1,721,218 research outputs found
Solution structure of chiral lanthanide complexes
No full textThe determination of solution structure of small to medium size chiral lanthanide complexes through paramagnetic NMR and circular dichroism is briefly reviewed. The main focus is on ytterbium as the rare earth, because of its negligible contact contribution to the hyperfine shift and of its intense CD spectrum in the near IR. The structures discussed contain various stereogenic elements: classical chiral centres, atropisomeric axes, slowly interconverting conformations, which gives rise to a manifold of situations to be identified, classified, and characterised through spectroscopic tools. The fallout of these structural properties are in enantioselective catalysis, in molecular recognition, or even in biomedicine, on account of the role of Gd3+ complexes as MRI contrast agents. Moreover, the information encoded in the NMR and CD spectra of Ln(3+) complexes may be used to extract original data on the solution stereochemistry of organic molecules used as ligands. The first part summarises some basic theoretical aspects, with special emphasis onto those which have practical consequences in the experimental design. A discussion of selected applications can be found in the second part
Multiple Solution Species of Titanium(IV) 1,1'-Bi-2-naphtholate Elucidated by NMR and CD Spectroscopy
No full textThe solution mixtures of (S)-BINOL and Ti(OiPr)4, with various titanium/BINOL ratios, have been characterized by NMR and CD spectroscopies. In the presence of 1-8 metal equivalents, two main complexes (species I and II) were detected. Species II, identified as (BINOLate)Ti(OiPr)2âTi(OiPr)4, is dominant in the conditions employed for the catalysis of enantioselective addition of alkylzinc reagents to aldehydes. NMR T1 and DOSY results confirmed that species I is dimeric in solution and may be formulated as [(BINOLate)Ti-(OiPr)2]2. The two complexes undergo relatively fast dynamic processes at room temperature, making all naphthoate rings equivalent at NMR; spectrum decoalescence is observed at low temperature. An equilibrium process between I and II, depending on Ti(OiPr)4, was ascertained and quantitatively followed by titration of BINOL with Ti(OiPr)4 and variable temperature experiments. CD spectra offered ultimate proof of titanium-chelating BINOL structures
Synthesis and CD spectra of isocyanide polymers: some new aspects about their stereochemistry
No full textStereochemistry and Near-Infrared Circular Dichroism of a Chiral Yb Complex
No full textThe near-infrared CD spectrum of ytterbium complexed by DOTMA, [DOTMA = (1R,4R,7R, 10R)-alpha, alpha', alpha ", alpha'''-tetramethyl-1,4,7, 10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)] is presented. This is the first report of a near-infrared CD spectrum in solution of the f-f transitions of ytterbium in a complex whose solution structure and dynamics are completely defined. The spectrum shows several well-resolved transitions around 980 nm with very high dissymmetry factors. In water and in methanol, there are two diastereomeric conformations of YbDOTMA in equilibrium, Lambda(delta delta delta delta) or n and Lambda(lambda lambda lambda lambda) or p, being the latter always in a large excess. The relative proportion depends on temperature and solvent composition, but the CD spectrum is rather insensitive to these parameters, which demonstrates that neglecting the minor form contribution is an acceptable approximation. On lowering the temperature, the presence of hot bands is highlighted, which leads to a tentative assignment of the transitions. The absorption counterpart compares very well with that of YbDOTA, an achiral analogue, largely studied and well understood
(S)-Leucine and (S)-1-(1-naphthyl)ethylamine as chiral building blocks of a bifunctional system: synthesis of a new chiral stationary phase and evaluation of its biselector properties in the HPLC resolution of racemic compounds
No full textThe s-triazine moiety as scaffold for connecting different chiral auxiliaries: synthesis of new biselector CSPs for enantioselective chromatography
No full textDetermination of Absolute Configuration of Acyclic 1,2-Diols with [Mo2(OAc)4], Part 2: New Structural Evidence toward a Rationale of the Method: What Remains of [Mo2(OAc)4] in DMSO Solution?
No full textThe nature of the CD-active species obtained by mixing dimolybdenum tetraacetate and a chiral 1,2-diol in DMSO has been studied by different techniques (1 D and 2D H-1 NMR, CD, UV/Vis) with two substrates, (R)phenyl-1,2-ethanediol (1) and (RR)butane-2,3-diol (2). The diol/dimolybdenum adducts have diagnostic CID spectra whose sign correlates with the absolute configuration of the organic substrate. It is demonstrated that, in DMSO solution, the acetate ligands of [Mo-2(OAc)(4)] dissociate to a large extent under the action of the dissolved water, yielding acetic acid and a poly-hydrated dimolybdenum species. Addition of a 1,2-diol leads to chelation with formation of one main active species for 2 and two for 1, all with 1:1 stoichiometries at diol/dimolybdenum molar ratios less than 1.5. Only a small fraction (less than 20%) of the 1,2-diol is bound. The structures of the active complexes are estimated on the basis of NMR spectra, by correlating the observed chemical shifts with the quadruple bond diamagnetic anisotropy. In the predominating complexes for 1 and 2, the 1,2-diol moiety bridges the Mo-2, core forming a six-membered ring
Linear alkylbenzensulphonic acids (LAS) oxidation by H2O2 and O2: an investigation by gas- and liquid-chromatography coupled with mass spectrometry
No full textLigand binding to a human serum albumin stationary phase: use of same-drug competition to discriminate pharmacologically relevant interactions
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