Electrophilic Substitutions of Five-Membered Rings

Marino, Gianlorenzo

doi:10.1016/s0065-2725(08)60351-7

Cited by 107 publications

(26 citation statements)

References 135 publications

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“…With bromine (or chlorine) in an alcohol solvent at low temperatures, reaction leads to the formation of a that bromination of methyl furoate in 50% aqueous acetic acid gives the substitution product, eq. [3], the same product as can be obtained under other reaction conditions (12).…”

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confidence: 61%

On the reaction of furan with bromine in aqueous solution. Observation of the slow hydration of malealdehyde

Tee¹,

Swedlund²

1983

Can. J. Chem.

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Thc rate of rcaction of furan with brominc in aqueous solution has bcen measurcd and thc ratc constant obtaincd by Williamson and Collcr is confirmed. Howcvcr. it is now found that thc rcaction of brominc with furan in watcr has two distinct stages: attack of brominc leading to the formation of malealdehyde 3 and hydration of 3 to a 2: 1 mixture of thc geometrical isomers of its cyclic hydrate 4. This second, slower process is observable by stopped-flow uv spectrophoton~etry occurring aftcr the consumption of bromine. The observed kinetics are vcry similar to thosc rcccntly found for thc analogous hydration of phthalaldchydc by McDonald and Martin.OSWALD S. TEE et B. E. SWEDLUND. Can. J. Chem. 61, 2171Chem. 61, (1983.On a mesuri la vitesse de la rCaction du furanne avec Ic brome en solution aqueusc et on a confirm6 la constantc de vitesse obtenue par Williamson et Coller. Cependant on trouve maintenant que la rCaction du brome avec le furanne dans I'cau a deux Ctapes distinctes: I'attaque du brome qui conduit B la formation du malCaldChyde 3 et I'hydratation du composC 3 qui conduit ? I un mClange 2: 1 des isomkres gComCtriques de son hydrate cyclique 4. Cc sccond processus. le plus lent, peut Ctre obscrvd par la spectrophotomdtrie uv a flux stoppd que I'on utilise apres la disparition du brome. Les cindtiqucs observees sont scmblables h cclles trouvCes rCcemment pour I'hydratation analogue du phthalaldehyde par McDonald ct Martin.[Traduit par le journal]The halogenation of furans is complex with the nature of products varying widely with the substrate, reagent, solvent, and reaction conditions (1 -4). However, two broad classes of reaction may be recognized, viz. addition and substitution, with the former often leading to ring-opening (1 -4).Bromination of furan in CCI, gives a mixture of 2-bromoand 2,5-dibromofuran (5). Using ' Hmr spectroscopy, Baciocchi et nl. (6) observed the formation of 1,4-and 1,2-dibromo adducts from furan and bromine in the nonnucleophilic solvent carbon disulphide, at low temperature. With time, and on warming, the adducts underwent elimination to 2-bromofuran (6). In hydroxylic solvents the reaction of furan (1) follows a different course. With bromine (or chlorine) in an alcohol solvent at low temperatures, reaction leads to the formation of a that bromination of methyl furoate in 50% aqueous acetic acid gives the substitution product, eq. [3], the same product as can be obtained under other reaction conditions (12).We have begun to study the bromination of furans systematically to try to ascertain why some derivatives undergo addition, whereas others show substitution. Our initial approach to this problem was to study the effect of substituents and solvents upon the rate of br~mination.',~ To anchor a correlation of results for aqueous solution a reliable value for furan was required.' During measurement of this value we found that another, slower process is observable, which is the subject of the present paper. Results Kinetic studies 1 2We first measured the second-order rate c...

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confidence: 61%

On the reaction of furan with bromine in aqueous solution. Observation of the slow hydration of malealdehyde

Tee¹,

Swedlund²

1983

Can. J. Chem.

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“…It should be emphasized that this ratio changes in the series furan > thiophene > pyrrole, which does not correlate with the above series of the reactivities (substrate selectivity) in electrophilic substitution reac tions. 1,2 Positional selectivity in reactions of furan, thiophene, selenophene, pyrrole, and their derivatives with electrophiles A hypothesis that rationalized the discrepancy between the substrate and positional selectivities of electrophilic substitution reactions in five membered heterocycles con taining one heteroatom was proposed twenty five years ago. 3, 4 Its essence is that the formation of β substituted compounds occurring through σ complexes B where only the heteroatom and one of the α carbon atoms are in volved in charge delocalization depends more substan tially on the ability of the elements X to exist in the onium state than the formation of α substituted derivatives where all atoms of the rings, except for the geminal C atom, are involved in charge delocalization in the cations A. Actu ally, a decrease in stability of the onium states in the series N + > S + > O + correlates well with experimental data on the formation of β substituted pyrrole, thiophene, and furan derivatives.…”

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confidence: 99%

Substrate and positional selectivity in electrophilic substitution reactions in pyrrole, furan, thiophene, and selenophene derivatives and related benzoannelated systems

et al. 2005

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Data on the relative reactivities (substrate selectivity) of five membered heterocycles in electrophilic substitution reactions and positional selectivity (α : β ratio) in these reactions were analyzed. Unlike the substrate selectivity (pyrrole >> furan > selenophene > thiophene) determined by the position of heteroatoms in the Periodic Table, the positional selectivity decreases in the order corresponding to the change in the relative stability of the onium states of the elements (O + < Se + ≤ S + < N + ) and reflects the predominant role of heteroatoms in the stabilization of σ complexes formed upon β substitution. These differences in the positional selectivity of the parent heterocycles have a substantial effect on the orientation in electrophilic substitution reactions in their derivatives and the corresponding benzoannelated systems. This interpretation was confirmed by ab initio quantum chemical calculations (RHF/6 31G(d) and MP2/6 31G(d)//RHF/6 31G(d)) and density functional theory calculations (B3LYP/6 31G(d)). Quantum chemical calculations were performed by the above mentioned methods for model N R pyrroles (R = Me, Et, Pr i , Bu t , CH=CH 2 , C≡CH, Ph, PhSO 2 , and 4 O 2 NC 6 H 4 ) and their α and β protonated σ complexes. The results of these calculations demonstrated that it is the steric factors and charges on the β C, α C, and N atoms and the substituents at the N atom (the kinetic control), as well as the nature of the electrophile, rather than the difference in the relative stabilities of the onium states of N + (which depends on the nature of the substituent at the N atom and reflects the role of the heteroatom in stabilization of σ complexes formed via β substitution; the thermodynamic control) that are responsible for the type of orientation (α or β) that prevails. , relative stability of onium states of chalcogens, quantum chemical calculations, ab initio methods (RHF/6 31G(d), MP2/6 31G(d)), DFT B3LYP/6 31G(d).Electrophilic substitution is the most important class of reactions of five membered heterocycles with one het eroatom, which allows one to prepare compounds with various substituents. The present paper summarizes the results of theoretical studies of the features of the sub strate and positional selectivities in electrophilic substitu tion reactions in pyrrole, furan, thiophene, and seleno phene derivatives and the corresponding benzoannelated systems that have been rationalized only recently.It is well known that the effect of the heteroatom is manifested in the higher reactivities of the α positions. This is generally interpreted as the result of higher stabil ity of the corresponding σ complex (A) due to favorable charge delocalization compared to its isomer (B) result ing from the attack at the β position (Scheme 1).The reactivities and positional selectivities of pyrrole, furan, and thiophene and the reactivity of selenophene in electrophilic substitution reactions were studied quantita tively. 1,2 There are tremendous differences in the reactiv ity, which drops by approximately ...

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Section: Methodsmentioning

confidence: 99%

“…1,2 It should be noted that the difference in reactivity, which falls by approximately 10 orders of magnitude in the series pyrrole >> furan > thiophene, does not correlate with the sequence of positional selectivity (the ratio of products of α-and β-substitution), which is furan > thiophene > pyrrole. 1,2 About 20 years ago one of us put forward a hypothesis explaining the reason for the noncorrespondence of the series of substrate and positional selectivities of electrophilic substitution reactions of five-membered heterocycles with one heteroatom. 3,4 The gist of it was that the formation of β-substituted compounds proceeding through a σ-complex of type B, in which only the heteroatom and one of the α-carbon atoms participate in charge delocalization, must depend more on the ability of element X to exist in the onium state, than for the formation of α-substituted compounds.…”

Section: Methodsmentioning

confidence: 99%

Positional selectivity in reactions of pyrrole and its N-substituted derivatives with electrophiles

Беленький¹,

Kim²,

Suslov³

et al. 2003

Arkivoc

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Experimental data on the positional selectivity (α:β-ratios) in reactions of N-substituted pyrroles with electrophiles have been considered. Based on the results of quantum chemical calculations of model N-R-pyrroles (R=H, Me, Et, i-Pr, t-Bu, CH=CH 2 , C≡CH, Ph, PhSO 2 , 4-O 2 NC 6 H 4 ) and their α-or β-protonated σ-complexes, carried out using ab initio methods (RHF/6-31G(d), MP2/6-31G(d)//RHF/6-31G(d)), and within the framework of density functional theory (B3LYP/6-31G(d)), it has been shown that the predominant α-or β-orientation is determined by steric factors and charges on the atoms β-С, α-С, N, and on the substituents at the N atom. We conclude that it is not determined by differences in the relative stabilities of the onium state N + depending on the nature of a substituent at the N atom, or reflecting the role of the heteroatom in the stabilization of σ-complexes formed by β-substitution.

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Electrophilic Substitutions of Five-Membered Rings

Cited by 107 publications

References 135 publications

On the reaction of furan with bromine in aqueous solution. Observation of the slow hydration of malealdehyde

On the reaction of furan with bromine in aqueous solution. Observation of the slow hydration of malealdehyde

Substrate and positional selectivity in electrophilic substitution reactions in pyrrole, furan, thiophene, and selenophene derivatives and related benzoannelated systems

Positional selectivity in reactions of pyrrole and its N-substituted derivatives with electrophiles

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