Alkane Radical Cations Research Articles

Radical contraction of 1,3,2-dioxaphosphepanes to 1,3,2-dioxaphosphorinanes: a kinetic and (17)O NMR spectroscopic study.

Two diastereomeric 5-bromo-4-phenyl-2-phenoxy-2-oxo-1,3,2-dioxophosphepanes have been synthesized and used to study the contraction of 4-phenyl-2-phenoxy-2-oxo-1,3,2-dioxophosphorinan-5-yl radicals. Kinetics were determined by competition methods and demonstrate Arrhenius parameters typical of rearrangements of this kind. Isotopic labeling reveals that all rearrangements are formally of the 1,2-type with retention of configuration at phosphorus. Analysis of the stereochemistry of the rearrangements, however, reveals the two diastereomers to take different paths with respect to the geometry of the presumed alkene radical cation intermediate.

ChemInform Abstract: Generation and Reaction of Alkene Radical Cations under Nonoxidizing Conditions: Synthesis of the Pyrrolizidine Nucleus.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Photoinduced Electron-Transfer Oxidation of Olefins with Molecular Oxygen Sensitized by Tetrasubstituted Dimethoxybenzenes: A Non-Singlet-Oxygen Mechanism

α-Methylstyrene (1) was photo-oxidized in the presence of a series of alkylated dimethoxybenzenes as sensitizers in an oxygen-saturated MeCN solution to afford the cleaved ketone 2, epoxide 3, as well as a small amount of the ene product 4 in ca. 1 : 1 : 0.04 ratio. The relative rate of conversion was well-correlated with the fluorescence quantum yield of sensitizers. Thus, a non-singlet-oxygen mechanism is proposed, in which an excited sensitizer is quenched by (ground-state) molecular oxygen to produce a sensitizer radical cation and a superoxide ion (O), the former of which oxidizes the substrate, while the latter reacts with the resulting olefin radical cation (1+.) to give the major oxidation products. Photodurability of such electron-donating sensitizers is dramatically improved by substituting four aromatic H-atoms in 1,4-dimethoxybenzene with Me or fused alkyl groups, which provides us with an environmentally friendly, clean method of photochemical functionalization with molecular oxygen, alternative to the ene reaction via singlet oxygenation.

Ab initio calculations on the isomerization of alkene radical cations. 2. Ring opening of cycloalkane radical cations

Ab initio calculations on the ring opening of the cyclopentane and cyclohexane radical cations do not give an explanation of previous experimental findings that, upon electron ionization, the cyclopentane ion ring opens whereas the cyclohexane ion is stable. The main difference between the two cases is that the cyclohexane ion first has to isomerize to the methylcyclopentane structure before ring opening. The barrier heights for ring opening in the two cases are comparable and significantly lower than the difference between the vertical and adiabatic ionization energies. The results suggest that, for an explanation of the experiments, accurate theoretical studies of the ionization process itself will be needed.

Generation and Reaction of Alkene Radical Cations under Nonoxidizing Conditions: Synthesis of the Pyrrolizidine Nucleus

[see reaction]. Stable beta-phosphatoxy nitroalkanes, readily assembled by the Henry reaction and subsequent phosphorylation, serve as good precursors to alkene radical cations on treatment with triphenyltin or tributyl hydride and AIBN in benzene at reflux. When the beta-phosphatoxy nitroalkane is suitably functionalized with nucleophilic groups, substitutions can be achieved with the formation of heterocyclic rings. When the nucleophile is an allylamine, tandem processes occur giving pyrrolizidines.

Product studies and laser flash photolysis on alkyl radicals containing two different beta-leaving groups are consonant with the formation of an olefin cation radical.

1-Bromo-2-methoxy-1-phenylpropan-2-yl (3) and 2-methoxy-1-phenyl-1-diphenylphosphatopropan-2-yl (4) were generated under continual photolysis from the respective PTOC precursors in a mixture of acetonitrile and methanol. The radicals undergo heterolytic fragmentation of the substituent in the beta-position to generate the olefin cation radical (5). Z-2-Methoxy-1-phenylpropene (15) is the major product formed in the presence of 1,4-cyclohexadiene, and is believed to result from hydrogen atom transfer to the oxygen of the olefin cation radical, followed by deprotonation. Laser flash photolysis experiments indicate that reaction between 5 and 1,4-cyclohexadiene occurs with a rate constant of approximately 6 x 10(5) M(-1) s(-1). 2,2-Dimethoxy-1-phenylpropane (18) is observed as a minor product. Laser flash photolysis experiments place an upper limit on methanol trapping of 5 at k <1 x 10(3) M(-1) s(-1) and do not provide any evidence for the formation of reactive intermediates other than 5. The use of two PTOC precursors containing different leaving groups to generate a common olefin cation radical enables one to utilize product analysis to probe for the intermediacy of other reactive intermediates. The ratio of 15:18 is dependent upon hydrogen atom donor concentration, but is independent of the PTOC precursor. These observations are consistent with the proposal that both products result from trapping of 5 that is formed via heterolysis of 3 and 4.

Identification of Radical Cations of n -Alkanes in Irradiated Solutions by Time-Resolved Magnetic Field Effect Methods

Alkane radical cations are the key transient species determining the composition of the products of radiolysis of alkane solutions. As probed by pulse radiolysis, the lifetimes of n -alkane radical cations under common conditions range from several nanoseconds to several tens of nanoseconds [1]. The same lifetime range for these species was obtained in studying stationary magnetic field effects by radioluminescence methods [2]. Short lifetimes of n -alkane radical cations render routine ESR methods impracticable for studying these species in solutions. Only such a highly sensitive ESR technique as optically detected ESR of spin-correlated radical ion pairs made it possible to observe a line with a width of 0.27 mT upon irradiation of pentadecane; it was assigned to the pentadecane radical cation [3]. No signals were observed for other alkanes [4]. We pioneered in obtaining ESR spectra of some n -alkane radical cations in solution by means of another method applicable to investigation into spin chemistry, namely, the method of magnetic field effects in recombination fluorescence of spin-correlated radical ion pairs [5, 6]. In nonpolar alkane solutions, fluorescence decay caused by recombination of radical ion pairs is determined by the relationship

Does the cationic or the radical character dominate the reactivity of alkene radical cations towards solvent molecules?

The reaction of ethylene radical cations with a broad selection of uncharged nucleophiles (hydrogen fluoride, dichloromethane, trifluoroethanol, water, formaldehyde, methanol, ethanol, ethylene glycol, dimethyl formamide, acetonitrile, ammonia) has been studied at the BHandHLYP/6-31 + G(d,p) level of theory. With the exception of hydrogen fluoride and dichloromethane, all of these nucleophiles form covalent addition products with ethylene radical cations. Transition states for these addition reactions could, however, not be identified. The reaction energies calculated for the addition reactions correlate well with the proton affinities of the respective nucleophiles. Based on this correlation the reactivity of ethylene radical cations can be characterized as intrinsically cationic. The reaction of ammonia, the water dimer, and the water trimer with the radical cations of some substituted alkenes (propene, trans-but-2-ene, butadiene, styrene) have been studied at the same level of theory. In all cases the most exothermic reaction occurs for the water trimer. Comparison of the reaction energies calculated for these radical cations with those calculated for the analogously substituted carbocations shows that alkene radical cations can best be understood as strongly stabilized carbocations. The stabilizing effect of the radical center on cationic intermediates is not constant and varies between 40 kcal mol−1 for the smaller systems and 20 kcal mol−1 for the larger systems investigated in this study.

A Study of the Reactions of Alkane Radical Cations with Alcohols in Solutions by MARY Spectroscopy

A Study of the Reactions of Alkane Radical Cations with Alcohols in Solutions by MARY Spectroscopy

Photochemical Reactions of Linear Alkane Radical Cations in Low-Temperature Matrices

The radical cations of linear alkanes (n-pentane, n-heptane) trapped in various matrices (Freon-11, Freon-113, Freon-113a, mixture of Freon-11 and Freon-114B2, and sulfur hexafluoride) were found to undergo the following types of photochemical reactions: (1) charge transfer to the matrix followed by neutralization, (2) isomerization and unimolecular decomposition, and (3) deprotonation. The absorption spectra of the radical cations were characterized, and the quantum yields of reactions occurring in different matrices at 77 K were determined. It was shown that the reaction pathway and efficiency of the photochemical processes observed for a given radical cation in different matrices with similar physical and chemical characteristics could considerably differ.

Laser flash photolysis evidence for styryl radical cation cyclization in the SET-induced photorearrangement of a p-methoxy-substituted 2-phenylallyl phosphite.

The SET-induced photorearrangement of dimethyl 2-(4-methoxyphenyl)allyl phosphite, 9 (UV light, uranium glass filter, 9,10-dicyanoanthracene (DCA), biphenyl), gives phosphonate 12 in 83% isolated yield. Laser flash irradiation at 355 nm of oxygen saturated solutions of phosphite 9 containing DCA and biphenyl generates the transient UV spectrum of the biphenyl radical cation that is quenched by electron transfer from phosphite 9 (k(q) = 8.9 x 10(9) M(-1) s(-1) at 20 degrees C) to form the 4-methoxystyryl cation 10. The UV spectrum of 10 decays by a measured first-order rate constant of 8.0 x 10(6) s(-1), presumably to generate the cyclic distonic radical cation 11. Intermediate 10 was further characterized by measurement of the second-order rate constants for its reaction with azide, chloride, and bromide ions and with the neutral nucleophile trimethyl phosphite. This study provides the first spectroscopic evidence regarding the proposed mechanism (Schemes 1 and 2) for the SET-induced photorearrangements of dimethyl 2-arylallyl phosphites to the corresponding 2-arylallylphosphonates. Moreover, absolute rate constants for the intramolecular trapping of alkene radical cations have seldom been measured. The removal of the electron from the styryl moiety of phosphite 9, rather than from phosphorus, and the detectability of 10 arise from the stabilizing effect of the 4-methoxy substituent. These results, however, do not allow conclusions to be made concerning the site of removal of an electron in the SET-induced photorearrangement of dimethyl 2-phenylallyl phosphite 1 to phosphonate 6.

Oxidative Single-Electron Transfer Activation of σ-Bonds in Aliphatic Halogenation Reactions

The reactions of a series of structurally related large-ring propellanes with iodine monochloride were studied experimentally and computationally. In the case of 1,3-dehydroadamantane (1) and [3.3.1]propellane (2) free-radical addition was observed. [3.3.2]Propellane (3) and 3,6-dehydrohomoadamantane (4), which are less prone to radical attack, selectively form products of formal double nucleophilic (oxidative) addition, e.g., dichloro (in ICl/CH2Cl2), dimethoxy (in ICl/CH3OH), and diacetamino (in ICl/CH3CN) derivatives under otherwise identical conditions. Single-electron transfer pathways involving the alkane radical cations are proposed for the activation step for aliphatic hydrocarbons with relatively low oxidation potentials such as cage alkanes. Similar mechanisms are postulated for the activation of the tertiary C−H bonds of adamantane based on H/D-kinetic isotope effect data. The latter compare well to the kH/kD value for hydrogen atom loss from the adamantane radical cation (measured 2.78 ± 0.21 ...

The 1,2 migration of NH 3 in protonated β-aminoalkyl radicals

The enthalpy barriers for 1,2-migration of NH 3 in protonated β-aminoalkyl radicals, β-distonic isomers of primary amine radical cations, were determined with composite ab initio quantum chemical methods of the Gaussian and CBS families. The rearrangements are close to thermoneutral for small, protonated β-aminoalkyl radicals, and the barriers range from 60 to 100 kJ mol −1, depending on the degree of substitution. The transition state resembles an NH 3 molecule symmetrically coordinated to an alkene radical cation. The heats of formation of the protonated C 2–C 4 β-aminoalkyl radicals are lower than those of the corresponding amine radical cations by 20–40 kJ mol −1. The geometry and conformations of primary amine radical cations and their distonic isomers are briefly discussed.

Carbon−Hydrogen vs Carbon−Carbon Protonation in the Proton Transfer from Alkane Radical Cations to Alkane Molecules. A Study in γ-Irradiated CCl3F/Decane at 77 K

A study is made of the formation of the isomeric chlorodecanes, and a search is made for shorter-chain 1-chloroalkanes (5 ≤ nC ≤ 9) formed in CCl3F/decane by γ-irradiation at 77 K and subsequent me...

Ab initio calculations on the isomerization of alkene radical cations

Ab initio calculations on the isomerization of butene and pentene radical cations indicate that, for all classical ion structures, the lowest barrier for a rearrangement to the most stable ion structure is below the dissociation limit. Isomerizations of linear butene radical cations to the isobutene structure take place via the CH 3CC 2H 5 ·+ structure, whereas in the pentene case the connection between linear and branched ion structures proceeds via the 1,2-dimethylcyclopropane radical cation. From the results a qualitative model is derived which suggests that for larger alkene radical cations an isomerization to structures with four alkyl substituents on the double bond may be in close competition with dissociation.

Structure-reactivity relationships in reactions of alkane radical cations: Study of the proton transfer from alkane radical cations to alkane molecules in γ-irradiated disordered CCl 3F/alkane systems by ESR spectroscopy at cryogenic temperatures

A summary is presented of ESR results obtained in γ-irradiated disordered CCl 3F/alkane systems at cryogenic temperatures, with respect to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The nature of the alkyl radicals formed by proton transfer is indicative for the site of proton donation and is derived unambiguously from ESR results by comparison with powder spectra of authentic isomeric alkyl radicals, obtained by γ-irradiation of various chloro and bromoalkanes in perdeuterated cis-decalin. The experiments can be divided into two main classes. (i) Experiments on n-alkane radical cations in the extended all- trans conformation, i.e. ESR results on the system CCl 3F/heptane. The ESR spectrum of γ-irradiated CCl 3F/heptane consists of a triplet due to heptane radical cations in the extended all- trans conformation. In this conformation, the unpaired electron is delocalized over the carbon-carbon σ-bonds as well as the two chain-end carbon-hydrogen bonds that are in the plane of the CC skeleton. Superimposed on the ESR triplet is a low-intensity spectrum due to heptyl radicals, which increases drastically with increasing heptane concentration. The formation of these heptyl radicals can be attributed unambiguously to proton transfer from heptane radical cations to heptane molecules, taking place in small heptane clusters to which positive-hole transfer still occurs efficiently. At the onset of proton transfer with increasing heptane concentration only primary heptyl radicals are present, clearly showing that the proton transfer takes place selectively from a chain-end position, in accordance with the electronic structure of the reacting radical cations. At higher heptane concentration secondary heptyl radicals also appear as a result of intermolecular radical-site transfer, i.e. the nature of the heptyl radicals becomes governed by their thermodynamic stability. (ii) Experiments on n-alkane radical cations in the gauche-at-C 2 conformation, i.e. ESR results on the system CCl 3F/octane. The ESR spectrum of γ-irradiated CCl 3F/octane indicates that octane radical cations are largely in the gauche-at-C 2 conformation in this matrix, with large unpaired-electron (and positive-hole) density on one planar chain-end CH bond and one planar penultimate CH bond at the other side of the radical cation. Careful investigation of ESR spectra with increasing octane concentration clearly reveals that in this case secondary octyl radicals are present from the very onset of proton transfer, in accordance with the electronic structure of the reacting radical cations. The results clearly point to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules and to a strict dependence of the site of proton donation on the electronic structure and conformation of the reacting radical cations.

EPR Spectra and Structure of the Radical Cations of Fluorinated Ethylenes and Propenes

Abstract EPR spectra for the radical cations of a series of fluorinated ethylenes and propenes, generated by irradiation with γ-rays in halocarbon solid matrices, have been observed at low temperature. The spectra consist of a hyperfine structure with a nearly axial symmetry mainly due to fluorine nuclei. For the trifluoro olefin cations, CF2=CFX+ (X = H, CH3, or CF3), the experimental spectra were successfully analyzed in terms of the three 19F nuclei with coaxial parallel components. This may strongly suggest a planar structure for the radical cations, similarly to the case of the CF2=CF2+ cation already reported. On the other hand, for mono- and di-fluoro olefin radical cations, it is difficult to conclude the planar structure unequivocally from the experimental 19F and 1H hf splittings. Thus, the optimized geometry of the radical cations was calculated by an ab initio MO method. In contrast with the ethylene cation and the propene cation, having nonplanar twisted structures, the fluorinated ethylene and propene cations are concluded to have planar structures. Calculations at the INDO levels including the evaluation of interatomic dipole-dipole interactions were done for the geometries of the radical cations optimized by the ab initio method. The calculated results strongly support the EPR spectra observed and assigned to the radical cations.

Cleavage of neutral alkenes and alkene radical cations; hybrid Hartree-Fock/density functional theory results

The cleavage reactions of the 1-butene and 4,4-dimethyl-1-pentene molecules and their cations, to form neutral and charged hydrocarbon products, are investigated using hybrid Hartree-Fock/density functional theory. In comparison with previous theoretical results (Du et al.), the density functional cleavage and ionization energies, including zero-point vibrational energy, lie in better agreement with experimental and thermochemical data. Assuming vertical ionization processes the mean absolute deviation (MAD) compared with experiment is 3.4 kcal/mol for the reaction sequences studied. Using adiabatic ionization processes instead gives a MAD of 5.2 kcal/mol. The largest deviation from experiment occurs for the cleavage reactions of the neutral parent molecules, where the difference between theory and experiment is up to 12.8 kcal/mol. In addition to reaction energies we also report optimized ground-state structures, and for the radicals studied, isotropic hyperfine coupling constants that are compared to experimental data. It is found that the experimental hyperfine properties of the 1-butene cation can be explained by rotational averaging caused by the flat potential surface for rotation about the C2—C3 bond.

Facial selectivity in the addition of nucleophiles to the radical cations of substituted 2-methyleneadamantanes

The diphenylmethylenecyclohexane, 2, and the substituted 2-methyleneadamantanes, 3-7, have been prepared. The radical cation of each was generated by photochemical oxidation using 1,4-dicyanobenzene as the sensitizer, and their reactivity was examined in methanol-acetonitrile mixtures with added tetraethylammonium cyanide. Although compound 2 reacted only by tautomerization, the other compounds all gave addition products. For the monoaryl alkenes 3, 4, and 5, comparable yields of both methanol and HCN addition products were obtained, resulting from nucleophilic attack at either end of the alkene radical cation. For the diphenylalkenes 6 and 7, the regioselectivity favored nucleophilic attack at the adamantyl carbon, and the addition products were predominantly those of HCN addition. For the 5-methoxy compound 7, HCN addition was facially selective with a syn:anti ratio of 58:42.Key words: methyleneadamantanes, facial selectivity, radical cations.

Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction, Part 15. Investigations involving fluoride anion as the nucleophile and the effect of fluorine substitution on the relative stability of the reaction intermediates

The scope of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include fluoride anion as the nucleophile. The 4-cyanophenyl substituted fluoroalkanes were obtained in moderate yields with 2,3-dimethyl-2-butene and 2-methyl-2-butene reacting as the olefin. The yields were lower with 2-methylpropene, and no photo-NOCAS products were detected when conjugated dienes were used as the olefin. The selectivity of the fluoride anion towards alkenes can be explained in terms of the Hard–Soft-Acid–Base (HSAB) principle. The regiochemistry of this reaction can be described as predominantly following the anti-Markovnikov mode of addition. The fluoride anion adds preferentially to the less alkyl-substituted end of an unsymmetrical alkene radical cation to produce the more heavily substituted β-fluoroalkyl radical. Ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) were used to determine the relative stability of the β-fluoroalkyl radical intermediates. The more heavily substituted β-fluoroalkyl radical is NOT the more stable. The addition step is kinetically, rather than thermodynamically, controlled. Keywords: photochemistry, radical ions, electron transfer, alkyl fluorides, nucleophilic addition.