Stereoselective Halogenations of Alkenes and Alkynes in Ionic Liquids

Abstract

[reaction: see text]. Room-temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bromide, and 1-butyl-3-methylimidazolium chloride, are used as "green" recyclable alternative to chlorinated solvents for the stereoselective halogenation of alkenes and alkynes.

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).…”

“…We recently reported [5] that addition of Br 2 to alkynes and alkenes in [bmim]Br was anti-stereospecific, independently of the nature of the substituents at the double or triple bond, while Br 2 addition in [bmim][PF 6 ] gave mixtures of syn and anti adducts.…”

“…Thus, at lower temperatures (high η) one would have Equation (5), while at high temperatures (low η) Equation (6) holds.…”

“…In this context, we have recently investigated some ILs as clean, environmentally benign alternatives to chlorinated solvents for the stereoselective halogenation of alkenes and alkynes. [5] Electrophilic bromination of double and triple bonds is a fundamental reaction in organic chemistry, [6] providing products that are key intermediates in organic synthesis and in analytical chemistry. [7] The mechanistic and synthetic aspects of bromination in molecular solvents have been very thoroughly studied over the years, and important features [ have been identified through kinetic and product distribution studies.…”

Halogenation of organic compounds in ionic liquids

“…The utility of ILs as solvents and reagents for the halogenation of alkenes and alkynes was studied by Chiappe et al 68 Reaction of cisstilbene (41), trans-stilbene (42) and styrene (43) with Cl 2 in [bmim][Br] at room temperature gave the corresponding 1,2-bromochlorides (41a-43a) in an anti-stereospecific manner (Scheme 20). Based on the regiochemistry of the addition product to styrene, the authors proposed that BrCl 2 À was the reactive species, accounting for the ability of the anion formed to react with the double bond, yielding electrophilic bromine and nucleophilic chloride.…”