Chiral Complexes Research Articles

Simple synthetic access to [Au(IBiox)Cl] complexes.

Green and sustainable access to chiral and achiral gold-IBiox complexes is reported. The gold complexes were synthesized using a simple, air-tolerant, weak base protocol carried out in a green solvent. Their catalytic activity was examined in the hydroamination of alkynes. The steric protection afforded the gold center by these ligands was quantified using the %Vbur model and compared with the most commonly encountered NHCs.

Circularly polarized luminescence in the one-dimensional assembly of binaphtyl-based Yb(iii) single-molecule magnets.

Lanthanide ions have attracted great interest owing to their optical and magnetic properties. Single-molecule magnet (SMM) behavior has been a fascinating science for thirty years. Moreover, chiral lanthanide complexes allow the observation of remarkable circularly polarized luminescence (CPL). However, the combination of both SMM and CPL behaviors in a single molecular system is very rare and deserves attention in the design of multifunctional materials. Four chiral one-dimensional coordination compounds involving 1,1'-Bi-2-naphtol (BINOL)-derived bisphosphate ligands and the Yb(iii) centre were synthesized and characterized by powder and single-crystal X-ray diffraction. All the Yb(iii)-based polymers displayed field-induced SMM behavior with magnetic relaxation occurring by applying Raman processes and near infrared CPL in the solid state.

Enantioselective reductive allylic alkylation enabled by dual photoredox/palladium catalysis.

A dual photoredox/palladium catalyzed regio- and enantioselective reductive cross-coupling of allylic acetates with tertiary/secondary alkyl bromides has been achieved, and Hantzsch ester is used as a homogeneous organic reductant. This straightforward protocol enables the stereoselective construction of C(sp3)-C(sp3) bonds under mild reaction conditions. Mechanistic studies suggest that this reaction involves radical pathways and a chiral Pd complex enables the control of the regio- and enantioselectivities.

Asymmetric Friedel-Crafts reaction of unsaturated carbonyl-tethered heteroarenes via vinylogous activation of Pd0-π-Lewis base catalysis.

The alkyne group can undergo facile transformations under palladium catalysis, such as hydropalladation, Wacker reaction, etc. Here we demonstrate that a chiral Pd0 complex can chemoselectively dihapto-coordinate to the alkyne moiety of 2-indolyl propiolates, and raise the Highest Occupied Molecular Orbital (HOMO)-energy ofthe deactivated heteroarenes via π-Lewis base catalysis. As a result, asymmetric C3-selective Friedel-Crafts addition to activated alkenes occurs, finally affording [3 + 2] or [3 + 4] annulation products with high enantioselectivity and exclusive E-selectivity. Moreover, this π-Lewis base vinylogous HOMO-activation strategy can be extended to remote Friedel-Crafts reaction of diverse five-membered heteroarenes tethered to a 2-enone or 2-acrylate motif with imines or 1-azadienes, and excellent enantiocontrol is generally achieved for the multifunctional adducts, which can be effectively converted to diverse frameworks with higher molecular complexity. In addition, NMR and density functional theory calculation studies are conducted to elucidate the catalytic mechanism.

Synthesis, characterisation and reactivity studies of chiral β-diketiminate-like supported aluminium Lewis acid complexes towards difficult Diels Alder cycloadditions.

Synthesis and characterisation of several chiral, oxazoline containing β-diketiminate type ligand supported-aluminium compounds are reported. Together with 1 equiv. of Na(BArCl4) (ArCl = 3,5-Cl2-C6H3), these chiral Lewis acid complexes, which possess an "achiral end" and "chiral end" have been successfully utilised as catalysts in asymmetric Diels-Alder reactions of 1,3-cyclohexadiene and several different chalcones. Systematic increase in the steric demand of the ligand's "achiral end" of these complexes resulted in enhanced enantioinduction for the cyclisation of 1,3-cyclohexadiene and chalcone. Further structural modifications of the "chiral end" clearly established that having a tert-butyl group connected to the stereogenic centre of the oxazoline fragment yielded the highest enantioselectivity value for the examined cyclisation. A substrate scope was then expanded using several different dienophiles (i.e. chalcones) generating an enantiomeric excess range of 24-68%.

Light-driven redox-relay deracemization of cyclic sulfonamides catalyzed by a chiral-at-metal iridium complex

A visible-light-driven redox-relay deracemization of cyclic sulfonamides catalyzed by a chiral-at-metal iridium complex as a single catalyst is described.

Synthesis of amphiphilic chiral salen complexes and their conformational manipulation at the air-water interface.

A series of amphiphilic salen complexes, [L1a,bM] and [L2a,bM], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic π-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [L1bM] (M = Ni, Cu, Zn), formed a monolayer at the air-water interface, while the monocationic [L1bCo(MeNH2)2](OTf) did not form a well-defined monolayer. The number of hydrophilic TEG chains also had an influence on the monolayerformation behavior; the tetra-TEG derivatives, [L1bNi] and [L2bNi], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [L1aNi] and [L2aNi]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [L1bNi] and [L2bNi], are more flexible than the corresponding bis-TEG analogues, [L1aNi] and [L2aNi], and that the phenanthrene derivatives [L1a,bNi] were more rigid than the corresponding naphthalene analogues, [L2a,bNi]. The Langmuir-Blodgett (LB) films of one of the complexes, [L1bNi], showed CD spectra slightly different from that in solution, which may originate from the unique anisotropic environment of the air-water interface. Thus, we demonstrated the possibility of controlling the chiroptical properties of metal complexes by mechanical compression.

Synthesis, stereochemical assignment, and enantioselective catalytic activity of late transition metal planar chiral complexes.

Planar chirality is an important form of molecular chirality that can be utilized to induce enantioselectivity when incorporated into transition metal catalysts. However, due to synthetic constraints, the use of late transition metal planar chiral complexes to conduct enantioselective transformations has been limited. Additionally, the published methods surrounding the stereochemical assignment of planar chiral compounds are sometimes conflicting, making proper assignment difficult. This review aims to provide clarity on the methods available to assign planar chirality and provide an overview on the synthesis and use of late transition metal planar chiral complexes as enantioselective catalysts.

Cage-based sensors for circular dichroism analysis.

Quantitative chiral sensing relying on circular dichroism (CD) is very important for determining the enantiomeric excess or concentration of small molecules without strong chromophores, because they form chiral complexes with sensors, yielding strong CD signals. Three-dimensional cages are promising platforms for chiral CD due to their stereochemical flexibility and their variety of cavity and external binding sites that can be used as chiral CD sensors. In this minireview, we discuss recent advances, future challenges, and opportunities in the quantitative sensing of small molecules in host-guest and peripheral complexes with cage sensors by chiral CD. We aim to provide inspiration for the rational design of cage sensors for quantitative chiral sensing of small molecules based on CD.

Chiral metallic anticancer drugs: A brief-review

Chiral metallic drugs are becoming the hottest point of discussion in the field of medicinal chemistry. As we know that more than 80% drugs are chiral in nature, and prescribed in the racemic form. The main problem with chiral drugs is the different biological activities of different enantiomers. This is because the human body has a chiral environment, as there is the presence of protein, carbohydrates, enzymes, and other chiral macromolecules. Hence, if a chiral anticancer drug is being prescribed to the patient in the racemic form, it means two or more drugs are being prescribed. Therefore, the chiral separation and analysis of chiral anticancer drugs are important for improving the quality of chiral drug medication. Many metal complexes are used as anticancer drugs, but the conditions become more critical if they have chirality or a chiral moiety, because of which they exist in two or more forms. Because of the presence of chirality or chiral moiety, the complex of metals is termed a chiral metallic complex. Of course, the enantioseparation of the chiral metallic complexes must be done before their prescription. Enantioseparation of the chiral metallic complex will not only provide a pharmaceutically active form to the patient but also reduce the side effects caused by the racemic mixture. Hence, the accessible article reviews the chiral metallic complexes having ruthenium, osmium, palladium, gold, silver, and platinum, etc. as central metal atoms. Besides, the future perspectives regarding the chiral metallic anticancer drugs and the role of their enantioseparation are also discussed.

Asymmetric Synthesis of Trifluoroethyl-Based, Chiral 3-Benzyloxy-1- and -2-Phenyl-quinazolinones of Biomedicinal Interest by Radical Type Cross-Coupling to Olefins

Several 2-substituted (H, Ph, and S-Me) and 1-substituted (H, Ph, and Bn), 3-hydroxy-1,3-quinazolin(di)ones were utilized for the first time as radical trapping agents in asymmetric 1,2-oxytrifluoromethylation of styrenes catalyzed by chiral vanadyl methoxide complexes bearing 3,5-disubstituted-N-salicylidene-t-leucinate templates. The effects of catalysts and solvents on the asymmetric induction were systematically examined. The best and complementary scenarios involved the use of vanadyl complexes V(O)-1 and V(O)-2, which bear 3-(2,5-dimethyl)phenyl-5-bromophenyl and 3-t-butyl-5-bromophenyl groups in an i-propanol solvent at ambient temperature. The corresponding (R)-cross-coupling products by V(O)-1 were obtained in 45-71% (for 2-substituted series) and 59-93% yields (for 1-substituted series) for p-/m-methylstyrenes and m-halo/CF3/CO2Me-styrenes in 38-63% ees (the best in 2-H case) and 60-84% ees (the best in 1-benzyl cases), respectively. The corresponding (S)-cross-coupling products by V(O)-2 were obtained in 28-55% (for 2-substituted series) and 45-72% yields (for 1-substituted series) for the same substrate class in 50-91% ees (85-91% ees in 2-phenyl cases) and 64-75% ees (up to 74-75% ees for each 1-H, Ph, and Bn cases), respectively. Theoretical calculations were carried out to explain the origin and extent of enantiocontrols. They both may serve as potential inhibitors of acetohydroxyacid synthase and epidermal growth factor receptor (EGFR) kinases.

Catalytic Enantioselective 6π Photocyclization of Acrylanilides.

Controlling absolute stereochemistry in catalytic photochemical reactions is generally challenging owing to high rates of background reactivity. Successful strategies broadly rely on selective excitation of the reaction substrate when associated with a chiral catalyst. Recent studies have demonstrated that chiral Lewis acid complexes can enable selective energy transfer from a photosensitizer to facilitate enantioselective triplet state reactions. Here, we apply this approach to the enantioselective catalysis of a 6π photocyclization through the design of an iridium photosensitizer optimized to undergo energy transfer to a reaction substrate only in the presence of a chiral Lewis acid complex. Among a group of iridium(III) sensitizers, enantioselectivity and yield closely correlate with photocatalyst triplet energy within a narrow window enabled by a modest reduction in substrate triplet energy upon binding a scandium/ligand complex. These results demonstrate that photocatalyst tuning offers a means to suppress background reactivity and improve enantioselectivity in photochemical reactions.

Circularly polarized luminescence of lanthanide complexes: From isolated individuals, discrete oligomers, to hierarchical assemblies

AbstractCircularly polarized luminescence (CPL) is the emission featured spatial orientation equivalent to circular dichroism. Owning to the unique magnetic dipole‐allowed but electric‐dipole‐forbidden 4f→ 4ftransitions, chiral lanthanide complexes have been recognized as ideal candidates to achieve unprecedented luminescence dissymmetry factor (glum). Their inherent electric‐dipole‐forbidden transition can be partly allowed, through the rational design of coordination ligands to lower the surrounding symmetry, further endowing them with relatively high quantum yields (QYs). Moreover, the blossom of supramolecular chemistry in lanthanide complexes allows for the construction of discrete oligomers and hierarchical assemblies, rendering them with enhanced chiroptical activities. Here, we deploy this review by summarizing recent advances in the chiroptical properties of lanthanide complexes, spanning multiple scales from isolated mononuclear individuals, discrete polynuclear oligomers, to infinite hierarchical assemblies. We first introduce the basic concept and several important parameters for the assessment of CPL activity. Then, isolated mononuclear lanthanide complexes coordinated with different types of antennas, along with the unique chirality transfer and induction in achiral lanthanide complexes were discussed. Next, the systematical discussion on chiroptical properties in discrete polynuclear lanthanide oligomers and infinite hierarchical lanthanide assemblies was presented. Meanwhile, the state‐of‐the‐art applications of CPL‐active lanthanide complexes were revisited. We end up this review with the conclusion and prospects, involving the rational design of ligands, directed and hierarchically precise self‐assembly, and extending applications of chiral lanthanide complexes.image

Optically active polymer charge‐transfer complex as a form of charge‐transfer chiralions

AbstractPoly(bis‐3,4‐ethylenedioxythiophene) was synthesized by electrochemical polymerization in cholesteric liquid crystal (Ch*LC) in the presence of 7,7,8,8‐tetracyanoquinodimethane (TCNQ). TCNQ acted as a supporting salt and a molecular dopant against the resultant poly(bisEDOT) to form a donor–acceptor‐type polymer charge‐transfer (CT) complex. Because a three‐dimensional (3D) one‐handed chiral structure was imprinted from the matrix chiral liquid crystal, the resulting polymer with TCNQ molecular doping displayed optical activity in the circular dichroism absorptions. This result indicates the production of optically active CT species comprising the polymer and the low‐molecular‐weight acceptor. The intermolecular chiral CT complex formed a one‐handed helical structure. This method is referred to as “CT complex liquid crystal (LC) electrochemical polymerization” to produce the helical polymer CT complex as atropisomers. Further, the optically active CT of the polymer is referred to as “CT chiralions.” The CT formation resulted from the molecular doping of TCNQ in the chiral polymer prepared in the helical LC matrix. As a result, molecular doping and molecular‐imprinting polymerization in the LC produced CT chiralions. The two‐step electrochemical polymerization in one‐pot polymerization developed in this study allowed the results to form a double‐layer fingerprint with a moiré‐like pattern.

Concise Synthesis of Functionalized Cyclopentadienyl Rhodium(III) Complexes via Suzuki–Miyaura Cross Coupling

AbstractHerein, we report a concise method for the preparation of functionalized cyclopentadienyl rhodium complexes via Suzuki–Miyaura cross-coupling. We successfully converted 2,3,4,5-tetramethylcyclopent-2-en-1-one into the corresponding cyclopentadienylboronic acid pinacol ester, which could be used as a coupling partner with aryl halides. Several functionalized cyclopentadienyl rhodium complexes were synthesized in two steps from readily available aryl bromides. We also demonstrated the rapid synthesis of a novel chiral cyclopentadienyl rhodium complex that could be used as a precatalyst for an asymmetric C–H functionalization reaction.

Palladium-Catalyzed Ring-Closing Aminoalkylative Amination of Unactivated Aminoenynes.

An efficient strategy for preventing the β-hydride elimination of alkylpalladium species by ligation of the palladium with adjacent amino-group was developed, which enabled a novel palladium-catalyzed ring-closing aminoalkylative amination of unactivated aminoenynes. The reaction is amenable to aminals, as well as aliphatic aldehydes with secondary amines, which provides straightforward access to structurally diverse exocyclic allenic amines bearing 5 to 12-membered N-heterocycles. With chiral phosphoramidite-ligated palladium complex as the catalyst, an enantioselective variant was achieved with up to 93 % ee. Simultaneously, synthetic transformations of the chiral products were also conducted to afford structurally unique spirodiamines including one pharmaceutically active molecule via axial-to-central chirality transfer.

Chirality Transfer from Polylactide to Achiral Fluorophore in Hierarchical Crystallization for Realizing Handedness-Tunable and Nonreciprocal Circularly Polarized Luminescence

Realizing chiral crystals from achiral compounds is an important but challenging issue. Here, a kebab-structured chiral supra-crystallization complex is constructed in accomplishing hierarchical chiral crystallization of achiral fluorescent compound (1,8-naphthalic anhydride, NA) employing synthetic chiral macromolecules (polylactide, PLA) as a template. Chirality transfer efficiently takes place from PLA to NA crystal during the co-crystallization process, endowing the initial achiral NA crystal with intense optical activity. More importantly, the obtained chiral supra-crystals show remarkable circularly polarized luminescence (CPL) properties, with the highest luminescence dissymmetric factor up to 5 × 10–2. Moreover, handedness-tunable and nonreciprocal CPL emissions are further realized in one single sample by simply flipping and rotating the supra-crystallization composite film, following a polarization mechanism. The work presents the first polymer-based hierarchical chiral supra-crystal with switchable CPL emissions and provides a simple and universal strategy for constructing nonreciprocal CPL materials.

Chiral zinc complexes with terpene derivatives of ethylenediamine: synthesis and biological activity

Chiral zinc complexes with terpene derivatives of ethylenediamine: synthesis and biological activity

3D-printed complex-shaped chiral organosilica porous monoliths that display enantioselectivity

We describe a new process for fabricating chiral organosilica 3D complex structures by combining digital light processing 3D printing with a sol-gel polycondensation process. The fabricated low-density objects have a high surface area with hierarchical porosity based on micropores resulting from the materials’ design, and on macropores in the structure resulting from the 3D printing design. Thus, several 3D objects having complex shapes were printed by the polycondensation of 3-acryloxypropyltrimethoxysilane (APTMS) and chiral silane monomers that were obtained by reacting (1R,2R)-cyclohexane-1,2-diamine or (1S,2S)-cyclohexane-1,2-diamine with (3-Isocyanatopropyl)triethoxysilane. The dual-function monomer APTMS enabled both localized photopolymerization and polycondensation. Printed gyroids, cubes, and disk-shaped chiral monoliths successfully revealed the enantioselective adsorption of tryptophan enantiomers. It was found that the macroscopic shape of the monolith affects the adsorption performance and its enantioselectivity. High enantioselectivity was obtained when the objects were formed from a chiral silane synthesized from (1R,2R)-cyclohexane-1,2-diamine: L-tryptophan was adsorbed ~10 fold higher than D-tryptophan. When the chiral object was fabricated using a chiral silane monomer prepared from (1S,2S)-cyclohexane-1,2-diamine, the enantioselectivity of the adsorption was reversed towards the D-tryptophan isomer. The new approach utilizes the 3D printing methodologies developed here for all-printed enantioselective separation columns; the printed macroporosity facilitates efficient flow, and the meso/microporous walls facilitate enantioselectivity. • A new UV-curable chiral hybrid material was synthesized. • 3D printing of complex structures of chiral organosilica material by stereolithography. • The printed objects exhibit highly selective enantiomeric adsorption. • The printed objects have high macro and micro surface areas. • The process enables the fabrication of new improved customized columns for enantiomeric separation.

Chiral-at-Metal Silver-Mediated Base Pairs: Metal-Centred Chirality versus DNA Helical Chirality.

When covalently incorporating ligands capable of forming chiral metal complexes into a DNA oligonucleotide duplex, an enantiospecific formation of metal-mediated base pairs is possible. We have been investigating the chirality of the silver-mediated base pair P-AgI -P (P, 1H-imidazo[4,5-f][1,10]phenanthroline) depending on the number of consecutive P : P pairs within a series of duplexes. Towards this end, both enantiomers of the nucleoside analogue 3-(1H-imidazo[4,5-f][1,10]phenanthrolin-1-yl)propane-1,2-diol comprising an acyclic backbone were introduced into DNA duplexes, resulting in diastereomeric metal-mediated base pairs. The same chiral-at-metal complex is formed inside the duplex for up to five neighbouring P-AgI -P pairs, irrespective of whether (S)-P or (R)-P is used. With six silver-mediated base pairs, the chirality of the metal complex is inverted for (S)-P but not for (R)-P. This indicates an intricate balance of what determines the configuration of the metal complex, the intrinsically preferred metal-centred chirality or the DNA helical chirality.