ChemInform Abstract: OXIDATION ANIONISCHER SIGMA‐KOMPLEXE

The feasibility of carrying out nucleophilic displacement of hydrogen from highly electron-deficient heteroaromatics has been addressed through a detailed investigation of the interaction of a series of nitrobenzofuroxans 3a-i and two related heterocycles 5 and 6, with the 2-nitropropenide anion. Although this series corresponds to a large modulation in the electrophilic properties of the six-membered ring, all reactions first lead to the quantitative formation of the sigma-adducts C-3a-i, C-5 and C-6 arising from covalent addition of the nucleophile to the C-7 carbon. With the exception of the 4-aza substituted member C-5, all the adducts have been isolated as pure and very stable alkali salts. Measurements of the oxidation potentials by cyclic voltammetry reveal that the ease of subsequent hydrogen substitution at carbon-7 strongly decreases with increasing electron deficient character of the six-membered ring. The measured E0 values are in the range 0.5-0.6 V (vs. SCE) for the 4-nitro-benzofuroxan and -benzofurazan adducts (C-3e, C-3i) but they go up to 1.20-1.33 V for the 4,6-dinitro- and 4-nitro-6-trifluoromethanesulfonylbenzofuroxan adducts (C-3a,d) in acetonitrile. Consistently with these E0 values, only very powerful oxidants such as the Ce4+/Ce3+ or the MnO4-/Mn2+ couples can successfully oxidize the adducts leading to the expected substitution products in moderate to good yields (35-72%). Interestingly, the rearomatization of the 4-nitro substituted benzofuroxan adducts proceeds with a partial Boulton-Katritzky rearrangement of the resulting products. Another noteworthy result is that the 4-nitrobenzofuroxan and 4-nitrobenzofurazan molecules suffer competitive addition of the (CH3)2C-NO2- anion to the 5- and 7- positions under some experimental conditions. This represents the first well-defined example of isomeric addition of a carbon nucleophile to these heterocycles.

show abstract

“…1 H and 13 C NMR data for S-3e and S-3i are given in Tables 3, S3 and S4 : C,34.52;N,22.36;H,2.25. Found: C,34.59;N,22.06; H, 2.15%.…”

Section: Oxidation Of the Monosubstituted Adducts C-3e And C-3imentioning

confidence: 99%

σ-Complex formation and oxidative nucleophilic aromatic substitution in 4-nitro-2,1,3-benzoxadiazoles

et al. 2003

View full text Add to dashboard Cite

show abstract

“…Then, the standard potential can be obtained as the midpoint of the cathodic Ic and anodic Ia peaks: E °=+1.15 V for 1 H − in CH 3 CN and E °=+0.99 V for 5 H − in DMSO (these and all other potentials given herein were recorded against that of the saturated calomel electrode (SCE)). For these last two compounds, it is also possible28 to derive the rate constant of the first‐order process following the electron transfer from the E p /log v plot: 130 s −1 for 1 H − and ≈40 s −1 for 5 H − .…”

Section: Resultsmentioning

confidence: 99%

“…Activation of the benzene ring with a third nitro group causes a 10 7 increase in stability of the σ adduct 9 H − f as compared with that of the 1,3‐dinitrobenzene analogue 9 H − c 1. This complex becomes therefore more reluctant to oxidation ( E °=0.82 V), requiring the use of much stronger oxidants to undergo conversion into the substituted products 27, 28. In accord with the experimental evidence that 4‐nitrobenzofuroxan and 4‐nitrobenzofurazan give rise to σ adducts of the same stability as that of TNB σ adducts,40 the E ° values for oxidation of the two nitropropenide complexes 2 H − and 3 H − compare well with those for oxidation of 9 H − f .…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Electrochemical Oxidation ofσ-Complex-Type Intermediates in Aromatic Nucleophilic Substitutions

et al. 2001

View full text Add to dashboard Cite

A series of sigma-adducts (1H-...7H-) derived from the addition of 2-nitropropenide ion to various nitrobenzofuroxans and nitrobenzofurazans have been oxidized electrochemically. The results show that the rearomatization of the carbocyclic ring of these adducts as well as that of a few additional 4,6-dinitrobenzofuroxan adducts (8 H- a-c) is associated with much higher oxidation potentials than found for the same process in the dinitro- and trinitrobenzene series. Especially high Eo values are measured for the oxidation of the 2-nitropropenide 4,6-dinitro- and 4-nitro-6-trifluoromethylsulfonylbenzofuroxan adducts 1H- and 4H- in acetonitrile: E (1H-)= 1.15 V versus SCE; Eo(4H-)=1.33V versus SCE. These values fit well with the available evidence that the chemical oxidation of these adducts requires the use of very strong oxidizing agents to proceed efficiently. The mechanism for the oxidation process has been established. It is shown to involve transfer of two electrons and liberation of one proton per sigma-complex precursor with no evidence whatsoever for the intermediacy of radical anionic species.

show abstract

ChemInform Abstract: OXIDATION ANIONISCHER SIGMA‐KOMPLEXE

Cited by 2 publications

References 0 publications

σ-Complex formation and oxidative nucleophilic aromatic substitution in 4-nitro-2,1,3-benzoxadiazoles

σ-Complex formation and oxidative nucleophilic aromatic substitution in 4-nitro-2,1,3-benzoxadiazoles

Electrochemical Oxidation ofσ-Complex-Type Intermediates in Aromatic Nucleophilic Substitutions

Contact Info

Product

Resources

About