Formation of Bromocarbenium Bromide Ion Pairs in the Electrophilic Bromination of Highly Reactive Olefins in Chlorinated Aprotic Solvents

Abstract

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(Br)()3()-) measured for the latter olefins deviated considerably from 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 pi-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.

1‐exo‐Alkylidene‐2,3‐anhydrofuranoses: Valuable Synthons in the Preparation of Furanose‐Based Templates

“…[17] Accordingly, treatment of 1a with bromine and with IDCT gave the halo-exo-glycals 2a and 2b in a stereocontrolled manner and in 63 % and 70 % yields, respectively. The Z derivatives were obtained as the sole isomers (Scheme 3, a and b).…”

Bromination of Alkynes in Ionic Liquids − A Kinetic Investigation

“…This species is known to add to alkenes [19] and alkynes [20] solvents by a mechanism that does not involve ionic intermediates and occurs through a product-and rate-determining attack by bromide on the olefinϪBr 2 π complex (Scheme 1). In contrast, the formation of both diastereoisomers in Br 2 addition to alkynes 1aϪd in [bmim][PF 6 ] indicated [5] that open vinyl cations (a) had been formed as intermediates, similarly to what has been observed [21] in 1,2-dichloroethane (DCE).…”

“…If we assume that the reaction in [bmim]Br occurs through the mechanism generally accepted for the reaction in molecular solvents, [19,21] shown in Scheme 1, the process should involve as the first step an equilibrium between two complexes of Br 2 : [Br 3 ] Ϫ and 1:1 alkyneϪBr 2 π complex.…”

Competing kinetic pathways in the bromine addition to allylic ethers in 1,2‐dichloroethane: Opposite temperature effects

“…Alkene (1) was prepared as reported by Bellucci et al [7]. Br 2 and DCE were purchased from Sigma Aldrich.…”

“…More recently, mixed third-and second-order kinetics have been proposed in the case of p, p -dimethoxy-disubstituted stilbenes in chlorinated solvents [7]. The second-order pathway was rationalized considering the strong electron-donating tendency of the substituents of the phenyl rings, which makes them able to stabilize, even in nonprotic solvents, the opened bromocarbenium-bromide ion pair.…”