Preassociation, free-ion, and ion-pair pathways in the electrophilic bromination of substituted cis- and trans-stilbenes in protic solvents

被引:41
作者
Ruasse, MF
Lo Moro, G
Galland, B
Bianchini, R
Chiappe, C
Bellucci, G
机构
[1] Univ Paris 07, Inst Topol & Dynam Syst, CNRS, URA 34, F-75005 Paris, France
[2] Univ Pisa, Dipartimento Chim Bioorgan, I-56126 Pisa, Italy
[3] Univ Florence, Dipartimento Chim Organ U Schiff, I-50100 Florence, Italy
关键词
D O I
10.1021/ja9700461
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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 (Delta(AcOH) and a Delta(TFE) 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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页码:12492 / 12502
页数:11
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