Different Pathways in the Base-Promoted Isomerization of Benzyl Oxiranyl Ethers

The base-promoted isomerization of benzyl oxiranyl ethers was investigated. In particular it was shown that the reaction may proceed toward two main regioisomeric products: a benzyl vinyl ether or a 2-aryl-3-(hydroxyalkyl)oxetane, depending on subtle variations in the substitution on the phenyl ring. Disubstituted oxetanes were obtained in a stereoselective manner, thus providing a good entry to this class of synthetically useful compounds.     

Ring-disk electrode studies of the formation,growth and transformation of iodine films formed during the anodic oxidation of iodide on platinum

The kinetics and mechanism of the formation, growth and dissolution of iodine films on platinum during the electrooxidation of iodide have been investigated using rotating-disk and ring-disk techniques. The dissolution-precipitation mechanism of film formation and a linear growth law for film growth has been confirmed. The iodine film on platinum is believed to be made up of an ionically insulating barrier layer covered by a porous overlayer. The oxidation behavior of iodide and ferrous species shows that the iodine film is predominantly an iodide-ion conductor. Also, a film transformation, responsible for the transient features observed during iodide oxidation has been confirmed. This transformation changes the mechanism of iodide transport through the film from a partly “pore”-type to a Grotthus-type mechanism. The ring current maximum, which occurs at the same instant as the disk current minimum, reveals the mechanism of dissolution of iodine film during the transient period as involving complexation with iodide. Evidence for the mass-transport-controlled component of the potentiostatic transient response has been obtained from the sinusoidal hydrodynamic modulation response.     

Photodissociation dynamics of small aromatic molecules studied by multimass ion imaging

The photodissociation dynamics of various aromatic molecules, studied using multimass ion imaging techniques, is reviewed. The experimental data reveals new isomerization and dissociation mechanisms. Our investigation of benzene, pyridine, and pyrimidine finds that H-atom elimination thresholds remain the same for the three molecules. We also notice that ring-opening dissociation thresholds decrease rapidly with the increase of the number of nitrogen atoms in the aromatic ring. Hydrogen atom elimination is the sole dissociation channel for benzene at 193 nm. Along with H-atom elimination, we observe five distinct ring-opening dissociation channels for pyridine at 193 nm. No dissociation channels were observed for benzene and pyridine at 248 nm. Ring-opening dissociation channels are the major channels for pyrimidine, which dissociates at 193 nm and also at 248 nm. A six-membered to seven-membered ring isomerization was observed for photodissociation processes involving toluene, m-xylene, aniline, 4-methylpyridine, alpha-fluorotoluene, and 4-fluorotoluene, indicating a general isomerization mechanism for all such aromatic molecules. What is significant, is that during the isomerization, atoms (i.e., carbon, nitrogen, fluorine, and hydrogen) belonging to respective alkyl or amino groups are involved in an exchange with atoms within the aromatic ring. This type of isomerization is not observed in other aromatic isomerization mechanisms. For small tyrosine chromophores, such as phenol, 4-methylphenol, and 4-ethylphenol, H-atom elimination from a repulsive excited state plays a key role. However, dissociation is quenched in large chromophores like 4-(2-aminoethyl)-phenol. Our work demonstrates the capability and high sensitivity of multimass ion imaging techniques in the study of aromatic compounds.     

GROUP TRANSFER POLYMERIZATION OF ETHYL ACRYLATE WITH LEWIS ACID CATALYST

This paper reports the kinetics of group transfer polymerization (GTP)of ethyl acrylate (EA)with zinc iodide catalyst in 1,2-dichloroethane using dimethyl ketene methyl trimethylsilyl acetal (MTS) as initiator at 0℃and above 0℃. The amount of catalyst used was studied. When zinc iodide catalyst used is more than 10mol% relative to monomer, the rate of polymerization is proportional to the concentration of monomer, whereas zinc iodide catalyst used is less than 10 mol% of the monomer, the rate of polymerization is independent of the monomer concentration.In the GTP of EA an induction period was observed when the zinc iodide contents are less than l0mol%. If the reaction temperature is over 0℃, living species become unstable and diminish, leading to incomplete monomer conversion. The reaction curves equations are obtained. The polymers have narrow molecular weight distributions which are not changed as decreasing zinc iodide contents. The polydispersity is about 1.2.     

Highly efficient redox isomerization of allylic alcohols at ambient temperature catalyzed by novel ruthenium-cyclopentadienyl complexes--new insight into the mechanism

A range of ruthenium cyclopentadienyl (Cp) complexes have been prepared and used for isomerization of allylic alcohols to the corresponding saturated carbonyl compounds. Complexes bearing CO ligands show higher activity than those with PPh3 ligands. The isomerization rate is highly affected by the substituents on the Cp ring. Tetra(phenyl)methyl-substituted catalysts rapidly isomerize allylic alcohols under very mild reaction conditions (ambient temperature) with short reaction times. Substituted allylic alcohols have been isomerized by employing Ru-Cp complexes. A study of the isomerization catalyzed by [Ru(Ph5Cp)(CO)2H] (14) indicates that the isomerization catalyzed by ruthenium hydrides partly follows a different mechanism than that of ruthenium halides activated by KOtBu. Furthermore, the lack of ketone exchange when the isomerization was performed in the presence of an unsaturated ketone (1 equiv), different from that obtained by dehydrogenation of the starting allylic alcohol, supports a mechanism in which the isomerization takes place within the coordination sphere of the ruthenium catalyst.     

Model Study of the Photochemical Rearrangement Pathways of 1,2,4‐Oxadiazole

The mechanisms of photochemical isomerization reactions are investigated theoretically by using a model system of 1,2,4‐ oxadiazole with the CAS(14,9)/6‐311G(d) and MP2‐CAS‐(14,9)/ 6‐311++G(3df,3pd)//CAS(14,9)/6‐311G(d) methods. Three reaction pathways are examined, including 1) the direct mechanism, 2) the ring contraction–ring expansion mechanism, and 3) the internal cyclization–isomerization mechanism, which lead to two types of photoisomers. The theoretical findings suggest that conical intersections play a crucial role in the photorearrangement of 1,2,4‐oxadiazoles. These model investigations also indicate that the preferred reaction route for 1,2,4‐oxadiazole, which leads to phototransposition products, is as follows: reactant → Franck‐Condon region → conical intersection → photoproduct. In other words, the direct mechanism is a one‐step process that has no barrier. These theoretical results agree with the available experimental observations.     

Photoisomerization in different classes of azobenzene

Azobenzene undergoes trans→cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis→trans isomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.     

Enhancement of electron-capture detection of methyl bromide in air by iodination.

An instrumentally simple and cost-effective method for the direct analysis of methyl bromide in ambient air is described. The method is based on the separation of sample components by gas chromatography, the conversion of methyl bromide to methyl iodide by reaction with an inorganic iodide salt, and the detection of the methyl iodide thereby produced by an electron-capture detector. Of the 20 different inorganic salts investigated here for conversion of methyl bromide to methyl iodide, zinc iodide was found to provide the greatest conversion efficiency. In addition, zinc iodide was found to provide high conversion efficiency at a modest reaction temperature, thereby minimizing both the thermal decomposition of compounds within the reaction volume and the level of column bleed introduced to the detector. The reactions of several other brominated and chlorinated organic compounds with zinc iodide have also been characterized. The successful application of this instrument to the quantitative determination of methyl bromide in a local background air sample is then demonstrated.     

Theoretical study of cyclization of 2′-hydroxychalcone

The isomerization mechanism of 2′(OH)chalcone (1) in flavanone (2) was studied. The calculations were performed with the semiempirical method AM1, using totally optimized molecular geometries. A 6-step mechanism including several equilibrium states was proposed. It was concluded that: (a) At the conformational equilibrium of 1 there could be 43.9% of s-cis conformer; (b) The acid dissociation of 1 trans-s-trans is considerable; (c) The E_E, ΔH_f and net charges show that the rotation of ring A of 1 and the formation of ring C of 2 occurs without greater impairments; (d) Although the keto structure is the most stable one, the enolate of 2 is present in the reaction medium; (e) The conversion of enol of 2 in the keto form would be the limiting step of the analyzed isomerization rate.     

柠檬酸中锌防蚀剂（1,3-Dioxolan-2-ylmethyl）三苯基溴化磷和碘离子的协同效应研究

采用称重法、动电位极化法和电化学交流阻抗技术研究了30 ^oC时(1,3-Dioxolan-2-ylmethyl)三苯基溴化磷（DTPB）对0.5 mol·L^-1柠檬酸中锌腐蚀行为的影响. 通过在环保型电解槽中对锌进行酸洗,这在文献中是不常用的. 结果表明,DTPB作为一种有效的防蚀剂,添加浓度仅为3×10^-3 mol·L^-1时,锌在柠檬酸溶液中的防蚀效率可达98.9%. 由于DTPB和碘化钾存在协同效应,两者联用时的防蚀效果要比单独使用DTPB强,防蚀参数为1.2,并随温度升高而减小. 本文提出了碘化钾作为吸附媒介,可使金属表面与DTPB结合的防蚀机制.     

Stereoselective Isomerization of Unsymmetrical Diallyl Ethers to Allyl (E)-Vinyl Ethers by a Cationic Iridium Catalyst

The stereoselective isomerization of unsymmetrical diallyl ethers to allyl (E)-vinyl ethers was carried out in the presence of a cationic iridium(I) catalyst. The catalyst prepared in situ by treating [Ir(cod)(PPh₂Me)₂]PF₆ with hydrogen was found to be an excellent catalyst to selectively isomerize the less substituted allyl group to an (E)-vinyl ether.     

Kinetic Study of the Polymerization of Alkyl Vinyl Ethers by HI/12 Catalyst

The polymerization of alkyl vinyl ethers initiated by hydrogen iodide and iodine catalyst has been studied in detail. The polymerization showed living behavior in nonpolar solvents at low (-15°) temperatures as reported earlier by others. The observed rate of polymerization under the reaction conditions was linearly dependent on the initial concentration of hydrogen iodide and iodine, respectively. However, the monomer concentration did not influence the rate of polymerization (apparent zero order). From the observed rate equation, two possible reaction schemes were proposed and attempts were made to distinguish them by using UV/visible spectroscopy and ¹³ C-NMR spectroscopy. In both schemes a reversible interaction between monomer and an iodine molecule was postulated as a necessary elemental reaction to fit the observed expression for rate of polymerization. From the spectroscopic analysis results, the interaction between the iodide compound (chain end) and the iodine molecule seems to be very weak compared to the interaction between monomer and the iodine molecule.     

The first azacyclopentenyl carbinyl radical isomerizations (ACCRI): independent use of steric and electronic (polarization) effects as gating elements

The first examples of the azacyclopentenyl carbinyl radical isomerization are described within a series of enantiomerically enriched 2-substituted indolines, a substructure found extensively in both heterocyclic and natural product chemistry. The isomerization was identified by the varying loss of enantiomeric enrichment (ee) of imines during aryl radical cyclizations to azomethine nitrogen. Independent modification of the steric and electronic nature of the ring substituents revealed the full spectrum of sensitivity to these variables and ultimately defined the use of these effects as gating elements. An example is also given in which a 1,4-amino group transfer is effected via the isomerization mechanism. Analogies are drawn between the title isomerization and the azacyclopropyl carbinyl radical isomerization that has been studied in both chemical and biological contexts.     

Regiospecific opening of oxetanes with trimethylsilyl cyanide - zinc iodide. a general approach to γ-amino alcohols.

Trimethylsilyl ethers of γ-hydroxy isonitriles were formed regiospecifically in the opening of oxetanes with trimethylsilyl cyanide - zinc iodide. Deprotection and hydrolysis of the initially formed ring cleavage products gave γ-amino alcohols.     

醚的初始暗氧化反应机理的研究

有机化合物的空气氧化是一个历史悠久的研究课题。早在1851年Schonbein[1]就注意到醚在空气中会被氧化产生过氧化物。本世纪初,Clover和Milas分析了醚的空气氧化产物,并且对醚的氧化机理,进行了研究。     

The van Alphen rearrangement. Thermal,catalytic, and photolytic transformation of 3,3,5-triphenylpyrazolenine-4-carboxylic acid esters

The thermal, catalytic, and acid-catalyzed transformation of 3,3,5-triphenylpyrazolenine-4-carboxylic acid esters was studied. The thermal and acid-catalyzed isomerization of the pyrazolenines leads not only to pyrazoles but also to isopyrazoles. A mechanism involving a 1,5-suprafacial sigmatropic shift is proposed for the isomerization. It is shown that the migration of substituents from the 4 position of the pyrazolenine ring, which has been previously proposed as a variant of the van Alphen rearrangement, does not occur.     

Tandem Isomerization and Cationic Copolymerization of Allyl Ethers

Abstract

A new catalyst system comprising a combination of dicobalt octacarbonyl and a hydridosilane has been successfully employed for the tandem isomerization and cationic copolymerization of 1-allyloxy-2-phenoxyethane (M₁) and l-allyloxyoctane (M₂). Reactivity ratios of r₁ = r₂ = 0.58 ± 0.01 were calculated for these monomers by both the Mayo-Lewis and the Fineman-Ross methods which are indicative of a statistical copolymerization. A GPC study of the copolymerization showed that only copolymers were obtained. This data further indicates that the isomerization of the individual monomers and their subsequent copolymerization proceed at very nearly identical rates. The copolymerization of allyl ether monomers presents an opportunity for designing novel polymers with tailored properties.     

Ring-hydrogen participation in the keto--enol isomerization of the acetophenone radical cation

Molecular ions obtained from acetophenone have been observed to undergo proton transfer reactions in competition with unimolecular blackbody dissociation in a Fourier transform ion cyclotron resonance spectrometer provided with an in situ high temperature blackbody source. The ionizing energy dependence of these two processes and generation of the enol molecular ion by fragmentation of butyrophenone reveal that the keto ion undergoes blackbody dissociation exclusively while the enol ion promotes fast proton transfer reactions and undergoes very slow blackbody induced dissociation. Experiments with labeled acetophenone either on the methyl group or on the ring reveal that the enol ions can transfer both H+ and D+ suggesting that the mechanism responsible for the tautomerization process of these radical cations may involve scrambling of the methyl and ring hydrogens, or more than one mechanism. Theoretical calculations at the B3LYP level predict that the most favorable pathway for unimolecular isomerization of the keto ion involves initial migration of an ortho hydrogen to the carbonyl. The subsequent rearrangement to the enol form is calculated to require enough internal energy that would allow hydrogen walk around the benzene ring in agreement with the experimental results. The possibility that isomerization may also occur by a direct 1,3-hydrogen migration is also explored in terms of possible excited electronic states of the ion.     

Kinetics and mechanism of benzyl chloride reaction with zinc in dimethylacetamide

Oxidative dissolution of zinc in the system of benzyl chloride-dimethylacetamide was investigated. The reaction stereochemistry as well as intermediates and reaction products formed were studied. The kinetic and thermodynamic parameters of the process were measured. The process was shown to follow the Langmuir-Hinshelwood mechanism with the formation of benzyl radicals and mono-solvated organozinc compound on the zinc surface. The components of mixture are adsorbed at various sites of the zinc surface, while recombination and the isomerization of the benzyl radicals occurs in solution.