Specific recognition of chiral amino alcohols via lanthanide coordination chemistry: structural optimization of lanthanide tris(beta-diketonates) toward effective circular dichroism/fluorescence probing

Imine-based chiroptical sensing for analysis of chiral amines: from method design to synthetic application

A robust chiroptical method for fast enantiopurity determination of chiral amines utilizing HPLC-CD, applied to monitoring crude transamination reactions.

Chiroptical spectra of tetrakis (+)-3-heptafluorobutylrylcamphorate Ln(III) complexes with an encapsulated alkali metal ion: solution structures as revealed by chiroptical spectra

The preparation of tetrakis((+)-hfbc) lanthanide(III) complexes with an encapsulated alkali metal and ammonium ions M[Ln((+)-hfbc)(4)] (hereafter abbreviated as M-Ln : (+)-hfbc, (+)-heptafluorobutyrylcamphorate; M, ammonium or benzyl ammonium ions as well as alkali metal ions) was reported and discussed. The electronic circular dichroism (CD) spectra in the intraligand π-π* transition of M-Ln were examined in view of the solvent effect. Here, the concentration, alkali metal, and ammonium ion dependences are compared with the solid CD, (5)D(0)←(7)F(0) (Eu(III)) excitation spectra, circularly polarized luminescence, and vibrational circular dichroism. It has been revealed that the dodecahedral eight coordinate DD-8-M-Ln complexes in crystals are equilibrated between the diastereoselectively formed square antiprism eight coordinate SAPR-8-M-Ln and [Ln((+)-hfbc)(3)] in EtOH and CH(3) CN solutions or between the SAPR-8-M-Ln and DD-D(2d) (mmmm)-8-M-Ln complexes in CHCl(3) solution. The observed CD couplets are found to reflect the exciton CD couplets which are useful to determine the four-bladed SAPR-(llll) absolute configuration around the lanthanide(III) ion.

Luminescent chiral lanthanide(III) complexes as potential molecular probes

This perspective gives an introduction into the design of luminescent lanthanide(iii)-containing complexes possessing chiral properties and used to probe biological materials. The first part briefly describes general principles, focusing on the optical aspect (i.e. lanthanide luminescence, sensitization processes) of the most emissive trivalent lanthanide ions, europium and terbium, incorporated into molecular luminescent edifices. This is followed by a short discussion on the importance of chirality in the biological and pharmaceutical fields. The second part is devoted to the assessment of the chiroptical spectroscopic tools available (typically circular dichroism and circularly polarized luminescence) and the strategies used to introduce a chiral feature into luminescent lanthanide(iii) complexes (chiral structure resulting from a chiral arrangement of the ligand molecules surrounding the luminescent center or presence of chiral centers in the ligand molecules). Finally, the last part illustrates these fundamental principles with recent selected examples of such chiral luminescent lanthanide-based compounds used as potential probes of biomolecular substrates.

Racemic atropisomeric N,N-chelate ligands for recognizing chiral carboxylates via Zn(II) coordination: structure, fluorescence, and circular dichroism

Two racemic atropisomeric N,N'-chelate ligands, bis{3,3'-[N-Ph-2-(2'-py)indolyl]} (1) and bis{3,3'-N-4-[N-2-(2'-py)indolyl]phenyl-2-(2'-py)indolyl} (2), have been found to be able to distinguish the enantiomers of Zn((R)-BrMeBu)2 and Zn((S)-BrMeBu)2 where BrMeBu = O2CCH(Br)CHMe2, with a distinct and intense CD spectral response at approximately the 10 microM concentration range. Computational studies established that the (R)-1-Zn((R)-BrMeBu)2 or (S)-1-Zn((S)-BrMeBu)2 diastereomer is more stable than (R)-1-Zn((S)-BrMeBu)2 or (S)-1-Zn((R)-BrMeBu)2. In addition, computational studies showed that the CD spectra of (S)-1-Zn((S)-BrMeBu)2 and (S)-1-Zn((R)-BrMeBu)2 are similar. (1)H NMR spectra confirmed that these two diastereomers exist in solution in about a 2:1 ratio for both complexes of 1 and 2. The distinct CD response of the racemic ligands 1 and 2 toward the chiral zinc(II) carboxylate is therefore attributed to the preferential formation of one diastereomer. The binding modes of the zinc(II) salt with ligands 1 and 2 were established by the crystal structures of the model compounds 1-Zn(tfa)2 and 2-Zn(tfa)2 (tfa = CF3CO2(-)), where the Zn(II) ion is chelated by the two central pyridyl groups in the ligand. Fluorescent titration experiments with various zinc(II) salts showed that the fluorescent spectrum of the atropisomeric ligand displays an anion-dependent change. The zinc(II) binding strength to the N,N'-chelate site of the atropisomeric ligand has been found to play a key role in the selective recognition of different chiral zinc(II) carboxylate derivatives by the racemic atropisomeric ligands.

Time-resolved fluorescent chirality sensing and imaging of malate in aqueous solution

Chiral discrimination of malates in aqueous solutions at near-neutral pH is achieved through fluorescence measurement and imaging using the europium-tetracycline complex (EuTc) as a fluorescent probe. The method is based on the significantly different fluorescence properties of the ternary complexes (Eu-Tc-malate) formed between EuTc and the enantiomeric malates. The enantiomeric excess (ee) of chiral malates can be quantified by both steady-state and time-resolved fluorescence, using either a conventional fluorescence microplate reader or fluorescence imaging. It offers a facile and sensitive method for high-throughput chiral discrimination.

Rigidified tripodal chiral ligands in the asymmetric recognition of amino compounds

Chiral rigidified piperidine and quinuclidine analogues of tris(2-pyridylmethyl)amine (TPA) derivatives were examined for asymmetric recognition of amino compounds by cyclic voltammetry and fluorescence. A Cu(II) complex of a piperidine analogue discriminated the enantiomers of some chiral amines and amino alcohols, giving differences in electrochemical potential for diastereomeric complexes. Protonated piperidine and quinuclidine analogues were able to differentiate the two enantiomers of certain amino alcohols by fluorescence spectroscopy. The quinuclidine analogue gave a 3-fold difference in response to the two enantiomers of phenyglycinol.

Lipophilic Lanthanide Tris(β-diketonate) Complexes as an Ionophore for Cl- Anion-Selective Electrodes

A series of novel anion-selective electrodes were developed by incorporating lipophilic lanthanide tris(beta-diketonates) into plasticized poly(vinyl chloride) membranes. The new electrodes exhibited high selectivity toward Cl(-) anion in the concentration range of Cl(-) anion between 1.0 x 10(-5) and 1.0 x 10(-1) mol/L with near-Nernstian slopes and practically low detection limits. They offered non-Hofmeister anion selectivity, and interestingly discriminated Cl(-) anion from NO(3)(-), ClO(4)(-), and other anions. Since the employed lanthanide tris(beta-diketonates) were confirmed to form 1:1 highly coordinated complexes with Cl(-) anion, the observed high selectivity for Cl(-) anion was attributed to the characteristics of lanthanide coordination chemistry. All the prepared sensors worked well at neutral pH with quite short response time, <30 s, and could be used for longer than four months without any significant divergence in performance.

Derivatization, complexation, and absolute configurational assignment of chiral primary amines: application of exciton-coupled circular dichroism

We report here a sensitive method for the determination of the absolute configurations of primary amines using exciton-coupled circular dichroism (ECCD). The method works on a microgram scale by derivatization of chiral amines with quinoline chromophores. Complexation of the chiral ligands with metal ion fixes the geometry of the chromophores, resulting in a twist that is governed by the asymmetric carbon configuration and steric environment of the amine. The absolute configurations of the primary amines can be interpreted from the couplets of the ECCD spectra of the derivatized complexes. Crystal structures, 2D NMR studies, and semiempirical calculations provide structural evidence for our model.

Chiral information transfer by solid-solid interaction: application for absolute configuration assignment

Host-guest complexes of calix[4]resorcarene with chiral molecules were efficiently formed by solid-solid grinding and exhibited CD Cotton effects reflecting the absolute configuration of the guest.

Anion receptor functions of lanthanide tris(β-diketonate) complexes: naked eye detection and ion-selective electrode determination of Cl−anion

Lanthanide tris(fluorinated beta-diketonates) acted as effective receptors of Cl- anion in luminescence sensing and ion-selective electrode systems via highly coordinated complexation.

Asymmetric autocatalysis and its application to chiral discrimination

Chiral pyrimidyl, quinolyl, and pyridyl alkanols act as asymmetric autocatalysts with significant amplification of enantiomeric excess (ee) in the enantioselective addition of diisopropylzinc to pyrimidine-5-, quinoline-3-, and pyridine-3-carbaldehydes, respectively. 2-Alkynyl-5-pyrimidyl alkanol with as low as 0.6% ee automultiplies during the consecutive asymmetric autocatalysis with increasing ee to as high as >99.5%. Asymmetric autocatalysis is applied to chiral discrimination of organic compounds. In the presence of methyl mandelate or 2-butanol with very low ee's (0.05-0.1%) as chiral initiators, the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc affords pyrimidyl alkanol with higher ee's with the correlated absolute configurations to those of the chiral initiators. Chirality of amino acids (such as leucine) and helicenes with very low ee's are also discriminated by asymmetric autocatalysis, affording pyrimidyl alkanol with very high ee's. Asymmetric autocatalysis also discriminates the chirality of primary alcohols-alpha-d, monosubstituted [2.2]paracyclophanes and octahedral cobalt complex with achiral ligands of which the chirality is due to the topology of coordination of the achiral ligand. Even the chirality of inorganic crystals such as quartz and sodium chlorate is discriminated by asymmetric autocatalysis of pyrimidyl alkanol. Thus, asymmetric autocatalysis provides a unique method for the discrimination of chiral compounds and crystals.