Abstract

Abstract

Abstract

Abstract

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. Chirality 17:S227–S233, 2005. © 2005 Wiley-Liss, Inc.

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. Chirality 15:180–189, 2003. © 2003 Wiley-Liss, Inc.

We report a method to determine the absolute configuration of α-amino acids by exciton coupled circular dichroism (ECCD). Naturally occurring amino acids were successfully derivatized with 2-bromomethylquinoline. Complexation of these conformationally flexible ligands with Cu(II) salts yielded defined propeller-like structures. The direction of the twist (i.e., the relative orientation of the chromophores to each other) is governed by the asymmetric amino acid carbon center. The transition moments of the chromophores couple and yield a bisignate circular dichroism spectrum, the sign of which corresponds to the absolute configuration of the chiral center of the amino acid. Enantiomeric excess (e.e.) of amino acid derivatives is linearly related to the differential extinction coefficient Δε and can be assessed easily utilizing a standard curve. This efficient, sensitive technique requires low analyte concentrations, offers several advantages over established methods, and could be applied in medicinal, pharmaceutical, or chemical retail and manufacturing industry. Chirality 14:471–477, 2002. © 2002 Wiley-Liss, Inc.

The ligand in [Cu(6)CH₃CN](ClO₄)₂ adopts two conformations that may be described as “pseudo-enantiomers” in that they possess approximately mirror-image molecular helicity, although they differ in the orientation of a single methyl substituent. The two conformations differ in energy as judged by analogy to other compounds studied previously, computation of the relative free energies of formation of the isomers, and measurement of solution circular dichroism spectra. The solid-state structures of both single enantiomer and racemic forms of the complex were determined by X-ray crystallography. In the chiral complex, a quasi-racemate was observed with both pseudo-enantiomeric conformers present in the asymmetric unit. Packing forces induce a higher energy conformation in order to achieve higher apparent symmetry in the solid state. In contrast, the racemic complex only displayed a single conformation corresponding to the lower energy one of the two observed in the single enantiomer structure.

Tripodal tetradentate ligands may act as chemosensor molecules. Their ability to torque a nematic into a cholesteric phase increases upon complexation with copper ion. Moreover, changes in overall shape of the complexes induced by different metals and counter ions were transferred sensitively to the supramolecular level, observed by proportionate changes in the degree of twisting. Modification of the oxidation state of the metal center also gave large changes in twisting power; this suggests potential application in electrochemical molecular switches. The handedness of the induced cholesteric phase is related to the stereochemistry of the ligand: The small amount of chiral dopant needed for the LC technique (less than 2 nmol) suggests the possible determination of the absolute configuration of the parent primary amines of the ligands.

Ligandi tetradentati tripodali possono agire da chemosensori; la loro capacità di trasformare una fase nematica in colesterica aumenta a seguito della complessazione con ioni rame. Inoltre le variazioni di forma dei complessi indotte dai diversi metalli e controioni sono state apprezzabilmente trasferite a livello supramolecolare e osservate attraverso le corrispondenti variazioni del grado di twist. Modifiche dello stato di ossidazione del centro metallico hanno portato a grosse variazioni di potere torcente suggerendo possibili applicazioni in dispositivi molecolari elettrochimici. Il senso del colesterico indotto è in relazione con la stereochimica del ligando: la piccola quantità di soluto chirale necessaria per la tecnica a cristalli liquidi (meno di 2 nmol) ne suggerisce il possibile uso nella determinazione della configurazione assoluta delle ammine primarie da cui i ligandi sono ottenuti.

Intense chiroptical properties and efficient reversibility of the chemical redox cycle support the adaptability of the Cu^I/Cu^II complex system 1/2 for the development of a binary molecular device. Reduction proceeds with ascorbic acid, and oxidation with ammonium persulfate.

Intensive chiroptische Eigenschaften und hervorragende Reversibilität der chemischen Redoxcyclen empfehlen das Cu^I/Cu^II-Komplexpaar 1/2 als Basis für die Entwicklung eines molekularen Schalters. Reduziert wird mit Ascorbinsäure, oxidiert mit Ammoniumpersulfat.

The solution configuration of labile coordination complexes may be difficult to determine, even in cases in which the solid state structure is known. We have previously synthesized a series of chiral ligands which form pseudo-C₃-symmetric complexes with Zn^II and Cu^II salts that possess an available electrophilic coordination site. Molecular modeling of Zn^II complexes of the chiral ligand N,N-bis[(2-quinolyl)methyl]-1-(2-pyridyl)ethanamine (α-MeBQPA) showed that the spatial arrangement of the heterocyclic arms is controlled by a substituent on one methylene arm, resulting in the adoption of an enantiomeric conformation displaying a propeller-like asymmetry. In this paper we report the application of the exciton chirality method to the determination of the conformation of asymmetric metal-ligand complexes in solution. There is a good correlation between the predicted and the observed Cotton effects, demonstrating that the geometry in solution closely resembles that predicted by computational simulations and those obtained by X-ray crystallographic studies of metal complexes with racemic and enantiomerically pure ligands. The X-ray crystallographic structure of the first optically pure complex in this series is reported. Chirality 9:616–622, 1997. © 1997 Wiley-Liss, Inc.