Pulse radiolysis study on the initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene

Initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene have been studied by means of microsecond pulse radiolysis. For styrene, absorption bands caused by the monomer cation radical St⁺? appear at 630 and 350 nm in a mixture of isopentane and n-butyl chloride at about ?165°C. In parallel with the decay of St⁺?, three absorption bands appear in the near-infrared (IR) region, and at 600 and 450 nm. The IR and 600 nm bands are assigned to the associated dimer cation radical St₂⁺?, and the 450 nm band to the bonded dimer cation radical St-St⁺?. The kinetic behavior of these species shows that reaction of St⁺? with styrene monomer forms both St₂⁺? and St-St⁺?. With the decay of St-St⁺?, another absorption band appears at 340 nm, and the lifetime of this band is relatively long. The 340 nm band may be due to carbonium ions of the growing polystyrene. For α-methylstyrene, the monomer cation radical (at 690 and 350 nm), the associated dimer cation radical (in the near-IR region and at 620 nm) and the bonded dimer cation radical (at 480 nm) behave in a manner similar to that of the corresponding styrene species. The absorption band caused by carbonium ions of growing poly(α-methylstyrene) appears at 340 nm.     

Radical and Electrophilic Reactions of Anodic Oxidation Intermediates of Organophosphorus Compounds

Abstract

Cationic radicals generated by anodic oxidation of organophosphorus(III) compounds are superelectrophilic radicals. This determines the pattern of their reactivity towards various substrates. The oxidation of (RO)₂ PO^? and (RO)₂PS^? gives corresponding radicals. Interaction of the mentioned radical species with ArH and olefines includes a common stage of radical adduct formation. It has been stated that the oxidation of the adduct into corresponding carbonium ion plays an important role in the following adduct conversions. This is particularly confirmed by the difference in the composition of products obtained in conditions of homogeneous (via photolysis or with stable radical cations) and heterogeneous (on the anode surface) generation of phosphorylating species.     

Contiguous concerted participation in the rearrangement of anti-tricyclo[4.2.1.1.2,5]deca-3,7,-diene-9,10-diol,a highly hindered system.

The acid catalysed peclppangement of anti-tricyclo[4.1.1.^2,5]deca-3,7-diene-9, 10-diol ($\text{1a}$) to $\text{2a}$ is shown to proceed through concommitant electrophilic and nucleophilic attack on an alkene by an incipient carbonium ion and the rernaining alcohol function.     

A 13C nuclear magnetic resonance study of cyclobutadieneirontricarbonyl-substituted carbonium ions

The ¹³C NMR spectra of (hydroxymethyl)-, (α-hydroxyethyl)- and (α-hydroxybenzyl)cyclobutadieneiron tricarbonyl (in CDCl₃) and of their derived carbonium ions (in concentrated H₂SO₄) are reported. The data suggest extensive donation of electron density from the metal, via the cyclobutadiene unit, to the exocyclic, electron-deficient carbon atom, thus giving highly stabilized carbonium ions.     

Formation of an intermediate radical cation in the nanosecond pulse radiolysis of Malachite Green Leucocyanide in organic solvents

The malachite green leucocyanide (MGCN) was irradiated in argon or oxygen saturated solutions of n-butyl chloride, 1.2-DCE, CCl₄ and acetone with 13 ns electron pulses. Two species with absorption maxima at 620 and 480 nm were observed. The latter was attributed to the malachite green leucocyanide radical cation (MGCN^+.) and the former to the known carbonium ion of malachite green dye (MG⁺). Observation of the consecutive charge transfer via the schemes: DCE^+. → BPh^+. → MGCN^+. and DCE^+. → MGCN^+. → TMPD^+., allowed to estimate the ionization potential of MGCN molecule in the range 6.9 eV < Ip_MGCN < 8.27 eV. Presented results and literature data suggest that positive charge in MGCN^+. radical cation is located in the “aniline” part of the molecule.     

Studies in sesquiterpenes—LVII: Isolongifolene (part 8): solvolysis of 8-bromoneoisolongifolene

Acetolysis of 8-bromoneoisolongifolene generates, besides two normal products (of elimination and S_N2 displacement), which were minor, one rearranged elimination product and a tertiary acetate. The rearranged diolefine arises from acetic acid-catalysed rearrangement of the normal elimination product, while the tertiary acetate is a Wagner substitution product arising from a syn-migration. In this context various factors controlling such migrations are discussed and, the possible role of energetics of the incipient carbonium ion at the migration origin highlighted.     

CHOLESTERYL DELTA-AMINOBUTYL SULFIDES FROM CYCLIC SULFONIUM SALTS

Abstract

In the course of studies with cholesterol sulfur derivatives, cholesteryl 3β-sulfonium salts were observed to act as alkylating agents toward amines.¹ 3β-Dimethyl-sulfonio-5-cholestene salts and aniline gave N-methyl-aniline, and not N-cholesterylaniline. Thus, the reaction seemed to be controlled more by steric requirements than by stabilizing factors involved in incipient formation of the homoallylic carbonium ion.     

Remote functionalisation by ferrous ion-cupric ion induced decomposition of alkyl hydroperoxides

By decomposition of alkyl hydroperoxides 1-8 with ferrous sulfate-cupric acetate reagent, intramolecular functionalisation of remote non-activated C atom takes place and unsaturated alcohols with double bond mainly at δ-position are obtained. The reaction proceeds involving the corresponding alkoxy radical 9 and δ-carbon radical 10 as intermediates. One-electron oxidative interception of δ-carbon radical by cupric acetate does not involve the corresponding carbonium ion; instead, the alkyl-copper intermediate 11 is formed and by elimination affords olefinic alcohols. The isotope effect for this elimination process was found to be k_h/k_d=6.1.     

Side-chain oxidation of some phenyl-substituted alcohols in aqueous solution: Formation and fragmentation of radical-cations

The reactions of the hydroxyl radical (at low pH) and SO₄^−. have been employed to oxidize a number of phenyl-substituted alcohols in aqueous solution; ESR spectroscopy has been employed to study directly the radicals formed by fragmentation of first-formed radical-cations. Examples of deprotonation (to give benzyl radicals), C_α−C_β bond scission (e.g. PhCH₂CH₂OH to PhCH₂ ^.) and longer-range fragmentation (e.g. PhCH₂CH₂CMe₂OH to PhCH₂CH₂ ^.) are described and discussed: evidence is obtained for intramolecular nucleophilic attack as a route for overall electron-transfer from side-chain to aromatic ring. Solvation of the proton and of oxygen-conjugated carbonium ions is thought, at least in part, to account for differences between fragmentations induced in the gaseous and aqueous phases.     

Ritter reactions. II. Reductive deamidation of N-bridgehead amides.

Adamantyl- or homoadamantyl-derived N-bridgehead amides are converted in high yields into hydrocarbon derivatives on prolonged reflux in ethanol and 50% sulphuric acid (1:1 by volume). This process probably involves A_AL¹ hydrolysis to the tertiary carbonium ion, followed by hydride abstraction from the ethanol solvent.     

Biphotonic origin of photodissociative processes in all-trans retinol

The photolytic transformation of all-trans retinol into retinyl carbonium ion and a transient species (in certain solvents) that is best characterized as retinyl ketyl radical (λ_max = 380–395 nm) is shown to be biphotonic in origin. The results are discussed in terms of mechanism(s) involving photoejection of an electron as the primary step.     

Anion induced regulation of the absorption maximum of the twenty two carbon analog of the N-retinylidene-n-butylammonium cation.

Abstract— This study was undertaken to further investigate the way in which the counter anion controls the Λ_max of the absorption spectrum of compounds similar to N-retinylidene-n-butylammonium salts (NRBA). The following relationship had been found: ΔE =ΔE^o -F d₀e²/εd²; here ΔE is the observed excitation energy, e the charge on the electron, ε the dielectric constant, d₀ a constant and d the distance between centers of opposite charge as estimated from crystallographic radii. Resonance theory implies that ΔE^o should be of the same numerical value as the corresponding carbonium ion which can be generated readily from the corresponding alcohol. The C₂₂SB analog of NRBA was prepared and then converted to the halide salts. The Δ_max of these salts was determined in several halohydrocar-bon solvents, and ΔE^o was determined by least squares for each solvent. The average value of ΔE^o was found to be 653 nm, while the Λ_max, for the carbonium ion was previously found to be 644 nm. The results are supportive of previous work.     

Dissociation of the N-C(alpha) bond and competitive formation of the [z(n) - H](+) and [c(n) + 2H](+) product ions in radical peptide ions containing tyrosine and tryptophan: the influence of proton affinities on product formation

Dissociations at the N-C_α bond of tryptophan and tyrosine residues are the prevalent pathways in the fragmentations of radical cations of tripeptides that contain such as residues. This process involves a proton transfer from the β-carbon of the tryptophan or tyrosine residue to the carbonyl oxygen of the amide group, followed by cleavage of the N-C_α bond, generating low-lying proton-bound dimers that dissociate to give each an ionic and a neutral product. Formation of the [z _n −H]^∢+ or [c _n +2H]⁺ ion is a competition between the two incipient fragments for the proton in a dissociating proton-bound dimer.     

CPh3⊕[Ph3CNWCl4(μ-F)WNCl2(μ-F)Cl4WNCPh3]⊖; Synthese und Kristallstruktur eines dreikernigen Nitrido-Nitren-Komplexes des Wolframs

CPh₃^⊕[Ph₃CNWCl₄(μ-F)WNCl₂(μ-F)Cl₄WNCPh₃]^?; Synthesis and Crystal Structure of a Trinuclear Nitrido Nitrene Complex of Tungsten Dark red crystals of the title compound are formed in the reaction of triphenyl carbonium tetrafluoro borate with AsPh₄[WNCl₄] in dichloro methane. They are characterized by a structural analysis by the aid of X-ray diffractional data as well as by it's IR spectrum. The complex crystallizes in the space group P_mmm with two formula units per unit cell. The cell dimensions are a = 2307, b = 1330 and c = 968 pm. Intensity measurements were carried out with 2 255 reflexions. Because of a disorder of the phenyl groups the structural parameters could only be refined to a final R-value of 9.3%. The structure consists of triphenyl carbonium cations and trinuclear anions [WNF₂Cl₂(Cl₄WNCPh₃)₂]^?. The anion can be regarded as a distorded trigonal-bipyramidal ion [WNF₂Cl₂]^?, coordinating to two molecules of Cl₄W?N? CPh₃ in trans-position to the nitrene ligands via W? F…?W bonds with different bond lengths.     

Electrochemical study of the methylcyclopentane oxidation in anhydrous hydrogen fluoride and HF−SbF5 superacid solutions

The electrochemical study of methylcyclopentane (MCPH) oxidation in anhydrous hydrogen fluoride and related superacid media leads to determination of the standard potentials of the MCPH redox couples and to the establishment of the potential acidity diagram of MCPH. Fromtthese results the oxidation by H⁺ ion of MCPH into the carbenium ion MCP⁺ is observed in acidic media: pH<6 or R(H)<?21.9 (pH values are reported on the acidity scale in HF, pH=0 for a molar solution of SbF₅, and the acidity levels are referred to H₂O by means of an R(H) function: for pH=0, R(H) is ?27.9). In HF?SbF₅ mixtures, pH<2.3 or R(H) <?25.6, the protonation of MCPH to form the carbonium ion MCPH₂⁺ has to be taken into account and the acidity constant is determined. In basic media, pH>8.1 or R(H)>?19.8, condensation reactions are enhanced through a radical oxidation state; the disproportionation constant of the dimer (MCP)₂ is determined. Moreover, in the most acidic media, pH<2 or R(H)<?25.9, evidence for the cracking of MCPH through the protolysis of a C?C bond is shown.     

Double-shift rearrangements of secondary ferrocenylcarbonium ions

In CF₃CO₂H solution, secondary ferrocenylcarbonium ions (Fc$\text{C}\text{+}$HCR¹R²R³) undergo a quantitative double-shift rearrangement to generate isomeric tertiary carbonium ions (Fc$\text{C}\text{+}$R¹CHR²R³).     

Control of Product Distribution and Mechanism by Ligation and Electric Field in the Thermal Activation of Methane

An unexpected mechanistic switch as well as a change of the product distribution in the thermal gas‐phase activation of methane have been identified when diatomic [ZnO]^.+ is ligated with acetonitrile. Theoretical studies suggest that a strong metal–carbon attraction in the pristine [ZnO]^.+ species plays an important role in the rebound of the incipient CH₃^. radical to the metal center, thus permitting the competitive generation of CH₃^., OH^., and CH₃OH. This interaction is drastically weakened by a single CH₃CN ligand. As a result, upon ligation the proton‐coupled single electron transfer that prevails for [ZnO]^.+/CH₄ switches to the classical hydrogen‐atom‐transfer process, thus giving rise to the exclusive expulsion of CH₃^.. This ligand effect can be modeled quite well by an oriented external electric field of a negative point charge.     

A quantum chemical study on electrophilic addition

Non-empirical SCF-MO calculations were carried out on two limiting structures of C₂H₄F⁺, corresponding to the cyclic and open valence tautomers, both of which are possible reaction intermediates of the electrophilic addition reaction of F₂ to CH₂ =CH₂. It was found that both species had thermodynamic stability, corresponding to two distinct minima on the energy surface. However, the 2-fluoroethyl carbonium ion showed a greater stability than the fluoronium ion by about 10 kcal/mole.     

Reaktionen von Bleitetraacetat mit alicyclischen Alkoholen,III. Cyclopropyl- und Cyclobutylcarbinole. 18. Mitteilung über Reaktionen mit Bleitetraacetat

When treated with lead tetraacetate, cyclopropylcarbinol and cyclopropylmethylcarbinol do not give β-fragmentation products resulting from the intermediate formation of cyclopropyl radicals; however, cyclopropylmethylcarbinol affords a small amount of a fragmentation product which arises from C_α? C_β bond cleavage involving removal of a methyl radical. In contrast, cyclobutylcarbinol undergoes β-fragmentation in 18% yield with formation of both the unrearranged cyclobutyl acetate and the rearranged cyclopropylmethyl acetate. These results suggest the following order of increasing radical stability: cyclopropyl < methyl < cyclobutyl, whereas the isolation of the isomeric fragmentation acetates in the lead tetraacetate reaction of cyclobutylcarbinol represents further evidence that in the β-fragmentation process the initially produced carbon radical fragment is in major part oxidized to the corresponding carbonium ion before final product formation.     

Simulate methylation reaction of arsenic(III) with methyl iodide in an aquatic system

The methylation reaction of inorganic arsenic occurring in aquatic systems was studied by HPLC‐HG‐AFS as a method to separate and detect soluble methylarsenic species. Transformation from inorganic arsenic to methylarsenic was essential for major changes in toxicity to organisms. Monomethylarsenic [AsOCH₃(OH)₂] was the only product in the methylation reaction of inorganic arsenic(III) with methyl iodide (MeI). This process can be described as an oxidative carbonium‐ion transfer, with MeI acting as a methyl donor. From a thermodynamic point of view, the activity of the carbonium ion and pH were the two major influencing factors. The pH dependence of redox potential of As(III) was the reason for the effect of pH on methylation of arsenic. The influences of salinity and concentration of the methyl donor may be explained by their effects on the activity of carbonium. Moreover, kinetics experiments demonstrated that the methylation reaction was first‐order for both As(III) and methyl iodide. First‐order reaction rates were also calculated at different pH, salinity and MeI, and were found to be in the range 0.0026–0.0123 h^?1. The methylation rate varied largely under different reaction conditions. Copyright © 2005 John Wiley & Sons, Ltd.