,2]dithiin were synthesized from oligophenyls bearing multiple methylthio groups. Single‐crystal X‐ray analyses revealed their nonplanar structures and helical enantiomers of higher meta‐congener 6. Such dibenzo[1,2]dithiin homologues are demonstrated to be applicable to lithium‐ion batteries as cathode, displaying a high capacity of 118 mAh g−1 at a current density of 50 mA g−1.

Controlled Crystal Transformations of a Chiral Conglomerate with Heterotactic Helical Coordination Arrays

Helix represents the most essential molecular chiral symmetry at a supramolecular level. Here, a unique quasiracemate conglomerate {[Cd(L)I(H2O)3][Cd(HL)I2(H2O)2](H2O)4}n (1, HL = N-(3,5-di(pyridin-3-yl)-4H-1,2,4-triazol-4-yl)nicotinamide), which crystallizes in P21 space group and consists of two different sets of helical enantiomers, has been successfully constructed. Remarkably, crystal transformations from the chiral quasiracemate 1 to two different achiral racemates {[Cd(L)I(H2O)3]·(H2O)3}n (2) and {[Cd(HL)I2(H2O)2](H2O)}n (3) can be achieved controllably by introducing acid and base additives, via solvent-mediated crystal transformations.

Diastereoselective Synthesis and Catalytic Activity of Two Chiral cis‐Dioxidomolybdenum(VI) Complexes

Two enantiomerically pure chiral dioxidomolybdenum(VI) complexes (1 and 2) of the type [MoO2L] (L = dianionic, tetradentate ONNO‐ligand) were synthesized and investigated in enantioselective oxidation reactions. The solid‐state structures of complex 1 and 2, determined via single‐crystal X‐ray diffraction analysis revealed two fundamentally different coordination geometries: a C1‐symmetric cis‐β isomer (Λ‐1), and a C2‐symmetric cis‐α isomer (Δ‐2). In both cases, only one of the two possible helical enantiomers (Λ‐ or Δ‐helix) was formed. The complexes were examined as precatalysts in the epoxidation of the challenging prochiral substrate trans‐stilbene, using either tert‐butyl hydroperoxide (TBHP) or cumyl hydroperoxide (CHP) as oxidants. The asymmetric cis‐β complex 1 was found to be significantly more active in the epoxidation than its cis‐α counterpart 2, asymmetric induction was, however, negligible for both complexes. The complexes were also tested in catalytic enantioselective sulfoxidation reactions where chiral induction could be achieved, albeit small. The observed putative molybdenum oxido‐peroxido intermediate 1‐O2 could be identified as an important precomplex before formation of the active catalyst in sulfoxidation.

Dutch Resolution of a configurationally stable [5]helquat.

Synthesis and nontrivial optical resolution of a helicene-like dication, helquat 1, has been accomplished. Starting with gram scale of the racemic helquat 1 sample, Dutch Resolution using family of 3 tartrate anions was key to achieve successful separation of M and P helical enantiomers of 1. Hundreds of milligrams of each enantiomer of this configurationally stable C2 -symmetric helquat have been obtained. Racemization barrier of 1 has been determined. To our knowledge this is the first report on Dutch Resolution performed with a helicene-like compound. Moreover, there are no literature precedents for Dutch Resolution of chiral quaternary ammonium cations.

Azobenzene as a photoregulator covalently attached to RNA: a quantum mechanics/molecular mechanics-surface hopping dynamics study.

The photoregulation of nucleic acids by azobenzene photoswitches has recently attracted considerable interest in the context of emerging biotechnological applications. To understand the mechanism of photoinduced isomerisation and conformational control in these complex biological environments, we employ a Quantum Mechanics/Molecular Mechanics (QM/MM) approach in conjunction with nonadiabatic Surface Hopping (SH) dynamics. Two representative RNA-azobenzene complexes are investigated, both of which contain the azobenzene chromophore covalently attached to an RNA double strand via a β-deoxyribose linker. Due to the pronounced constraints of the local RNA environment, it is found that trans-to-cis isomerization is slowed down to a time scale of ∼10-15 picoseconds, in contrast to 500 femtoseconds in vacuo, with a quantum yield reduced by a factor of two. By contrast, cis-to-trans isomerization remains in a sub-picosecond regime. A volume-conserving isomerization mechanism is found, similarly to the pedal-like mechanism previously identified for azobenzene in solution phase. Strikingly, the chiral RNA environment induces opposite right-handed and left-handed helicities of the ground-state cis-azobenzene chromophore in the two RNA-azobenzene complexes, along with an almost completely chirality conserving photochemical pathway for these helical enantiomers.

Doubly N‐Confused [36]Octaphyrin(1.1.1.1.1.1.1.1): Isomerization, Bis‐Metal Coordination, and Topological Chirality

AbstractA novel [36]octaphyrin analogue embedding two N‐confused pyrrole units demonstrated unique prototropy‐coupled isomerization between the Figure‐of‐eight and dumbbell conformers. Upon bis‐metal coordination, fixation of fully π‐conjugated Figure‐of‐eight structures was achieved as referred from the X‐ray crystal structure. Chirogenesis of the helical enantiomers was proved by intense circular dichroism (CD) response in the near infrared (NIR) region.

Doubly N-Confused [36]Octaphyrin(1.1.1.1.1.1.1.1): Isomerization, Bis-Metal Coordination, and Topological Chirality.

A novel [36]octaphyrin analogue embedding two N-confused pyrrole units demonstrated unique prototropy-coupled isomerization between the Figure-of-eight and dumbbell conformers. Upon bis-metal coordination, fixation of fully π-conjugated Figure-of-eight structures was achieved as referred from the X-ray crystal structure. Chirogenesis of the helical enantiomers was proved by intense circular dichroism (CD) response in the near infrared (NIR) region.

Sterically Congested Chiral 7,8‐Dioxa[6]helicene and Its Dihydro Analogues: Synthesis, Regioselective Functionalization, and Unexpected Domino Prins Reaction

AbstractA C2‐symmetric 7,8‐dioxa[6]helicene‐2,13‐diol was synthesized from readily available 2,7‐dihydroxynaphthalene on a gram scale. A high‐yielding synthetic strategy for the regioselective hydroxymethylation at the sterically most hindered C1 position of diol analogues was investigated. The dioxa[6]helicene backbone with configurationally stable helical enantiomers was synthesized, and these enantiomers were separated by HPLC on a chiral stationary phase. We also observed an unexpected domino Prins reaction. This is the first report of a domino Prins reaction occurring on a helicene. Along with the high‐yielding regioselective functionalization of dioxa[6]helicene analogues, the synthesis of a few cis‐7a,14c‐dihydro‐functionalized helicenoid diols substituted at the sterically most hindered C1 and C14 positions in their racemic forms was explored.

Enantioselective DNA condensation induced by heptameric lanthanum helical supramolecular enantiomers

DNA condensation induced by a pair of heptameric La(III) helical enantiomers M-[La7(S-L)6(CO3)(NO3)6(OCH3)(CH3OH)7]·2CH3OH·5H2O and P-[La7(R-L)6(CO3)(NO3)6(OCH3)(CH3OH)5(H2O)2]·2CH3OH·4H2O (M-La and P-La, L=2-(2-hydroxybenzylamino)-3-carbamoylpropanoic acid) has been investigated by UV/vis spectroscopy, fluorescence spectroscopy, CD spectroscopy, EMSA, RALS, DLS, and SEM. The enantiomers M-La and P-La could induce CT-DNA condensation at a low concentration as observed in UV/vis spectroscopy. DNA condensates possessed globular nanoparticles with nearly homogeneous sizes in solid state determined by SEM (ca. 250nm for M-La and ca. 200nm for P-La). The enantiomers bound to DNA through electrostatic attraction and hydrogen bond interactions in a major groove, and rapidly condensed free DNA into its compact state. DNA decompaction has been acquired by using EDTA as disassembly agent, and analyzed by UV/vis spectroscopy, CD spectroscopy and EMSA. Moreover, the enantiomers M-La and P-La displayed discernible discrimination in DNA interaction and DNA condensation, as well as DNA decondensation. Our study suggested that lanthanum(III) enantiomers M-La and P-La were efficient DNA packaging agents with potential applications in gene delivery.

ChemInform Abstract: Atropisomeric Chiral Dienes in Asymmetric Catalysis: C2‐Symmetric (Z,Z)‐2,3‐Bis[1‐(diphenylphosphinyl)ethylidene]tetralin as a Highly Active Lewis Base Organocatalyst.

Diene catalysts with a twist: The title C2 -symmetric tetralin-fused 1,3-butadiene derivative is atropisomeric and can be resolved into the two helical enantiomers. The optically pure compound showed excellent enantioselectivity as well as unusually high catalytic activity as a chiral Lewis basic organocatalyst in the asymmetric allylation of various aldehydes with β-substituted allyltrichlorosilanes.

Atropisomeric Chiral Dienes in Asymmetric Catalysis: C2‐Symmetric (Z,Z)‐2,3‐Bis[1‐(diphenylphosphinyl)ethylidene]tetralin as a Highly Active Lewis Base Organocatalyst

Diene catalysts with a twist: The title C2 -symmetric tetralin-fused 1,3-butadiene derivative is atropisomeric and can be resolved into the two helical enantiomers. The optically pure compound showed excellent enantioselectivity as well as unusually high catalytic activity as a chiral Lewis basic organocatalyst in the asymmetric allylation of various aldehydes with β-substituted allyltrichlorosilanes.

Metal-Assisted Ring-Closing/Opening Process of a Chiral Tetrahydroquinazoline

The ring-chain tautomerism of a 2-aryl-1,2,3,4-tetrahydroquinazoline has been exploited to induce reversible changes in the aminal-imine equilibrium, as desired, by coordination of a suitable metal ion. This process was studied by NMR and UV-vis spectroscopies, X-ray crystallography, and molecular modeling approach. The results obtained show that the imine H(2)L(i) undergoes a selective ring-closing reaction upon complexation with Ni(2+). As a result, complexes of the type Ni(HL(a))(2) are obtained, whose chirality arises from the chiral ligand H(2)L(a) and the helicity of the structure. Hence, helical enantiomers form the following racemates: [Δ-C(R,R)N(S,S),Λ-C(S,S)N(R,R)]-Ni(HL(a))(2)·2HOAc and [Δ,Λ-C(S,R)N(R,S)]-Ni(HL(a))(2)·4MeOH. In contrast to the situation observed for Ni(2+), the cyclic tautomer of the ligand, H(2)L(a), undergoes a selective ring-opening reaction upon complex formation with Pd(2+), ultimately yielding Pd(HL(i))(2)·MeOH, in which the open-chain imine ligand is bidentate through the N,O donor set of the quinoline residue. Density functional theory calculations were conducted to provide insight into the different behavior of both coordinated metals (Ni(2+) and Pd(2+)) and to propose a mechanism for the metal-assisted opening/closing reaction of the tetrahydroquinazoline ring.

Solution Structure and Dynamics, Stability, and NIR Emission Properties of Lanthanide Complexes with a Carboxylated Bispyrazolylpyridyl Ligand

The complexation behavior of the ligand 2,6-bis{3-[N,N-bis(carboxymethyl)aminomethyl]pyrazol-1-yl}-pyridine, L, toward lanthanide cations was investigated throughout the series. Potentiometric titration experiments on L (0.1 M KCl) revealed the presence of four protonation steps in the 2-12 pH domain, associated with the protonation of the two tertiary amine nitrogen atoms and with two of the four carboxylate functions. The stability constants for the formation of the [LnL](-) complexes (Ln = La, Nd, Eu, Ho, and Lu) were determined in water and evidenced a hill-shaped complexation trend along the series, with log K increasing from 14.56(9) (La) to 16.68(2) (Ho) and decreasing to 15.42(2) (Lu). Geometry optimizations showed the [LnL](-) complexes (Ln = La, Nd, Eu, Ho, Yb, and Lu) adopting a C(2) symmetry with the symmetry axis going through the metal atom and the nitrogen atom of the central pyridine ring, leading to the presence of Delta and Lambda helical enantiomers. Analysis of the calculated Ln-O and Ln-N bond lengths showed a marked deviation from the expected values at the end of the series, which accounts for the observed decreased complexation affinity. (1)H and (13)C NMR experiments in D(2)O (room temperature) showed the shortening of the bond distances in [LnL](-) complexes from La to Lu to be accompanied by a rigidification of the structure, leading to a decreased C(2) symmetry for the Lu complex compared to C(2v) for La. This decreased symmetry was attributed to a slow Delta <--> Lambda interconversion that was followed by variable-temperature (13)C NMR experiments on the Lu complex. The activation parameters determined by line broadening analysis for this interconversion process point to an associatively assisted interconversion process. The (1)H NMR spectrum of the paramagnetic [YbL](-) complex was investigated in D(2)O, and a lanthanide induced shift analysis showed good agreement between the observed paramagnetic chemical shifts and those calculated from the DFT optimized structures using a dipolar model, especially when solvent effects are taken into account. The UV-vis absorption and near-infrared luminescence spectra of the Pr, Nd, Er, and Yb complexes were measured in water and showed the complexation pocket provided by the ligand to be well-suited for the protection of the cations, all of them displaying typical Ln-centered emission spectra, the Yb complex having a relatively long lifetime of 3.0 micros in water.

Helical Diastereomerism in Self-organization of Molecular Strands

The polyheterocyclic molecule 2 reported in this paper is a helically-wrapped molecular strand presenting two helical substrands of same helicity separated in space thus forming P-P and M-M helical enantiomers. It represents the case of a molecule presenting two chiral centers of helical type. The dimerization process of the compound described in this paper appears to give rise to heterodimeric and homodimeric species. At high and room temperature the formation of dimeric species is accompanied by folding-unfolding interconversion of single molecular strands. Sliding of the monomers along one another without dissociation at low temperature allows each of two helical partners to associate in dimers stabilized by pronounced p�p stacking interactions between the internal heterocycles of the strands.