Conformational analysis of substituted polyacetylenes

A conformational analysis of isolated chains of polymethylacetylene (PMA), polypentylacetylene (PPA), and poly(t-butylacetylene) (PTA) was carried out taking into account interactions between nonbonded atoms and torsional potentials. It was found that the trans configuration of all three polymers is more stable than the cis configuration, the difference in potential energy between the trans and the cis isomers however being very small for PTA, leading to the possibility of observing a trans^→ cis isomerization in some solvents. The calculations show that the substituted polyacetylenes are not found in a planar conformation, the larger deviations from planarity being found with the bulkier substituents: PTA > PPA > PMA. A correlation could be established between the UV absorption limit of the samples and the minimum torsional angle of the potential-energy functions. This relation predicts that the absorption limit is shifted to long wavelengths on increasing the planarity of the molecule. Moreover, UV spectra could be calculated from the potential-energy functions, and it is shown that the potential-energy functions of other substituted polyacetylenes can be calculated from their experimental UV spectra.     

Light-induced cis/trans isomerization of cis-[Pd(L-S,O)2] and cis-[Pt(L-S,O)2] complexes of chelating N,N-dialkyl-N′-acylthioureas: key to the formation and isolation of trans isomers

Irradiation cis-[M(Lⁿ-S,O)₂] complexes (M = Pt^II, Pd^II) derived from N,N-dialkyl-N′-benzoylthioureas (HLⁿ) with various sources of intense visible polychromatic or monochromatic light with λ < 500 nm leads to light-induced cis?→?trans isomerization in organic solvents. In all cases, white light derived from several sources or monochromatic blue-violet laser 405 nm light, efficiently results in substantial amounts of the trans isomer appearing in solution, as shown by ¹H NMR and/or reversed-phase HPLC separation in dilute solutions at room temperature. The extent and relative rates of cis/trans isomerization induced by in situ laser light (λ = 405 nm) of cis-[Pd(L²-S,O)₂] was directly monitored by ¹H NMR and ¹⁹⁵Pt NMR spectroscopy of selected cis-[Pt(L-S,O)₂] compounds in chloroform-d; both with and without light irradiation allows the δ(¹⁹⁵Pt) chemical shifts cis/trans isomer pairs to be recorded. The cis/trans isomers appear to be in a photo-thermal equilibrium between the thermodynamically favored cis isomer and its trans counterpart. In the dark, the trans isomer reverts back to the cis complex in what is probably a thermal process. The light-induced cis/trans process is the key to preparing and isolating the rare trans complexes which cannot be prepared by conventional synthesis as confirmed by the first example of trans-[Pd(L-S,O)₂] characterized by single-crystal X-ray diffraction, deliberately prepared after photo-induced isomerization in acetonitrile solution.     

Selective formation of thecis isomer of the nitridotechnetium(V) complex with 2,2′:6′,2″-terpyridine

The reaction of [TcNCl₂(PPh₃)₂] with 2,2′:6′,2″-terpyridine producedcis-[TcNCl₂(terpy)] selectively. The resulting complexes were characterized by¹H NMR and IR spectroscopy. The geometries of thecis andtrans isomers were estimated by theoretical calculations following a density functional method. Thecis isomer is likely more stable than thetrans one with respect to thetrans influence of the nitrido ligand. Furthermore, the behavior of nitridotechnetium complexes in polar solvents was compared to Os-analogues.     

A Study of the Cis-Trans Isomerization of a Square Planar pt Ylide Complex

The isomerization of cis-Pt(PPh₃)₂(I)(CH₂P(O)(OCH₃)₂), 1 , was studied by an NMR technique. An Arrhenius plot for the isomerization gives an activation energy of 99.2 KJ/mol, ΔH^≠ = 97 KJ/mol and ΔS^≠ = ?8.3 J/mol-K. Under a CO atmosphere the cis isomer catalytically isomerized to its trans form. Free PPh₃ did not catalyze the cis-trans isomerization. In the proposed isomerization mechanism the reaction goes through an intramolecular assisted phosphine dissociation, followed by dimer formation. The addition of phosphine to the dimer then completes the isomerization of the original monomer from cis to trans.     

Cis-trans isomerization and 13C-NMR chemical shift of polyphenylacetylene

Before and after cis-trans isomerization, the observed ¹³C-NMR chemical shifts of poly(phenylacetylene) (PPA) in the solid state were investigated on the basis of ¹³C-NMR chemical shift calculations within AM1 for the cis-transoidal and deflected trans-transoidal forms. Two ¹³C-resonance peaks in the observed CP/MAS ¹³C-spectrum were assigned theoretically by the ¹³C chemical shifts of the main and side chains. After thermal isomerization, the ¹³C peak of the main chain for PPA shifted upfield by 3.5 ppm, in contrast to the downfield shift of the ¹³C peak for polyacetylene. This upfield shift of trans-PPA largely was attributed to the increases of the excitation energy from the ground state to the lowest φ_π–π* state in the paramagnetic terms of ¹³C chemical shift on the main chain carbons with the increase in deflected angle τ of 0 to 80°. The ±80° deflected conformation of the trans-transoidal chain due to the cis-trans isomerization was confirmed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1657–1664, 1999     

Mechanistic insights into the cis-trans isomerization of ruthenium complexes relevant to catalysis of olefin metathesis

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single‐step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by ¹H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square‐pyramidal trans isomer, with the Ru?CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high‐energy structure collapses into the cis isomer. Then, the influence of the N‐heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square‐pyramidal metallacycle with the N‐heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis–trans isomerization of these Ru complexes.     

Synthesis and characterization of polymers of 1,4-hexadienes

The homopolymerization of trans-1,4-hexadiene, cis-1,4-hexadiene, and 5-methyl-1,4-hexadiene was investigated with a variety of catalysts. During polymerization, 1,4-hexadienes undergo concurrent isomerization reactions. The nature and extent of isomerization products are influenced by the monomer structure and polymerization conditions. Nuclear magnetic resonance (NMR) and infrared (IR) data show that poly(trans-1,4-hexadiene) and poly(cis-1,4-hexadiene) prepared with a Et₃Al/α-TiCl₃/hexamethylphosphoric triamide catalyst system consist mainly of 1,2-polymerization units arranged in a regular head-to-tail sequence. A 300-MHz proton NMR spectrum shows that the trans-hexadiene polymer is isotactic; it also may be the case for the cis-hexadiene polymer. These polymers are the first examples of uncrosslinked ozone-resistant rubbers containing pendant unsaturation on alternating carbon atoms of the saturated carbon-carbon backbone. Polymerization of the 1,4-hexadienes was also studied with VOCl₃- and β-TiCl₃-based catalysts. Microstructures of the resulting polymers are quite complicated due to significant loss of unsaturation, in contrast to those obtained with the α-TiCl₃-based catalyst. In agreement with the literature, there was no discernible monomer isomerization with the VOCl₃ catalyst system.     

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC → TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S₁→S₀ internal conversion (major), intersystem crossing S₁→T₁, pyran ring formation, photoenolization and TC → TT isomerization. Computations support the S₁→S₀ and T₁→S₀ channels as responsible for photostabilization of the TC form.     

Synthesis and structural characterization of syndiotactic trans‐1,2 and cis‐1,2 polyhexadienes

The (E) isomer in mixtures of (E) and (Z) 1,3‐hexadiene was polymerized with the system CoCl₂(PⁱPrPh₂)₂‐MAO, a highly active and stereospecific catalyst for the preparation of 1,2 syndiotactic polybutadiene. A new crystalline polymer with a melting point of 109 °C was obtained. The polymer was characterized by IR, NMR (¹³C, ¹H in solution and ¹³C in the solid‐state), X‐ray diffraction, DSC, GPC and it was found to have a trans‐1,2 syndiotactic structure with a 5.18 ± 0.04 Å fiber periodicity. Since only the (E) isomer was polymerized, at the end of the reaction we were able to separate the (Z) isomer, which was ultimately polymerized with CpTiCl₃‐MAO at low temperature, obtaining a low molecular weight, stereoregular polymer that, characterized by IR and NMR methods, was found to exhibit a cis‐1,2 syndiotactic structure, never reported before. Molecular mechanics calculations were carried out on the trans‐1,2 syndiotactic polymer and structural models consistent with the X‐ray diffraction data are proposed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5339–5353, 2007     

Kinetics and mechanisms of the reactions of thiosulphato-amine complexes of cobalt(III). Part I. Isomerization and base hydrolysis ofcis andtrans isomers of dithiosulphatobis(ethylenediamine) cobalt(III) ion in aqueous solution

Summary Investigations were carried out on the isomerization and base hydrolysis ofcis andtrans forms of dithiosulphatobis-(ethylenediamine)cobalt(III) ions. Thecis form isomerizes to thetrans form in neutral aqueous medium, rates being 1.15, 2.30 and 4.0×10^–5s^–1, respectively at 42, 50 and 58 °C. Thetrans complex isomerizes to thecis form in basic solution only, the rate varying with pH in a sigmoid pattern. In presence of OH^–, an acid-base equilibrium of the complex ion sets in, but only the basic form takes part in the isomerization reaction. Hydrolysis of thecis isomer proceeds through a base-dependent path only, but that of thetrans isomer proceeds both through base-dependent and base-independent paths. The mechanisms are associative in nature. Thetrans form reacts faster thancis in all cases.     

Dinuclear Tin(II) Complex of a Bulky cis‐Oxamide: Synthesis,Characterization, Crystal Structure,and DFT Studies

The reaction of the di‐lithiated oxamide of 1 with two equivalents of SnCl₂ provided the tin trans‐oxamide 3 . In solution, spectroscopic analysis suggests exclusively the formation of a trans‐oxamide (trans‐ 3 ). However, the solid state shows an atypical cis‐oxamide (cis‐ 3 ), where the oxamide fragment acts as an anti‐Janus head ligand. An ¹¹⁹Sn‐NMR variable temperature experiment ([D₈]THF) of the trans‐oxamide (trans‐ 3 ) was performed however, at lower temperature no additional signal was observed, which confirmed the absence of a dynamic equilibrium. Dispersion‐corrected density functional calculations revealed that the cis conformation of this tin(II) oxamide complex is more stable than the trans isomer by 1.4 kcal · mol^–1.     

1H NMR study of cis-trans isomerization in two analogs of the thiol form of thiamine

Proton magnetic resonance (¹H NMR) was used to study cis-trans isomerization in N-methyl-N-(1-methylthio-2-propenyl)formamide and N-benzyl-N-(1-methylthio-2-propenyl)formamide, two analogs of the thiol form of thiamine. Benzene dilution studies and shift reagent studies were used to make resonance assignments, which indicate that the predominant isomer for each analog has the C? C bond trans to the carbonyl oxygen. Shift reagent studies, using Pr(fod)₃ in CCl₄ or CDCl₃, suggest that the reagent may be bonding to both the nitrogen and oxygen atoms of the substrate. For some of the systems studied, varying ρ at constant temperature had the same spectral effect as varying temperature at constant ρ.     

Coinage Metal Complexes of a Tellurium Diimide: cis→trans Isomerization and Metal–Metal Interactions

The different coordination behavior of the ligand tBuN=Te(μ-NtBu)₂Te=NtBu (L) towards Cu⁺ and Ag⁺ results from a cis→trans isomerization. The two Cu⁺ ions in [Cu₂L₃]²⁺ (shown schematically) bridge trans and cis isomers of the ligand, whereas the Ag⁺ ions in [Ag₂L₂]²⁺ link two trans ligands and exhibit a weak Ag⋅⋅⋅Ag interaction.     

Polymerization of acetylenic derivatives. XXX. Isomers of polyphenylacetylene

Cis-transoidal (orange, soluble, and of low crystallinity) and cis-cisoidal (red, insoluble, and highly crystalline) polyphenylacetylenes (PPA) were prepared by Ziegler-Natta catalysts and trans-cisoidal (yellow, soluble, and amorphous) polyphenylacetylenes were prepared by using phosphine complexes, TiCl₄ and by thermal initiation. The cis-transoidal and cis-cisoidal structures isomerize thermally in the solid state above 100°C. In solution the cis-transoidal structure isomerizes above 80°C. The polymers obtained by thermal isomerization are soluble, amorphous, and have a trans-cisoidal structure. At temperatures higher than 120°C the cis–trans isomerization is accompanied by cyclization and by scission of the polymer chain. A method was developed for determination of cis content of cis-transoidal and cis-cisoidal polyphenylacetylenes.     

Cationic copolymerization of anethole: Reactivity of geometric isomers of α,β-disubstituted ethylenes

Cationic copolymerizations of anethole were carried out under various conditions in order to confirm the relative reactivities of its geometric isomers. trans-Anethole was more reactive than cis-anethole in copolymerizations with p-methoxystyrene or styrene, but less reactive in the mutual copolymerization of cis- and trans-anethole; i.e., the trans isomer was more reactive to a growing chain end with little steric hindrance. Thus the intrinsic reactivity of an olefinic double bond to carbonium ion is greater for the trans isomer than for the cis isomer. This idea is supported by ¹³C NMR spectra, since the signal of the olefinic β-carbon of the trans isomer is at higher field than that of the cis isomer. The behavior of anethole was compared with the results observed in vinyl ethers, where the cis isomer was always more reactive irrrspective of the structure of the growing chain end. In addition, the dependence of monomer reactivity ratios on polymerization conditions is discussed.     

Trans-cis isomerization of 4-(2-(9-anthryl)vinyl) pyridine.Molecular structures and ~1H NMR kinetic studies

Both cis- and trans-isomers of 4-(2-(9-anthryl)vinyl)pyridine were isolated and their molecular structures established by X-ray crystallographic method. Variable temperature 1H NMR spectroscopy was used to study the trans to cis isomerization of the title compound. The kinetic study of the reaction was based on the ratio of the NMR integration heights in toluene-d8 of the double doublet due to the cis-isomer at δ 8.51 to that of the multiplet at δ 8. 15 which was kept constant during the whole experiment. The isomerization process was found to be first order and the Arrhenius activation parameters Ea , In A ,△ H≠ and △ S≠ were calculated as 27.84kJ/mol, 6.71, 25.23 kJ/mol and - 197.89 J/(K·mol) , respectively. Besides,conformational analyses of both compounds based on molecular modelling were carried out and the results were used to compare with the experimental data.     

Dynamic Inversion of Stereoselective Phosphate Binding to a Bisurea Receptor Controlled by Light and Heat

A chiral bisurea anion receptor, derived from a first‐generation molecular motor, can undergo photochemical and thermal isomerization operating as a reconfigurable system. The two possible cis configurations in the isomerization cycle are opposite in helicity, as is shown by CD spectroscopy. ¹H NMR titrations demonstrate that the P and M helical cis isomers hold opposite enantioselectivity in the binding of binol phosphate, while anion complexation by the intermediate trans isomer is not selective. The difference in the binding affinity of the enantiomers was rationalized by DFT calculations, revealing very distinct binding modes. Thus, the enantiopreferred substrate binding in this receptor can be inverted in a dynamic fashion using light and heat.     

Conformational analysis of <Emphasis Type="Italic">cis-</Emphasis> and <Emphasis Type="Italic">trans</Emphasis>-isomers of 2,5-dimethyl-1,3-dioxane

The conformational isomerization of cis- and trans-isomers of 2,5-dimethyl-1,3-dioxane was investigated by means of ab initio RHF//6-31G(d) and PBE//ccpVDZ quantum-chemical methods. It has been shown that in comparison with cis-form the potential energy surface of trans-isomer has a less number of stationary points and includes the main minimum belonging to the diequatorial chair form. In the case of cisisomer the main minimum corresponds to the C 2e5a chair form. The values of potentional barriers of conformational isomerization were also determined. On the basis of experimental (NMR ¹H) and theoretical vicinal coupling constants we determine the value ΔG ⁰ of chair-chair inversion for the trans-isomer molecule.     

Photoinduced isomerization of 4-(4′-dimethylaminostyryl) pyridine N-oxide

The effects of photoirradiation on the conformation state of 4-(4′-dimethylaminostyryl)pyridine N-oxide was studied by spectral (EAS, ¹H NMR, IR) and thermochemical methods. Trans→cis isomerization was found to proceed in 4-(4′-dimethylaminostyryl)pyridine N-oxide irradiated with light (λ ≤ 400 nm) in chloroform. The conversion of the trans form of 4-(4′-dimethylaminostyryl)pyridine N-oxide to the cis form depends on the time and intensity of irradiation. Maximum conversion was achieved when a photostationary mixture formed with a ratio of components trans:cis = 40:60. Further irradiation of the solutions of 4-(4′-dimethylaminostyryl)pyridine N-oxide in chloroform led to salification with HCl formed during the decomposition of CHCl₃.     

Conformational studies of new pseudotripeptide with pyrazine amidoxime motif and simplified analogs using IR,NMR spectroscopy,and molecular dynamic simulations

Solution structures of new pyrazine-based pseudotripeptide with amidoxime function and simplified pseudodipeptide analogs were determined by a combination of IR and NMR spectroscopic studies and molecular dynamic simulations using explicit chloroform as a solvent. It was found that proline-phenylalanine dipeptide residue and amidoxime moiety in o-position are essential for intramolecular hydrogen bonding including a seven-membered γ-turn formation. In addition, a cis/trans equilibrium study was present for prolyl amides in polar solvents (D₂O and DMSO). A phenylalanine substituent was found to exhibit profound effect on thermodynamic parameters in prolyl peptides. The presence of intramolecular hydrogen bonds dramatically increases the amount of trans isomer in non-hydrogen-bonding CHCl₃ and significantly favor cis isomer in hydrogen-bonding solvents such as DMSO and D₂O. All molecules are not cytotoxic therefore they can be further studied in relation to potent biological activities.