The kinetics and the products of bromination of several substituted stilbenes with tetrabutylammonium tribromide (TBAT) have been investigated in aprotic solvents at different temperatures, Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products. The reactions followed a second-order rate law, and an inverse kinetic isotope effect (KIE), K-H/k(D) = 0.85(0.05), was found for the bromination of cis-stilbene. The reactions of cis- and trans-4,4-dimethoxystilbenes yielded mixtures of meso and d,l dibromides both in chloroform and 1,2-dichloroethane. The rate constants (k(Br3)-) measured for the latter olefins deviated considerably fi om the Hammett correlations, and added bromide had a significant effect on the rates. The reactions of these activated stilbenes with molecular Br-2, carried out at low Br-2 concentration, followed a mixed second/third-order rate law. The kinetic and product distribution data for the reaction, with TBAT, of stilbenes bearing electron-withdrawing or moderately electron-donating substituents are interpreted on the basis of the known mechanism involving a product- and rate-determining nucleophilic attack by bromide on the olefin-Br-2 pi-complex. The data related to the bromination of the more activated methoxystilbenes are rationalized considering that, for these olefins, even in aprotic solvents, the ionization of the initially formed 1:1 rr-complex to a bromocarbenium bromide ion pair can compete both with the formation of a bromonium-tribromide ion pair and with the nucleophilic attack by Br-. For this second-order process (first order in Br-2), the kinetic constants and the activation parameters have been measured in chloroform and 1,2-dichloroethane and the activation parameters have been compared with those related to the third-order Br-2 addition and to the reaction with TBAT.
|Autori:||Bellucci G; Chiappe C; LoMoro G|
|Titolo:||Formation of bromocarbenium bromide ion pairs in the electrophilic bromination of highly reactive olefins in chlorinated aprotic solvents|
|Anno del prodotto:||1997|
|Digital Object Identifier (DOI):||10.1021/jo9620526|
|Appare nelle tipologie:||1.1 Articolo in rivista|