Efficient Method for Generation of Radical Anions from Benzene Derivatives of Low Electron Affinity

Gerson, Fabian; Heckendorn, René

doi:10.1002/anie.198305561

Cited by 4 publications

(2 citation statements)

References 5 publications

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“…The values 1.23 and 0.95 mT tended to an average | a (H-5,6)| = 1.08 mT upon raising the temperature from 195 to 240 K. Hyperfine splittings from the γ-protons of the 1,4- tert -butyl substituents and from the 39 K nucleus of the counterion were not perceivable (<0.03 mT), but the two Rb isotopes gave rise to detectable coupling constants, | a ( 85 Rb)| = 0.040 and | a ( 87 Rb)| = 0.136 mT. With cesium as the reducing agent, the ESR spectrum of the radical anion of 1,4-di- tert -butylbenzene was observed, due to the “aromatization” of the six-membered ring.…”

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confidence: 96%

Unusually Tight Ion Pairing of the 1,4- and 2,3-Di-tert-butylbuta-1,3-diene Radical Anions with Alkali-Metal Cations: An ESR and ENDOR Study

et al. 1998

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Radical anions of several alkyl-substituted buta-1,3-dienes, 4 − 9, and of 1,4-di-tert-butylcyclohexa-1,3-diene (11) were characterized by their hyperfine data with the use of ESR and ENDOR spectroscopy. Some of these radical anions, which are protected by bulky alkyl substitutents, could be generated by reaction of the corresponding neutral compounds with a potassium, rubidium, or cesium mirror in 1,2-dimethoxyethane (DME) and, mostly, tetrahydrofuran (THF). In particular, the radical anions of 1,4- and 2,3-di-tert-butylbuta-1,3-dienes (6 and 7, respectively) prepared by this method proved to be fairly persistent, and their ESR and ENDOR spectra were thoroughly studied in a wide temperature range. Characteristic of the hyperfine patterns of 6 •- and 7•- are very large coupling constants of the alkali-metal nuclei in the counterion: a(39K) = 0.12−0.15, a(85Rb) = 0.40−0.84, a(87Rb) = 1.4−2.8, and a(133Cs) = 0.70−2.6 mT. Values of this size, unusual for counterions of hydrocarbon radical anions, point to a tight or contact ion pairing of 6 •- and 7 •- with the alkali-metal cations M+ (M = K, Rb, Cs). Whereas for 6 •-/M+ in DME and THF only such tight ion pairs were observed, their coexistence with loose or solvent-separated ion pairs was noticed for 7 •-/K+ in DME at very low temperatures. Apart from the large hyperfine splittings due to the alkali-metal nuclei, a striking feature of the tight ion pairs 7 •-/M+ is the coupling constant of two protons in the 1,4-positions; its absolute value (ca. 0.4 mT) is much smaller than that (ca. 0.7 mT) of the corresponding protons in the radical anions of buta-1,3-dienes. Assignment of this reduced value to the endo-protons is compatible with the ESR and ENDOR spectra of 7-d 2 •-/M+ dideuterated in the 1-position. The structures of 6 •-/M+ and 7 •-/M+ were discussed with the aid of theoretical calculations. Undoubtfully, the planar trans-conformation with the bulky 1,4-tert-butyl substituents in the sterically unhindered exo-positions is generally favored for 6 •-/M+. The counterion M+ should be situated on the 2-fold axis above or below the molecular plane of 6 •-. In 7 •-, the steric overcrowding by the 2,3-tert-butyl substituents is relieved by a strong twist about the C(2)−C(3) bond; loose and tight ion pairs 7 •-/M+ may have different conformations. For the tight ion pairs 7 •-/M+, a conformation should be preferred in which the counterion M+ is situated on the 2-fold axis with the two tert-butyl substituents “on the other side” of the molecule. The close contact of the alkali-metal cation M+ with either 6 •- or 7 •- must be promoted by the spatial arrangement of these substituents.

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Section: Resultsmentioning

confidence: 96%

Unusually Tight Ion Pairing of the 1,4- and 2,3-Di-tert-butylbuta-1,3-diene Radical Anions with Alkali-Metal Cations: An ESR and ENDOR Study

et al. 1998

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“…In a modification of this procedure, the direct contact of the compound with the metallic mirror is avoided by first dissolving the alkali metal in DME or HMPT at low temperatures and subsequently carrying out the reaction with solvated electrons thus formed (36). The reducing power of such electrons is enhanced by simultaneous irradiation with visible light, so that even benzene derivatives with very low electron affinity could be converted to their radical anions (37). On the other hand, quinones are already reduced in an alkaline alcohol by mild reagents like glucose (38).…”

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Organic Radical Ions as Studied by Epr Spectroscopy

Gerson¹

1998

Foundations of Modern EPR

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Radicals and Radical Ions Derived from 1,3‐Dienes

Courtneidge

Davies

1984

Bulletin des Soc Chimique

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Recent improvements in experimental methods have made it possible to qenerate radicals and radical ions derived from simple 1,3-dienes, so that their 0.s.r. spectra can be observed in fluid.solution.PhOtOly8is of various cyclopentadienylmetallic compounds, o r even of p ntamethylcyclopentadiene itself, serves as a source of the corresponding cyclopentadienyl radicals, and the former method can be extended to the preparation of the fulvalene and pentalene radical anions.Cyclobutadiene radical cations are formed when the complexes formed between dimerimed dialkylalkynes and aluminium chloride are photolysed. ethyne may also undergo a double nucleophilic rearrangement when it is treated with aluminium chloride, resulting in the formation of the hexamethylbutadiene radical cation, which can also be prepared from hexamethylbutadiene itself. and acids, respectively, of neutral radicals, and this principle has been exploited in the generation of cyclopentadiene radical cations from the photolysis of cyclopentadienes in acid solution. Di-t-butyl- Radical anions and cations can be regarded as being the conjugate bases INTRODUCTIONExcited states provide readily accessible electronically modified molecules, with their own distinctive chemistry, and which provide the basis of photochemical processes. ted to their precursors by the simplest of all chemical reactions, that of addition or removal of one electron. The radical ions of arenes, particularly the polycyclic arenas, have for long been familiar.' of simpler unsaturated molecules are now being rapidly developed. As yet, these new species have attracted the interest of, mainly, physical organic chemist$, but it is easy to forsee the development of synthetic methods with radical ions as intermediates.We have been interested in the generation, from simple unsaturated compounds, of both neutral radicals, and radical ions, and the relation between these species, and we review here some of our recent work on l,3-dienes. Radical ions constitute

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Efficient Method for Generation of Radical Anions from Benzene Derivatives of Low Electron Affinity

Cited by 4 publications

References 5 publications

Unusually Tight Ion Pairing of the 1,4- and 2,3-Di-tert-butylbuta-1,3-diene Radical Anions with Alkali-Metal Cations: An ESR and ENDOR Study

Unusually Tight Ion Pairing of the 1,4- and 2,3-Di-tert-butylbuta-1,3-diene Radical Anions with Alkali-Metal Cations: An ESR and ENDOR Study

Organic Radical Ions as Studied by Epr Spectroscopy

Radicals and Radical Ions Derived from 1,3‐Dienes

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