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Configurational Stability (configurational + stability)
Selected AbstractsChemInform Abstract: Stereospecific Formation of Optically Active Trialkylsilyllithiums and Their Configurational Stability.CHEMINFORM, Issue 2 2001Masakatsu Omote Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Configurational stability of 2-benzoylcyclohexanone: Unexpected solvent effects on the rate of racemizationCHIRALITY, Issue 4 2005Gilbert E. Tumambac Abstract The kinetics of the racemization of 2-benzoylcyclohexanone 1 in hexanes, ethanol, and mixtures thereof have been investigated by time dependence of enantiomeric purity using enantioselective HPLC. In pure hexanes and ethanol, the racemization half-lives were determined as 552 and 23.8 min, respectively, at 66°C. Surprisingly, racemization of 1 in mixtures of hexanes and ethanol was found to involve an induction period followed by a sigmoidal decrease of the enantiomeric excess with half-lives varying between 11.5 and 24.0 min. This unexpected solvent influence on the rate of racemization of 1 was attributed to complex isomerization mechanisms involving three possibly interconverting enol tautomers of 1.Chirality 17:171,176, 2005. © 2005 Wiley-Liss, Inc. [source] Multi-Input/Multi-Output Molecular Response System Based on the Dynamic Redox Behavior of 3,3,4,4-Tetraaryldihydro[5]helicene Derivatives: Reversible Formation/Destruction of Chiral Fluorophore and Modulation of Chiroptical Properties by Solvent PolarityCHEMISTRY - A EUROPEAN JOURNAL, Issue 37 2009Takanori Suzuki Prof. Abstract 3,3,4,4-Tetaaryldihydro[5]helicenes (1) and 1,1,-binaphthyl-2,2,-diylbis(diarylcarbenium)s (22+) can be reversibly interconverted upon electron transfer, which is accompanied by a vivid color change (electrochromism) as well as by the formation/cleavage of a CC bond ("dynamic redox behavior"). Because only the neutral donor 1 exhibits strong fluorescence, electrochemical input can further modify the fluorescent properties of the pair. Due to the configurational stability of the helicity in 1 and axial chirality in 22+, the redox reaction of optically pure material proceeds stereospecifically, which induces a chiroptical change such as circular dichroism (CD) as an additional output. The CD spectra of dications 22+ exhibit solvent dependency (chiro-solvatochromism), which is accompanied by solvatochromic behavior based on the ,,, interaction of the two cationic chromophores as well as coordinative interaction of the Lewis basic solvent to the Lewis acidic triarylcarbenium moieties. Thus, the present system is endowed with multi-input functionality for modifying multiple output signals. [source] On the Dichotomic Reactivity of Lithiated Styrene Oxide: A Computational and Multinuclear Magnetic Resonance InvestigationCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2009Vito Capriati Prof. Abstract A multinuclear magnetic resonance investigation, supported by density functional theory calculations, has been synergically used to investigate the configurational stability, reactivity and aggregation states of ,-lithiated styrene oxide in THF at 173,K. NMR studies on ,-lithiated [,,,- 13C2]styrene oxide (also in an enantiomerically enriched form) proved that in THF this oxiranyllithium is mainly present as a solvated monomeric species in equilibrium with a complex mixture of stereoisomeric dimeric aggregates, as well as with bridged and tetrameric aggregates. The fact that some C,Li bonds are partially broken in some stereoisomers reduces their symmetry and complicates the NMR spectra: two diastereoisomers each having a pair of diastereotopic carbon atoms slowly inverting at the lithium atom in absence of tetramethylethylenediamine (TMEDA) have been detected. A (13C,7Li)-HMQC experiment to correlate 7Li and 13C resonances of the various aggregates has been performed for the first time. From natural bond analysis, the monomeric aggregate was proven to have a lower carbenoid character with respect to bridged O-coordinated dimeric aggregates. The employment of suitable experimental conditions in terms of concentration, temperature and the presence or not of TMEDA are crucial to mitigate at the best the "carbene-like" reactivity of lithiated styrene oxide toward intermolecular CLi insertions, eliminative dimerisation reactions and ring-opening reactions. A two-step mechanism for the deprotonation of styrene oxide by sBuLi in THF has been proposed and discussed as well as competitive side reactions. [source] [2,3]-Wittig Rearrangement of Enantiomerically Enriched 3-Substituted 1-Propenyloxy-1-phenyl-2-propen-1-yl Carbanions: Effect of Heteroatoms and Conjugating Groups on Planarization of an ,-Oxy-Benzylcarbanion through a Double BondCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2009Michiko Sasaki Dr. Abstract Don't get trapped: The effect of conjugating electron-withdrawing groups and ,-anion-stabilizing heteroatom substituents on configurational stability of chiral carbanions through a double bond was examined on the basis of extent of chirality transfer in intramolecular trapping in [2,3]-Wittig rearrangement of chiral 3-substituted 1-propenyloxy-1-phenyl-2-propen-1-yl carbanions (see scheme). The effect of conjugating electron-withdrawing groups and ,-anion-stabilizing heteroatom substituents on configurational stability of chiral carbanions through a double bond was examined on the basis of extent of chirality transfer in intramolecular trapping in [2,3]-Wittig rearrangement of chiral 3-substituted 1-propenyloxy-1-phenyl-2-propen-1-yl carbanions. [source] The Synthesis and Characterization of all Diastereomers of a Linear Symmetrically Fused Tris-Tröger's Base Analogue: New Chiral Cleft CompoundsCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2006Josep Artacho Abstract The synthesis and characterization of all diastereomers of a linear symmetrically fused tris-Tröger's base analogue are described. The diastereomers are unambiguously assigned as syn,anti1,a, anti,anti1,b, and syn,syn1,c isomers, by using X-ray diffraction analysis and NMR spectroscopy. For the first time, the anti,anti and the syn,syn diastereomers of a linear symmetrically fused tris-Tröger's base analogue have been synthesized. Molecules 1,a and 1,c are new cleft compounds and analysis of compound 1,a in the solid state shows inclusion of one molecule of CH2Cl2 in the larger aromatic cleft, whereas in isomer 1,c disordered solvent molecules are trapped in the extended aromatic cleft. Furthermore, in the solid state, isomer 1,c forms infinite open channels along one of the crystallographic axes and perpendicular to this axis there are infinitely extending "wedged-ravines". Importantly, each of the diastereomers 1,a,c is resistant to inversion at the stereogenic nitrogen atoms under strongly and weakly acidic conditions in the range from room temperature (RT) to 95,°C. This observed configurational stability at the stereogenic nitrogens of 1,a,c is unique for analogues of Tröger's base in general to date. Finally, the ratio of cleft compounds 1,a and 1,c significantly increased relative to cavity compound 1,b when ammonium chloride was used as an additive in the Tröger's base condensation to 1,a,c suggesting a templating effect of the ammonium ion. [source] |