Control synthesis of isocyanate and alkoxy-silane terminated macromers

The kinetics of the reactions in bulk of 4,4′-dicyclohexyl methane diisocyanate (H₁₂ MDI) and 5-isocyanato-1,3,3-trimethylcyclohexylmethyl isocyanate or isophorone diisocyanate (IPDI) with benzylic alcohol (BZA) and α-hydroxy-ω-methyl ether-terminated polyethylene oxide PEO (M?_w = 350) were studied by size exclusion chromatography (SEC) and ¹³C nuclear magnetic resonance (¹³C-NMR). The substitution effect is exhibited in the case of H₁₂ MDI reactivity. The kinetic constants were calculated by a numerical method. The second-order kinetic mechanism was shown to be valid. In the IPDI case, the cycloaliphatic isocyanate group is shown to be more reactive than the aliphatic group in our conditions, without catalysis, in agreement with previous results from the literature, in our obtained by ¹H-and ¹³C-NMR without any catalyst. The reactivity ratio is found to be on the order of 3. This difference in reactivity of the two isocyanate groups is used for the control synthesis of isocyanate and alkoxy-silane-terminated macromers.     

Isolation by HPLC. and Identification by NMR. Spectroscopy of 11 mono-, di- and tri-cis isomers of an aromatic analogue of retinoic acid,ethyl all-trans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethyl-nona-2,4,6, 8-tetraenoate

Photoisomerization of an aromatic analogue of retinoic acid, ethyl all-trans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethyl-nona-2,4,6, 8-tetraenoate 1 in dilute solutions of hexane, benzene, and ethanol yielded multi-component mixtures of cis isomers which were separated by HPLC. FT-¹H-NMR. at 270 MHz and, in some cases, homonuclear decoupling and Overhauser experiments as well as ¹³C-NMR. were applied to establish the structures of 4 mono-cis, 4 (of 6 possible) di-cis, and 3 (of 4 possible) tri-cis isomers. The structures of 3 isomeric esters, namely (2Z, 4E, 6E, 8E) 6 , (2Z, 4Z, 6E, 8E) 9 , and (2Z, 4Z, 6Z, 8E) 7 were independently confirmed by direct syntheses. The ¹H-NMR. data of all these compounds and the ¹³C-NMR. data of the all-trans and of 6 cis isomers available in sufficiently large quantities are discussed.     

Syntheses and Absolute Configuration of (E)- and (Z)-α-Bisabolenes

Stereospecific syntheses of (E)- and (Z)-α-bisabolenes have made revision of the configuration of natural (+)-α-bisabolene from Opoponax oil necessary. ¹³C-NMR. spectroscopy has been used for the differentiation of these isomers.     

Synthesis and NMR-Spectroscopic Structure Determination of Novel 7,7′-Diphenyl-7,7′-diapocarotenoids

Wittig reaction of crocetindial ( 1 ) and benzylidenetripenylphosphorane ( 2 ) gave (7E, 7′Z)-7,7′-diphenyl-7-7′-diapocarotene ( 3 ), instead of the previously reported (7E, 7′E)-isomer. Similar reaction of 8,9-didehydrocrocetindial ( 4 ) with 2 yielded the three acetylenic isomers 5a–c which differ in the configuration of the terminal double bonds. Structures were established by 1D- and 2D-NMR studies. Illustrative spectra and their interpretation are presented. Most chemical shifts of corresponding protons in 3 and 5 and nearly identical, but ¹³C shifts differ considerably.     

Conformations of Z- and E-isomers of some chiral (1R,4R)-2-arylidene-p-menthan-3-ones

The ¹Í NMR method in combination with molecular simulation was used to study conformations of Z- and E-isomers of (1R,4R)-cis-2-(4-methoxyphenyl)benzylidene-p-menthan-3-one. In solutions the Z-isomer, unlike the conformationally uniform Å-isomer, is an equilibrium mixture of chair conformers with the substantial predomination of one form with the axially oriented methyl and equatorial isopropyl groups (75—78%). The enone group is more nonplanar in the Z-isomer than in the Å-isomer. For the isopropyl fragment, the equiprobable existence of trans- and two gauche-rotamers for the Z-isomer and a substantial predomination of gauche-forms in the case of the E-isomer were established.     

Structural studies on 9-hydrazono-6,7,8,9-tetrahydro-4-oxo-4H-pyrido[1,2-a]pyrimidines by 1H, 13C and 15N NMR spectroscopy

¹H, ¹³C and ¹⁵N NMR studies demonstrated that 9-hydrazono-6,7,8,9-tetrahydro-4-oxo-4H-pyrido-[1,2-a] pyrimidlnes exist as an equilibrium mixture of Z-E isomers in the hydrazono–imino tautomeric form having an exocyclic double bond. Proton-catalysed Z-E interconversion is fast. Substituent and solvent effects revealed that the decisive factors controlling the Z:E ratio are internal hydrogen bonding in the Z-isomer, stabilization by solvation and steric interaction.     

Synthesis and Stereoisomerization of 2-(1-Alkoxyimino-2,2,2-trifluoroethyl)-5-trimethylsilylfurans

2-(1-Alkoxyimino-2,2,2-trifluoroethyl)-5-trimethylsilylfurans were synthesized by the condensation of 2-(trifluoroacetyl)-5-trimethylsilylfuran with alkoxyamines. According to ¹H and ¹⁹F NMR spectroscopic data, the alkoxyimino group in the E-isomers descreens the H-3 and H-4 protons of the furan ring more strongly than in the Z-isomers, shifting their signals downfield. The fluorine atoms of the α-trifluoromethyl group in the Z-isomer are characterized by a downfield shift in relation to the E-isomer. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 834–838, June, 2005.     

Study of alkaloids from the flora of the Siberian and Altai regions. 6. Crystal and molecular structure of songorine Z-oxime

Oximation of songorine afforded a mixture of its Z- and E-oximes. The crystal and molecular structure of the Z-isomer was established by X-ray diffraction analysis. Its structure was also confirmed by the spectral data (2D ¹H—¹H and ¹³C—¹H NMR spectroscopy and mass spectrometry). The structure of isomeric E-oxime was established by comparing its NMR spectroscopic data (¹H and ¹³C) with the data for the Z-isomer.     

On the Mechanism of a Modified Perkin Condensation Leading to a-Phenylcinnamic Acid Stereoisomers – Experiments and Molecular Modelling

 A modified Perkin condensation leading to α-phenylcinnamic acid stereoisomers affords predominantely or exclusively the (E)-isomer. Reaction duration, temperature, and polarity of the solvent affect the isomeric distribution only to a minor extent. However, geometry optimization of the stereoisomers by semiempirical quantum chemical methods revealed that their thermodynamic stabilities in the absolute minimum are the same. (E)-(Z)-isomerization under the conditions of the Perkin condensation resulted in an equilibrium mixture of nearly 1:1 composition, thus corroborating the results of the computations. Comparing the detailed potential energy maps of the isomers obtained at the semiempirical level of theory may explain the controversy. The (Z)-isomer has a structured potential energy surface with few but well-defined minima, whereas that of the (E)-isomer shows an extended flat plateau enabling this isomer to approach a minimum (which is close to the global one) right after its formation.     

Crystal structure and quantum chemical calculations of (E)1-benzyl-3-((4-methoxyphenyl)imino)-5-methylindolin-2-one

The known Schiff base compound, (E)1-benzyl-3-((4-methoxyphenyl)imino)-5-methylindolin-2-one, was prepared as before by reacting 1-benzyl-5-methylindoline-2,3-dione with 4-methoxyaniline. The product was unambiguously characterized using elemental analysis, ¹H and ¹³C-NMR spectroscopy, and its new single-crystal X-ray structural analysis. Molecular orbital calculations were conducted in order to investigate the structures and relative stabilities of the (E) and (Z) isomers of 1-benzyl-3-([4 methoxyphenyl]-imino)-5-methylindolin-2-one. Specific attention was paid to the (E) isomer. The available crystallographic experimental data for the latter ensured also validation of the model structures computationally derived at the theoretical B3LYP/6-31G(d,p) level.     

Synthesis,characterization and selective Cu2+ recognition of novel E– and Z–stilbenophanes

We have demonstrated the synthesis and characterization of novel E- and Z-stilbenophanes via four reaction steps. X-ray structure analysis showed that the p-electrons of the double bonds and the oxygen atoms pointing towards the center of a cavity of Z-isomer. Both isomers recognize selectively Cu²⁺ ions over other competing metal ions as measured by UV-vis titration by the detection limit 10–12 µM. The 1:1 ratio complex formation was confirmed by HRMS analysis. Theoretical calculations based on GIAO and CMAD approaches and ¹H NMR analysis confirmed the UV-vis data of Z-isomer.     

Studies on chiral thiophosphoric acids and their derivatives: 4. The stereospecific synthesis of optically active O-ethyl O-phenyl O-(1-methyl-2-ethoxycarbonyl) vinyl phosphorothionate

The stereoisomers (3 and 4) of O-ethyl O-phenyl O-(1-methyl-2-ethoxycarbonyl) vinyl phosphorothionate have been synthesized by the reaction of optically active O-ethyl O-phenyl phosphorothiochloride 2 with ethyl acetoacetate under different conditions. 3 (100% Z-isomer, determined by ¹H NMR) was synthesized by the reaction of 2 with ethyl sodio-acetoacetate in the mixed solvent of 1:3 toluene-dioxane at 50°C. 4 (>95% E-isomer) was obtained by the reaction of 2 with ethyl acetoacetate in presence of t-BuOK in DMSO at 15°C. 100% E-isomer 4 was separated from crude 4(>95% E-isomer) by column chromatography on silica gel (petroleum ether-ether 6:1). By this reaction either Z- or E-isomers were formed with inversion of the configuration at phosphorus atom. Thus, six stereoisomers of 3 and 4 which were prepared from 2 (RS, S, R) by the above method namely (RS)-Z, (R)-Z, (S)-Z and (RS)-E, (R)-E, (S)-E.     

Rh(I)/(III)-N-Heterocyclic Carbene Complexes: Effect of Steric Confinement Upon Immobilization on Regio- and Stereoselectivity in the Hydrosilylation of Alkynes

Rh(I) NHC and Rh(III) Cp* NHC complexes (Cp*=pentamethylcyclopentadienyl, NHC=N-heterocyclic carbene=pyrid-2-ylimidazol-2-ylidene (Py−Im), thiophen-2-ylimidazol-2-ylidene) are presented. Selected catalysts were selectively immobilized inside the mesopores of SBA-15 with average pore diameters of 5.0 and 6.2 nm. Together with their homogenous progenitors, the immobilized catalysts were used in the hydrosilylation of terminal alkynes. For aromatic alkynes, both the neutral and cationic Rh(I) complexes showed excellent reactivity with exclusive formation of the β(E)-isomer. For aliphatic alkynes, however, selectivity of the Rh(I) complexes was low. By contrast, the neutral and cationic Rh(III) Cp* NHC complexes proved to be highly regio- and stereoselective catalysts, allowing for the formation of the thermodynamically less stable β-(Z)-vinylsilane isomers at room temperature. Notably, the SBA-15 immobilized Rh(I) catalysts, in which the pore walls provide an additional confinement, showed excellent β-(Z)-selectivity in the hydrosilylation of aliphatic alkynes, too. Also, in the case of 4-aminophenylacetylene, selective formation of the β(Z)-isomer was observed with a neutral SBA-15 supported Rh(III) Cp* NHC complex but not with its homogenous counterpart. These are the first examples of high β(Z)-selectivity in the hydrosilylation of alkynes by confinement generated upon immobilization inside mesoporous silica.     

Réaction d'ylures stabilisés sur des aldéhydosucres: influence de divers facteurs dont la structure de l'aldéhydosucre sur le pourcentage d'isomères géométriques obtenu

Reactions of stabilized ylides with aldehydosugars: influence of some factors, particularly the structure of the aldehydosugar, on the ratio of the geometrical isomers formed Aldehydosugars bearing no cis-substituent on the C-atom β to the carbonyl group ( 2, 3, 6, 7 ) or bearing a substituent without an electron lone-pair ( 5 ) gave, upon treatment with acetylmethylidenetriphenylphosphorane, exclusively the E-isomer, this representing the classical behaviour of aldehydes. On the other hand, aldehydosugars having a lone-pair bearing substituent ( 1, 4, 8, 9 ) gave a mixture of E- and Z-isomers. In the case of most of the aldehydosugars of the latter type (giving some Z-isomer) a partial epimerization of the C-atom α to the carbonyl group took place when the solvent of the Wittig reaction was HCONMe₂. It is probable that the presence of an electron lone-pair in the adequate position inhibits the reversion of the kinetic erythro-betaine, allowing the formation of the Z-isomer and, in particular solvent conditions, a cycloelimination leading to the epimeric aldehydosugar which ultimately reacts to give the epimeric E-enone.     

On the Mechanism of a Modified Perkin Condensation Leading to a-Phenylcinnamic Acid Stereoisomers – Experiments and Molecular Modelling

Summary.  A modified Perkin condensation leading to α-phenylcinnamic acid stereoisomers affords predominantely or exclusively the (E)-isomer. Reaction duration, temperature, and polarity of the solvent affect the isomeric distribution only to a minor extent. However, geometry optimization of the stereoisomers by semiempirical quantum chemical methods revealed that their thermodynamic stabilities in the absolute minimum are the same. (E)-(Z)-isomerization under the conditions of the Perkin condensation resulted in an equilibrium mixture of nearly 1:1 composition, thus corroborating the results of the computations. Comparing the detailed potential energy maps of the isomers obtained at the semiempirical level of theory may explain the controversy. The (Z)-isomer has a structured potential energy surface with few but well-defined minima, whereas that of the (E)-isomer shows an extended flat plateau enabling this isomer to approach a minimum (which is close to the global one) right after its formation. Received May 7, 2000. Accepted (revised) June 13, 2000     

Capricious Stereoselectivities of Alkenylpotassium Formation under Kinetic and Thermodynamic Control Oxygen-Triggered Configurational Equilibration of an Allyl Type Organometallic Compound

(Z)-1-Alkylated propenes undergo metallation at an allylic site more rapidly than do their (E)-isomers, no matter in which configuration the resulting organometallic is thermodynamically more stable. A striking illustration is given by (Z)-4,4-dimethyl-2-pentenyl potassium which is formed 15 times faster than its (E)-isomer, although the latter is clearly favored under equilibrium conditions. The configurational reorganization of alkenylpotassium compounds in tetrahydrofuran solution is very slow. At least in one case, however, traces of oxygen efficiently catalyse the Z/E equilibration.     

NMR spectroscopic study of the (Z)/(E)-isomerism of 1-aryl-3-arylamino-2-propen-1-ones in solution and in the crystalline state

Summary ¹H,¹³C, and CP/MAS¹³C NMR spectra of sixp-substituted 1-aryl-3-arylamino-2-propen-1-ones in solution and in the solid state are reported and discussed. In the proteon-accepting solvent dimethylsulfoxide, electronegative substituents shift the isomeric equilibrium to the (E)-isomer. Bulky substituents promote crystallization of the (Z)-form.

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NMR-Spektroskopische Untersuchung der (Z)/(E)-Isomerie von 1-Aryl-3-arylamino-2-propen-1-onen in Lösung und im Kristallzustand (Kurze Mitt.)

Zusammenfassung ¹H-,¹³C- und CP/MAS-¹³C-NMR Spektren von sechsp-substituierten 1-Aryl-3-arylamino-2-propen-1-onen in gelöstem und festem Zustand werden berichtet und diskutiert. In protonenakzeptierendem Dimethylsulfoxid verschieben elektronegative Substituenten das Gleichgewicht zum (E)-Isomer. Große Substituenten begünstigen die Kristallisation in der (Z)-Form.

     

13C NMR studies of conformational isomerism in thioureas: Signal assignments via chemical shift and population trends

The 22.63 MHz ¹³C NMR spectra of a series of alkylated thioureas are reported. Characteristic Z and E spectral regions were found for the ¹³C ? S resonances. The two regions were generally found to be non-overlapping for the series, with the region of the Z, Z resonances occurring more downfield than those of either the Z, E or E, Z conformers in the cases of 1,3-disubstitution. The Z, Z configuration became favored and the relative chemical shift difference (Rδ) increased linearly with increasing substituent size. At 217 K, hindered internal rotation caused a multiplicity of resonances which were normally single peaks in the broad band ¹H decoupled 62.86 MHz ¹³C spectrum of CH₃NHCSNH(CH₂)₂NHCSNHCH₃ (2MTE) at room temperature. The trends in chemical shifts and populations were employed to assign tentatively the resonances of five of the six possible configurational isomers contributing to the 2MTE spectra at 217 K. The isomer populations are given. The ¹³C NMR spectra reported here led to signal assignments of Z and E isomers which supported prior ¹H NMR results and contradicted more recent results of another ¹³C NMR study of N-methylthiourea. The major peak of the exchange doublet occurs at relatively high field strengths in both methanol-d₅.     

Absolute Konfiguration von Loroxanthin (=(3R, 3′R, 6′R)-β, ϵ-Carotin-3, 19, 3′-triol)

Absolute Configuration of Loroxanthin (=(3R, 3′R, 6′R)-β, ?-Carotene-3, 19, 3′-triol) ‘Loroxanthin’, isolated from Chlorella vulgaris, was separated by HPLC. methods in two major isomers, a mono-cis-loroxanthin and the all-trans-form. Solutions of the pure isomers easily set up again a mixture of the cis/trans-isomers. Extensive ¹H-NMR. spectral measurements at 400 MHz allowed to establish the 3′, 6′-trans-configuration at the ?-end group in both isomers and the (9E)-configuration in the mono-cis-isomer. The absolute configurations at C(3) and C(6′) were deduced from CD. correlations with synthetic (9Z, 3R, 6′R)-β, ?-carotene-3, 19-diol ( 5 ) and (9E, 3R, 6′R)-β, ?-carotene-3, 19-diol ( 6 ), respectively. Thus, all-trans-loroxanthin ( 3 ) is (9Z, 3R, 3′R, 6′R)-β, ?-carotene-3, 19, 3′-triol and its predominant mono-cis-isomer is (9E, 3R, 3′R, 6′R)-β, ?-carotene-3, 19, 3′-triol ( 4 ). Cooccurrence in the same organism and identical chirality at all centers suggest that loroxanthin is biosynthesized from lutein ( 2 ).     

Stereospecific adduct ion formation in the chemical ionization mass spectra of E- and Z-1,2,3-triaryl-2-propen-1-ones

Stereospecific adduct ion formation has been observed in the chemical ionization mass spectra (positive and negative) of certain E- and Z-1,2,3-triaryl-2-propen-1-ones. The Z isomers are found to give higher relative abundances of adduct ions than the E isomers. This has been interpreted in terms of the differences in the proton affinities of the isomers originating from their different degrees of enone resonance. Halide ion (CI^? and Br^?) attachment spectra of these compounds also show stereochemical differences in the relative abundances of [M]^?˙ and [M+halide]^? ions, though the effect is not as pronounced as in the case of the positive ion spectra.