20857-17-4Relevant academic research and scientific papers
Electron transfer. 33. Rate enhancement by external pyridinedicarboxylato derivatives
Fanchiang,Heh, Jack C.-K.,Gould, Edwin S.
, p. 1142 - 1145 (1978)
The reductions, using Eu2+ and Cr2+, of such outer-sphere oxidants as Co(NH3)5py3+ and Co(en)33+ are strongly accelerated by derivatives of pyridinedicarboxylic acids. In the Cr2+ systems, the carboxylate species is consumed during the course of the reaction and thus does not function as a true catalyst. The Eu2+ reductions are straightforward and, as in earlier work, 4 are interpreted in terms of a sequence in which the catalyst is reduced (k1) to a radical intermediate, after which this intermediate may participate in reversal of the initial step (k-1) or may react with Co(III) (k2). Differences in catalytic effectiveness exhibited by the various dicarboxylato derivatives reflect principally differences in k1, which is greatest when both carboxyl groups are conjugated with ring nitrogen and least when neither group is conjugated. In the Cr2+ sequence, the active intermediate appears to be a Cr(III)-bound radical which, because of slow substitution at the Cr(III) center, is not in rapid equilibrium with Cr(H2O)63+. Kinetic data for Cr2+ reductions as accelerated by the methyl ester of 2,4-pyridinedicarboxylic acid (Est) allow calculation of: k1, pertaining to formation of the radical-ion intermediate CrIIIEst·; the ratio k-1/k2, which governs the competition between reversal of the initial step and reaction of CrIIIEst· with Co(III); KA, the acidity constant of the catalytically inactive conjugate acid of the diester. The selectivity exhibited by CrIIIEst· toward the various Co(III) oxidants in this study corresponds closely to that displayed by unbound pyridyl radicals and also by reducing metal ion centers in uncatalyzed reactions. Evidence is presented that the reaction of Cr2+ with 2,4-pyridinedicarboxylic acid can yield two different radical ions, one of which is much more reactive toward Co(III) centers than is the other.
Electron and Hydrogen Transfer Reductions of Some 2,4-Disubstituted Pyridines
Attia, A.,El-Salam, O. I. Abd,Youssef, T. E.
, p. 351 - 362 (2007/10/03)
New synthesis of 2,4-diacetylpyridine 1 was undertaken by Claisen condensation of 2,4-diethoxycarbonylpyridine 2a with ethyl acetate, followed by hydrolysis of the intermediate 3. Reduction of 1 with zinc or its salt in hydrochloric, formic or acetic acids afforded mixtures of the ketocarbinol 4 dicarbinol 5 or diethyl 6 pyridine derivatives. Formation of 6 as the complete reduction product was chemically proved to proceed via 4 and 5. Electrochemical reduction of 2a leads to 2,4-dimethylpyridine 7, and that of 1 gave rise to a mixture of 5 and 6 along with 2,4-diethyl-1,2,5,6-tetrahydropyridine 8 and 2,4-diethylpiperidine 9. Finally, metal hydride reduction of 1 gave 5, and that of diesters 2 or diacid chloride 10 derivatives afforded 2,4-dihydroxymethylpyridine 11.
Do high-spin topology rules apply to charged polyradicals? Theoretical and experimental evaluation of pyridiniums as magnetic coupling units
West Jr.,Silverman,Dougherty
, p. 1452 - 1463 (2007/10/03)
Ab initio calculations on pyridine and pyridinium analogues of m-xylylene indicate that the neutral heterocycle is essentially equivalent to benzene as a ferromagnetic coupling unit, while the cationic pyridiniums behave much differently. Depending on the substitution pattern, a protonated pyridine can serve as a ferromagnetic coupling unit or an antiferromagnetic coupling unit. Both valence bond and molecular orbital arguments provide qualitative rationalizations of these results. In an effort to test the theoretical predictions, bis(trimethylenemethane) analogues of the pyridine and pyridinium biradicals were synthesized and analyzed by electron paramagnetic resonance spectroscopy. General support for the theoretical predictions is obtained.
Homolytic Acylation of Protonated Pyridines and Pyrazines with α-Keto Acids: The Problem of Monoacylation
Fontana, Francesca,Minisci, Francesco,Barbosa, Maria Claudia Nogueira,Vismara, Elena
, p. 2866 - 2869 (2007/10/02)
The silver-catalyzed decarboxylation of α-keto acids by persulfate leads to acyl radicals, which can effect the selective homolytic acylation of pyridine and pyrazine derivatives.Compared with the previously developed source of acyl radicals by hydrogen abstraction from aldehydes, this procedure is more effective in monoacylation when multiple positions of high nucleophilic reactivity are available in the heterocyclic ring.Although the introduction of an acyl group strongly activates the heterocyclic ring toward further substitution, monoacylation can be achieved by taking advantage of the difference in basicity and lipophilicity between the starting base and the monoacylation products in a two-phase system.
SELECTIVE SUBSTITUTION OF UNPROTONATED PYRIDINES BY ALKYL RADICALS
Chianelli, D.,Testaferri, L.,Tiecco, M.,Tingoli, M.
, p. 657 - 663 (2007/10/02)
The reactions of dioxanyl and cyclohexyl radicals with 2- and 3-X-pyridines (X=CN, COMe, CO2Me) give a single substitution product deriving by addition at the 5- and 6-position respectively; with 4-X-pyridines substitution occurs preferentially at the 3-position.If the reactions are carried out with protonated pyridines other positional isomers are obtained.From the synthetic point of view the two procedures are therefore complementary.The change in positional selectivity on passing from unprotonated to protonated aromatic substrates is discussed and interpreted on the basis of the different nature of the transition state of the addition step.
