Sterically Crowded Trianglimines—Synthesis,Structure, Solid‐State Self‐Assembly,and Unexpected Chiroptical Properties

The chiral, triangular‐shape hexaimine macrocycles (trianglimines), bearing bulky alkynyl or aryl substituents were synthesized and studied by means of experimental and theoretical methods. The macrocyclization reactions are driven by the extraordinary stability of the trianglimine ring and provided products with high yields. Electrostatic repulsion between imine nitrogen atoms and the substituents forced an anti conformation of the aromatic linkers. Although the DFT‐optimized structure of 7 is D₃ symmetrical, in the crystal, the macrocycle adopts a bowl‐like molecular shape. The macrocycle self‐assembles into tail‐to‐tail dimers by mutual interdigitation of aromatic moieties. In contrast, macrocycle 8 adopts a rigid pillararene‐like conformation. The nature of the substituent significantly affects the electronic properties of the linker. As a result, unexpectedly high exciton Cotton effects are observed in the electronic circular dichroism (ECD) spectra. The origin of these effects was subject of an in‐depth study.     

Chiral Phosphoric Acids in Metal–Organic Frameworks with Enhanced Acidity and Tunable Catalytic Selectivity

Chiral phosphoric acids are incorporated into indium‐based metal–organic frameworks (In‐MOFs) by sterically preventing them from coordination. This concept leads to the synthesis of three chiral porous 3D In‐MOFs with different network topologies constructed from three enantiopure 1,1′‐biphenol‐phosphoric acid derived tetracarboxylate linkers. More importantly, all the uncoordinated phosphoric acid groups are periodically aligned within the channels and display significantly enhanced acidity compared to the non‐immobilized acids. This facilitates the Brønsted acid catalysis of asymmetric condensation/amine addition and imine reduction. The enantioselectivities can be tuned (up to >99 % ee) by varying the substituents to achieve a nearly linear correlation with the concentrations of steric bulky groups in the MOFs. DFT calculations suggest that the framework provides a chiral confined microenvironment that dictates both selectivity and reactivity of chiral MOFs.     

Synthesis and chiral recognition of helical poly(phenylacetylene)s bearing l‐phenylglycinol and its phenylcarbamates as pendants

A series of novel stereoregular one‐handed helical poly(phenylacetylene) derivatives ( PPA‐1 and PPA‐1a～g ) bearing l ‐phenylglycinol and its phenylcarbamate residues as pendants was synthesized for use as chiral stationary phases (CSPs) for HPLC, and their chiral recognition abilities were evaluated using 13 racemates. The phenylcarbamate residues include an unsubstituted phenyl, three chloro‐substituted phenyls (3‐Cl, 4‐Cl, 3,5‐Cl₂), and three methyl‐substituted phenyls (3‐CH₃, 4‐CH₃, 3,5‐(CH₃)₂). The acidity of the phenylcarbamate N‐H proton and the hydrogen bonds formed between the N‐H groups of the phenylcarbamate residues were dependent on the type, position, and the number of substituents on the phenylcarbamate residues. The chiral recognition abilities of these polymers significantly depended on the dynamic helical conformation of the main chain with more or less regularly arranged pendants. The chiral recognition abilities seem to be improved by the introduction of substituents on the phenylcarbamate residues, and PPA‐1d bearing the more acidic N‐H groups due to the 3,5‐dichloro substituents, exhibited a higher chiral recognition than the others. PPA‐1d showed an efficient chiral recognition for some racemates, and baseline separation was possible for racemates 5 , 11 , 12 , and 15 . © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 809–821     

Double-stranded ladderphanes with C2-symmetric planar chiral ferrocene linkers

Eight ladderphanes with C₂-symmetric planar chiral ferrocene linkers are synthesized by ring opening metathesis polymerization of bisnorbornene monomers using Grubbs-I catalyst. The aminobenzoate in both monomers and polymers shows absorption maximum around 320 nm. Both monomers and polymers are Circular dichroism (CD) active. Little enhancements of CD profiles around 320 nm are observed for ladderphanes having chiral chloro- or phenyl-substituted ferrocene linkers. However, ladderphanes with a phenyl substituent on the cyclopendienyl ring exhibits enhancement of CD curves aound 240–300 nm. The congested phenyl moieties in adjacent linkers in this polymer might be well oriented such that interactions between these aromatic substituents on different monomeric units might provoke the enhancement of the CD curves in this region. When the methyl-substituted cyclopendienyl ligand is used for chiral ferrocene linkers, the ladderphanes exhibit two-fold enhancement of CD spectrum around 320 nm. This enhancement is further increased when the cyclopentadienyl ligand contains an additional phenyl substituent, owing to exciton coupling between aminobenzoate moieties in adjacent monomeric units. Moreover, the intensity of the CD curves in the region of 240–300 nm is significantly increased. These results suggest that the later polymer may adopt a posible helical structure. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2999–3010     

Mechanochemical Encapsulation of Fullerenes in Peptidic Containers Prepared by Dynamic Chiral Self‐Sorting and Self‐Assembly

Molecular capsules composed of amino acid or peptide derivatives connected to resorcin[4]arene scaffolds through acylhydrazone linkers have been synthesized using dynamic covalent chemistry (DCC) and hydrogen‐bond‐based self‐assembly. The dynamic character of the linkers and the preference of the peptides towards self‐assembly into β‐barrel‐type motifs lead to the spontaneous amplification of formation of homochiral capsules from mixtures of different substrates. The capsules have cavities of around 800 ų and exhibit good kinetic stability. Although they retain their dynamic character, which allows processes such as chiral self‐sorting and chiral self‐assembly to operate with high fidelity, guest complexation is hindered in solution. However, the quantitative complexation of even very large guests, such as fullerene C₆₀ or C₇₀, is possible through the utilization of reversible covalent bonds or the application of mechanochemical methods. The NMR spectra show the influence of the chiral environment on the symmetry of the fullerene molecules, which results in the differentiation of diastereotopic carbon atoms for C₇₀, and the X‐ray structures provide unique information on the modes of peptide–fullerene interactions.     

Synthesis of ortho‐Phenylenebis(guanidine) Derivatives with Potential Chirality

We report the synthesis and potential chirality of ortho‐phenylenebisguanidines (BGs) with substituents at C(3) and C(6). Guanidinylation of 3,6‐disubstituted benzene‐1,2‐diamines with 2‐chloro‐4,5‐dihydro‐1,3‐dimethyl‐1H‐imidazolium chloride gave the corresponding BGs. X‐Ray crystallography showed that the two guanidine moieties occupy different faces of the benzene ring, creating potential chirality, although optical resolution of ^tBu‐substituted BG by chiral HPLC failed. However, a methylated acyclic bisguanidinium salt (BGms) was obtained as a chiral crystal with a space group of P2₁2₁2₁.     

Isochroman derivatives and their tendency to crystallize in chiral space groups

In methyl [5‐methoxy‐4‐(4‐methoxy­phenyl)­isochroman‐3‐yl]­acetate, C₂₀H₂₂O₅, (I), and methyl [4‐(2,5‐di­methoxy­phenyl)‐8‐methoxy­isochroman‐3‐yl]­acetate, C₂₁H₂₄O₆, (II), the heterocyclic rings adopt half‐chair conformations. The substituents at the 3‐ and 4‐positions are in a trans configuration in both (I) and (II), being in an axial conformation in (I) and in an equatorial conformation in (II). The crystal structure of (I) is stabilized by weak C—H⋯O hydrogen bonding, leading to the formation of an infinite three‐dimensional network. Compound (II) crystallizes in a chiral space group. This feature, which was also found in previously investigated isochroman derivatives, is related to the arrangement of substituents attached to the isochroman moiety.     

Control of Conformation and Chirality of Nonplanar π‐Conjugated Diporphyrins Using Substituents and Axial Ligands

Nonplanar conformations of pyrazine‐fused Zn^II diporphyrins could be controlled by the choice of the meso‐aryl substituents and an axial ligand on the central metals. Zn^II diporphyrins bearing sterically demanding meso‐aryl groups with ortho‐substituents led to a twisted chiral D₂ conformation, while an achiral C_2h form was preferred in the case of aryl groups without ortho‐substituents. Helical chirality induction on Zn^II diporphyrins in the twisted conformation was achieved by controlling their handedness of the molecular twist through coordination of optically active 1‐phenethylamine.     

Synthesis and Properties of 2,3‐Dihydro‐1H‐corannuleno[2,3‐cd]pyridine (=2,3‐Dihydro‐1H‐dibenzo[1,10 : 6,7]fluorantheno[3,4‐cd]pyridine) Derivatives: Heterocyclic peri‐Anellated Corannulenes

The anellation of a 6‐membered ring to the 2,3‐position of corannulene (=dibenzo[ghi,mno]fluoranthene; 1 ) leads to curved aromatic compounds with a significantly higher bowl‐inversion barrier than corannulene (see Fig. 1). If the bridge is −CH₂−NR−CH₂−, a variety of linkers can be introduced at the N(2) atom, and the corresponding curved aromatics act as versatile building blocks for larger structures (see Scheme). The locked bowl, in combination with an amide bond (see 9 and 10 ), gives rise to corannulene derivatives with chiral ground‐state conformations, which possess the ability to adapt to their chiral environment by shifting their enantiomer equilibrium slightly in favor of one enantiomeric conformer. Rim annulation of corannulene seems to display a significantly lower electron‐withdrawing effect than facial anellation on [5,6]fullerene‐C₆₀‐I_h, as determined by an investigation of the basicity at the N‐atom of CH₂−NR−CH₂ (see 4 vs. 15 in Fig. 2).     

Copper‐Complex‐Catalyzed Asymmetric Aerobic Oxidative Cross‐Coupling of 2‐Naphthols: Enantioselective Synthesis of 3,3′‐Substituted C1‐Symmetric BINOLs

A novel chiral 1,5‐N,N‐bidentate ligand based on a spirocyclic pyrrolidine oxazoline backbone was designed and prepared, and it coordinates CuBr in situ to form an unprecedented catalyst that enables efficient oxidative cross‐coupling of 2‐naphthols. Air serves as an external oxidant and generates a series of C₁‐symmetric chiral BINOL derivatives with high enantioselectivity (up to 99 % ee) and good yield (up to 87 %). This approach is tolerant of a broader substrates scope, particularly substrates bearing various 3‐ and 3′‐substituents. A preliminary investigation using one of the obtained C₁‐symmetric BINOL products was used as an organocatalyst, exhibiting better enantioselectivity than the previously reported organocatalyst, for the asymmetric α‐alkylation of amino esters.     

Synthesis,Structural Characterisation and Stereochemical Investigation of Chiral Sulfur‐Functionalised N‐Heterocyclic Carbene Complexes of Palladium and Platinum

Palladium and platinum complexes containing a sulfur‐functionalised N‐heterocyclic carbene (S‐NHC) chelate ligand have been synthesised. The absolute conformations of these novel organometallic S‐NHC chelates were determined by X‐ray structural analyses and solution‐phase 2D ¹H–¹H ROESY NMR spectroscopy. The structural studies revealed that the phenyl substituents on the stereogenic carbon atoms invariably take up the axial positions on the Pd‐C‐S coordination plane to afford a skewed five‐membered ring structure. All of the chiral complexes are structurally rigid and stereochemically locked in a chiral ring conformation that is either (R_s,S,R)‐λ or (S_s,R,R)‐δ in both the solid state and solution.     

Reversal of Regioselectivity of Nitrone Cycloadditions by Lewis Acids

The regio‐ and stereoselectivity of cycloadditions of the nitrone 1a and the chiral, sugar‐derived nitrones 13a and 13b with 3‐(prop‐2‐enoyl)‐1,3‐oxazolidin‐2‐one ( 2 ) depends on the nature of the Lewis acid catalyst used. Addition of Lewis acid reverses the regioselectivity of the cycloaddition, and improves the anti‐diastereoselectivity in the case of chiral nitrones. The sterically favored isoxazolidin‐5‐yl‐substituted adducts 3, 4 , and 14 – 17 are produced as the major products in the absence of Lewis acid, while the electronically favored regioisomers with isoxazolidin‐4‐yl substituents ( 5, 6 , and 18 – 21 , respectively) are obtained as major products in the [Ti(OⁱPr)₂Cl₂] catalyzed reactions. The reactions of nitrone 13b with 2 in the presence of other Lewis acids such as ZnCl₂, ZnBr₂, ZnI₂ and MgI₂/I₂ gave both regioisomeric pairs of the diastereoisomers, favoring the 4‐substituted congeners. The diastereoisomeric isoxazolidines 3a – 6a were reduced with NaBH₄ in THF/H₂O with subsequent desilylation to yield the separable diols 9 – 12 . Reduction of the diastereoisomeric isoxazolidines 19a and 18a afforded the chiral alcohols 23 and 22 , the latter of which was analyzed by X‐ray crystallography.     

Highly Enantiomerically Enriched Planar Chiral Naphthalene Tricarbonylchromium Complexes

Lithiation/electrophile trapping reactions were carried out with the highly enantiomerically enriched complex [Cr(5‐bromonaphthalene)(CO)₃]. Electrophile quenching with ClPPh₂, PhCHO, and (Me₃SiO)₂ afforded the enantiomerically enriched (>97 % ee) planar chiral 5‐substituted naphthalene complexes with PPh₂, CH(Ph)OH, and OH substituents, respectively. Very mild Pd‐catalyzed Suzuki–Miyaura cross‐coupling reactions were developed and applied to the highly labile [Cr(5‐bromonaphthalene)(CO)₃] to give nine new planar chiral aryl‐, heteroaryl‐, alkynyl‐, and alkenylnaphthalene chromium complexes with high enantiomeric purity. The efficient ambient‐temperature coupling reactions with borinates prepared in situ were also applied to a number of chlorobenzene complexes and to aryl and vinyl halides.     

C1‐Symmetric Bicyclo[2.2.2]octa‐2,5‐diene (bod*) Ligands in Rhodium‐Catalyzed Asymmetric 1,4‐Addition of Arylboronic Acids to Enones and 1,2‐Addition to N‐[(4‐Nitrophenyl)sulfonyl]imines

A set of ten C₁‐symmetric chiral bicyclo[2.2.2]octa‐2,5‐dienes (bod*) 2 (Fig. 1) were tested as ligands in Rh‐catalyzed arylation reactions. The 1,4‐addition of arylboronic acids to cyclohex‐2‐en‐1‐one, cyclopent‐2‐en‐1‐one, and tert‐butyl cinnamate proceeded smoothly with excellent enantioselectivities (up to 99% ee; Tables 1–3). The challenging 1,2‐addition of triphenylboroxine to N‐[(4‐nitrophenyl)sulfonyl]imines yielded the product in high yield and in good enantioselectivity (up to 92% ee; Table 4). Generally, the use of C₁‐symmetric chiral bod* ligands bearing bulky substituents resulted in lower enantioselectivities, whereas several electron‐poor and electron‐rich bod* ligands gave higher enantioselectivities than the benchmark ligands reported in literature.     

Phase‐Transfer‐Catalyzed Asymmetric Synthesis of Axially Chiral Anilides

Catalytic asymmetric synthesis of axially chiral o‐iodoanilides and o‐tert‐butylanilides as useful chiral building blocks was achieved by means of binaphthyl‐modified chiral quaternary ammonium‐salt‐catalyzed N‐alkylations under phase‐transfer conditions. The synthetic utility of axially chiral products was demonstrated in various transformations. For example, axially chiral N‐allyl‐o‐iodoanilide was transformed to 3‐methylindoline by means of radical cyclization with high chirality transfer from axial chirality to C‐centered chirality. Furthermore, stereochemical information on axial chirality in o‐tert‐butylanilides could be used as a template to control the stereochemistry of subsequent transformations. The transition‐state structure of the present phase‐transfer reaction was discussed on the basis of the X‐ray crystal structure of ammonium anilide, which was prepared from binaphthyl‐modified chiral ammonium bromide and o‐iodoanilide. The chiral tetraalkylammonium bromide as a phase‐transfer catalyst recognized the steric difference between the ortho substituents on anilide to obtain high enantioselectivity. The size and structural effects of the ortho substituents on anilide were investigated, and a wide variety of axially chiral anilides that possess various functional groups could be synthesized with high enantioselectivities. This method is the only general way to access a variety of axially chiral anilides in a highly enantioselective fashion reported to date.     

Host–guest complexes between cryptophane‐C and chloromethanes revisited

Cryptophane‐C is composed of two nonequivalent cyclotribenzylene caps, one of which contains methoxy group substituents on the phenyl rings. The two caps are connected by three OCH₂CH₂O linkers in an anti arrangement. Host–guest complexes of cryptophane‐C with dichloromethane and chloroform in solution were investigated in detail by nuclear magnetic resonance techniques and density functional theory (DFT) calculations. Variable temperature proton and carbon‐13 spectra show a variety of dynamic processes, such as guest exchange and host conformational transitions. The guest exchange was studied quantitatively by exchange spectroscopy measurements or by line‐shape analysis. The conformational preferences of the guest‐containing host were interpreted through cross‐relaxation measurements, providing evidence of the gauche+2 and gauche?2 conformations of the linkers. In addition, the mobility of the chloroform guest inside the cavity was studied by carbon‐13 relaxation experiments. Combining different types of evidence led to a detailed picture of molecular recognition, interpreted in terms of conformational selection. Copyright © 2012 John Wiley & Sons, Ltd.     

Optically active derivatives of terephthalic acid: four crystal structures from two powder patterns

A novel important class of nanoporous crystalline solids, metal–organic frameworks (MOFs), composed of organic ligands (linkers) and metal ions, is now considered as a platform for the development of various functional hybrid materials. In order to design new MOF‐based asymmetric catalysts, two terephthalic acid derivatives, namely 2‐{[1‐(1‐tert‐butoxycarbonyl)‐l ‐prolyl]amino}terephthalic acid, C₁₈H₂₂N₂O₇, (1), and 2‐(l ‐prolylamino)terephthalic acid, C₁₃H₁₄N₂O₅, (2), which could find potential applications as chiral linkers for the construction of enantioselective MOFs, were synthesized and their powder samples were measured at synchrotron station ID22 (ESRF). Each sample contained two unknown crystalline phases, so four new crystal structures were determined, namely, the 2.24‐hydrate of (1), (1a) (space group C222₁), and the 2.08‐hydrate of (1), (1b) (P222₁), which are crystallohydrates, and two polymorphs of (2), i.e. (2a) (C222₁) and (2b) (P2₁2₁2₁), and were validated with DFT‐d (dispersion‐corrected density functional theory) optimizations.     

Synthesis of Molecular Frameworks Containing Two Distinct Heterocycles Connected in a Single Molecule with Enhanced Three‐Dimensional Shape Diversity

Herein, we report the synthesis of fused‐triazole scaffolds that are connected by pyrimidines, pyrazoles, or pyrazolopyrimidines through carbohydrate‐derived stereodivergent linkers. Pyrimidine‐, pyrazole‐, or pyrazolopyrimidine‐based carbohybrids were constructed through condensations of the key intermediates, 2‐C‐formyl glycals, with various dinucleophiles. Fused‐triazole scaffolds were obtained through intramolecular 1,3‐dipolar cycloadditions after selective functionalization of the carbohybrid polyol moieties with azide and alkyne functionalities using S_N2‐type alkylations or Mitsunobu reactions. Overall, this synthetic method affords two distinct privileged substructures in a single molecule, connected by stereodivergent diol linkers derived from abundant natural chiral sources, namely, carbohydrates. The resulting vicinal diols in the linker were further modified to achieve unique connectivities between the two privileged structures for maximized three‐dimensional shape diversity, which we called the linker diversification strategy.     

Modular amino acid amide chiral ligands for enantioselective addition of diethylzinc to aromatic aldehydes

Enantioselective addition of diethylzinc to a series of aromatic aldehydes was developed using a modular amino acid amide chiral ligand (2S)‐3‐phenyl‐N‐((R)‐1‐phenyl‐ethyl)‐2‐(tosylamino)propanamide without using titanium complex. The catalytic system employing 10 mol% of 1g was found to promote the addition of diethylzinc (ZnEt₂) to a wide range of aromatic aldehydes with electron‐donating and electron‐withdrawing substituents, giving up to 82% ee of the corresponding secondary alcohol under mild conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Me‐BIPAM for the Synthesis of Optically Active 3‐Aryl‐3‐hydroxy‐2‐oxindoles by Ruthenium‐catalyzed Addition of Arylboronic Acids to Isatins

A chiral O‐linked C₂‐symmetric bidentate phosphoramidite (Me‐BIPAM) was found to be efficient for the ruthenium‐catalyzed addition of arylboronic acids to isatins. Asymmetric synthesis of 3‐aryl‐3‐hydroxy‐2‐oxindoles by 1,2‐addition of arylboronic acids to isatins was carried out in the presence of [RuCl₂(PPh₃)₃]/(R,R)‐Me‐BIPAM and KF, resulting in an enantioselectivity as high as 90 % ee. It was found that the reaction with N‐protected isatins proceeds with high yields and good enantioselectivities. The best protective groups on the nitrogen atom were different depending on the substituents on the aromatic ring. The use of a N‐benzyl group resulted in excellent enantioselectivities in many substrates compared with other groups.