Initiation of Radical Chain Reactions of Thiol Compounds and Alkenes without any Added Initiator: Thiol‐Catalyzed cis/trans Isomerization of Methyl Oleate

A kinetic study of the dodecanethiol‐catalyzed cis/trans isomerization of methyl oleate (cis‐ 2 ) without added initiator was performed by focusing on the initiation of the radical chain reaction. The reaction orders of the rate of isomerization were 2 and 0.5 for 1 and cis‐ 2 , respectively, and an overall kinetic isotope effect k_H/k_D of 2.8 was found. The initiation was shown to be a complex reaction. The electron‐donor/‐acceptor (EDA) complex of dodecanethiol ( 1 ) and cis‐ 2 formed in a pre‐equilibrium reacts with thiol 1 to give a stearyl and a sulfuranyl radical through molecule‐assisted homolysis (MAH) of the sulfur–hydrogen bond. Fragmentation of the latter gives the thiyl radical, which catalyzes the cis/trans isomerization. A computational study of the EDA complex, MAH reaction, and the sulfuranyl radical calculated that the activation energy of the isomerization was in good agreement with the experimental result of E_A=82 kJ M ^?1. Overall, the results may explain that the thermal generation of thiyl radicals without any initiator is responsible for many well‐known thermally initiated addition reactions of thiol compounds to alkenes and their respective polymerizations and for the low shelf‐life stability of cis‐unsaturated thiol compounds and of mixtures of alkenes and thiol compounds.     

Catalytic activity of the VIII Group metals in the hydrogenation and isomerization of α- and β-pinenes

The kinetic regularities of the liquid-phase hydrogenation and isomerization of α- and β-pinenes over the Pd/C, Ru/C, Rh/C, Pt/C, and Ir/C catalysts were studied at temperatures ranging from 20 to 100 °C and at hydrogen pressures of 1–11 bar using n-octane as the solvent. The hydrogenation and isomerization of α- and β-pinenes occur simultaneously on the Ru/C, Rh/C, Pt/C, and Ir/C catalysts, and the reaction mixture contains the products of double bond hydrogenation, viz., cis- and trans-pinanes. The Ru, Rh, and Pd metals have a higher catalytic activity in β-pinene isomerization than Ir and Pt. Among the VIII Group metals studied, the Pd-based catalyst has the highest catalytic activity in double bond isomerization of α- and β-pinenes. The general scheme of the mechanism of hydrogenation and isomerization of α- and β-pinenes on the Pd/C catalyst was proposed.     

Initiation of radical chain reactions of thiol compounds and alkenes without any added initiator: thiol-catalyzed cis/trans isomerization of methyl oleate

A kinetic study of the dodecanethiol-catalyzed cis/trans isomerization of methyl oleate (cis-2) without added initiator was performed by focusing on the initiation of the radical chain reaction. The reaction orders of the rate of isomerization were 2 and 0.5 for 1 and cis-2, respectively, and an overall kinetic isotope effect k(H)/k(D) of 2.8 was found. The initiation was shown to be a complex reaction. The electron-donor/-acceptor (EDA) complex of dodecanethiol (1) and cis-2 formed in a pre-equilibrium reacts with thiol 1 to give a stearyl and a sulfuranyl radical through molecule-assisted homolysis (MAH) of the sulfur-hydrogen bond. Fragmentation of the latter gives the thiyl radical, which catalyzes the cis/trans isomerization. A computational study of the EDA complex, MAH reaction, and the sulfuranyl radical calculated that the activation energy of the isomerization was in good agreement with the experimental result of E(A)=82?kJ M(-1). Overall, the results may explain that the thermal generation of thiyl radicals without any initiator is responsible for many well-known thermally initiated addition reactions of thiol compounds to alkenes and their respective polymerizations and for the low shelf-life stability of cis-unsaturated thiol compounds and of mixtures of alkenes and thiol compounds.     

The β-carotene dilemma: ESR study with coordinated peroxyl radicals

The apparently unpredictable behaviour of β-carotene in the supplementation of the diet of smokers is discussed in the light of the reactions of peroxyl radicals with β-carotene in the absence of oxygen. The decay of tert-butylperoxyl radicals in the presence of β-carotene was studied at ambient temperature in non-polar solvents by ESR spectroscopy. The primary reaction in the absence of oxygen is interpreted as a spin-trapping effect of a peroxyl radical by β-carotene producing an intermediate labile free radical, which disappears after recombination with a second tert-butylperoxyl radical. The result is the transformation of β-carotene to a diamagnetic compound with two peroxy bonds. In the presence of chelating transition metals with unpaired d-electrons as electron donors the peroxy group of the oxidized β-carotene can be split to alkoxyl free radicals. The primary attack of tert-butylperoxyl radicals is completely inhibited in the presence of vitamin E followed by production of free aryloxy radicals and the presence of oxygen has no significant effect on this reaction. Spin-trapping of peroxyl radicals by the double bond of vitamin A leads to its oxidation in the absence of vitamin E. Transition metal ions such as Co, Cr, Fe, and Mn, known to be present in the aerosol of cigarette smoke, homolyse the peroxyl bonds of peroxidised β-carotene, which results in cell damage.     

Photoinduced Isomerization of Potassium Polyfluoroalk‐1‐enyltrifluoroborates

The UV irradition of K [RCF=CFBF₃] [R = C₄F₉ (trans), C₂F₅ (cis), C₆F₁₃ (cis), Cl (cis/trans 1 : 1)] in acetone led to cis/trans‐isomerization with a final cis/trans composition 7 : 3. In the case of R = C₄H₉ (trans) or C₃F₇O (cis/trans 25 : 75) the photoisomerization was accompanied by a partial decomposition.     

Electron-Transfer-Catalyzed cis → trans Isomerization of 1,1′-Azonorborane. Prototype of a reversible two-stage storage system

ESR and cyclic voltammetry investigations show that isomerization of the radical cation of cis-1,1′-azonorbornane (cis- 1 ) to the trans-radical ion proceeds too fast in solution for direct investigation of the cis-radical ion even at ?78°. The facile isomerization of the radical cation is in agreement with PM 3 calculations proposing an activation barrier of only 17 kJ/mol. As a consequence, quantitative cis → trans isomerization of 1,1′-azonorbornane can effectively be accomplished by addition of catalytic amounts of one-electron oxidants. This is the first evidence for a radical-cation-catalyzed cis → trans isomerization of azo compounds.     

Structure elucidation of β-carotene isomers by HPLC-NMR coupling using a C30 bonded phase

The separation of cis/trans isomers of β-carotene has been performed with a C₃₀ stationary phase employing ¹H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison of simulated 1D ¹H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of the separated compounds is inhibited. Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996     

Photochromism of 4-acryloxyazobenzene/(−)-menthyl acrylate copolymers

Copolymers of trans-4-acryloxyazobenzene ( AAB ) with (?)-menthyl acrylate ( MtA ) have been prepared by free radical initiation; comonomer reactivity ratios have been found to be r _AAB = 0.89 and r _MtA = 0.53. Dependence of chiroptical properties on copolymer composition has been investigated and contribution to circular dichroism by azobenzene chromophores of isolated and sequence AAB units, as well as the influence of the trans to cis photoisomerization, have been determined. Kinetics of photoinduced trans → cis and thermally induced cis → trans isomerization of azobenzene side chains have also been examined and the kinetic parameters evaluated with relation to copolymer structure. The results have been discussed in terms of light-induced secondary structure reversible modifications.     

Structure elucidation of β-carotene isomers by HPLC-NMR coupling using a C30 bonded phase

The separation of cis/trans isomers of β-carotene has been performed with a C₃₀ stationary phase employing ¹H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison of simulated 1D ¹H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of the separated compounds is inhibited. Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996     

Diethoxy‐azobis(pyridinium) Salt as Photoinitiator for Cationic Polymerization: Towards Wavelength Tunability by Cis–Trans Isomerization

N,N′‐diethoxy‐4,4′‐azobis(pyridinium) hexafluorophosphate (DEAP) has been synthesized by alkylation of the corresponding N‐oxide and characterized. DEAP exhibits UV induced cis–trans isomerization with absorptions at around λ = 459 and 360 nm, respectively. The ability of the DEAP ion to act as a photoinitiator for the cationic polymerization of cyclohexene oxide and N‐vinylcarbazole is demonstrated. The initiation step involves the decay of the excited state of the trans form of the salt with homolytic bond rupture of the nitrogen–oxygen bond. Its potential use as a photoinitiator for free radical polymerization is also demonstrated using methyl methacrylate monomer as the example.

     

Unusually facil cis to trans isomerization of a polypropiolate bearing a long alkyl chain polymerized using a [Rh(norbornadiene)Cl]2 catalyst: An ESR study

Alkylpropiolate having n-hexadecyl moiety in the ester group was stereospecifically polymerized with a [Rh(norbornadiene)Cl]₂ complex catalyst in alcohol to give rise to cis-transoid polyacetylenes in high yields. The unusually facile cis to trans isomerization for the polymer was found to be induced when the polymer was warmed to even less than ca. 57°C. Under these conditions ESR spectra bearing hyperfine structures due to the radicals generated by the rotational scission in the cis–π bond were observed. This isomerization induced a large g value shift in the ESR spectra together with an increase of the radical concentration during the isomerization, indicating formation of planar conjugated trans sequences that stabilize mobile unpaired electrons as solitons. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2457–2461, 1998     

Catalytic isomerization of allylbenzene on organomagnesium clusters

Catalytic isomerization of allylbenzene to form trans-β-methylstyrene quantitatively occurs on the magnesium-anthracene cluster adduct under mild conditions. A low-stability organomagnesium compound, presumably of the cluster nature and active in catalytic allylbenzene isomerization, is formed by cocondensation of magnesium and allylbenzene vapors at the liquid nitrogen temperature. The products of low-temperature solid-phase reactions of magnesium with hydrocarbons containing aromatic rings exhibit high catalytic activity in the allyl isomerization of allylbenzene. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1155–1158, May, 2005.     

Tracking radical migration in large hydrogen deficient peptides with covalent labels: Facile movement does not equal indiscriminate fragmentation

Photodissociation of iodo-tyrosine modified peptides yields localized radicals on the tyrosine side chain, which can be further dissociated by collisional activation. We have performed extensive experiments on model peptides, RGYALG, RGYG, and their derivatives, to elucidate the mechanisms underlying backbone fragmentation at tyrosine. Neither acetylation nor deuteration of the tyrosyl phenolic hydrogen significantly affects backbone fragmentation. However, deuterium migration from the tyrosyl β carbon is concomitant with cleavage at tyrosine. Substitution of tyrosine with 4-hydroxyphenylglycine, which does not have β hydrogens, results in almost complete elimination of backbone fragmentation at tyrosine. These results suggest that a radical situated on the β carbon is required for a-type fragmentation in hydrogen-deficient radical peptides. Replacement of the αH of the residue adjacent to tyrosine with methyl groups results in significant diminution of backbone fragmentation. The initial radical abstracts an αH from the adjacent amino acid, which is poised to “rebound” and abstract the βH of tyrosine through a six-membered transition-state. Subsequent β-scission leads to the observed a-type backbone fragment. These results from deuterated peptides clearly reveal that radical migration in peptides can occur and that multiple migrations are not infrequent. Counterintuitively, close examination of all experimental results reveals that the probability for fragmentation at a particular residue is well correlated with thermodynamic radical stability. A-type fragmentation therefore appears to be most likely when favorable thermodynamics are combined with the relevant kinetic control. These results are consistent with ab initio calculations, which demonstrate that barriers to migration are significantly smaller in magnitude than probable dissociation thresholds.     

Quantum-chemical simulation of the allyl isomerization of allylbenzene in the presence of gold atom

The isomerization of allylbenzene into trans- and cis-β-methylstyrene was simulated by the PBE density functional method for the allylbenzene-Au ^z (z = 0, +1) system. It was discovered that the activation energy falls and the rate constant of allylic isomerization of allylbenzene rises upon the transition from Au⁰ to Au⁺. The experimentally observed preferential formation of trans-isomer is verified by our calculations.     

Feruloyl,caffeoyl, and flavonol glucosides from <Emphasis Type="Italic">Equisetum hyemale</Emphasis>

Feruloyl, caffeoyl, and flavonol glucosides were isolated from the stems of Equisetum hyemale L. The structures of the compounds were elucidated as trans-feruloyl-4-β-glucoside (1), cis-feruloyl-4-β-glucoside (2), trans-caffeoyl-3-β-glucoside (3), kaempferol-3-sophoroside (4), kaempferol-3-sophoroside-7-β-glucoside (5), and herbacetin-3-sophoroside-8-β-glucoside (6) based on the spectral evidence.     

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.     

Thermal- and UV-induced cis/trans isomerization of undoped polyacetylene films. An ESR and IR investigation

The UV- and thermal-induced cis→trans isomerization of undoped polyacetylene (PA) films has been investigated. The results have shown that temperature and UV light promote the isomerization of PA with a similar mechanism. We suggest that the formation of paramagnetic defects in trans PA takes place by bond rehybridization, in agreement with previous hypotheses, and that both spin concentration and spin delocalization depend on the temperature of isomerization. It was found that under UV irradiation thermal cis→trans isomerization of polyacetylene also takes place at temperatures at which the sole thermal treatment isomerization is much lower or nil, and we suggest that the possibility of preparing trans PA under relatively mild conditions may lead to a better material.     

Rhodium-Katalysierte Isomerisierungen von 2-Methylidenglutarsäureestern zu Methylglutaconsäureestern

The Rhodium(III)-catalyzed thermal isomerization of 2-methylidenglutaric acid esters affords predominantly mixtures of cis/trans-2-methylglutaconic acid esters ( 2, 3 ) which upon slow distillation isomerize completely into the cis-esters ( 2 ). Saponification of 2 yields trans-2-methylglutaconic acid ( 5 ). Attempts to prepare the acid chloride of 5 produces 6-chloro-5-methylpyrone-2 ( 9 ) or 6-chloro-3-methylpyrone-2 ( 10 ) which react with anilines to N-substituted derivatives of trans-4-methylglutaconic acid amides ( 11 ). The thermal isomerizations of the respective esters are discussed in terms of 1,5-hydrogen shifts in their ester enol structures.     

Study of <Emphasis Type="Italic">cis</Emphasis>–<Emphasis Type="Italic">trans</Emphasis> isomerization mechanism of 3,3′-azobenzene disulphonate in the lowest singlet and triplet electronic states by density functional theory

Abstract  

The cis–trans isomerization pathways of 3,3′-azobenzene disulphonate in the S₀ and T₁ states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S₀ state, the cis–trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis–trans isomerization of 3,3′-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S₀ state is 22.79 kcal mol⁻¹. In the T₁ state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T₁ state is related to a lower energy barrier, 5.02 kcal mol⁻¹, but requires a change in spin-multiplicity.     

Degradation processes in the cellulose/<Emphasis Type="Italic">N</Emphasis>-methylmorpholine-<Emphasis Type="Italic">N</Emphasis>-oxide system studied by HPLC and ESR. Radical formation/recombination kinetics under UV photolysis at 77 K

Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC) and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature, radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide type radicals ∼CH₂–NO^•–CH₂∼ and/or ∼CH₂–NO^•–CH₃∼ at the first and methyl ^•CH₃ and formyl ^•CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g., Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO.