Stereochemistry of chromium complexes with monosubstituted allenes

Previously unknown arenechromium dicarbonyl complexes with different allenes including functionally substituted ones were synthesized. The formed compounds represent a mixture of all three possible isomers and exhibit stereochemical nonrigidity. The isomerization process and stereochemical peculiarities of the complexes synthesized were studied using the ¹H NMR method.     

Model reactions for stereochemistry of Ziegler-Natta polymerization: Asymmetric hydroisomerization of α-olefins by chiral titanium tetra-alkoxides/tri-isobutyl aluminum catalytic systems

Homogeneous catalytic systems based on optically active low-molecular-weight and macromolecular titanium alcoholates and tri-isobutyl aluminum were used in the isomerization and hydrogenation of chiral and prochiral olefins. The reactions performed in a nitrogen and hydrogen atmosphere show a different behavior in regard to reaction rate, composition of reaction mixture, and stereochemical control. The catalyst based on macromolecular titanium alcoholate displays lower activity and a varying influence on the stereochemical pathway with respect to the catalysts based on low- molecular-weight alcoholates. The data obtained in both cases strongly support the absence of a stereocontrol by the growing chain in the stereoelective polymerization of racemic α-olefins by Ziegler-Natta catalysts based on optically active precursors. Finally evidence is provided in favor of the intermediate step responsible for asymmetric isomerization which does not occur via η-allyl complex.     

Asymmetric cyclopropane synthesis via phosphine oxide mediated cascade reactions

A silyloxy-THF has been converted into a cyclopropane containing three stereocentres as mixture of diastereoisomers. The mechanism of the reaction has been established and the source of stereochemical leakage proposed. An alternative stereospecific cascade reaction has been discovered.     

Can enantiomers be separated in achiral chromatographic systems?

Consideration of chromatography of a nonracemic mixture on an achiral sorbent from a stereochemical point of view allows the claim that partial separation of the excess enantiomer zone from the racemate zone is possible only with analytes capable of self-associating under the conditions of the chromatographic column. It is from these positions that features of this process can be explained and conditions for its maximal proceeding formulated.     

pH variation as the switch for chiral recognition in a biomembrane model

Chiral recognition was observed in a biomembrane model. Micellar aggregates formed by N-dodecyl-l-proline were, in fact, able to convert the racemic mixture of a biphenyl derivative into an enantiomerically enriched mixture. The extent of deracemization was established by HPLC on a chiral stationary phase and, in some conditions, resulted in the highest ever observed in these systems. The configuration of the exceeding enantiomer and the extent of enantiomeric enrichment depend on the medium pH. In fact, the exceeding enantiomer has the same configuration at pH 1 and 7, being more abundant at pH 7, whereas it has the opposite configuration at pH 13. The changes in the stereochemical bias are reversible.     

Reactions de substitution homolytique—III : Etude stereochimique de la decarboxylation de peracides a structure bicyclo[2.2.1]heptanique

The decarboxylation of either 2-norbornane or 2-apobornane percarboxylic acids gives, by a radical process, a mixture of isomeric alcohols (2-norbornanols and 2-apobornanols). The exo entrance is always favoured (95/5 and 70/30 respectively for the norbornane and apobornane series). In this reaction the steric hindrance of 3-methyl cyclohexane and 7-syn-bicyclo (2.2.1) heptane is the same. By comparison with the stereochemical results of non radical reactions we consider that the pyramidal structure of the 2-bicyclo [2.2.1] heptyl radical influences the stereochemical course of the reaction.     

Trivinylphosphine and trivinylphosphine chalcogenides: stereochemical trends of ³¹P-¹H spin-spin coupling constants

A combined theoretical and experimental study of the stereochemical behavior of (31)P-(1)H spin-spin coupling constants has been performed in the series of trivinylphosphine and related trivinylphosphine oxide, sulfide and selenide. Theoretical energy-based conformational analysis of the title compounds performed at the MP2/6-311G** level reveals that each of the four compounds of this series exists in the equilibrium mixture of five true-minimum conformers, namely s-cis-s-cis-s-cis, s-cis-s-cis-gauche, syn-s-cis-gauche-gauche, anti-s-cis-gauche-gauche and gauche-gauche-gauche, which were taken into account in the conformational averaging of (31)P-(1)H spin-spin couplings calculated at the second-order polarization propagator approach/aug-cc-pVTZ-J level of theory. All (31)P-(1)H spin-spin coupling constants involving phosphorus and either of the vinyl protons are found to demonstrate a marked stereochemical dependences with respect to the geometry of the coupling pathway and internal rotation of the vinyl group around the P-C bond which is of major importance in the stereochemical studies of the unsaturated phosphines and phosphine chalcogenides.     

Glycosidation of an ambident organic phosphoroselenothioate in the presence of a Lewis acid

An ambident O,O‐dialkylphosphoroselenothioate is glycosidated in the presence of boron trifluoride etherate to give both Se‐ and S‐glycosyl‐phosphoroselenothioates, the former predominating. The stereochemical course of this reaction depends on the kind of sugar substrates. By selective oxidation of the mixture of products, the Se‐glycosyl derivative is isolated. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 259–262, 1999     

Dynamic Covalent Optical Chirality Sensing with a Sterically Encumbered Aminoborane

A sterically encumbered aminoborane sensor is introduced and used for quantitative stereochemical analysis of monoalcohols, diols and amino alcohols. The small-molecule probe exhibits a rigid ortho-substituted arene scaffold with a proximate boron binding site and a triarylamine circular dichroism (CD) reporter unit which proved to be crucial for the observed chiroptical signal induction. Coordination of the chiral target molecule produces strong Cotton effects and UV changes that are readily correlated to its absolute configuration, enantiomeric composition and concentration to achieve comprehensive stereochemical analysis within a 5 % absolute error margin. The sensing method was successfully applied in the chromatography-free analysis of less than one milligram of a crude asymmetric reaction mixture and the advantages of this chiroptical sensing approach, which is amenable to high-throughput experimentation equipment and automation, over traditional methods is discussed.     

Kinetically controlled ring-closing metathesis: synthesis of a potential scaffold for 12-membered salicylic macrolides

For the synthesis of a 12-membered salicylic macrolide scaffold, ring-closing metathesis (RCM) of a omega-diene compound was planned. The stereochemical outcome of the RCM reaction changed depending on the type of Ru catalyst that was used; a "first-generation" Grubbs catalyst produced exclusively the E isomer and "second-generation" catalysts provided a mixture of the E and Z isomers under kinetic control (not thermodynamic control). Considerations for the E/Z selectivity are described.     

Divinyl selenide: conformational study and stereochemical behavior of its 77Se-1H spin-spin coupling constants

Theoretical energy-based conformational analysis of divinyl selenide performed at the MP2/6-311G** level is substantiated by the second-order polarization propagator approach (SOPPA) calculations and experimental measurements of its (77)Se-(1)H spin-spin coupling constants, demonstrating marked stereochemical behavior in respect of the internal rotation of both vinyl groups around the Se-C bonds. Based on these data, divinyl selenide is shown to exist in an equilibrium mixture of three nonplanar conformers: one the preferred syn-s-cis-s-trans and two minor anti-s-trans-s-trans and syn-s-trans-s-trans forms.     

Highly enantioselective stereo-inverting sec-alkylsulfatase activity of hyperthermophilic Archaea

rac-sec-Alkyl sulfate esters 1a-8a were resolved in low to excellent enantioselectivities with E-values up to >200 using whole cells of aerobically-grown hyperthermophilic sulfur-metabolizers, such as Sulfolobus solfataricus DSM 1617, Sulfolobus shibatae DSM 5389 and, most notably, Sulfolobus acidocaldarius DSM 639. Significantly enhanced selectivities were obtained using cells grown on sucrose-enriched Brock-medium. The stereochemical course of this biohydrolysis was shown to proceed with strict inversion of configuration, thus the preferred (R)-enantiomers were converted into the corresponding (S)-sec-alcohols to furnish a homochiral product mixture.     

Poly(ethylene glycol)-supported tetrahydroxyphenyl porphyrin: a convenient,recyclable catalyst for photooxidation reactions

[reaction: see text] A new poly(ethylene glycol)-supported porphyrin has been prepared and its ability as a promoter in photooxidation reactions has been studied. The PEG-supported catalyst exhibits high activity, comparable to that of a nonanchored sensitizer, and it is easily removable by filtration from the reaction mixture. The polymer-bound porphyrin has been recycled up to six times with no loss of chemical and stereochemical efficiency.     

Enantiodivergence in alkylation of 1-(6-methoxynaphth-2-yl)ethyl acetate by potassium dimethyl malonate catalyzed by chiral palladium-DUPHOS complex

Alkylation of racemic 1-(6-methoxynaphth-2-yl)ethyl acetate by potassium dimethyl malonate catalyzed by a chiral palladium-DUPHOS complex afforded the substitution product with 87% ee, along with 6-methoxy-2-vinylnaphthalene that arose from an elimination process, in a 43/57 substitution/elimination ratio. The reaction performed on a mixture of quasi-enantiomeric substrates provided insight into the stereochemical course of the reaction, establishing that—for a given enantiomer of the catalyst, one enantiomer of the substrate afforded mainly the substitution product whereas the other enantiomer underwent elimination.     

Highly enantioselective sec-alkyl sulfatase activity of Sulfolobus acidocaldarius DSM 639

[reaction: see text] rac-sec-Alkyl sulfate esters 1a-4a were resolved in high enantioselectivities with E-values up to >200 using whole cells of aerobically grown Sulfolobus acidocaldarius DSM 639. The stereochemical course of this biohydrolysis was shown to proceed with strict inversion of configuration; thus, the preferred (R)-enantiomers were converted into the corresponding (S)-sec-alcohols to furnish a homochiral product mixture.     

Identification of a stereoblock poly(methyl methacrylate) sample with a stereocomplex

Acid hydrolysis of a stereoblock poly(methyl methacrylate) sample leads to a mixture of isotactic and syndiotactic poly(methacrylic acid) which can be separated by electrophoresis. The experiment confirms the stereochemical identity between the so-called “stereoblock” poly(methyl methacrylate) and the stereocomplex which syndiotactic and isotactic poly(methyl methacrylate) form in the ratio 2:1. A possible mechanism of replica polymerization is suggested to account for this effect.     

Ozonolysis of 3,4-dehydroproline

Ozonolysis of protected 3,4-dehydro- DL -proline ( 2 ) in methanol gives as the initially isolable product, a seven-membered ring cyclic peroxide ( 5 ). Compound 5 undergoes rearrangement thermally to give methyl N-tosylglycinate ( 8 ) and a stereochemical mixture of oxazolidine aldehyde esters ( 7 ). Structural evidence for 7 came from detailed ¹³C nmr studies of 7 and its reduction product ( 9 ).     

Syntheses and structural analysis of 10-monoxy- and -dioxy-5-N-substituted iminothianthrene derivatives and the stereochemical change on their sulfur atom under acidic and thermal conditions

5-(N-p)-Toluenesulfonyl)iminothianthrenes, whose sulfur atoms are oxidized to a sulfoxide or sulfone at the 10-position, were hydrolysed readily in high yield to N-unsubstituted-sulfilimines by using concentrated H2SO4. During hydrolysis, 10-monoxy-5-N-unsubstituted-sulfilimines were obtained as a separable mixture of the cis and trans isomers. The stereochemical interconversion of these compounds was studied under both hydrolytic and thermal conditions and their structures were elucidated by using X-ray crystallography.     

Diastereoselective cycloadditions of 1,3-thiazolium-4-olates with chiral 1,2-diaza-1,3-butadienes

A series of 2-(N-methyl)benzylamino-1,3-thiazolium-4-olates (2-aminothioisomunchnones) react with chiral 1,2-diaza-1,3-butadienes derived from carbohydrates to afford a diastereomeric mixture of (4R,5S)- and (4R,5R)-4,5-dihydrothiophenes. These substrate-controlled cycloadditions are chemoselective, regiospecific, and proceed with a high facial diastereoselection. A theoretical rationale at semiempirical level does justify the stereochemical outcome observed in the experiments.     

Polymerization of optically active isoamyl glycidyl ether in the presence of the catalyst system diethylzinc — (-)l-menthol

Conclusions The polymerization of optically active isoamyl glycidyl ether in the presence of diethylzinc with the addition of an optically active alcohol (-)l -menthol has been investigated. The isomers contained in the monomer mixture are polymerized at different rates and give two extremes on the curve relating specific rotation and yield which, evidently, is associated both with the specific properties of the catalyst and also with the stereochemical effect of the optically active substituent.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 923–924, April, 1970.