The molecular structure of cyclic methylsiloxanes

A series of methylcyclosiloxanes from the trimer to the hexamer has been investigated by gas phase electron diffraction. The Si-O bond distance varies between 1.635 and 1.620 Å, the SiOSi angle has values from 132° to 149° within the series. The trimer is substantially planar, the best agreement for the tetramer was obtained with a model of S₄. symmetry. As a consequence of large amplitude vibrations, the larger rings do not possess a well-defined conformation. The results are discussed in relation to previous spectroscopic investigations.     

Development of chromatographic methods by using direct-sampling procedure for the quantification of cyclic and linear volatile methylsiloxanes in biogas as perspective for application in online systems.

By empirically examining the persistent theme, we hope to produce a more complete understanding of methods for determination of volatile methylsiloxanes in biogas stream. Therefore, we made an attempt to investigate a rapid and sensitive method for simultaneous analysis of linear and cyclic volatile methylsiloxanes in sewage biogas in the context of the perspective for application in online systems. The gas chromatographic (GC) parameters were optimised, and sampling of volatile methylsiloxanes from biogas was performed using novel direct sampling procedure with applying of three kinds of liquid-media. Through application of well-established gas chromatography technique coupled with two types of detector – flamed ionisation detector and mass spectrometer detector – we developed the characterisation of the presented methods. Moreover, during the samples preparation the extraction procedure was consistently excluded, as well as the time of analysis was significantly reduced. The analyses were carried out by applying special constructed sampling train where the absorbed VMSs were trapped and analysed directly by GC technique, afterwards. The instrumental analytical protocol was found to yield a linear calibration in the range 0.1–55.13 (µg g^?1) with R² values 0.996 and in the range from 0.1 to 65.17 µg g^?1) with R² values > 0.99 for GC-FID and GC-MS method respectively. In all analysed samples linear and cyclic VMSs were found in sewage gas with quantities exceeding 4.6 mg Nm^?3 and 19.9 mg Nm^?3, respectively Furthermore, estimation of VMSs solvent absorption efficiency was tested and the highest absorption efficiency was obtained when acetone was used as a primary solvent. High range of linearity (0.1–65.17 µg/g), and low values of limit of detection (0.01 µg/g), limit of quantification (0.04 µg/g) clearly indicate that the analysis can be successfully repeated in other independent laboratory. The proposed method creates the real perspective for analyis of VMSs in on-line system.     

Stereoselective radical reactions

For many years, the reputed lack of selectivity in some free-radical reactions has resulted in this class of reaction being overlooked in the stereoselective synthesis of important target molecules. More recently, however, the situation has changed as new and milder methods of radical generation have been developed, which have helped researchers to gain a better understanding of the key factors that influence selectivity in radical transformations. As a consequence, the use of radicals in stereoselective synthesis is increasing and there are a number of important intra- and intermolecular additions, where high levels of stereoselectivity have been achieved in the formation of carbon-carbon bonds.     

Decomposition of cyclic and linear N-acetyl-N-trimethylsilyl-amidophosphites

Conclusions N-Acetyl-N-trimethylsilylamidophosphites when heated undergo decomposition to give the corresponding silyl phosphites, which decomposition is facilitated by adding electron-acceptor substituents to the phosphorus atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 672–675, March, 1977.     

Azanorbornenes in radical polymerization reactions

Activity of azanorbornenes: N-benzyl-2-azanorborn-5-ene, (2-azanorborn-5-en-2-yl)methyl acetate, and N-allyl-2-azanorborn-5-ene in radical homo-and copolymerization with vinyl monomers and sulfur dioxide was studied.     

Kinetics of radical reactions in freons

Gaseous mixtures containing hydrochlorofluorocarbons (HCFC) and hydrofluorocarbons (HFC) were pulse radiolized and the kinetics of the radical (OH, RF) reactions has been studied. The obtained rate coefficients are also given.     

Electrocatalytic reactions: anion radical cyclobutanation reactions and electrogenerated base reactions

Intermolecular anion radical [2+2] cyclobutanation reactions have been observed between vinyl sulfones and vinyl/propenyl ketones. The products are novel and formed electrocatalytically, although yields are at best modest (11-45%). Competing mechanisms are discussed. Additionally, the electrochemical reduction of vinyl alkyl sulfones in acetonitrile leads to near quantitative formation of cyanomethylation product cyano-sulfones. The single step approach has electrocatalytic factors in excess of thirty.     

Synthetic applications in radical/radical cationic cascade reactions

Oxidative photoinduced electron transfer (PET) reactions have been performed with various cyclic cyclopropyl(vinyl) silyl ethers bearing an olefinic or acetylenic side chain. The reactions result in bi- to tetracyclic ring systems via a fragmentation-radical/radical cationic addition reaction pathway with well defined ring juncture. The mode of cyclisation (endo/exo) can be partially controlled by addition of nucleophiles due to the suppression of radical cationic reaction pathways. Quantum chemical calculation of the cyclisation transition states underline the experimentally found selectivities. Additional mechanistic studies concerning the saturation step reveal that the final radical is saturated mostly by the solvent and traces of water in the solvent.     

N-heterocyclic carbene-catalyzed radical reactions

While N-heterocyclic carbene(NHC)catalyzed electron-pair-transfer processes have been developed into an important tool for synthetically important bond formations during the past decades,the corresponding radical reactions via NHC catalysis have only received growing attention in the past six years.Taking into account the advantages NHC-catalyzed radical reactions might bring,such as creating new activation modes that were previously unobtainable,it is worthwhile to provide a conceptual understanding of this emerging area.Therefore,herein we give an overview of NHC-catalyzed radical reactions via different synthetic techniques.     

Heisenberg models of radical reactions

Interactions between a pair of two-electron (singlet or triplet) systems undergoing abstraction, insertion, exchange and association reactions are treated on the basis of the quantum Heisenberg model. The local spin-permutation-symmetry properties of these reactions can well be visualized from the continuous or discontinuous variations of Penney's bond orders for the interacting electron pairs. In reactions where such a local permutation symmetry is not conserved, there is an apparent transition between two distinct valence-bond structures. It is shown that the concept of the local spin-permutation-symmetry conservation is useful for understanding the nature of the electron pairing requirement inherent in reactions of biradical species.     

Selection rule in free radical reactions

The insertion and abstraction reactions are investigated on the basis of the Heisenberg model. These reactions are characterized as spin-symmetry forbidden and allowed reactions, respectively. It is pointed out that free radical reactions occur readily when there is no spin degeneracy in the course of these reactions. The rule obtained is discussed in relation to the orbital symmetry conservation rule for concerted reactions.     

Nuclear spin polarization in radical reactions

An extension of the Closs-Kaptein-Oosterhoff theory concerning nuclear spin polarization resulting during free radical reactions is presented. This extension is based on the Merrifield model for the magnetic field dependence of triplet-triplet annihilation.     

Remote aromatic stabilization in radical reactions

The rates of free radical reduction of a series of anthracene derivatives and 1-phenyl-4-bromodecane with tributyltin hydride are mediated by the remote aromatic substituent in an apparent through-space interaction. Density functional calculations suggest that this enhancement is not due to direct stabilization of the free radical intermediate, and is likely to be achieved through the interaction of the aromatic moiety with the polarized transition state leading to the intermediate.     

Diallylaminophosphonium salts in radical polymerization reactions

The activity of tris(diethylamino)diallylaminophosphonium tetrafluoroborate and chloride in radical polymerization reactions was studied. The kinetic relationships of the copolymerization with sulfur dioxide were examined.     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.