Spectroscopic and Theoretical Investigations of Electrophilic Bromination Reactions of Alkynes: The First Evidence for π Complexes as Reaction Intermediates

Abstract

A bromine ± alkyne p complex (l max 294 nm) of 1:1 stoichiometry has been observed for the first time in the course of the bromination of 1-phenylpropyne by means of a diode-array stopped-flow technique. ); this demonstrates that the complex is actually an essential intermediate on the reaction coordinate. The bromination of a series of nine alkynes has been studied. Bromination reactions with negative apparent activation parameters lead to mixtures of E and Z vinyl dibromides, whereas reactions with positive activation energy yield the E isomers exclusively. The reason for the difference in reactivity of these alkynes compared with structurally similar alkenes most likely lies in the stability of these 1:1 charge-transfer complexes. Usually open arylvinyl cations correspond to the energetically favored product-determining intermediates; bridged bromirenium ions are formed from deactivated alkynes and react to give E isomers. The kinetic effect of alkyl groups and of p-OCH 3 , p-CN, and p-NO 2 substituents at the aryl group on the bromination of arylalkylacetylenes is discussed. Density functional calculations provide insight into the geometries, energies, and bonding of the intermediate 1:1 and 2:1 Br 2 ± acetylene complexes involved. These theoretical investigations demonstrate that the most stable trimolecular Br 2 ± Br 2 ± acetylene adduct possesses a structure very similar to a crystallographically characterized Br 2 ± Br 2 ± alkene species, which can directly yield the ionic products, Br À 3 and vinyl cation, driven by the heterolytic action of a solvent.

Bromination of silatranyl‐, germatranyl‐, silyl‐, and germyl‐phenylacetylenes

“…It seemed to be interesting for us to examine tetra-n-butylammonium tribromide (TBAT) as a brominating reagent. Literature data review reveals wide range of acetylenes being treated with this reagent give trans-1,2-dibromalkenes as the only products, result of the reaction being independent of its conditions and ratio of reagents [9].…”

“…43 Earlier we reported that the treatment of N(CH 2 CH 2 O) 3 GeC CPh (1) with NBS/DMSO system affords N(CH 2 CH 2 O) 3 GeCBr 2 C(O)Ph and the reaction with bromine leads to the formation of cis-N(CH 2 CH 2 O) 3 GeC(Br) C(Br)Ph [7]. In contrast to this most of alkylatylacetylenes react with bromine in chloroalkanes under conditions of kinetic control to give a mixture of cis-and trans-dibromoalkenes, the latter considerably prevailing [8,9]. The addition of bromine to alkylphenylacetylenes containing strong electron-withdrawing para-substituents in the phenyl group is stereoselective and yields only the trans-dibromo adducts.…”

Bromination of Alkynes in Ionic Liquids − A Kinetic Investigation

“…Although the stereochemical behaviour of the reactions of 1b and 1c was similar to that observed [21] in addition of Br 2 to the same alkynes in chlorinated solvents ( Table 2), that of 1a and 1d was significantly different. In particular, the reaction of 1a was characterised by a higher anti stereoselectivity.…”

“…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.…”