The significance of a cyclic π-septet in some reactions of pyridine and its salts

Kuthan, J.; Ferles, M.; Volke, J.; Koshmina, N. V.

doi:10.1016/s0040-4020(01)93082-0

Cited by 16 publications

(7 citation statements)

References 27 publications

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“…Our results vary little across basis sets and regardless of whether implicit solvation is included. Our results are consistent with calculations performed as early as 1970 in which Hückel theory and SCF quantum calculations were used to show that the electron density of the 7-(π-electron) pyridine anion resides primarily at the 4 position 34. Semi-empirical methods AM1 and MNDO have also been used to show that the kinetic regioselectivity for nucleophilic attack on the pyridinium ring is governed by the electron density at each carbon 8…”

Section: Resultssupporting

confidence: 88%

Using a Two-Step Hydride Transfer To Achieve 1,4-Reduction in the Catalytic Hydrogenation of an Acyl Pyridinium Cation

et al. 2008

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The stoichiometric reduction of N-carbophenoxypyridinium tetraphenylborate (6) by CpRu(P-P)H (Cp = η 5 -cyclopentadienyl; P-P = dppe, 1,2-bis(diphenylphosphino)ethane or dppf, 1,1′-bis (diphenylphosphino)ferrocene) and Cp*Ru(P-P)H (Cp* = η 5 -pentamethylcyclopentadienyl; P-P = dppe) gives mixtures of 1,2-and 1,4-dihydropyridines. The stoichiometric reduction of 6 by Cp*Ru (dppf)H (5) gives only the 1,4-dihydropyridine, and 5 catalyzes the exclusive formation of the 1,4-dihydropyridine from 6, H 2 , and 2,2,6,6-tetramethylpiperidine. In the stoichiometric reductions, the ratio of 1,4 to 1,2 product increases as the Ru hydrides become better one-electron reductants, suggesting that the 1,4 product arises from a two-step (e − /H•) hydride transfer. Calculations at the UB3LYP/6-311++G(3df,3pd)//UB3LYP/6-31G* level support this hypothesis, indicating that the spin density in the N-carbophenoxypyridinium radical (13) resides primarily at C4, while the positive charge in 6 resides primarily at C2 and C6. The isomeric dihydropyridines thus result from the operation of different mechanisms: the 1,2 product from a single-step H − transfer and the 1,4 product from a two-step (e − /H•) transfer.

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Section: Resultssupporting

confidence: 88%

Using a Two-Step Hydride Transfer To Achieve 1,4-Reduction in the Catalytic Hydrogenation of an Acyl Pyridinium Cation

et al. 2008

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“…On the other hand, the aromatization of the 1,4-dihydropyridine ring to the corresponding pyridine derivative constitutes the most important in vivo reaction to terminate the pharmacological effect in the current clinically used drugs. 37 Finally, on the basis of findings presented here and other previous studies 27,29,31,33,34 Scheme 2 concerning the oxidation mechanism in an aprotic medium can be postulated. This mechanism is supported by two experimental facts: ͑i͒ the generation of the neutral dihydropyridyl radical intermediates C-centered was demonstrated by the trapping of these species with PBN and their characteristic spin adducts were determined by the ESR technique for the three compounds and ͑ii͒ the pyridine derivatives were identified by GC-MS as the final oxidation products in the aprotic medium.…”

Section: F26supporting

confidence: 58%

“…Considering the present ESR results, it seems likely that the species added to the PBN spin trap is a dihydropyridyl radical. From previous quantum chemical calculations, 34 it follows that the unsubstituted pyridyl radical, Py • , which has the unpaired electron density in positions 2-, 4-, or 6-, with the highest electron density at the 4-position 34 seems to be the preferred one. Taking into account the above-described information and our experimental data on hyperfine splitting constants, Scheme 1 is proposed for the trapping of neutral dihydropyridyl radicals by PBN.…”

Section: F26mentioning

confidence: 90%

Electrolytic Oxidation of C4-Nitrofuryl 1,4-Dihydropyridines in Nonaqueous Medium

Núñez‐Vergara¹,

Salazar²,

Navarrette-Encina

et al. 2007

J. Electrochem. Soc.

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A study of the electrolytic oxidation of three new C-4 nitrofuryl 1,4-dihydropyridines in nonaqueous aprotic medium is presented. Controlled-potential electrolysis ͑CPE͒ in dimethylformamide +0.1 M tetrabutyl-ammonium hexafluorophosphate ͑TBAPF 6 ͒ was followed by UV-visible spectroscopy, high-performance liquid chromatography ͑HPLC͒-photodiode array ͑PDA͒, and gas chromatography-Mössbauer spectroscopy ͑GC-MS͒ chromatography, and electron spin resonance and electrochemical techniques. Carbon-centered radical intermediates produced in the electrochemical oxidation of C-4 nitrofuryl substituted 1,4dihydropyridines were trapped with N-benzylidene-tert-butylamine-N-oxide ͑PBN͒ and their splitting constants were calculated. The neutral pyridine derivatives were identified by GC-MS techniques as final oxidation products. HPLC-PDA and GC-MS chromatographic techniques were used to follow the time-course of CPE of both parent 1,4-DHP derivatives and their respective oxidation products. Also, an overall oxidation mechanism of C-4 nitrofuryl-1,4-DHP derivatives is presented.

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“…At higher temperatures, 2,3 0 -, 3,3 0 -and 3,4 0 -bipyridines are also formed. If the reaction is carried out with Zn in the presence of acetic anhydride, the dihydropyridinium intermediate 263 is trapped and 4,4 0 -bidihydropyridine 265 is obtained [211]. However, pyridines under dimerization conditions using Ni afford 2,2 0 -(266) instead of 4,4 0 -bipyridine (264).…”

Section: Reactivity J1489mentioning

confidence: 99%

Six‐Membered Heterocycles: Pyridines

González‐Bello

Castedo

2011

Modern Heterocyclic Chemistry

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Figure 16.5 Examples of agrochemical pyridine derivatives and their applications.1434j 16 Six-Membered Heterocycles: Pyridines IR DataThe pyridine IR spectrum is quite similar to the corresponding vibrations of benzene. Pyridine has C-H stretching frequencies in the range 3020-3070 cm À1 as well as C¼C and C¼N stretching frequencies at 1590-1660 cm À1 and near 1500 cm À1 . N J 2,3 = 4-6 J 2,4 = 0-2.5 J 2,5 = 0-2.5 2 3 4 5 6 J 2,6 = 0-0.6 J 3,4 = 7-9 J 3,5 = 0.5-2 1436j 16 Six-Membered Heterocycles: Pyridines R + Metal catalyst ∆ R = alkyl, vinyl, aryl Scheme 16.2 16.

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The significance of a cyclic π-septet in some reactions of pyridine and its salts

Cited by 16 publications

References 27 publications

Using a Two-Step Hydride Transfer To Achieve 1,4-Reduction in the Catalytic Hydrogenation of an Acyl Pyridinium Cation

Using a Two-Step Hydride Transfer To Achieve 1,4-Reduction in the Catalytic Hydrogenation of an Acyl Pyridinium Cation

Electrolytic Oxidation of C4-Nitrofuryl 1,4-Dihydropyridines in Nonaqueous Medium

Six‐Membered Heterocycles: Pyridines

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