Cyclohexenyl Radical Research Articles

Intermolecular C-H Activation at the Allylic/Benzylic and Homoallylic/Homobenzylic Positions of Cyclic Hydrocarbons by a Stable Divalent Silicon Species.

Direct activation of inert C(sp3 )-H bonds by main group element species is yet a formidable challenge. Herein, the dehydrogenation of cyclohexene and 1,2,3,4-tetrahydronaphthalene through the allylic/benzylic and homoallylic/homobenzylic C-H bond activation by cyclic (alkyl)(amino)silylene 1 in neat conditions is reported to yield the corresponding aromatic compounds. As for the reaction of cyclohexene, allylsilane 3 and 7-silanorbornene 4 were also observed, which could be interpreted as a direct dehydrogenative silylation reaction of monoalkenes at the allylic positions. Experimental and computational studies suggest that the dehydrogenation of cyclohexene at the homoallylic position was accomplished by a combination of silylene 1 and radical intermediates such as hydrosilyl radical INT1 or cyclohexenyl radical H, which are generated in the initial step of the reaction.

Radical Cation Diels‐Alder Reactions of Non‐Conjugated Alkenes as Dienophiles by Electrocatalysis

Summary of main observation and conclusionRadical cation Diels‐Alder reactions provide a powerful method for the construction of six‐membered ring systems between both electron‐rich dienes and dienophiles. However, the most recent examples of this class have been limited to β‐methylstyrenes as dienophiles; the use of non‐conjugated alkenes remains challenging. The present study describes the serendipitous development of novel radical cation Diels‐Alder reactions by electrocatalysis that use non‐conjugated alkenes as dienophiles. The key to successful transformation involves highly substituted cyclohexenyl radical cations that are stable enough to be reduced by intermolecular single electron transfer.

A new strategy toward indole alkaloids involving an intramolecular cycloaddition/rearrangement cascade.

The intramolecular Diels-Alder reaction between an amidofuran moiety tethered onto an indole component was examined as a strategy for the synthesis of Aspidosperma alkaloids. Furanyl carbamate 23 was acylated using the mixed anhydride 26 to provide amidofuran 22 in 68% yield. Further N-acylation of this indole furnished 27 in 88% yield. Cyclization precursors were prepared by removing the carbamate moiety followed by N-alkylation with the appropriate alkyl halides. Large substituent groups on the amido nitrogen atom causes the reactive s-trans conformation of the amidofuran to be more highly populated, thereby facilitating the Diels-Alder cycloaddition. The reaction requires the presence of an electron-withdrawing substituent on the indole nitrogen in order for the cycloaddition to proceed. Treatment of N-allyl-bromoenamide 48 with n-Bu(3)SnH/AIBN preferentially led to the 6-endo trig cyclization product 50, with the best yield (91%) being obtained under high dilution conditions. The initially generated cyclohexenyl radical derived from 48 produces the pentacyclic heterocycle 50 by either a direct 6-endo trig cyclization or, alternatively, by a vinyl radical rearrangement pathway.

Low-temperature oxidation and autoignition of cyclohexene: A modeling study

A detailed thermokinetic model of the low-temperature oxidation of cyclohexene has been built. It comprises 136 species and 1064 reactions. The oxidation mechanism is based on the experimental identification of C6 intermediate species and on the peroxidation scheme of acyclic alkanes. Reactions related to addition of HO2 and OH radicals to the double bond were added, as well as the reactions linked to the association of the allylic cyclohexenyl radical with HO2. Known kinetic parameters were taken from published results. Unknown parameters were assigned by using values published for similar reactions or thermochemical calculations. Particular attention was devoted to the isomerization reactions of cyclohexenylperoxy radical in relation to the bicyclic structure of the transition states. The model was tested by simulating results obtained in a rapid compression machine. The measured autoignition delay times are well reproduced between 650 and 900 K in the pressure range 0.76 to 1.58 M Pa. The concentration-time profiles of major CB intermediates at 727 K are also well reproduced. An analysis of the fluxes of OH formation identified the most important pathways for the production of branching agents. The absence of a cool flame phenomenon is explained by the delayed production of these branching agents compared to acyclic alkanes. The reaction sequences linked to HO2 addition to cyclohexene and cyclohexa-1,3-diene have a key role in the preignition chemistry, the former having the highest rate of OH formation while the latter leads to the production of a major branching agent.

The production of benzene in the low-temperature oxidation of cyclohexane, cyclohexene, and cyclohexa-1,3-diene

The oxidation and auto-ignition of cyclohexane, cyclohexene, and cyclohexa-1,3-diene have been studied by rapid compression between 600 K to 900 K and 0.7 MPa to 1.4 MPa to identify the low-temperature pathways leading to benzene from cyclohexane. Auto-ignition delay times were measured and concentration-time profiles of the C6 intermediate products of oxidation were measured during the auto-ignition delays. Cyclohexane showed two-stage ignition at low temperatures, but single-stage ignition at higher temperatures, and a well-marked negative-temperature coefficient. By contrast there was neither a cool flame, nor a negative-temperature coefficient for cyclohexa-1,3-diene. Cyclohexene behaved in an intermediate way without a cool flame, but with a narrow, not very marked negative-temperature coefficient. The identified C6 products belong to three families: the bicyclic epoxides and cyclic ketones, the unsaturated aliphatic aldehydes, and the conjugated alkenes, which are always the major products. The formation of C6 products from cyclohexane is explained by the classical scheme for low-temperature oxidation, taking into account addition of O2 to cyclohexyl radicals and the various isomerizations of the resulting peroxy radicals. Most of the C6 products from cyclohexene are predicted by the same scheme, beginning with the formation of the allylic cyclohexenyl radical. However, addition of HO2 to the double bond has to be included to predict the formation of 1,2-epoxycyclohexane. For cyclohexa-1,3-diene, the classical scheme is not valid: the C6 oxygenated products are only formed by addition of HO2 to the double bond. For all three hydrocarbons, the pathways to benzene are those leading to conjugated alkenes, and they are always more efficient than those producing oxygenated products, either by adding HO2 to double bonds, or by addition of O2 to the initial cyclic radical.

Direct observation of the equilibrium between cyclohexenyl radicals, O 2, and cyclohexenylperoxy radicals

Reaction of cyclohexenyl radicals with O 2 has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. These radicals could be generated either by photolyzing 3-bromocyclohexene or by the two step reaction of carbon tetrachloride photolysis followed by the H atom abstraction reaction of Cl atom with cyclohexene. The equilibrium between cyclohexenyl radicals, O 2 and cyclohexenylperoxy radicals could be observed at a temperature of 361 K. An analysis of the temporal signal of cyclohexenyl radicals was used to evaluate both the forward and reverse rate constants as well as the equilibrium constant. Use of the latter together with an estimated Δ S 0 for the reaction yields a C–O bond energy of 80.4±4.2 kJ mol −1 in the RO 2 adduct.

Free radicals from biogenic volatile organic compounds (VOCs): an electron spin resonance investigation

Electron spin resonance has been used to characterise free radicals formed by the reactions of photochemically generated tert-butoxyl radicals with α-pinene, β-pinene, α-terpinene, γ-terpinene, car-2-ene, car-3-ene and limonene. The spectra have been interpreted using computer analysis and simulation and assigned to substituted cyclohexenyl or cyclohexadienyl radicals formed by abstraction of a hydrogen atom from the parent terpene.

ChemInform Abstract: ESR Study of Free Radicals in Irradiated Thiourea Canal Complexes. Part 4. Structure and Ring Inversion of Cyclohexenyl and Cycloheptenyl Radicals

ChemInform Abstract: ESR Study of Free Radicals in Irradiated Thiourea Canal Complexes. Part 4. Structure and Ring Inversion of Cyclohexenyl and Cycloheptenyl Radicals

ESR study of free radicals in irradiated thiourea canal complexes. Part 2. Structure and ring inversion of cyclohexyl and cycloheptyl radicals

ESR spectra have been observed for the cyclohexenyl radical in the temperature range between 77 and 360 K and for the cycloheptenyl radical in both of the host complexes at temperatures between 77 and 390 K. Temperature-dependent line broadening and shifting effects have been interpreted in terms of a ring inversion process and a torsional oscillation

Use of organic molecules as mechanistic probes for semiconductor-mediated photoelectrochemical oxidations: bromide oxidation

Cyclohexene has been used as an organic probe for mechanism in the semiconductor-photocatalyzed oxidation of bromide in acetonitrile. Products derived from bromine addition and from cyclohexenyl radical mediated autooxidation were isolated. These results implicate a mechanism in which the photoexcited semiconductor effects a one-electron oxidation of adsorbed bromide, producing surface-bound bromine atoms. These potentially could abstract hydrogen from cyclohexene to initiate autooxidation or could migrate along the semiconductor surface, producing bromine (Br/sub 2/), which migrates into solution where it is rapidly trapped in conventional electrophilic addition. 15 references, 1 figure, 1 table.

Reaktionen von Cyclanyl‐Radikalen in der Gasphase. I. Cyclohexyl‐Radikale aus Azocyclohexan

Reactions of Cyclanyl Radicals in the Gas Phase. I. Cyclohexyl Radicals from AzocyclohexaneBy theorem cyclothexyl radicals can react to yield the three isomeric C6H11‐radicals: 5‐hexenyl‐, resonance‐stabilized propyl allyl‐, and cyclopentylmethyl radical.This is not sufficient a basis to explain the amount of propylene by the conversion of cyclohexenyl radicals at high temperatures. For this reason we have studied the decomposition behaviour and all other types of reactions of cyclohexyl radicals from azocyclohexane as radical source in a flow system between 400 and 480°C and in part more.The most important result is the evidence of the 3‐methylcyclopentyl radical as an essential reaction intermediate.

Ring inversion in the cyclohexenyl radical

Activation parametes for ring inversion in the cyclohexenyl radical (ΔH‡= 6·81 ± 0·58 kcal mol–1, ΔS‡=–0·04 ± 2·40 cal K–1 mol–1, and ΔG‡298= 6·82 ±0·98 kcal mol–1) have been determined by variable temperature e.s.r. spectroscopy.

The γ-Radiolysis of a Cyclohexene–Carbon Dioxide Mixture in the Liquid Phase at −18 °C

Abstract The γ-radiolysis of a cyclohexene–carbon dioxide mixture in the liquid phase at −18 °C has been studied. When the mole fraction of carbon dioxide was low, carbon dioxide behaved as an electron scavenger and slightly suppressed the formation of the hydrogen and hydrocarbon products: 2,2′-bicyclohexenyl, cyclohexane, 3-cyclohexylcyclohexene, bicyclohexyl, 1-cyclohexylcyclohexene, and 1,3-cyclohexadiene. Along with this suppression, the formation of cyclohexanol was observed. At the high mole fraction of carbon dioxide, the main products were carbon monoxide and the oxygen atom-addition products: cyclohexene oxide, cyclohexanone, and cyclopentane-carbaldehyde. The material balance for oxygen atoms was good. The formation ratio of the addition products suggests that the precursors of the addition products are the ground-state oxygen atoms. These oxygen atoms are probably produced by the direct radiolysis of carbon dioxide. As the CO2 mole frcation approached unity, the G-values of the addition products and other oxygenated compounds including 2-cyclohexenone and 2-cyclohexenol sharply increased. This suggests that there is another source of oxygen atoms which is suppressed by the presence of cyclohexene. The reactions may be CO2++CO2−→CO+O+CO2 and CO2++cyclohexene→CO2+cyclohexene+. The formation of 2-cyclohexenone and 2-cyclohexenol may be explained in terms of the reaction between the cyclohexenyl radicals and oxygen molecules eventually produced. To support the elucidation of the reaction mechanism, the radiolysis in the solid phase at 77K and the mercury-photosensitized reaction of a nitrous oxide–cyclohexene mixture in the gas phase have also been studied.

The absorption spectra of cyclohexyl and cyclohexenyl radicals

The absorption spectrum of the cyclohexyl radical exhibits a pronounced shoulder in the region of 250 nm in contrast to the absorption of the cyclopentyl radical which decreases monotonically with increasing wavelength above 220 nm. In this region the cyclohexyl radical absorbs considerably more strongly than does the cyclopentyl radical, e.g., the extinction coefficients of the two radicals are, respectively, 920 and 430 M −1 cm −1 at 250 nm. The closer proximity of the unpaired electron orbital to the radical skeleton in the non-planar cyclohexyl radical may account for these differences. The cyclohexenyl radical has an intense absorption band centered at 240 nm ( E 240 ≈ 6500 M −1 c −1). This band, which is obviously associated with a transition involving the π system in this conjugated radical, is considerably broader than the similar but more intense band for the cyclopentenyl radical (widths, respectively of 35 and 20 nm at half maxima). The second order rate constants for disappearance of cyclohexyl and cyclohexenyl radicals have been measured to be (1.2 ± 0.4) and (1.6 ± 0.2) × 10 9 M − sec −1, respectively.

The absorption spectra of cyclohexyl and cyclohexenyl radicals

The absorption spectra of cyclohexyl and cyclohexenyl radicals

The absorption spectra of cyclohexyl and cyclohexenyl radicals

The absorption spectra of cyclohexyl and cyclohexenyl radicals

E.S.R. study of allyl type radicals observed in irradiated polycrystalline thiourea adducts of cycloheptane and cyclohexene

Allyl type radicals have been observed as the final irradiation products of both cyclohexene and cycloheptane included in thiourea. In the cyclohexene adduct the cyclohexenyl radical appears to be formed straight away, whilst in the cycloheptane adduct the cycloheptyl radical is formed first and, except at low temperatures where it is stabilized, changes to an allyl type radical without further irradiation. Consideration of the temperature dependence of the β splittings together with a comparison between the splittings observed in these two radicals and those of other alkyl radicals, leads to the conclusion that the motion of the β methylene groups about the bonds joining them to the π-bonded section of the radical is anisotropic, even when all four β protons appear to be equivalent.

ESR Study of Free Radicals Produced in Irradiated Cyclopentadiene and Cyclohexene: The Cyclopentadienyl and Cyclohexenyl Radicals

The ESR spectra of irradiated cyclopentadiene and cyclohexene were measured at various temperatures. The spectra showed motional narrowing on raising temperature and the free radicals were identified from analysis of the narrowed spectra. Cyclopentadiene showed a well-resolved sextet spectrum of 5.6-G splitting, which was assigned to the cyclopentadienyl radical. Cyclohexene gave a spectrum consisting at least of 26 lines, which can be reasonably attributed to the cyclohexenyl radical by assuming appropriate hyperfine constants.

PHOTOCHEMICAL SYNTHESES: I. THE REACTION OF ALDEHYDES AND KETONES WITH CYCLOALKENES

The irradiation of dialkyl ketones in cyclohexene leads to the formation of cyclohexenyl-dialkylcarbinols. The use of aldehydes leads to the formation of the corresponding secondary alcohols, but the reaction is more complex because of concomitant Kharasch addition which can, however, be suppressed. The mechanism of the alcohol formation has been elucidated by the use of an optically active substrate, carvomenthene, and has been shown to involve the intermediacy of a cyclohexenyl radical. The structure and genesis of a biscarbinol by-product is discussed.