Quantum-Chemical Study on Positional Selectivity in the Trimethylsilylation and Sulfonation of Pyrrole and N-Alkylpyrroles

Nesterov

2006

Chem Heterocycl Compd

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A detailed quantum-chemical study of the sulfonation of pyrrole with regard to the effect of the solvent (the model of overlapping spheres) on the energy characteristics of the formation of the σ-complexes produced during attack on the α-and β-positions of the heterocycle and their possible transformation paths was made by density functional theory . The possibility of mutual transformations between the isomeric σ-complexes by α/β-migration of the SO 3 is examined. The formation of pyrrolesulfonic acids was studied for the case of the intramolecular rearrangement of the complexes. Comparison of the activation energies shows that in contrast to the gas-phase reaction the formation of the β-sulfonic acid is preferred in methylene chloride: the solvation energy of the α-isomer of the σ-complex is higher than the energy for the transition state of its rearrangement and its product, α-pyrrolesulfonic acid, leading to an increase in the kinetic barrier and to a decrease of the energy gain on the path to the formation of the latter. The opposite variation of the energy characteristics on the path to the β-isomer with regard to solvation leads to agreement between the calculated data and the experimentally observed preferred formation of the β-pyrrolesulfonic acid.Keywords: quantum-chemical calculations, B3LYP/6-31G(d) method, position selectivity of substitution, sulfonation of pyrrole.On the whole the results of calculations [1] carried out by ab initio [MP2/6-31G(d)//RHF/6-31G(d)] and density functional theory [B3LYP/6-31G(d)] methods for the molecules of pyrrole, furan, thiophene, selenophene, and the corresponding benzannulated systems and hetarenium ions formed during C-protonation agree with existing experimental data on the positional selectivity not only for acid-catalyzed isotope exchange but also for other electrophilic substitution reactions of five-membered heterocycles containing one heteroatom and some of their derivatives. Disagreement with experiment is only observed for the most active and least selective molecules of pyrrole and its N-substituted derivatives. In all cases investigation of the molecules of various N-substituted pyrroles and of the hetarenium ions formed during their α-and β-protonation by the RHF/6-31G(d), MP2/6-31G(d)//RHF/6-31G(d), and B3LYP/6-31G(d) methods predicts a preference for α-substitution [2], although cases of preferred β-substitution are known in a series of reactions of similar N-substituted pyrroles and in pyrrole itself (see, in particular, the summarizing paper [3]).In [3] it was suggested that agreement between the calculated and experimental data can be achieved if particles closer to real particles and not the proton are used as model electrophile and the effect of the solvent is taken into account. The results from MP2/6-31G(d)//RHF/6-31G(d) and B3LYP/6-31G(d) quantum-chemical _______ * Dedicated to Mikhail Grigor'evich Voronkov. __________________________________________________________________________________________

supporting

confidence: 70%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Quantum-chemical investigation of the mechanism of the sulfonation of pyrrole

Nesterov

2006

Chem Heterocycl Compd

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“…However, investigation of the sulfonation of the same pyrroles by the B3LYP/6-31G(d) method using the SO 3 molecule as electrophile [6] did not give entirely unambiguous results. On the one hand they indicated a preference for the σ-complexes formed during attack at the α-position; on the other, they indicated greater thermodynamic preference for the β-sulfonic acids and a stronger preference in the transition from pyrrole to N-methylpyrrole and then to N-(tert-butyl)pyrrole.…”

mentioning

confidence: 95%

“…In fact, the results of quantum-chemical calculations by the MP2/6-31G(d)//RHF/6-31G(d) and B3LYP/6-31G(d) methods for the σ-complexes formed by pyrrole, N-methylpyrrole, and N-(tert-butyl)pyrrole with the trimethylsilyl cation and not the proton as model electrophile [6] agree with experimental data [3] on the formation of pyrrole and its N-substituted derivatives, β-substituted during trimethylsilylation with trimethylsilyltriflate, even if the solvent effect is not taken into account.…”

mentioning

confidence: 99%

Quantum-chemical investigation of the effect of solvent polarity on the direction of sulfonation of pyrrole

Nesterov

2007

Chem Heterocycl Compd

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The relative stability of the isomeric σ-complexes formed in the sulfonation of pyrrole at the α-or β− position (the α-isomer is energetically more favorable) does not agree with the experimentally established positional selectivity of substitution (the formation of β-pyrrolesulfonic acid). However, quantum-chemical calculations of the energy parameters for the reaction of pyrrole and SO 3 with due regard to the solvation effect in the model solvent methylene chloride (ε = 8.93) lead to the conclusion that the calculated activation energy of the rearrangement to the more favorable β-pyrrolesulfonic acid for the less favorable β-isomer of the σ-complex is lower than on the path to the formation of the α-pyrrolesulfonic acid. It was shown that the significant increase in the polarity of the model medium in the transition to DMSO (ε = 46.7) does not lead to substantial change in the energy parameters of the reaction. The explanation for the positional selectivity during the sulfonation of pyrrole using Py⋅SO 3 , according to previous data, involves the participation of the pyridine in the transformation of the σ-complexes into the products. The calculations were made by the B3LYP/6-31G(d) and HF/3-21+G methods using the model of overlapping spheres to take account of solvation.Keywords: quantum-chemical calculations, B3LYP/6-31G(d) method, positional selectivity of substitution, sulfonation of pyrrole.Earlier [1] we undertook quantum-chemical calculations for the molecules of pyrrole, furan, thiophene, selenophene, and the corresponding benzannelated systems and hetarenium ions formed during their C-protonation by ab initio methods [MP2/6-31G(d)//RHF/6-31G(d)] and by density functional theory [B3LYP/6-31G(d)]. On the whole the results of these calculations agree with existing experimental data on the positional selectivity not only for the acid-catalyzed isotope exchange of hydrogen but also for other electrophilic substitution reactions of five-membered heterocycles with one heteroatom and their various derivatives. Only for the most active and least selective compounds (pyrrole and its N-substituted derivatives) are there discrepancies with experiment; calculations by these methods for the molecules of N-substituted pyrroles (substituents at the nitrogen atom R = Me, Et, i-Pr, t-Bu, CH═CH 2 , C≡CH, Ph, PhSO 2 , 4-O 2 NC 6 H 4 ) and the hetarenium ions formed during their -and -protonation predict a preference for -substitution [2], although cases of preferred or even exclusive -protonation are known for a series of reactions of similar _______ * Dedicated to E. J. Lukevics on his 70th birthday __________________________________________________________________________________________

Development of quantum-chemical investigations of heterocycles at the Institute of Organic Chemistry, Russian Academy of Sciences (review)

2011

Chem Heterocycl Comp

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of attack by electrophile (position selectivity), relative stability of onium ions, stable thiophenium ions, their transformations.This review is dedicated to the memory of the distinguished organic chemist, one of the "founding fathers" of the journal "Chemistry of Heterocyclic Compounds", Professor, Doctor of chemical sciences, eminent man of science and technology of the RSFSR Ya. L. Gol'dfarb. As a scientist Prof. Gol'dfarb invariably showed a keen interest in all that was new and progressive in science, and the last 30 years of Yakov Lazarevich's life, when instrumental methods and theoretical concepts were being vigorously developed, were exceptionally prolific. It is no wonder that even in 1970-1971 Ya. L. Gol'dfarb became the initiator of the use of quantum-chemical methods in work at the synthesis laboratories of the N. D. Zelinsky Institute of Organic Chemistry. The possibilities of quantum-chemical methods have certainly increased substantially over the last 40 years, but the pioneering work still retains its significance in that appropriate models were carefully selected and the results of the calculations were always matched with the experimental data.Typically, the quantum-chemical calculations were no way to "dress up" the work carried out in the laboratory but were directed toward a meaningful study of the reactivity of heteroaromatic compounds, and to this day investigations are conducted with close collaboration between specialists in organic synthesis and specialists in the field of quantum chemistry. It is no coincidence that even in 1972 experimental data existing at that time on the reactivity and structure of compounds of the thiophene and furan series carrying electronaccepting substituents were being analyzed in detail on the basis of the results of quantum-chemical calculations performed at the Institute of Organic Chemistry, USSR Academy of Sciences [1], while in1977 the _______ * In happy memory of Yakov Lazarevich Gol'dfarb.