Conformational analysis and stereodynamics of primary acyclic alkyl radicals by EPR spectroscopy

Ingold, K. U.; Nonhebel, D. C.; Walton, John C.

doi:10.1021/j100404a017

Cited by 14 publications

(3 citation statements)

References 9 publications

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“…This spectrum is readily simulated by assuming coupling to two inequivalent nitrogen and single hydrogen atoms (Figure b), attributable to the neutral radical [S 2 N 2 H] • through a proton transfer . Significantly, the hfs values for N2 and H show significant variation with changes in temperature and environment (Table ) as expected for a non-planar radical . Since adventitious moisture is the most likely source of protons for forming [S 2 N 2 H] • , this radical was generated deliberately by electrolysis of “wet” 2:1 mixtures of CH 2 Cl 2 /CH 3 CN (where the latter was degassed but not dried).…”

Section: Resultsmentioning

confidence: 97%

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S₄N₄]^−•, EPR Spectroscopic Characterization of the [S₂N₂H]^• Radical, and X-ray Structure of [Na(15-crown-5)][S₃N₃]

et al. 2009

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Voltammetric studies of S(2)N(2) employing both cyclic voltammetry (CV) and rotating disk electrode (RDE) methods on GC electrodes at room temperature (RT) revealed two irreversible reduction processes at about -1.4 V and -2.2 V in CH(3)CN, CH(2)Cl(2), and tetrahydrofuran (vs ferrocene) and no observable oxidation processes up to the solvent limit when the scan is initially anodic. However, after cycling the potential through -1.4 V, two new couples appear near -0.3 V and -1.0 V due to [S(3)N(3)](-/0) and [S(4)N(4)](-/0) respectively. The diffusion coefficient D for S(2)N(2) was determined to be 9.13 x 10(-6) cm(2) s(-1) in CH(2)Cl(2) and 7.65 x 10(-6) cm(2) s(-1) in CH(3)CN. Digital modeling of CVs fits well to a mechanism in which [S(2)N(2)](-*) couples rapidly with S(2)N(2) to form [S(4)N(4)](-*), which then decomposes to [S(3)N(3)](-). In situ electron paramagnetic resonance (EPR) spectroelectrochemical studies of S(2)N(2) in both CH(2)Cl(2) and CH(3)CN resulted in the detection of strong EPR signals from [S(4)N(4)](-*) when electrolysis is conducted at -1.4 V; at more negative voltages, spectra from transient adsorbed radicals are observed. In moist solvent or with added HBF(4), a longer-lived spectrum is obtained due to the neutral radical [S(2)N(2)H](*), identified by simulation of the EPR spectrum and density functional theory (DFT) calculations. The chemical reduction of S(2)N(2) with Na[C(10)H(8)] or Na[Ph(2)CO] produces [Na(15-crown-5)][S(3)N(3)], while reduction with cobaltocene gives [Cp(2)Co][S(3)N(3)]. The X-ray structure of the former reveals a strong interaction (Na...N = 2.388(5) A) between the crown ether-encapsulated Na(+) cation and one of the nitrogen atoms of the essentially planar six-membered cyclic anion [S(3)N(3)](-).

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Section: Resultsmentioning

confidence: 97%

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S₄N₄]^−•, EPR Spectroscopic Characterization of the [S₂N₂H]^• Radical, and X-ray Structure of [Na(15-crown-5)][S₃N₃]

et al. 2009

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“…[46,47,50,[57][58][59][60][61][62][63][64][65][66] Products 2a and 2b were prepared according to literature protocols. [67][68][69][70]…”

Section: Methodsmentioning

confidence: 99%

Catalytic Oxidation of Primary C–H Bonds in Alkanes with Bioinspired Catalysts

Dantignana

Costas

2020

Chimia

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Catalytic oxidation of primary C–H bonds of alkanes with a series of iron and manganese catalysts is investigated. Products resulting from oxidation of methylenic sites are observed when hexane (S1) is used as model substrate, while corresponding primary C–H bonds remain unreactive. However, by using 2,2,3,3-tetramethylbutane (S2) as model substrate, which only contains primary alkyl C–H bonds, oxidation takes place catalytically using a combination of hydrogen peroxide, a manganese catalyst and acetic acid as co-catalyst, albeit with modest yields (up to 4.4 TON). Complexes bearing tetradentate aminopyridine ligands provide the best yields, while a related pentadentate ligand provides smaller product yields. The chemoselectivity of the reaction is solvent dependent. Carboxylic acid 2b is observed as major product when the reaction takes place in acetonitrile, because of the facile overoxidation of the first formed alcohol product 2a. Instead the corresponding primary alcohol 2a becomes dominant in reactions performed in 2,2,2-trifluoroethanol (TFE). Polarity reversal of the hydroxyl moiety arising from the strong hydrogen bond donor ability of the latter solvent accounts for the unusual product chemoselectivity of the reaction. The significance of the current results in the context of light alkane oxidation is discussed.

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“…CH 2 CHR 2 which show no exchange broadening. 11,12 Due to freezing of the propane solvent at lower temperatures, we were unable to determine the hfs of the nonequivalent H a . However, in analogous radicals .…”

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confidence: 99%

First EPR characterisation of the structures and reactivities of α- and β-boryl radicals derived from boronic esters

Walton¹,

McCarroll²,

Nziengui³

et al. 1997

Chem. Commun.

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The first EPR observations of carbon-centred radicals with aand b-boronic ester substitutents indicated moderate thermodynamic stabilisation for the former whereas the latter had an inclination towards b-scission.Boronic acids and esters are convenient synthetic intermediates which have been widely applied in stereocontrolled reactions. 1 Recent research has shown that a-boryl radicals, generated by addition of carbon-centred radicals to alkenyl-2 and alkynylboronic esters, 3 can take part in inter-and intra-molecular chain addition reactions. a-Boryl radicals have also been generated by iodine abstraction from a-iodoboronic esters 4 and high yields of useful homoallylboronic esters were obtained in allylation reactions. These developments led us to apply EPR spectroscopy to identify the radical intermediates and examine key reaction rates for several types of carbon-centred radicals possessing a boronic ester functionality. Little is known about the EPR spectra of boron-containing species in solution, apart from the impressive spectroscopic experiments of Roberts and co-workers with amine-boryl and other ligated-boryl radicals, 5 a report on the Me 2 BCH 2. radical in the solid state 6 and some aminoxyl species. 7 The a-bromoboronic esters 1a,b were prepared according to the protocols developed by Matteson and co-workers from B(OPr i ) 3 , CH 2 Br 2 and Bu n Li for 1a, 8 from pinacol n-hexyl boronate, CH 2 Br 2 and Pr i 2 NLi for 1b. 9 EPR spectra, recorded during abstraction of the bromine atoms of these boronic esters by photochemically generated Me 3 Sn . radicals (or Et 3 Si . radicals) in tert-butylbenzene, cyclopropane or propane could be positively assigned to the corresponding a-boryl radicals 2a,b (EPR parameters in Table 1). The hyperfine splittings (hfs) from the a-Hs are somewhat smaller than in analogous unsubstituted primary and secondary alkyl radicals [e.g. for . CH 2 CH(OEt) 2 a(H a ) = 22.2 G] 10 which suggests there is some electron delocalisation into the vacant boron p-orbital (3). This is also consistent with the comparatively large 11 B hfs.If 3 makes a significant contribution to the ground state there should be an appreciable barrier to rotation about the C a -B bond. To try and detect this we examined the EPR spectra of radicals generated from the unsymmetrical boronic esters 4a,b prepared by transesterification of 1a,b with (+)-pinanediol.

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Conformational analysis and stereodynamics of primary acyclic alkyl radicals by EPR spectroscopy

Cited by 14 publications

References 9 publications

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S₄N₄]^−•, EPR Spectroscopic Characterization of the [S₂N₂H]^• Radical, and X-ray Structure of [Na(15-crown-5)][S₃N₃]

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S₄N₄]^−•, EPR Spectroscopic Characterization of the [S₂N₂H]^• Radical, and X-ray Structure of [Na(15-crown-5)][S₃N₃]

Catalytic Oxidation of Primary C–H Bonds in Alkanes with Bioinspired Catalysts

First EPR characterisation of the structures and reactivities of α- and β-boryl radicals derived from boronic esters

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Conformational analysis and stereodynamics of primary acyclic alkyl radicals by EPR spectroscopy

Cited by 14 publications

References 9 publications

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S4N4]−•, EPR Spectroscopic Characterization of the [S2N2H]• Radical, and X-ray Structure of [Na(15-crown-5)][S3N3]

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S4N4]−•, EPR Spectroscopic Characterization of the [S2N2H]• Radical, and X-ray Structure of [Na(15-crown-5)][S3N3]

Catalytic Oxidation of Primary C–H Bonds in Alkanes with Bioinspired Catalysts

First EPR characterisation of the structures and reactivities of α- and β-boryl radicals derived from boronic esters

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Product

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Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S₄N₄]^−•, EPR Spectroscopic Characterization of the [S₂N₂H]^• Radical, and X-ray Structure of [Na(15-crown-5)][S₃N₃]

Electrochemical and Chemical Reduction of Disulfur Dinitride: Formation of [S₄N₄]^−•, EPR Spectroscopic Characterization of the [S₂N₂H]^• Radical, and X-ray Structure of [Na(15-crown-5)][S₃N₃]