Abdelkrim Meddour

On the assignment of 119Sn resonances of bis[dicarboxylatotetraorganodistannoxanes] in solution and solid state 119Sn NMR spectra

Abstract The scope and limitations of 119Sn resonance assignment strategies for structural characterizations of bis[dicarboxylatotetraorganodistannoxanes] by 2D gradient assisted 1H–119Sn HMQC NMR in solution and MAS 119Sn NMR in the solid state are discussed for three compounds, the structure of which was already known by X-ray diffraction, bis[diacetatotetramethyldistannoxane] (1), bis[bis(2,2-dimethylpropanoato)tetramethyldistannoxane] (3), bis[bis(pentafluorobenzoato)tetra-n-butyldistannoxane] (4) and for the novel compound bis[bis(4-methylbenzoato)tetramethyldistannoxane] (2), for which an X-ray crystal structure is also reported. The structures of {R′COO(R2Sn)–O–(SnR2)OOCR′}2 each feature a central R8Sn4O2 core, and in addition to the μ3-oxo links between the endo- and exo-cyclic tin atoms, the carboxylate ligands associating in a variety of structural motifs. Their relationship to 119Sn MAS NMR patterns, in particular, the principal components of the 119Sn shielding tensors, is discussed.

Deuterium NMR stereochemical analysis of threo–erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents

Abstract We report the proton-decoupled deuterium NMR study of labelled diastereomers with remote stereogenic carbons dissolved in various mixtures of poly-γ-benzyl- l -glutamate (PBLG) and poly-γ-benzyl- d -glutamate (PBDG) liquid crystalline solutions. The evolution of quadrupolar splitting as well as the diastereomeric and the enantiomeric discrimination versus the proportion of PBLG and PBDG in the liquid crystalline phase is studied. It is shown that racemic liquid crystalline solutions of PBLG and PBDG may be used to measure diastereomeric excess ( de ). Thereafter the spectrum in PBLG solution allows for measuring the enantiomeric excess ( ee ) of each diastereomer. These first results suggest substantial prospects in the field of the analysis of diastereomers with remote stereogenic carbons.

MEASUREMENT OF THE OPTICAL PURITY OF FLUORINATED COMPOUNDS USING PROTON DECOUPLED 19F NMR SPECTROSCOPY IN A CHIRAL LIQUID CRYSTAL SOLVENT

The use of trifluoroacetic anhydride as an achiral derivatizing agent for enantiomeric excess measurement through chiral liquid crystal NMR is described. Examples involving amines, amino acids and alcohols are provided. It is shown that the proton decoupled fluorine NMR spectra of the -CO-CF3 group allow good enantiomeric discrimination based on both the chemical shifts and the dipolar splitting. The spectra separation is due to the differential ordering effect of enantiomers in the oriented chiral solvent. The results are then compared to the discriminating power of the -CO-CD3 group that has been previously described using deuterium NMR in the same anisotropic medium.

Visualization of enantiomers in a polypeptide liquid-crystal solvent through carbon-13 NMR spectroscopy

High-resolution, proton-coupled and decoupled, carbon-13 NMR in natural abundance is used to observe the spectra of enantiomers in chiral molecules dissolved in the poly(γ-benzyl-L-glutamate)(PBLG)/CD2Cl2 liquid-crystalline system. Discrimination between enantiomers can be obtained either through differences in their dipolar interactions or in their chemical shift anisotropies. This technique provides a new tool for the visualization of enantiomers and the measurement of the enantiomeric ratio. Several specific examples are presented and discussed.

Enantioselective Intramolecular Hydroamination of Secondary Amines Catalyzed by Easily Accessible Ate and Neutral Rare‐Earth Complexes

Intramolecular catalytic asymmetric hydroamination of unactivated olefins is currently the subject of extensive research as one of the more elegant, environmentally friendly, and atomeconomical methods for the formation of enantioenriched and value-added nitrogen-containing heterocycles. Despite significant achievements in this challenging field, no general answer has yet emerged, and considerable improvements are still required to bring this attractive method into the organic chemist’s toolbox as a pivotal tool to create C N bonds. Indeed, amongst the diversity of reported catalytic systems, only a very limited number, based on rare-earth, lithium, zirconium, or rhodium, affords a high level of enantioselectivity (>90 % ee) for hydroamination/cyclization reactions of amines tethered to unactivated alkenes. 7, 8] Moreover, these highly enantioselective catalytic systems derived from neutral zirconium amidate and MOP–rhodium complexes (MOP = 2-(diarylphosphino)-2’-methoxy-1,1’-binaphthalene) are respectively restricted to primary or secondary amines, affording, correspondingly, no or poor reactivity with secondary and primary amines. 10] An alternative cationic zirconium complex bearing an aminophenolate ligand as a chiral scaffold was already reported for the hydroamination/cyclization of N-methylated amines to unactivated olefins with up to 82 % ee. The catalytic system derived from lithium and based on chiral (bis)oxazoline ligands has only been reported for the enantioselective hydroamination of N-methylated aminoalkenes. To date, the highest ee value (95 %) was achieved using a sophisticated substituted binaphtholate rare-earth complex for intramolecular hydroamination of a primary amine, whereas lower asymmetric induction was observed with a secondary amine. A chiral aminobis(thio)phenolate yttrium complex was previously demonstrated to catalyze the enantioselective intramolecular addition of primary and secondary amines to unactivated alkenes, exhibiting a similar trend in terms of level of enantioselectivity (80–87 % ee for primary amines and 69 % ee for a secondary amine). To our knowledge, this last ee value is the highest reported to date for a rare-earth-catalyzed hydroamination of a secondary amine. In this context, we report herein the in situ preparation of chiral binaphthylamido alkyl ate and neutral yttrium and ytterbium complexes and their use as highly active catalysts for the intramolecular hydroamination of secondary amines. Our group has long been interested in the development of chiral well-defined binaphthylamido ate and neutral rare-earth complexes as catalysts for the asymmetric intramolecular hydroamination of primary aminoalkenes. These ongoing studies have identified neutral and ate heteroleptic binaphthylamido alkyl yttrium and ytterbium complexes as the most promising candidates for the development of efficient and easily accessible chiral catalysts by a convenient procedure. Very recently, we reported a novel “single-pot” route for the rapid in situ generation of ate yttrium complexes from the stable and straightforwardly synthesized rare-earth precursor [Li(thf)4] [Y(CH2SiMe3)4] [12] 1-Y. This very reliable process showed its ability to afford active and enantioselective (pre)catalysts for the intramolecular hydroamination/cyclization of challenging substrates such as primary amines tethered to unactivated 1,2-disubstituted alkenes. Herein we report a similar strategy to easily access neutral binaphthylamido alkyl yttrium and ytterbium analogues from a related homoleptic rare-earth source [Ln(CH2SiMe3)3(thf)2] [13] (2-Ln; Ln = Y or Yb). We describe the potential of these neutral and ate rare-earth catalysts to promote the intramolecular hydroamination of secondary amines tethered to monosubstituted olefins. The catalytic activities of corresponding chiral species arising from coordination to an enantiopure binaphthyldiamine ligand were then evaluated for the transformation of the same substrates. To date, the highest enantiomeric excesses for the cyclization of primary aminoolefins catalyzed by ate tetraamido and ate amido alkyl rare earth complexes were measured using N-cyclopentyl binaphthyl amine ligand L (Scheme 2). This diamine was thus chosen for this study. The activities of the tetraalkyl ate yttrium precursor 1-Y and trisalkyl neutral precursors 2-Y and 2-Yb were firstly

An achiral deuterated derivatizing agent for enantiomeric analysis through NMR in a liquid crystalline solvent

Abstract The application of a ‘deuterium probe’ for the enantiomeric analysis of chiral amines, aminoacids and alcohols is proposed. The probe is the perdeuterated benzoyl fragment which can easily and reversibly be attached to the corresponding materials. The analysis is performed through measurements of the differential ordering effect (DOE) in the deuterium NMR of the chiral material dissolved in a cholesteric-nematic liquid crystalline system. Since all the chiral compounds which we studied were not racemic, the assignments of the observed DOEs to the enantiomers enabled us to draw tentative conclusions relating the results to the absolute configurations. This may open the way for the empirical derivation of absolute configurations of enantiomers from DOE measurements.

Gas chromatographic properties of two thermotropic Poly(L-glutamates) with long alkyl side chains

SummaryComparative gas chromatographic properties of two thermotropic Poly(L-glutamates) with long alkyl side chains (10 and 16 carbons called Poly 10 and Poly 16 respectively) are studied.Their thermal properties were established with differential scanning calorimetry (DSC) and gas chromatography. They present a cholesteric liquid crystal in a large temperature range.The chromatographic separation abilities of the two polymers in the liquid crystal state were studied using capillary glass columns. Interesting analytical performances were obtained in different fields: isomeric separation of alkanes, aromatics, polyaromatics, volatile aroma compounds and cis and trans isomers. Some differences were noticed in their behaviours; 2 and 3-methyloctane and xylene isomers are separated only on Poly 10 while limonene and eucalyptol are separated only on Poly 16.

Achiral deuterated derivatizing agent for enantiomeric analysis of carboxylic acids by NMR in a chiral liquid crystalline solvent

Abstract The use of a deuterated ‘probe’ for the enantiomeric analysis of chiral carboxylic acids is proposed. The probe is the perdeuterobenzyl fragment which can be easily attached to the acid and removed from the corresponding ester. The analysis is performed through the measurement of the proton-decoupled deuterium NMR spectrum of the chiral benzyl ester dissolved in poly-γ-benzyl- l -glutamate/dimethylformamide liquid crystal. Enantiomeric discrimination was observed for all the studied compounds on the para and/or α deuterons. There is no need for the deuterons to be located close to the stereogenic centre. Thus enantiomers were correctly distinguished from the signal of a deuterium located 12 bonds away from the asymmetric carbon. The major interest of this general technique is that no kinetic resolution should occur during the derivatization process.

Evolution of the Saupe order parameters of enantiomers from a racemic to a non-racemic liquid crystal solvent: an original light on the absolute configuration determination problem

Chiral liquid crystals used as solvents have been shown to be very powerful in giving separate nuclear magnetic resonance (NMR) signals for enantiomers. However, a fundamental problem remains: how to attribute absolute configurations to the various NMR responses? In this paper it is demonstrated that the absolute configuration problem could be solved in predicting the sign of the rotation of the principal axis system of the Saupe matrix in the molecular frame when going from an achiral (racemic) to a chiral anisotropic solvent.

Analysis of the diastereoselectivity of a Diels-Alder cycloaddition through 2H NMR in chiral liquid crystals

Abstract Deuterium NMR in chiral liquid crystal solvents is used to measure the diastereoselectivity in asymmetric Diels-Alder cycloadditions on 2-alkoxy-2-vinyl-1,3-dioxolanes issued from chiral 1,2 diols.