Detection of enantiomers in dissymmetric biaryls with chiral shift reagents

Methoxycarbonyl groups in racemic biphenyls show separation of the methyl proton magnetic resonance signals in the presence of large amounts of chiral shift reagents.     

Application of axially dissymmetric ligand recoverable with fluorous solvent as a chiral auxiliary

(Ra)-(R)₂-2,2′-Bis(1-hydroxy-1H-perfluorooctyl)biphenyl ((Ra)-(R)₂-1c), which is an axially dissymmetric ligand with two chiral centers, works as a good chiral auxiliary for asymmetric aldol reaction. Thus, the reaction of monopropanoyl ester of 1c (2) with benzaldehyde in the presence of triethylamine and titanium(IV) chloride gave (2R),(3S)- and (2R),(3R)-3-hydroxy-2-methyl-3-phenylpropanoic acid esters (3a) in an approximate ratio of 4:1 in a total high yield. This result shows that stereoselectivity at 2-position is quite high, while that at 3-position is moderate. Both isomers were easily separated by column chromatography. Methanolysis of the separated isomers gave nearly quantitative recovery of 1c by extraction with a fluorous solvent without any loss of ee, while methyl (2R),(3S)- or (2R),(3R)-3-hydroxy-2-methyl-3-phenylpropanoates were obtained by CH₂Cl₂ extraction quantitatively in >99% ee. Aldol reaction of 2 with various aldehydes gave similar results.     

Stereochemical models for rh-catalyzed amination reactions of chiral sulfamates

Oxidative C-H amination of chiral sulfamate esters using achiral Rh-carboxylate catalysts, PhI(OAc)(2), and MgO occurs in high yield and with good to excellent diastereocontrol. A number of examples are included to support a proposed transition state model that accounts for the observed stereoinduction. In addition, stereoselective intramolecular aziridination with substituted homoallyl sulfamates is demonstrated and is rationalized through an analogous stereochemical construct. [reaction: see text]     

Stereochemical versatility of nickel(II). Nickel(II) complexes of cyclohexanone thiosemicarbazone

Summary Using cyclohexanone thiosemicarbazone as ligand (L), nickel(II) complexes, NiL₂X₂ · nH₂O, have been prepared for various anions. Their properties are as follows: X = Cl, green paramagnetic, 5-coordinate; X = Br dimorphous; green, diamagnetic, square-red; paramagnetic, possibly square pyramidyl; X = nitrate, dimorphous-anhydrous, red, diamagnetic, planar; dihydrated, green, paramagnetic, traps-octahedral; X = acetate, intermediate; X = thiocyanate, paramagnetic, parrot green, isothiocyanate, octahedral, X = sulphate, dimorphous anhydrous, brown, diamagnetic. planar; trihydrate, blue, diamagnetic and planar.     

Stereochemical aspects of the controlled ring-opening polymerization of chiral cyclic esters

The stereochemical aspects of controlled polymerization of lactide and β-butyrolactone are discussed. The physicochemical and thermal properties of these polymers are shown to be depending on the stereochemical structure of the macromolecular chain. Different types of processes involving change of enantiomeric composition in the course of the polymerization reaction are examined in function of different initiators used. The formation of stereocomplexes from stereocopolymers of various enantiomeric compositions is reported.     

Stereochemical aspects of asymmetric diels-alder reaction catalyzed by chiral alkoxyaluminum dichlorides

Clear stereochemical relationships were observed between the structure of chiral sources of catalysts and absolute configuration of the Diels-Alder adducts.     

Stereochemical study of chiral tautomeric flavorous furanones by vibrational circular dichroism

2-Substituted-3(2H)-furanone derivatives are industrially significant aroma compounds possessing a unique keto-enol tautomeric feature causing their racemization. Absolute configurations of two flavorous furanones, which have remained unclear for the past 40 years since their discovery, were clarified by the vibrational circular dichroism technique as well as chemical relay reactions. Odor evaluation of each enantiomer revealed relationships between their configurations and their odor activities.     

Esterification reactions on syndiotactic poly(methallylalcohol). IV. Homo- and copolymers of N-protected methallyl-(L)-histidinate

Syndiotactic poly(methylallylalcohol) is fully esterified with N^α-protected (L )-histidine by carbodiimide in pyridine to yield the corresponding homopolymers, i.e., N^α-protected 2-methylallyl-(L )-histidinate monomer units and unreacted 2-methylallyl alcohol units are obtained, which in a second exhaustive esterification step are reacted with N^α-(benzyloxycarbonyl)-(L )-aspartic acid anhydride. The resulting copolymers consist of N^α-protected 2-methylallyl-(L )-histidinate and 2-methylally-N^α-(benzyloxycarbonyl)-(L )-hydrogen-α-aspartate monomer units. They are polyampholytes containing both imidazole and carboxyl groups. The structure, including composition of the copolymers, is determined by ¹H- and ¹³C-NMR, while water solubility and apparent pK_aa values are investigated by potentiometry.     

Stereochemical effects of chiral monolayers on enhancing the resistance to mammalian cell adhesion

This work describes the different durations of surface confinement of adhered mammalian cells by monolayers comprised of enantiomers of bio-inert polyol-terminated alkanethiols. Enhanced resistance to protein adsorption and cell adhesion is obtained on monolayers formed by a racemic mixture of the enantiomeric alkanethiols.     

Enantioselective catalytic epoxidation of nonfunctionalized prochiral olefins by dissymmetric chiral Schiff base complexes of Mn(III) and Ru(III) metal ions II

A series of new dissymmetric chiral Schiff base complexes has been obtained by a systematic condensation of (1S,2S)(+)-diaminocyclohexane and 3-acetyl-4-hydroxy-6-methyl-2-pyrone with salicylaldehyde, 5-chloro-, 5-methoxy-and 5-nitrosalicylaldehyde and by subsequent metallation with manganese and ruthenium. The characterization of the complexes 1–8 was accomplished by physico chemical studies viz. microanalysis, IR-, UV/VIS-, and CD spectral studies, optical rotation, molar conductance measurements and cyclic voltammetry. Enantioselective epoxidation of non functionalised olefins, viz. cis-stilbene, trans-3-nonene and trans-4-octene with iodosyl benzene as oxidant was demonstrated in the presence of catalytic amounts of chiral Mn(III) and Ru(III) dissymmetric Schiff base complexes. Good optical yields of epoxides were obtained for the catalyst 4 with the substrates trans-3-nonene and cis-stilbene.     

Stereochemical control in reactions of chiral ketone with 2-lithiated substituted ferrocene derivatives

The formation of a new chiral center in the reactions of ketone 4-MeOC₆H₄CH(Et)C(O)Et with planar-chiral 2-lithio-1-methyl- and 1-chloro-2-lithioferrocenes proceeded stereoselectively to give two diastereomers in a ratio of 3 : 2 or a single diastereomer, respectively, out of four possible diastereomeric 4-aryl-3-ferrocenylhexan-3-ols. The replacement of the hydroxy group in the resulting compounds by hydrogen under the conditions of ionic hydrogenation was studied. The configurations of the reaction products were established by X-ray diffraction analysis and ¹H NMR spectroscopy.     

Asymmetric reduction of the carbonyl group by chiral hydrides. 1. Stereochemical considerations

Conclusions A quadrant rule is proposed for the enantioselective reduction of acetophenone by chiral hydrides based on LiAlH₄, NaBH₄, LiBH₄, and BH₃. This enables the configuration of the product to be explained and predicted, depending on the preferred conformations of the ligands in these hydrides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 843–848, April, 1987.     

Two-component supramolecular helical architectures: creation of tunable dissymmetric cavities for the inclusion and chiral recognition of the third components

The inclusion and chiral recognition of racemic arylalkanols by supramolecular helical architectures consisting of enantiopure primary amines and achiral carboxylic acids were thoroughly studied. Among the architectures examined, a supramolecular helical architecture composed of the salt of enantiopure erythro-2-amino-1,2-diphenylethanol (1 b) and benzoic acid (2 a) was found to include a wide variety of racemic arylalkanols with recognition of their chirality. The helical architecture gave a dissymmetric 1D groove in the salt crystal, and the arylalkanols were enantioselectively included in the groove. The size and shape of the groove were tunable by proper selection of the achiral carboxylic acid component. The origin of the chiral recognition with the combination 1 b/2 a is discussed on the basis of X-ray crystallographic analyses.     

Acid-functionalized dissymmetric salen ligands and their manganese(III) complexes

Acid-functionalized symmetric and dissymmetric salen-type ligands were synthesized via a novel self-protection step in a quantitative yield. This synthetic method allows one to quickly prepare salen-based dissymmetric chiral compounds with tailorable coordinating properties. Therefore, this approach provides a blueprint for synthesizing and evaluating a new class of acid-functionalized salen ligands that can be used as chiral building blocks for a wide range of catalysts and coordination polymers with chemically tailorable properties.     

Scanning tunneling microscopy and scanning tunneling spectroscopy studies of planar and nonplanar naphthalocyanine on graphite (0001). Part 2: tip-sample distance-dependent I-V spectroscopy

Tip-sample distance-dependent current-voltage tunneling spectroscopy on monolayers of base-free naphthalocyanine (Nc), a planar molecule, and tin-naphthalocyanine (SnNc), a nonplanar molecule, has been studied on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface using a variable-temperature STM at 50 K under ultra-high vacuum conditions. The current-voltage curves show an unsymmetrical diode-like nature especially at large tip-sample distances in both cases. Normalized differential conductivity of all spectra has been considered for further analysis. The ionization and electron affinity levels are compared with the single-molecule local density of states (LDOS) near the Fermi energy using a theoretical calculation for Nc and SnNc. A tip-sample distance-dependent highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap shrinking is observed in the case of Nc, in which the filled levels of the molecules are pinned while the unfilled levels near the Fermi energy are shifting toward lower energy. In contrast, there is no such HOMO-LUMO gap shrinking in the case of the SnNc decreasing tip-sample distance. However, a subsequent increase in the tunneling current was observed by almost 1 order of magnitude compared with Nc. A model is proposed to explain this phenomenon where the Nc-graphite interface is considered as a pure capacitive interface.     

The First Butyltin(IV) Homo- and Heterobimetallic Diethanolaminates

Equimolar reactions of BuSn(OPrⁱ)₃ with diethanolamines, RN(CH₂CH₂ OH) ₂ (abbreviated as RdeaH₂, where R = H or Me), afford dimeric isopropoxo-bridged six-coordinate butyltin(IV) complexes [{Bu(η³-Rdea)Sn(μ-OPrⁱ)}₂] (R = H ( 1 ), Me ( 2 )). Interactions between BuSn(OPrⁱ)₃ and diethanolamines (RdeaH₂) in a 1:2 molar ratio yield monomeric derivatives of the type [BuSn(Rdea)(RdeaH)] (R = H ( 3 ), R = Me ( 4 )). These homometallic complexes on 1:1 reactions with an appropriate metal alkoxide form monomeric heterobimetallic complexes of the type [BuSn (Rdea)₂ {M(OR′)_n}] (R = H, M = Al, R′ = Prⁱ, n = 2 ( 5 ); R = H, M = Ti, R = Prⁱ, n = 3 ( 6 ); R = H, M = Zr, R′ = Prⁱ, n = 3 ( 7 ); R = Me, M = Al, R′ = Prⁱ, n = 2 ( 8 ); R = Me, M = Ti, R′ = Prⁱ, n = 3 ( 9 ); R = Me, M = Ge, R′ = Et, n = 3 ( 10 )). The driving force behind this work was (i) to explore the utility of homometal complexes ( 1 ) –( 4 ) in assembling a metal alkoxide fragment via a condensation reaction and (ii) to gain insights into the structures of new compounds by NMR spectral data. All of these derivatives have been characterized by elemental analysis, spectroscopic (IR, NMR; ¹H, ²⁷Al, and ¹¹⁹Sn) studies, and molecular weight measurements. ¹¹⁹Sn NMR spectral studies indicate that both the homometallic ( 3 ) and ( 4 ) and heterobimetallic ( 5 ) – ( 9 ) complexes exist in a solution in an equilibrium of six- and five-coordinated tin(IV) species.     

Stereochemical control of both C-C and C-N axial chirality in the synthesis of chiral N,O-biaryls

A Rh(I)-catalyzed asymmetric [2 + 2 + 2] cycloaddition of achiral ynamides is described here. This work demonstrates a unique concept of stereochemical control of both the C-C and C-N axial chirality and provides an approach to the synthesis of chiral N,O-biaryls as well as chiral anilides.