Synthesis of ionic liquids functionalized β-cyclodextrin-bonded chiral stationary phases and their applications in high-performance liquid chromatography

Four novel ILs functionalized β-cyclodextrins (β-CDs) were prepared by treating 6-tosyl-β-cyclodextrin with 1,2-dimethylimidazole or 1-amino-1,2,3-triazole, and bonded to silica gel to obtain chiral stationary phases (CSPs) to be used in high-performance liquid chromatography (HPLC). The separation performances of these CSPs were examined with 16 chiral aromatic alcohol derivatives and 2 racemic drugs in acetonitrile-based polar-organic mobile phase. Excellent enantioseparations were achieved for most of the analytes. The highest value of resolution factor calculated is 6.868. Comparison of the performance of 8a, 8b, 8c and 8d suggests that the positively charged imidazole group provides electrostatic interactions probably through strong H-bonding with the analytes, whereas the cationic triazole, which forms a weaker ion pair with its counter ion, is more capable of participating in ion-pairing interactions with acidic analytes. However, for compounds 12 and 13, which have larger molecular volumes than the other analytes, the interactions between analytes and both cationic imidazole and its counter ion of the selectors play an important role in the chiral resolution. Moreover, the high resolutions were found to depend on the properties of the cations and anions on the selectors in combination with the chiral recognition sites on the rim of the CD. The ionic strength in mobile phase affects the relative interactions between analytes and the chiral selector as well as between analytes and solvents.     

Preparation and Enantioseparation Property of Chiral Stationary Phases Based on Cellulose Tris(3,5-dimethylbenzoate)——A New Way to Prepare Polysaccharide-coating Type Chiral Stationary Phases

A new method to prepare polysaccharide-coating type chiral stationary phases （CSPs） was developed in this work. As a typical example, naked silica gel was coated by cellulose, which was then derivatized with 3,5-dimethylbenzoyl chloride to afford cellulose tris（3,5-dimethylbenzoate）-silica gel （CTDBS） complex. The silanols on CTDBS were end-capped with 3- aminopropyltriethoxysilane to obtain CSP 1. The amino groups on CSP 1 were further end-capped with 3,5-dimethylbenzoyl chloride to give CSP 2. The silanols on CTDBS were end-capped with methyltrimethoxysilane to yield CSP 3. CSPs 1-3 were characterized by FTIR, solid-state 13C-NMR and elemental analysis. The enantioseparation abilities of CSPs 1-3 were evaluated with structurally various chiral analytes. The enantioseparation results demonstrated that the end-capping moieties on CSPs 1 and 2 significantly affected enantioseparation. In addition, the effect of the structures of chiral analytes and end- capping moieties on the retention factors and the resolutions was discussed.     

Novel chiral solvating agents derived from natural amino acid: enantiodiscrimination for chiral α-arylalkylamines

Two benzo[de]isoquinoline 1,3-dione amino acids 1 and 2 were readily prepared, and their enantiodiscriminating ability were investigated by ¹H NMR spectroscopy. It was found that 1 exhibited an excellent chiral recognition ability toward chiral α-phenylethylamine and some of its derivatives, leading to clear baseline separation of the multiplet of the probe groups in two enantiomers. The stoichiometric ratio and association constants of some host-guest complexes were determined. The interactions between the hosts and guest 3 were further studied by intermolecular NOE experiment and ESI-MS.     

Chiral recognition of protected amino acids by means of fluorescent binary complex pyrene/heptakis-(6-amino)-(6-deoxy)-β-cyclodextrin

The ability of the binary complex pyrene (Py)/heptakis-(6-amino)-(6-deoxy)-β-cyclodextrin (am-β-CD) to act as a chiral selector was tested at two pH values (8.0 and 9.0). Phenylalanine (Phe), methionine (Met) and histidine (His) were used as chiral model molecules. The stability of ternary complexes Py/am-β-CD/amino acid was determined by means of spectrofluorimetric measurements. The data collected showed an increase in stability going from the binary to ternary complex and above all the possibility to use the binary complex as a chiral selector. Finally, data collected at two pH values showed that the binary complex is a better chiral selector when charged rather than in its neutral form.     

Structurally simple chiral thioureas as chiral solvating agents in the enantiodiscrimination of α-hydroxy and α-amino carboxylic acids

C₂-Symmetrical chiral thioureas (S,S)-1 and (S,S)-2 were prepared in good yield by the reaction of 2 equiv of inexpensive (S)-1-phenylethylamine, or the corresponding naphthyl analog, with 1 equiv of thiophosgene in the presence of excess triethylamine. The presence of asymmetric elements in (S,S)-1 and (S,S)-2, and their capacity to act as receptors for anionic species via hydrogen bonding were exploited in the development of ¹H NMR spectroscopic enantiodiscrimination of chiral carboxylic acids. In particular, the diastereomeric complexes derived from thioureas (S,S)-1 and (S,S)-2 with ammonium salts of the chiral acids gave rise to well separated signals of the α-hydrogens and simple integration provides the corresponding enantiomeric ratios. Furthermore, it was observed that C_α-H in the (R) enantiomers of the chiral α-hydroxy and α-amino carboxylic acids studied in this work consistently appears downfield relative to the same signals in the (S) enantiomers.     

Synthesis and characterization of a novel chiral azomethine diselenide

The first chiral diselenide 9 having an ortho-azomethine functional group has been synthesized by the reaction of bis(o-formylphenyl) diselenide with the chiral amine R(+)-(1-phenylethylamine). The chiral diselenide 9 was further characterized by derivatizing it into the corresponding selenenyl halides. The derivatives are characterized by single crystal X-ray diffraction studies. In the solid state, the bromide derivative 11 shows the strongest Se?N intramolecular interaction. The chiral azomethine diselenide 9 has been further reduced to afford the diselenide 13. The monoselenide analogues of 9 and 13 have also been synthesized.     

A new type of chiral porphyrin: Organopalladium porphyrins with chiral chelating diphosphine ligands

Peripherally palladated Ni(II) porphyrins have been prepared using enantiopure chiral chelating diphosphines as supporting ligands on the attached Pd(II) fragment. Both enantiomers of the following complexes have been obtained in good yields, using oxidative addition of the bromoporphyrin starting material 5-bromo-10,20-diphenylporphyrinatonickel(II) (NiDPPBr (1)) to the [Pd⁰L] complex generated in situ from Pd₂dba₃ and the chiral ligand L: [PdBr(NiDPP)(CHIRAPHOS)] (2a,b) [CHIRAPHOS = 2,3-bis(diphenylphosphino)butane], [PdBr(NiDPP)(Tol-BINAP)] (3a,b) [Tol-BINAP) = 2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl] and [PdBr(NiDPP)(diphos)] [diphos = 1,2-bis(methylphenylphosphino)benzene] (4a,b). The induced asymmetry in the porphyrin was readily detected by ¹H NMR and CD spectroscopy. The porphyrin chiroptical properties are strongly dependent upon the structure of the chiral ligand, such that a monosignate CD signal, and symmetric and asymmetric exciton couplets were observed for 4a, 2b, and 3a,b, respectively.     

Chiral organotin (IV) carboxylates complexes: Syntheses, characterization, and crystal structures with chiral (S)-(+)-6-methoxy-α-methyl-2-naphthaleneaceto acid ligand

Eight new organotin (IV) carboxylates, (R₃Sn)₄(nap)₄ (R = Me 1, n-Bu 2), [(R₃Sn) (nap)]_n (R = Ph 3, PhCH₂4), (R₂Sn) (nap)₂ (R = n-Bu 5, Ph 6, PhCH₂7) and {[R₂Sn(nap)]₂O}₂ (R = Me 8) (nap = (S)-(+)-6-methoxy-α-methyl-2-naphthaleneaceto anion) have been synthesized. All of the complexes have been characterized by elemental analysis, FT-IR, NMR (¹H, ¹³C and ¹¹⁹Sn) spectra. Among these complexes, complexes 1, 3, 5 and 8 were also characterized by X-ray crystallography diffraction analysis, and the data of X-ray crystallography diffraction indicated that complexes 1, 3 and 5 are new chiral organotin (IV) carboxylates complexes. The structural analyses show that complex 1 has a tetranuclear Sn₄O₈ macrocycle structure, complex 3 has a 1D spring-like chiral helical chain with a columnar channel, complex 5 possesses a dimer structure, and complex 8 has a supramolecular chainlike ladder structure through weak intermolecular non-covalent O^…O interactions.     

Chiral molecular polygons based on the Pt-alkynyl linkage: Self-assembly, characterization, and functionalization

Treatment of 2,2′-diacetoxy-1,1′-binaphthyl-6,6′-bis(ethyne), L-H₂, with one equiv of trans-Pt(PEt₃)₂Cl₂ led to a mixture of different sizes of chiral metallocycles [trans-(PEt₃)₂Pt(L)]_n (n = 3-8, 1-6). Each of the chiral molecular polygons 1-6 was purified by silica-gel column chromatography and characterized by ¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy, MS, IR, UV-Vis, and circular dichroism (CD) spectroscopies, size exclusion chromatography, and microanalysis. Chiral molecular square 2 was also characterized by single-crystal X-ray diffraction. The acetyl groups of 2 were readily deprotected under mild conditions to generate 2a which possesses exposed chiral dihydroxy functional groups. The dihydroxy groups were functionalized with n-octadecyl chains or Fréchet-type dendrons to generate dendritic molecules built on a chiral molecular square core. This work shows the potential of generating interesting functional supramolecular systems based on Pt-alkynyl chiral molecular polygons.     

Homohelicity-enriched zinc bilinone dimers with chiral aliphatic spacers. Synthesis and application to chiral induction of a nematic liquid crystal

The zinc bilinone (ZnBL) dimers 4 and 5 bearing chiral aliphatic spacers ((2S,4S)-2,4-pentanedioxy and (3S,5S)-2,6-dimethyl-3,5-heptanedioxy for 4 and 5, respectively) were newly prepared, and their conformational distribution was investigated. The ¹H NMR and circular dichroism spectra revealed that the present dimers predominantly adopted the homohelicity conformation (MM and PP for 4 and 5, respectively), although the reference monomers with the corresponding subunit structures exhibited poor helicity enrichment. The helical twisting powers of these ZnBL dimers for a nematic liquid crystal (N-(4-methoxybenzylidene)-4-butylaniline, MBBA) were also investigated. With the dimers doped into MBBA, highly efficient chiral nematic induction was achieved. Especially, the dimer 5 exhibited the β_M value of +1800 μm⁻¹.     

Chiral β-diketonate ligands of ‘pseudo planar chiral’ topology: enantioselective synthesis and transition metal complexation

A practical enantioselective synthesis of chiral β-diketonate ligands 1a-1d, which are of ‘pseudo planar-chiral’ topology, is described. Additionally, the first chiral bis(diketonates) 2a-2c, ligands of C₂-symmetry that are isoelectronic with respect to related salen ligand systems, have been prepared. Protocols for the metallation of ligands 1a-1d, 2b and 2c are reported.     

Rhodium complexes with chiral counterions: achiral catalysts in chiral matrices

The neutral complexes [Rh(I)(NBD)((1S)-10-camphorsulfonate)] (2) and [Rh(I)((R)-N-acetylphenylalanate)] (4) reacted with bis-(diphenylphosphino)ethane (dppe) to form the cationic Rh(I)(NBD)(dppe) complexes, 5 and 6, respectively, accompanied by their corresponding chiral counteranions. Analogously, 4 reacted with 4,4^′-dimethylbipyridine to yield complex 7. Complexes 5 and 6 disproportionated in aprotic solvents to form the corresponding bis-diphosphine complexes 8 and 9, respectively. 8 was characterized by an X-ray crystal structure analysis. In order to form achiral Rh(I) complexes bearing chiral countercations new sulfonated monophosphines 13-16 with chiral ammonium cations were synthesized. Tris-triphenylphosphinosulfonic acid (H₃TPPS, 11) was used to protonate chiral amines to yield chiral ammonium phosphines 14-16. Thallium-tris-triphenylphosphinosulfonate (Tl₃TPPS, 12) underwent metathesis with a chiral quartenary ammonium iodide to yield the proton free chiral ammonium phosphine 13. Phosphines 15 and 16 reacted with [Rh(NBD)₂]BF₄ to afford the highly charged chiral zwitterionic complexes [Rh(NBD)(TPPS)₂][(R)-N,N-dimethyl-1-(naphtyl)ethylammonium]₅ (17) and [Rh(NBD)(TPPS)₂][BF₄][(R)-N,N-dimethyl-phenethylammonium]₆ (18), respectively. Complexes 5, 6, and 18 were tested as precatalysts for the hydrogenation of de-hydro-N-acetylphenylalanine (19) and methyl-(Z)-(α)-acetoamidocinnamate (MAC, 20) under homogeneous and heterogeneous (silica-supported and self-supported) conditions. None of the reactions was enantioselective.     

Synthesis and dynamics of atropisomeric (S)-N-(α-phenylethyl)benzamides

The synthesis of atropisomeric 2-substituted benzamides 2a-e, 3a-e, and 4a-e, and characterization by X-ray structure analysis of 2d, 2e, 3c, 3e, 4c, and 4e are reported. Dynamic ¹H NMR spectroscopic studies of benzamides 2b-d, 3b-d, and 4b-d indicate that only two of the four possible rotamers are present in solution, with population ratios ranging between 1.5:1 and 4.1:1. The measured free energy of activation to interconversion of the rotamers ranged from 12.4 to 18.9 kcal mol⁻¹. Benzamides ArCON[(S)-phenethyl]₂ (2e, 3e, and 4e), exhibited atropisomer ratios between 1.7:1 and 1:1, and free energies of interconversion of the rotamers ranged from 11.5 to 17.6 kcal mol⁻¹. The highest rotation barriers were observed for the ortho-nitro derivatives 2a-e. Molecular calculations at the semiempirical level (PM3MM) gave free energies of activation for benzamides 2e and 3e of 23.6 and 12.4 kcal mol⁻¹, respectively, which are comparable to the experimental values.     

Bottom-up synthesis of three heterometallic coordination polymers with layered structures constructed from presynthesized [Sb2(tart)2] metalloligands

Self-assembly of presynthesized [Sb₂(tart)₂]²⁻ metalloligand as molecular building block with metal salts affords three unique heterometallic coordination polymers, namely, {[Ln(H₂O)₆][Sb₂(tart)₂]}Cl·5H₂O (Ln = La (1), Pr (2)) and {[Ba₂(H₂O)₇][Sb₂(tart)₂]₂}·4H₂O (3), (tart = tartaric acid). Their structures were determined by single crystal X-ray diffraction analyses and further characterized by elemental analyses, IR spectra, and TG analyses. Compounds 1 and 2 are isostructural and represent the first examples of lanthanide-organic open frameworks containing [Sb₂(tart)₂]²⁻ metalloligands. The structures of 1 and 2 contain left-handed and right-handed layer, each built up from the same-handed helical chains. Compound 3 consists of two kinds of arm-shaped chiral layers, which alternately stack in a heterochiral fashion to yield a racemic 3D hydrogen-bonded network with 1D channels along the a axis. To the best of our knowledge, compounds 1-3 are the first 2D chiral layer frameworks constructed from [Sb₂(tart)₂]²⁻ metalloligands and rare-earth or alkaline-earth metal ions.     

The investigation of stereogenic properties of cyclotriphosphazene derivatives with two different chiral centres

Hexachlorocyclotriphosphazene N₃P₃Cl₆ and gem-disubstituted cyclotriphosphazene derivatives N₃P₃Cl₄X₂ (X = Ph, PhS, PhNH) were reacted with N-methyl-1,3-propanediamine and 3-amino-1-propanol to give compounds (9a-12a, 9b-12b) which exist as cis and trans geometric isomers and are two different racemic isomers, respectively to describe the stereogenic properties of a series of chiral cyclotriphosphazene compounds with two different centres of chirality. The geometric isomers were separated by column chromatography on silica gel and analysed by elemental analysis, mass spectrometry, and ³¹P and ¹H NMR spectroscopies, and also the geometric forms (cis or trans) of 9b, 10a, 11a, 11b and 12a have been determined by the X-ray crystallography. The enantiomers of all racemic compounds have been analysed by the changes in ³¹P NMR spectra on addition of a Chiral Solvating Agent (CSA), (R)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. On the other hand, the racemic forms of chiral cyclotriphosphazene derivatives have been confirmed by contribution of chiral HPLC methods which have been developed for this study.     

Highly enantioselective synthesis and potential biological activity of chiral novel nucleoside analogues containing adenine and naturally phenol derivatives

This paper described an efficient synthetic strategy for chiral acyclic nucleoside analogues containing both the phenoxy components of some bioactive natural compounds and a heterocyclic base. The phenoxy components with adenine moiety were incorporated into the chiral acyclic nucleoside analogues through two key synthetic tactics. Chiron 5-(R)-menthyloxy-2(5H)-furanone 5 was obtained in good yield from the cheap starting material furfural via a valuable synthetic route. The asymmetric Michael addition of 5 with adenine and the subsequent reduction reaction afforded the key chiral intermediate, 2-(R)-(9′-adeninyl)-1,4-butanediol 8. The absolute configuration of 8 was established by X-ray crystallography. The intermolecular dehydration reaction between 2-(9′-adeninyl)-1,4-butanediol 8 and phenoxy components 9 on treatment with diethyl azodicarboxylate and triphenylphosphine was carried out to give the chiral acyclic nucleoside analogues 1a-1e. The regioselectivity of the reaction was established by NMR methods, especially through ¹³C NMR shifts and NOE effect observed in the target molecule 1c, as well as by HMBC/HMQC experiments. The target compounds were tested for inhibition of cytopathogenicity against different cancer cells and exhibited potential anticancer activity.     

Chiral supramolecular metal-organic architectures from dinuclear copper complexes

Two new chiral dinuclear Cu(II) complexes [Cu₂(μ-Cl)₂(HL¹)₂] · C₂H₅OH (1) and [Cu₂(μ-Cl)₂(HL²)₂] · CH₃OH (2), have been synthesized and structurally characterized, where the chiral ligands H₂L¹ and H₂L² are derived from the chiral amino alcohols (S)-(−)-2-amino-3-phenyl-1-propanol and (S)-(+)-2-phenylglycinol. Single-crystal X-ray crystallographic analyses revealed that in these complexes, the dominant hydrogen bonding property of metal bound chloride anion directs the self assembly of complex molecules through C–H···Cl hydrogen bonding interactions leading to the formation of intriguing hydrogen bonded metallo-supramolecular architectures in their respective crystal lattices. The supramolecular systems described here belong to the rare class of metal-organic architectures that are formed as a result of metal directed hydrogen bonding interactions among chiral complex molecules. Complexes 1 and 2 are further characterized by IR, ESR, UV–Vis and CD spectroscopy.     

Synthesis,crystal structure and reactivity of a new pentacoordinated chiral dioxomolybdenum(VI) complex

The novel tridentate chiral ligand 2,6-bis{[(1R,2S,4R)-2-hydroxy-1,3,3-trimethyl-bicyclo[2.2.1]hept-2-yl]}pyridine (1) was readily prepared by reaction of 2,6-dilithiopyridine with (R)-(−)-fenchone. Reaction of 1 with [MoO₂(acac)₂] resulted in the formation of the new metal-oxo five-coordinated complex [MoO₂(ONO)] (2) [ONO = (1 – 2H)]. The reactivity of 2 has been studied and the derivatives [MoS₂(ONO)] (3) and [MoO(O₂)(ONO)] (4) were prepared. The compounds 1–4 have been characterised by ¹H and ¹³C{¹H} NMR, microanalysis and IR spectroscopy. Furthermore, the molecular structures of 1 and 2 have been determined by single-crystal X-ray diffraction. The behaviour of 2 as catalyst in oxotransfer and in nucleophilic substitution of propargylic alcohols reactions has been tested.     

Comparison of enantiomer separation on two chiral stationary phases derived from (+)-18-crown-6-2,3,11,12-tetracarboxylic acid of the same chiral selector

Liquid chromatographic comparisons for enantiomer resolution of α-amino acids and chiral primary amino compounds were made using chiral stationary phases (CSPs) prepared by covalently bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA) of the same chiral selector. The resolution of all α-amino acids on CSP 1 developed in our group was found to be better than that on CSP 2 reported by Machida et al. All α-amino acids examined in this study were well enantioseparated on CSP 1 (α=1.22–2.47), while four analytes were not resolved or all the other analytes were poorly resolved on CSP 2 than on CSP 1. However, in resolving the primary amino compounds without a carbonyl group, CSP 1 was comparable with CSP 2. Although (+)-18-C-6-TA of the same chiral selector was used to prepare CSP 1 and CSP 2, this study showed that different connecting methods for the CSPs might influence their ability to resolve the analytes depending on their structures related to the chiral recognition mechanism.