The cis-trans isomerization of 1,2,5,6-tetrasilacycloocta-3,7-dienes: analysis by mechanistic probes and density functional theory

A series of alkyl- and aryl-substituted derivatives of cis,cis-1,2,5,6-tetrasilacycloocta-3,7-diene were prepared. Isomerization of these compounds to the corresponding trans,trans-1,2,5,6-tetrasilacycloocta-3,7-dienes by exposure to Ru and Zr hydride complexes was explored. Experimental probes of the isomerization were consistent with a stepwise mechanism involving metal hydride addition/elimination rather than one involving radical intermediates. Analysis of the low energy conformers of the various cis and trans isomers of 1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacycloocta-3,7-diene using density functional theory suggested the following trend in stability: trans,trans > cis,trans > cis,cis. The calculated trend in stability was consistent with the experimentally observed unidirectional isomerization of the carbon-carbon double bonds from all cis to all trans and supports a cis,trans isomer as a tenable intermediate.     

Polymeric organosilicon systems: XIII. Thermal polymerization of 1,2,5,6-tetrasilacycloocta-3,7-diynes

Treatment of 1,2,5,6-tetrasilacycloocta-3,7-diynes in a degassed sealed tube at 230°C afforded ring-opened polymers, poly[(disilanylene)ethynylenes] in high yields.     

Kinetics and equilibria of cis/trans isomerization of backbone amide bonds in peptoids

The biological activities of N-substituted glycine oligomers (peptoids) have been the subject of extensive research. As compared to peptides, both the cis and trans conformations of the backbone amide bonds of peptoids can be significantly populated. Thus, peptoids are mixtures of configurational isomers, with the number of isomers increasing by a factor of 2 with each additional monomer residue. Here we report the results of a study of the kinetics and equilibria of cis/trans isomerization of the amide bonds of N-acetylated peptoid monomers, dipeptoids, and tripeptoids by NMR spectroscopy. Resonance intensities indicate the cis conformation of the backbone amide bonds of the peptoids studied is more populated than is generally the case for the analogous secondary amide bond to proline residues in acyclic peptides. Rate constants were measured by inversion-magnetization transfer techniques over a range of temperatures, and activation parameters were derived from the temperature dependence of the rate constants. The rate of cis/trans isomerization by rotation around the amide bonds in the peptoids studied is generally slower than that around amide bonds to proline residues and takes place on the NMR inversion-magnetization transfer time scale only by rotation around the amide bond to the C-terminal peptoid residue.     

聚乙炔顺、反异构化机理研究

探讨了聚乙炔三种异构化-光、热、掺杂异构化的共性和特性,对光、热异构化过程提出了一种新的可能的"双自由基"机理,该机理在定性方面吻合了ESR实验结果,有限分子CNDO/2及INDO量子化学计算证明该机理在能量上最有利;估价了有限分子半经验MO方法在聚合物方面应用的可靠性,结果表明,这种简单易行的方法对一些规律性的预言是可靠的,从而为通常的半经验MO方法在高分子方面的推广应用提供了算例。     

Theoretical research of isomerization mechanisms from cis to trans form in polyacetylene

The characteristics of photo, thermal and doping isomerizations of polyacetylene were discussed. A new biradical mechanism for photo and thermal isomerizations is proposed, which is consistent with the ESR results. The CNDO/2 and INDO molecular orbital calculation results show that the mechanism is energetically the most favorable. The reliability of applying general semi-empirical MO methods in polymers is estimated and showed to be valuable in predicting properties.     

Thermal dimerization of 1,3-butadiene: Kinetics of the formation of cis,cis-cycloocta-1,5-diene

The thermal reaction of 1,3-butadiene (BD) has been studied between 464 and 557°K at pressures between 49 and 450 torr. The products are 4-vinylcyclohexene (VCH) and cis, cis-cycloocta-1,5-diene (COD), and their formations are second order. The rate constant (in 1/mol · sec) for VCH is given by and that for COD by The thermal reaction of COD has also been studied. The temperature was varied from 505 to 586°K and the pressure from 15 to 51 torr. The rate constants (in sec^?1) for the formations of VCH and BD are given by A biradical mechanism seems to fit these results. The heat of formation and the entropy of COD are estimated.     

Counter-current chromatographic estimation of hydrophobicity of Z-(cis) and E-(trans) enalapril and kinetics of cis/trans isomerization

The kinetics of Z-(cis)/E-(trans) isomerization of enalapril was investigated by reversed phase high-performance liquid chromatography (RP-HPLC) using a monolith ODS column under a series of different temperature and pH conditions. At a neutral pH 7, the rate (k(obs)) of Z-(cis)/E-(trans) isomerization of enalapril at 4 degrees C (9.4 x 10(-3)min(-1)) is much lower than at 23 degrees C (1.8 x 10(-1)min(-1)), while the fractional concentration of Z-(cis) isomer is always higher than that of E-(trans) isomer in the pH range 2-7. The fractional concentration of the E-(trans) isomer becomes a maximum (about 40%) in the pH range 3-6, where enalapril exists as a zwitterion. The hydrophobicity (logP(O/W)) of both isomers was estimated by high-speed counter-current chromatography (HSCCC). Normal phase HSCCC separation using a tert-butyl methyl ether-acetonitrile-20mM potassium phosphate buffer (pH 5) two-phase solvent system (2:2:3, v/v/v) at 4 degrees C was effective in partially separating the isomers, and the partition coefficient (K) of each isomer was directly calculated from the retention volume (V(R)). The logP(O/W) values of Z-(cis) and E-(trans) isomers were -0.46 and -0.65, respectively.     

Unconventional thermodynamically stable cis isomer and trans to cis thermal isomerization in reversibly photoresponsive [0.0](3,3')-azobenzenophane

Sterically hindered [0.0](3,3')-azobenzenophane exhibits thermal trans-to-cis isomerization to the thermodynamically stable cis-cis isomer, and reversible photochemical isomerization with good fatigue resistance.     

The conformations and vibrational spectra of 2,2,4,4,6,6,9,9-octamethyl-3,7-decadiyne

The IR spectra of the title compound as a solute in various solvents, as a melt and as a crystalline solid have been recorded. Raman spectra of the solutions, the melt and of the crystalline solid were obtained and semiquantitative polarization measurements carried out.The data have been interpreted in terms of one conformer (anti) present in the crystal. In the melt and in solutions an additional conformer (gauche) was present in low abundance, probably between 5 and 10%, and ΔH^o (gauche → anti) was estimated to be −7.9 ± 2 kJ mol⁻¹. Spectral correlations with the related molecule, 1,5-hexadiyne are pointed out.     

Unexpectedly fast cis/trans isomerization of Xaa-Pro peptide bonds in disulfide-constrained cyclic peptides

Acyclic dithiol and cyclic disulfide forms of the peptides Ac-Cys-Pro-Xaa-Cys-NH2 (Xaa = Phe, His, Tyr, Gly, and Thr) and Ac-Cys-Gly-Pro-Cys-NH2 and the peptide Ac-Ala-Gly-Pro-Ala-NH2 were synthesized and characterized by mass spectrometry and NMR spectroscopy. Rate constants kct and ktc for cis-to-trans and trans-to-cis isomerization, respectively, across the Cys-Pro or Gly-Pro peptide bonds were determined by magnetization transfer NMR techniques over a range of temperatures, and activation parameters were derived from the temperature dependence of the rate constants. It was found that constraints imposed by the disulfide bond confer an unexpected rate enhancement for cis/trans isomerization, ranging from a factor of 2 to 13. It is proposed that the rate enhancements are a result of an intramolecular catalysis mechanism in which the NH proton of the Pro-Xaa peptide bond hydrogen bonds to the proline nitrogen in the transition state. The peptides Ac-Cys-Pro-Xaa-Cys-NH2 and Ac-Cys-Gly-Pro-Cys-NH2 are model compounds for proline-containing active sites of the thioredoxin superfamily of oxidoreductase enzymes; the results suggest that the backbones of the active sites of the oxidized form of these enzymes may have unusual conformational flexibility.     

Mechanism of the photochemical cis⇌trans isomerization of thioindigo and 6,6′-diethoxy-thioindigo in solution

The light induced isomerization of thioindigo (I) and 6,6′-diethoxy-thioindigo(II) has been investigated by means of nanosecond flash photolysis and steady state methods. A mechanism is proposed in which a common triplet state generated from both the trans and cis isomers is responsible for the isomerization. Quantum yields at different oxygen concentrations     

Properties of astaxanthin/Ca2+ complex formation in the deceleration of cis/trans isomerization

Deceleration of the regioselective cis/trans isomerization of all-trans-astaxanthin (ASTX) in the presence of Ca2+ was shown by HPLC analysis. The NMR and ITC analyses provided evidence for complexation of ASTX with Ca2+ in 1:2 stoichiometry via chelation at both carbonyl and hydroxyl groups. The rotation across torsion omega6 (C5-C6-C7-C8) upon complexation is consistent with the NOE between 7-H and 5-CH3. This study supports the inhibitory effect of ASTX on calcium-induced turbidity of lens crystallins.     

Photochemical E (trans)-->Z (cis) isomerization in substituted 1-naphthylacrylates.

Substituted naphthylacrylates, 1-3, not showing rotamerism have been synthesized with a view to study photochemical E (trans)-->Z (cis) isomerization. Photostationary state composition of the isomers upon direct excitation, triplet sensitized isomerization, quantum yield of isomerization, and steady state and time-resolved fluorescence behavior have been studied for these naphthylacrylates. The direct excitations of the compounds yield high Z (approximately 80%) isomer composition, whereas the triplet sensitization results in less Z (approximately 20%) isomer composition. This indicates that the singlet pathway is very efficient in converting the E isomer to the Z isomer. The naphthylacrylates 1 and 2 exhibit structured fluorescence at room temperature in hexane and upon changing the solvent to CH3CN; the structure of the fluorescence is lost, indicating that the singlet excited-state develops a polar character in a polar environment. The polar nature of the singlet excited state becomes more clear in the case of 3 from its fluorescence solvatochromism. The naphthylacrylates did not exhibit excitation wavelength-dependent fluorescence at room temperature suggesting that the ground state conformers (rotamers) are not involved. Fluorescence lifetimes measured for these compounds displayed biexponential behavior, which is explained using a two-state model.     

Structural evolution of the chromophore in the primary stages of trans/cis isomerization in photoactive yellow protein

We have studied the structural changes induced by optical excitation of the chromophore in wild-type photoactive yellow protein (PYP) in liquid solution with a combined approach of polarization-sensitive ultrafast infrared spectroscopy and density functional theory calculations. We identify the nuC8-C9 marker modes for solution phase PYP in the P and I0 states, from which we derive that the first intermediate state I0 that appears with a 3 ps time constant can be characterized to have a cis geometry. This is the first unequivocal demonstration that the formation of I0 correlates with the conversion from the trans to the cis state. For the P and I0 states we compare the experimentally measured vibrational band patterns and anisotropies with calculations and find that for both trans and cis configurations the planarity of the chromophore has a strong influence. The C7=C8-(C9=O)-S moiety of the chromophore in the dark P state has a trans geometry with the C=O group slightly tilted out-of-plane, in accordance with the earlier reported structure obtained in an X-ray diffraction study of PYP crystals. In the case of I0, experiment and theory are only in agreement when the C7=C8-(C9=O)-S moiety has a planar configuration. We find that the carboxylic side group of Glu46 that is hydrogen-bonded to the chromophore phenolate oxygen does not alter its orientation on going from the electronic ground P state, via the electronic excited P state to the intermediate I0 state, providing conclusive experimental evidence that the primary stages of PYP photoisomerization involve flipping of the enone thioester linkage without significant relocation of the phenolate moiety.     

Diastereoselective cis to trans desymmetrization of dimethyl succinates

Meso-succinates, readily available by Diels–Alder cycloaddition of dimethyl maleate or maleic anhydride followed by esterification, can be isomerized quantitatively from the cis to the trans isomers in the presence of lithium alkoxides. The reaction performed with enantiopure chiral lithium alkoxides yields diastereomeric trans-succinates in good yield and selectivity.