Halogen abstraction studies. V. Abstraction of iodine by phenyl radicals from iodonaphthalenes, iodopyridines, and iodothiophenes. Question of polar effects

Danen, Wayne C.; Saunders, Donald G.; Rose, Kenneth A.

doi:10.1021/ja00821a033

Cited by 14 publications

(9 citation statements)

References 2 publications

(2 reference statements)

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“…Subsequently, the thus-introduced 4-(4‘-pyridyl)pyridinium functionality could be detected and quantified by cyclic voltammetry (not shown). We found a low surface coverage on the order of 0.7 × 10 -10 mol cm -2 , which could be an effect of the low reactivity of the alkyl radicals toward the surface or C 6 H 5 • reacting with already-grafted alkyl-Cl chains, possibly by performing halogen abstraction …”

Section: Resultsmentioning

confidence: 90%

Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts

et al. 2007

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The range of materials susceptible to electrochemically assisted grafting onto carbon materials has been expanded to include a new group of compounds. This new approach is based on the reduction of symmetrical or unsymmetrical triarylsulfonium salts and alkyldiphenylsulfonium salts. Our findings suggest that it is possible to form layers of aryl moieties on the surface and that the unsymmetrical triarylsulfonium salts cleave upon reduction in a direction dictated by the substituent on the rings (i.e., (4-methoxyphenyl)diphenylsulfonium salt leads to a film made predominantly of phenyl groups, whereas (4-chlorophenyl)diphenylsulfonium salt leads to a mixture of phenyl and chlorophenyl groups). These relationships may be understood by considering the inductive nature of the substituent with regard to the aryl-S bonds and are supported by preparative experiments. Upon reduction, the alkyldiphenylsulfonium salts are found to cleave almost exclusively to an alkyl radical and diphenyl sulfide. As judged from the electrochemical blocking properties of the films made from such species, either relatively thick or compact films are formed. The mass spectrometric analysis indicates that the films are made of a combination of alkyl and aryl groups and possibly related structural derivatives. Importantly, our findings provide evidence that it is possible to graft electrode surfaces with reactive aryl radicals even using precursors reduced at potentials that are substantially more negative than the estimated reduction potential of the grafting radical.

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Section: Resultsmentioning

confidence: 90%

Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts

et al. 2007

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“…The homolytic C−I bond dissociation energies of the 2-, 3-, and 4-iodopyridinium cations were calculated (B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p)) to be 68.7, 67.2, and 67.2 kcal/mol, respectively. The homolytic C−I bond dissociation energies of 3-and 4-iodopyridines have been shown 49 to be similar to that of iodobenzene. The calculated C−I bond dissociation energy of iodobenzene (at the same level of theory as the other calculated values) is 65.7 kcal/mol, which is in very good agreement with the experimentally determined 50 bond dissociation energy (66.7 kcal/mol).…”

Section: ■ Results and Discussionmentioning

confidence: 98%

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

et al. 2012

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In order to directly compare the reactivity of positively charged carbon-centered aromatic σ-radicals toward methanol in solution and in the gas phase, the 2-, 3-, and 4-dehydropyridinium cations (distonic isomers of the pyridine radical cation) were generated by ultraviolet photolysis of the corresponding iodo-precursors in a mixture of water and methanol at varying pH. The reaction mixtures were analyzed by using liquid chromatography/mass spectrometry. Hydrogen atom abstraction was the only reaction observed for the 3- and 4-dehydropyridinium cations (and –pyridines) in solution. This also was the major reaction observed earlier in the gas phase. Depending on the pH, the hydrogen atom can be abstracted from different molecules (i.e., methanol or water) and from different sites (in methanol) by the 3- and 4-dehydropyridinium cations/-pyridines in solution. In the pH range of 1 – 4, the methyl group of methanol is the main hydrogen atom donor site for both 3- and 4-dehydropyridinium cations (just like in the gas phase). At higher pH, the hydroxyl groups of water and methanol also act as hydrogen atom donors. This finding is rationalized by a greater abundance of the unprotonated radicals that preferentially abstract hydrogen atoms from the polar hydroxyl groups. The percentage yield of hydrogen atom abstraction by these radicals was found to increase with lowering the pH in the pH range of 1.0-3.2. This pH effect is rationalized by polar effects: the lower the pH, the greater the fraction of protonated (more polar) radicals in the solution. This finding is consistent with previous results obtained in the gas phase and suggests that gas-phase studies can be used to predict solution reactivity, but only as long as the same reactive species is studied in both experiments. This was found not to be the case for the 2-iodopyridinium cation. Photolysis of this precursor in solution resulted in the formation of two major addition products, 2-hydroxy- and 2-methoxypyridinium cations, in addition to the hydrogen atom abstraction product. These addition products were not observed in the earlier gas-phase studies on 2-dehydropyridinium cation. Their observation in solution is explained by the formation of another reactive intermediate, the 2-pyridyl cation, upon photolysis of 2-iodopyridinium cation (and 2-iodopyridine). The same intermediate was observed in the gas phase but it was removed before examining the reactions of the desired radical, 2-dehydropyridinium cation (which cannot be done in solution).

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“…Besides the steric effect, there are also purely electronic (through-bond) factors that influence the activation energy of the abstraction of iodine. ,− ,− As already mentioned, electron-withdrawing substituents accelerate the radical abstraction of halogen atoms; this is in contrast with the abstraction of hydrogen, where the substituent effect is just the opposite. , One would expect, on the basis of electronic effects only, the greatest acceleration of the rate with COOR, SO 3 Me, and CF 3 , which is not the case. Moreover, groups, such as C(CF 3 ) 2 OH and C(CF 3 ) 2 OMe should have approximately the same effect on the rate of abstraction; however, they do not.…”

Section: Resultsmentioning

confidence: 99%

Abstraction of Iodine from Aromatic Iodides by Alkyl Radicals: Steric and Electronic Effects^,

Dolenc

Plesničar

2006

J. Org. Chem.

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Abstraction of the iodine atom from aryl iodides by alkyl radicals takes place in some cases very efficiently despite the unfavorable difference in bond dissociation energies of C-I bonds in alkyl and aryl iodides. The abstraction is most efficient in iodobenzenes, ortho-substituted with bulky groups. The ease of abstraction can be explained by the release of steric strain during the elimination of the iodine atom. The rate of abstraction correlates fairly well with the strain energy, calculated by density functional theory (DFT) and Hartree-Fock (HF) methods as a difference in the total energy of ortho and para isomers. However, besides the steric bulk, the presence of some other functional groups in an ortho substituent also influences the rate. The stabilization of the transition state, resembling a 9-I-2 iodanyl radical, by electron-withdrawing groups seems to explain a positive sign of the Hammett rho value in the radical abstraction of halogen atoms.

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Halogen abstraction studies. V. Abstraction of iodine by phenyl radicals from iodonaphthalenes, iodopyridines, and iodothiophenes. Question of polar effects

Cited by 14 publications

References 2 publications

Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts

Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Abstraction of Iodine from Aromatic Iodides by Alkyl Radicals: Steric and Electronic Effects^,

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Halogen abstraction studies. V. Abstraction of iodine by phenyl radicals from iodonaphthalenes, iodopyridines, and iodothiophenes. Question of polar effects

Cited by 14 publications

References 2 publications

Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts

Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts

Direct Comparison of Solution and Gas-Phase Reactions of the Three Distonic Isomers of the Pyridine Radical Cation with Methanol

Abstraction of Iodine from Aromatic Iodides by Alkyl Radicals: Steric and Electronic Effects,

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Abstraction of Iodine from Aromatic Iodides by Alkyl Radicals: Steric and Electronic Effects^,