Radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichloroethane

Bellucci, Giuseppe; Chiappe, Cinzia

doi:10.1002/(sici)1099-1395(1998100)11:10<685::aid-poc17>3.0.co;2-c

Cited by 3 publications

(8 citation statements)

References 37 publications

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“…No isomerization to cis -1,2-dichloroethylene, which would have been produced in the presence of free radicals, was ever observed. This excluded that the transformations 9a → 9b , 10a → 10b , and 11a → 11b were due to a competing free-radical bromination process, and left the ionic process as the only one responsible for these isomerizations.…”

Section: Resultsmentioning

confidence: 99%

Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substitutedcis- andtrans-Stilbenes in Protic Solvents

et al. 1997

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Rates and products of electrophilic bromination of ring-substituted cis- and trans-stilbenes have been investigated in acetic acid, trifluoroethanol, ethanol, methanol, and water−methanol mixtures. The mY Br relationships (linear for nucleophilic solvents only, with m = 0.8), the deviations of the two nonnucleophilic solvents from the mY Br plots (ΔAcOH and ΔTFE positive, negative, or negligible), the kinetic solvent isotope effects (k MeOH/k MeOD = 1.1−1.6), the chemoselectivity (predominant dibromide, DB, or solvent-incorporated adducts, MA), and the high dependence of the stereochemistry on the solvent and the substituents (from stereoconvergency to stereospecificity) are discussed and interpreted in terms of a mechanistic scheme, analogous to the Jencks scheme for aliphatic nucleophilic substitutions, in which preassociation, free-ion, and ion-pair pathways compete. In particular, the stereochemical outcome of these reactions is consistent with a marked change in the nucleophilic partners of the product-forming ionic intermediate arising from different ionization routes. Return, i.e. change in the rate-limiting step from ionization to product formation, is shown to be related to substituent-dependent, but not solvent-dependent, bromine bridging.

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Section: Resultsmentioning

confidence: 99%

Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substitutedcis- andtrans-Stilbenes in Protic Solvents

et al. 1997

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“…However, the stability of the bromochlorides in the presence of TBACl was not checked since it may be inferred from the stereospecificity of these reactions. As previously reported [10] for the unsubstituted stilbenes 1b and 2b, the ad- [19,21] Ϫ [b] Diastereoisomeric ratio determined at very low reagent concentrations.…”

Section: Equilibrium Constant For Br 2 CL ؊ Formationmentioning

confidence: 99%

“…Finally, the two bromochlorides 3 and 4 were formed in different ratios from couples 1bϪ2b and 1cϪ2c, pointing to the involvement of partially bridged intermediates. With the exception of 1e, the ratios between bromochlorides for each olefin were very similar to those between dibromides found [19,21] for Br 2 addition to the same olefins in DCE, in which the reactions were carried out at high reagent concentrations, suggesting the involvement of very similar cationic intermediates. It is noteworthy, however, that the stereochemistry of bromochlorides was in general different to that of the dibromides formed in the same reaction medium.…”

Section: Equilibrium Constant For Br 2 CL ؊ Formationmentioning

confidence: 99%

“…Rate constants and apparent activation parameter for BrCl addition to stilbenes 1b, 1c, and 1e in DCE [a] From ref. [19,21] Scheme 4 sequence of the fact that the BrCl 2 Ϫ ion is in equilibrium with Br 2 Cl Ϫ and Br 3…”

Section: Full Papermentioning

confidence: 99%

“…Dibromides 5 and 6 were identified on the basis of their NMR spectra. [13,19,21] Bromochlorides 3؊4: All stilbenes 1؊2 were bromochlorinated as followed. DCE solutions containing 5 ϫ 10 Ϫ2  1aϪe or 2aϪe, 6 ϫ 10 Ϫ2  Br 2 , 6 ϫ 10 Ϫ2  Cl 2 , and 3 ϫ 10 Ϫ1  TBACl were left at room temperature for two days.…”

Section: Formation Constants and Molar Extinction Coefficients Of Br mentioning

confidence: 99%

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Equilibria and UV-Spectral Characteristics of BrCl, BrCl2−, and Br2Cl− Species in 1,2-Dichloroethane − Stereoselectivity and Kinetics of the Electrophilic Addition of these Species to Alkenes

Chiappe¹,

Moro²,

Raugi

2001

Eur. J. Org. Chem.

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UV/Vis spectrophotometric measurements have been used to determine the equilibrium constants and the absorption spectra of Bu 4 N + Br 2 Cl − and Bu 4 N + BrCl 2 − species in 1,2-dichloroethane (DCE) at 25°C. The formation constants and the absorption spectra of BrCl from Br 2 and Cl 2 and of Bu 4 N + Cl 3 − species from Bu 4 N + Cl − and Cl 2 were also determined in this solvent at 25°C. The kinetics and the products of bromochlorination of several aryl-substituted olefins with BrCl, with Br 2 in the presence of an excess of Bu 4 N + Cl − (Br 2 / Cl − ), and with BrCl in the presence of an excess of Bu 4 N + Cl − (BrCl/Cl − ) were investigated in DCE at different temperatures. The kinetics and product distribution data for the reactions with Br 2 /Cl − or BrCl/Cl − are interpreted on the basis

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Stereochemistry of Radical Halogenation Reactions. An ab Initio Molecular Orbital Study

Fan

Wong

2001

J. Phys. Chem. A

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The stereochemistry of radical halogenation of alkyl halides has been studied by ab initio molecular orbital theory. Two key elementary reactions, hydrogen abstraction reaction [XCH2CH3 + Y• → XCH2CH2 • + HY (R1)] and halogen abstraction reaction [XCH2CH2 • + Y2 → XCH2CH2Y + Y• (R2)], as well as rotational barrier of XCH2CH2 • radical, with X = H, F, Cl, and Br and Y = F, Cl, and Br, were studied using the G2(MP2,SVP) theory. Reactions R1 and R2 with X = F, Cl, and Br were found to be stereoselective. For X = F, both reactions prefer a gauche abstraction, whereas for X = Cl and Br, both reactions prefer a trans pathway. The high rotational barriers of ClCH2CH2 • and BrCH2CH2 • radicals and the low abstraction barriers of their reactions with Cl2 and Br2 are the two main factors that guarantee the retention of their radical configuration during the abstraction reactions. Thus, radical chlorination and bromination of alkenes and chlorine- and bromine-substituted alkanes are predicted to be stereospecific, in good accord with experimental results. We show that the stereochemical control observed in radical halogenation reactions can be explained without the use of Skell hypothesis. The trends of the calculated energy differences between the gauche and trans transition states of reactions R1 and R2, the rotational barriers of XCH2CH2 • radicals, and the gauche effect of XCH2CH3 can be rationalized in a uniform manner in terms of hyperconjugation interaction.

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Radical bromination of 1,1- and 1,2-diphenylethylenes in 1,2-dichloroethane

Cited by 3 publications

References 37 publications

Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substitutedcis- andtrans-Stilbenes in Protic Solvents

Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substitutedcis- andtrans-Stilbenes in Protic Solvents

Equilibria and UV-Spectral Characteristics of BrCl, BrCl2−, and Br2Cl− Species in 1,2-Dichloroethane − Stereoselectivity and Kinetics of the Electrophilic Addition of these Species to Alkenes

Stereochemistry of Radical Halogenation Reactions. An ab Initio Molecular Orbital Study

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