Abstract:The tetrazole-azide tautomerization of some halogen-substituted tetrazolo [1,5-a] N NMR spectroscopy at various temperatures. The tetrazolopyridines can exhibit equilibrium between the azide and the tetrazole forms. For some of them slow exchange occurs on the NMR time-scale, such that it is possible to estimate equilibrium constants. The position and nature of the substituent in the pyridine ring result in stabilization or destabilization of the tetrazole form and exert a strong influence on the values found … Show more
“…However, the two compounds reacted similarly with the nucleophiles A − G , the corresponding σ-adducts being only obtained in their tetrazolo form i.e., 8A − G and 9A − G . A similar situation prevailed upon σ-complexation of 8 and 9 with water (or OH - ) and methanol (or MeO - ). 16a,26d Going to eq 11, it has been established that the equilibrium lies far to the 1-oxide side in the case of benzofuroxans bearing electron-withdrawing groups at both the 4- and 6- positions, including for the 4-aza-6-nitro derivative. ,,28a,b,, In addition, in all reactions of 2 − 6 with A − G , no evidence could be obtained for an appreciable 1-oxide/3-oxide exchange in the resulting σ-adducts. In accord with the absence of a substituent at the 5-position, neither the 4-nitrobenzofuroxans 2 − 4 nor the related σ-adducts were found to undergo a Boulton−Katritzky rearrangement. ,, …”
Section: Resultsmentioning
confidence: 92%
“…The tautomerisms of eqs 10 and 11 are structural transformations which are intimately related to the chemistry of nitrotetrazolopyridines and nitrobenzofuroxans, respectively. − The question therefore arises of whether these equilibria could affect the σ-complexation of compounds 8 and 9 on one hand and 2 − 6 on the other hand. Regarding eq 10, the NMR evidence is that the conversion of 9 into 9-Az is not detectable while that of 8 into 8-Az is of about 30% in acetonitrile and 10% in Me 2 SO at room temperature . However, the two compounds reacted similarly with the nucleophiles A − G , the corresponding σ-adducts being only obtained in their tetrazolo form i.e., 8A − G and 9A − G .…”
The kinetics of the reactions of a series of reference carbon nucleophiles, consisting of N-methylpyrrole A, indole B, N-methylindole C, and enamines D-G, with 10 electron-deficient aromatic and heteroaromatic substrates (1-10), resulting in the formation of stable anionic sigma-adducts, have been investigated in acetonitrile at 20 degrees C. It is shown that the second-order rate constants k(1) pertaining to the carbon-carbon coupling step of these processes fit nicely the three-parameter equation log k (20 degrees C) = s(N + E), allowing the determination of the electrophilicity parameters E of 1-10 and therefore the ranking of these neutral electron-deficient compounds on the comprehensive electrophilicity scale defined for cationic electrophiles by Mayr et al. (Mayr, H.; Kempf, B,; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66). The E values of 1-10 are found to cover a range from -13 to -5, going from 1,3,5-trinitrobenzene 1, the least reactive molecule, to 4,6-dinitrotetrazolo[1,5-a]pyridine 8, 4-nitro-6-trifluoromethanesulfonylbenzofuroxan 3, and 4,6-dinitrobenzofuroxan 2, the three most reactive heterocycles. Of major interest is that the E value of 2 is essentially the same as that for 4-nitrobenzenediazonium cation (E = -5.1), approaching that of the tropylium cation family (E approximately -3 to -6) as well as a number of metal-coordinated carbenium ions. Such a ranking holds promise for expanding the range of coupling reactions which can be envisioned with such strongly electron-deficient neutral heteroaromatics as nitrobenzofuroxans and related compounds. Arguments are also given which exclude the possibility for the reactions studied to proceed via an electron-transfer mechanism.
“…However, the two compounds reacted similarly with the nucleophiles A − G , the corresponding σ-adducts being only obtained in their tetrazolo form i.e., 8A − G and 9A − G . A similar situation prevailed upon σ-complexation of 8 and 9 with water (or OH - ) and methanol (or MeO - ). 16a,26d Going to eq 11, it has been established that the equilibrium lies far to the 1-oxide side in the case of benzofuroxans bearing electron-withdrawing groups at both the 4- and 6- positions, including for the 4-aza-6-nitro derivative. ,,28a,b,, In addition, in all reactions of 2 − 6 with A − G , no evidence could be obtained for an appreciable 1-oxide/3-oxide exchange in the resulting σ-adducts. In accord with the absence of a substituent at the 5-position, neither the 4-nitrobenzofuroxans 2 − 4 nor the related σ-adducts were found to undergo a Boulton−Katritzky rearrangement. ,, …”
Section: Resultsmentioning
confidence: 92%
“…The tautomerisms of eqs 10 and 11 are structural transformations which are intimately related to the chemistry of nitrotetrazolopyridines and nitrobenzofuroxans, respectively. − The question therefore arises of whether these equilibria could affect the σ-complexation of compounds 8 and 9 on one hand and 2 − 6 on the other hand. Regarding eq 10, the NMR evidence is that the conversion of 9 into 9-Az is not detectable while that of 8 into 8-Az is of about 30% in acetonitrile and 10% in Me 2 SO at room temperature . However, the two compounds reacted similarly with the nucleophiles A − G , the corresponding σ-adducts being only obtained in their tetrazolo form i.e., 8A − G and 9A − G .…”
The kinetics of the reactions of a series of reference carbon nucleophiles, consisting of N-methylpyrrole A, indole B, N-methylindole C, and enamines D-G, with 10 electron-deficient aromatic and heteroaromatic substrates (1-10), resulting in the formation of stable anionic sigma-adducts, have been investigated in acetonitrile at 20 degrees C. It is shown that the second-order rate constants k(1) pertaining to the carbon-carbon coupling step of these processes fit nicely the three-parameter equation log k (20 degrees C) = s(N + E), allowing the determination of the electrophilicity parameters E of 1-10 and therefore the ranking of these neutral electron-deficient compounds on the comprehensive electrophilicity scale defined for cationic electrophiles by Mayr et al. (Mayr, H.; Kempf, B,; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66). The E values of 1-10 are found to cover a range from -13 to -5, going from 1,3,5-trinitrobenzene 1, the least reactive molecule, to 4,6-dinitrotetrazolo[1,5-a]pyridine 8, 4-nitro-6-trifluoromethanesulfonylbenzofuroxan 3, and 4,6-dinitrobenzofuroxan 2, the three most reactive heterocycles. Of major interest is that the E value of 2 is essentially the same as that for 4-nitrobenzenediazonium cation (E = -5.1), approaching that of the tropylium cation family (E approximately -3 to -6) as well as a number of metal-coordinated carbenium ions. Such a ranking holds promise for expanding the range of coupling reactions which can be envisioned with such strongly electron-deficient neutral heteroaromatics as nitrobenzofuroxans and related compounds. Arguments are also given which exclude the possibility for the reactions studied to proceed via an electron-transfer mechanism.
“…Previous studies have shown that many azidoazines readily undergo intramolecular cyclizations to form the corresponding bicyclic tetrazoles . As a rule, these reactions are typical of azidoazines bearing either electron‐donating substituents or only hydrogen atoms on the azine ring.…”
Section: Resultsmentioning
confidence: 97%
“…[46] Previous studies have shown that many azidoazines readily undergo intramolecular cyclizations to form the corresponding bicyclic tetrazoles. [47][48][49][50] As a rule, these reactions are typical of azidoazines bearing either electron-donating substituents or only hydrogen atoms on the azine ring. In 15 N NMR spectra, the transformations of azides in tetrazoles are manifested in considerable deshielding of the 15 N signals of the azido group approximately until δ -66 (N α ), +27 (N β ) and À30 (N γ ) ppm.…”
Section: Effect Of Aromatic Ring On 15 N Nmr Parameters Of Azido Groupsmentioning
2,4,6-Triazido-s-triazine, 2,4,6-triazidopyrimidine and six different 2,4,6-triazidopyridines were studied by (15)N NMR spectroscopy. The assignment of signals in the spectra was performed using the gauge-independent atomic orbital (GIAO)-Tao-Perdew-Staroverov-Scuseria exchange-correlation functional (TPSS)h/6-311+G(d,p) calculations on the M06-2X/6-311+G(d,p) optimized molecular geometries. The Truhlar and coworkers' continuum solvation model called SMD was applied to treat solvent effects. With this approach, the root mean square error in estimations of the (15)N chemical shifts for the azido groups was just 1.9 ppm. It was shown that the different reactivity of the α- and γ-azido groups in pyridines correlates well with the chemical shifts of the Nα signals of these groups. Of two nonequivalent azido groups of azines, the azido group with the most shielded Nα signal is the most electron-deficient and reactive toward electron-rich reagents. By contrast, the azido group of azines with the most deshielded Nα signal is the most reactive toward electron-poor reagents.
“…The position of the tetrazole-azide tautomerism in CDCl 3 solution can be simply assigned by employing 13 C [δ(C-8a)(ppm)] and 15 N chemical shifts δ (ppm) (Scheme 5.8) [24]. Selectively 15 N-labeled derivatives lead to the easy appearance of all 15 N, 13 C coupling constants which, together with 15 N chemical shifts, provide additional structural information [25].…”
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