Conformation Of Radicals Research Articles

Stereoselective C-C Bond Formation in Carbohydrates by Radical Cyclization Reactions; VI. Influence of the Conformation of Anomeric Radicals on the Stereoselectivity of Their Cyclization

Stereoselective C-C Bond Formation in Carbohydrates by Radical Cyclization Reactions; VI. Influence of the Conformation of Anomeric Radicals on the Stereoselectivity of Their Cyclization

Cycloalkylmethyl radicals. Part 7.—Electron paramagnetic resonance characterisation of axial and equatorial centres in seven-membered alicyclic molecules

The EPR spectra of cycloheptylmethyl, cyclohept-4-enylmethyl and 1,3-dioxacyclohept-5-enylmethyl radicals have been observed. The cycloheptylmethyl radical spectrum shows only a single, average conformation, but for the cycloheptene and dioxacycloheptene rings the CH˙2 group proved to be a useful probe of the conformations. The preferred conformation of the 1,3-dioxacycloheptene ring was found to depend on the type of substitution at C(2). For the 2-CH˙2 radical, twist-boat and chair conformers were observed with an Arrhenius activation barrier to inversion of 5 kcal mol–1. For the 2-CH3, 2-CH˙2 radical the main conformation was the twist-boat. For the 2-Ph, 2-CH˙2 radical the main conformation was the chair; the rotation of the CH˙2 group was found to be restricted by a barrier of ca. 7.5 kcal mol–1, probably because of steric interaction from two syn axial hydrogens on C(4) and C(7).

Study of the structure of free radicals in irradiated monocrystals of N-formyl-L-leucine at room temperature

A study was carried out on the structure of the radical formed upon γ-radiolysis of monocrystals of N-formyl-L-leucine by ESR spectroscopy and quantum chemical calculations. The calculated ESR parameters for the radical conformations, corresponding to an energy minimum, are in best accord with the experimental values.

Vibrational spectra of cyclic dithioethers and their dications. 2. Raman spectra and conformations of the monocation radicals and dications of 1,5-dithiacyclooctane and related dithioethers

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTVibrational spectra of cyclic dithioethers and their dications. 2. Raman spectra and conformations of the monocation radicals and dications of 1,5-dithiacyclooctane and related dithioethersMakiko Tamaoki, Masanori Serita, Yasushi Shiratori, and Koichi ItohCite this: J. Phys. Chem. 1989, 93, 16, 6052–6058Publication Date (Print):August 1, 1989Publication History Published online1 May 2002Published inissue 1 August 1989https://doi.org/10.1021/j100353a023RIGHTS & PERMISSIONSArticle Views49Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (824 KB) Get e-Alerts

Conformations of cycloalkenyl radicals: A dynamic EPR study of the cyclohept‐4‐enyl radical

AbstractCyclopent‐3‐enyl, cyclohex‐3‐enyl and cyclohept‐4‐enyl radicals were generated and their EPR spectra obtained in solution over a range of temperatures. The spectral data were consistent with a fully planar conformation for cyclopent‐3‐enyl radicals and a slightly flattened half‐chair conformation for cyclohex‐3‐enyl radicals. Cyclohept‐4‐enyl radicals showed exchange broadening in their EPR spectra which indicated inversion from one chair conformer to another with an Arrhenius activation barrier of 16.5 ± 0.3 kJ mol‐1.

ESR and ENDOR studies of radicals produced in hydroxyproline single crystals by x-irradiation at low temperatures. 2

In an extension of previous work, six additional radicals have been detected in crystals of hydroxyproline (NH/sub 2//sup +/C/sub delta/H/sub 2/C/sub ..gamma../HOHC/sub ..beta../H/sub 2/C/sub ..cap alpha../HCOO/sup -/) irradiated at 10 or 65 K. Five of these have been identified, with characteristics as follows. In radical I, the product of decarboxylation, the unpaired electron interacted with five hydrogen nuclei. One of the interactions was from a nonexchangeable ..cap alpha..-proton, two were from nonexchangeable ..beta..-protons, and two were from easily exchangeable ..beta..-protons. Radical I formed at temperatures below 10 K and was stable up to 160 K. In radical II, which was observed to anneal near 20 K, the unpaired electron interacted with two nonexchangeable hydrogen nuclei. On the basis of the characteristics of the couplings, radical II was tentatively identified as the product of electron loss from the side chain, followed by deprotonation at C/sub ..beta../. Radical III, which exhibited four nonexchangeable ..beta..-proton interactions, was identified as the product of net hydrogen abstraction from C/sub ..gamma../. Radical III was found after irradiation at 65 K, but not when crystals irradiated at 10 K were warmed. Radical IV, identified as an additional conformation of the deamination radical, was found preferentially (but notmore » exclusively) in deuteriated crystals in the temperature range 77-125 K. In radicals V and VI, the unpaired electron interacted with only one ..beta..-proton. Radical V was identified as another conformation of the primary carboxyl anion, but it was not possible to identify radical VI« less

The formation of β-muonium-substituted cyclopentyl and cycloheptyl radicals, and the significance of the A′µ/AH isotope ratio in relation to the conformations of muonium-substituted alkyl radicals

From the coupling constants obtained from a wide variety of β-muonium-substituted alkyl radicals and their protiated analogues, we conclude that the preference for the conformation in which the β-C—Mu bond eclipses the singly occupied orbital depends largely on hyperconjugative effects, although steric effects become more important as the size of the substituents at the radical centre is increased. Cyclic radicals, including the β-muonium-substituted cyclopentyl and cycloheptyl radicals, are discussed with respect to the ‘limiting isotope ratio’ towards which all alkyl radicals should tend at high temperatures, according to hyperconjugation theory. We conclude that the value of this limiting ratio is 1.18 ± 0.05.

Ab initio study of propyl radicals

The UHF method has been used to show that the most stable conformation of the n-propyl radical is when the methyl group is in a plane perpendicular to the plane of the methylene group, contrary to experiment and previous SCF calculations. The hindered rotation coordinate for the methylene group is calculated by including the zero-point energies of each conformation within the adiabatic approximation. The most stable conformation is now when one of the Cβ—H bonds is in the plane perpendicular to the plane of the methylene group. These calculations suggest that C—CH3 hyperconjugation is at least as significant as C—H hyperconjugation. A study is made on the isopropyl radical to investigate the motions of the equivalent methyl groups. This radical is interesting, since the muonated derivative has been studied.

ChemInform Abstract: Selective Formation and Conformational Analysis of Carbohydrate‐Derived Radicals.

Carbohydrate free radicals are generated in non-aqueous solution by abstraction of bromine, iodine or selenophenyl with photolytically generated stannyl radicals from appropriately substituted pyranose derivatives and their e.s.r. spectra are recorded in the temperature range from –30 to 70 °C. From the hyperfine splitting parameters the preferred conformations of these sugar radicals are derived. The findings are compared with results from investigations in aqueous solution and an attempt is made to identify the factors which determine the conformations. From the α-13C coupling constant of the tetra-acetylglucosyl radical it is concluded that pyranosyl radicals are of the π-type. A novel 1,2-migration of an acetoxy group in tetra-acetylgalactosyl radical is reported. The mechanism of interaction of nitrosugars and related compounds with tin radicals is discussed.

Selective formation and conformational analysis of carbohydrate-derived radicals

Carbohydrate free radicals are generated in non-aqueous solution by abstraction of bromine, iodine or selenophenyl with photolytically generated stannyl radicals from appropriately substituted pyranose derivatives and their e.s.r. spectra are recorded in the temperature range from –30 to 70 °C. From the hyperfine splitting parameters the preferred conformations of these sugar radicals are derived. The findings are compared with results from investigations in aqueous solution and an attempt is made to identify the factors which determine the conformations. From the α-13C coupling constant of the tetra-acetylglucosyl radical it is concluded that pyranosyl radicals are of the π-type. A novel 1,2-migration of an acetoxy group in tetra-acetylgalactosyl radical is reported. The mechanism of interaction of nitrosugars and related compounds with tin radicals is discussed.

On the conformation of bipyridine dications and cation radicals

Ab initio STO-3G calculations with partial geometry optimization were performed to determine the conformational structure of 2,2′- and 4,4′-bipyridine dications and their cation radicals. The results suggest a considerably twisted conformation for the dications, whereas the corresponding radical cations are nearly planar. Reasons for the structure differences are discussed and related to the herbicidal action of these compounds.

Thermal cycloaddition reaction of xanthene‐9‐thione with allenes: An ESR study

AbstractDuring the parallel (2 + 2)‐ and (4 + 2)‐cycloaddition reactions of aromatic thiones such as xanthene‐9‐thione with substituted allenes XCHCCH2 (X = OR, SR, NMe2, Ph), a strong ESR signal is observed. This signal is associated with the (4 + 2)‐cycloaddition reaction, affording thiopyran derivatives 3,11b‐dihydro‐2‐(X‐methylene)‐2H‐thiopyrano[4,3,2‐kl]xanthene F or 3,11b‐dihydro‐3X‐2‐methylene‐2H‐thiopyrano[4,3,2‐kl]xanthene E. Computer analyses of this signal made possible its identification as emanating from a 9‐xanthenyl radical substituted at the 1‐ and 9‐positions. The type of xanthenyl radical that is formed in the thermal reaction E rad or F rad depends on the nature of the substituent X at the allenic system H2CCCHX. This investigation shows that where X = Ph and NMe2, xanthenyl radicals of structure E rad are produced, and where X = O‐t‐Bu and S‐t‐Bu xanthenyl radicals of structure F rad are produced. It is striking that this production is so selective.On irradiation (λ>254nm), the production of xanthenyl radicals is enhanced. The conformation of xanthenyl radicals E rad and the role of E rad and F rad in the formation of addition products E and F are discussed.

Electron spin resonance spectroscopic investigation of carbohydrate radicals. Part 2. Conformation and configuration in pyranos-1-yl radicals

Carbohydrate free radicals wer regiospecifically generated at C-1 of acylated and alkylated pyranosyl derivatives in non-aqueous solution and their conformations were deduced from the hyperfine splittings of their e.s.r. spectra. The preferred conformations of the pyranosyl radicals are discussed in terms of a stabilizing interaction of the singly occupied p-orbital with the σ*-LUMO of the adjacent β-C–OR bond. From the α-13C coupling constant of the tetra-acetylglucosyl radical it is concluded that pyranosyl radicals are of π-type.

Esr studies of the ring opening of cyclopropane radical cations in freon matrices

The radical cations of cyclopropane and several of its methyl derivatives have been characterized by ESR spectroscopy following their generation by γ irradiation of dilute solutions of the parent compounds in Freon matrices at 77 K. In the CFCl 3, CF 3CCl 3, and CF 2ClCCl 3, matrices, only the ring-closed species is usually observed in the accessible temperature range up to ca 160 K. In the CFCl 2CF 2Cl matrix, however, the ring-closed radical cations initially formed at 77 K undergo ring opening between 83 and 110 K, the more highly substituted radical cations requiring a higher temperature for this transformation. The ring-closed radical cations are 2A 1 species for C 2v symmetry, the most substituted cyclopropane C-C bond being elongated with the spin density largely confined to the basal carbons in a face-to-face (90°, 90°) structure. In the ring-opened radical cations, the radical center is localized on the most substituted carbon atom following the breaking of the weakened C-C bond of the ring-closed species. The radical conformations of the ring-opened species have been determined, the RCH 2CH 2· center produced from cyclopropane having a bisected conformation while the RCH 2CMe 2· center obtained from 1,1,2,2-tetramethylcyclopropane is eclipsed, as expected for the presence of α-methyl substituents at the radical site. The nature of the putative carbocation center in the ring-opened radical cations is discussed with reference to recent proposals that this center is strongly coordinated to an electrophile (Cl - or RCl) thereby negating the requirement for an orthogonal structure. Consideration of the strong matrix dependence of the ring-opening reaction suggests a possible solvation effect, however, in which the CFCl 2CF 2Cl matrix assists the twisting of one of the CR 1R 2 groups at the most substituted bond, leading to the rupture of this one-electron σ bond. A strong solvation effect also explains why ring-opening can occur in a suitable polar solvent despite theoretical calculations of unfavourable energetics for a similar gas-phase reaction. Experiments are also described on spiro[2.5]octane, the cyclopropane ring undergoing scission at the CH 2-CH 2 bond of this radical cation to give an RCH 2· radical center. this radical then undergoes a H-atom abstraction with a neutral spiro[2.5]octane molecule in the CFCl 2CF 2Cl matrix at higher temperature to give the spiro[2.5]oct-6-yl radical.

Conformations of hexahydropyridazine cation radicals

Conformations of hexahydropyridazine cation radicals

ESR spectra and kinetics of the disproportionation of substituted phenoxy radicals Coomunication 2. Radicals of 2,4,6-trialkyl-substituted mono-, p-bis-, p-tris-, and p-tetraphenols

1. The parameters of the ESR spectra and the rate constants of the disproportionation of a series of 2,4,6-trialkyl-substituted phenoxyl radicals formed by the reaction of the corresponding phenols with alkoxyl radicals in benzene have been determined in the 25–80°C interval. 2. The HFS constants with the β-protons and the rate constants of disproportionation are practically the same for raononuclear phenoxyls and their polynuclear analogs. The predominant conformations of the radicals have been determined. 3. The change in the disproportionation rate constants is due mainly to a change in the activation energy. The connection of the kinetics of the disproportionation of the phenoxyls with their structure, spin density distribution, conformation, and intensity of intramolecular motions has been considered.

ENDOR spectra of nitroxide radicals in disordered matrices

An ENDOR study on several nitroxide radicals included in polycrystalline and glassy matrices is reported. A calculation program has been developed to simulate ENDOR spectra of radicals in disordered matrices. The shape of the computed spectra can be changed from crystal-like to powder-like type through the intermediate cases. The principal values of the AH hyperfine tensors have been obtained for tempone, tempol, tempyo and di-t-butyl-nitroxide. The influence of the host matrix on radical conformations is also discussed.

Reactivity and selectivity in the cyclization of sila-5-hexen-1-yl carbon-centered radicals

A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri- n -butyltin hydride (deuteride). The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially exo-trig ) cyclization compared to the all-carbon system. The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system. The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate via the expected exo-trig mode and and from a significantly enhanced rate of hydrogen abstraction from TBTH . Both the α- and γ-silyl radicals cyclized via the endo-trig mode at rates close to that of the parent 5-hexen-1-yl radical itself. The cyclizations studied were demonstrated to be irreversible. The kinetic control thus shown by the preferred formation of endo cyclized product from the α- and β-silyl radicals is highly unusual and represents the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule exo-trig cyclization preferred by 5-hexen-1-yl radicals). Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors. The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no exo mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet. Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.

Conformation of Glycosyl Radicals: Radical Stabilization by β‐CO Bonds

A transformation from the chair- into the boat conformation takes place when glucosides such as 1 are converted into glucosyl radicals 2. This preference for the boat conformation is due to the radical-stabilizing effect of the coplanar arrangement of the β-CO bond and semi-occupied orbital. The findings are of interest regarding stereoselective radical CC-linkage in the synthesis of C-glycosides.

Dynamic effects in the E.S.R. spectra of the 5-hydro-6-methyl-6-yl-uracil radical in different solid matrices

5-Hydro-6-methyl-6-yl-uracil (HMU) radicals generated from 5,6-dihydro-6-methyluracil or 6-methyluracil by X-irradication have been studied in single crystals, polycrystalline powders and aqueous glasses of both compounds. In all matrices the spectra of the HMU radicals show a strong temperature dependence attributable to the alternating linewidth effect. By simulation of the spectra exchange rates have been determined for the methylene protons at C5. It is shown that structure and conformation of the HMU radical are essentially independent of the precursor molecule and of the matrix. The differences observed in the temperature dependence of the spectra can be explained completely by thermal motions of the radicals, which are differently impeded by the different molecular packings of the respective matrices.