Anwendungen der Phasentransfer‐Katalyse, 23. Notiz über Bromiodcarben und zugehörige Halogenaustausch‐prozesse

Abstract

A rapid halogen exchange between HCBr,, HCBr21, HCBrI,, and HCI, occurred in the presence of conc. sodium hydroxide and a quaternary ammonium salt. Trapping experiments with various alkenes gave cyclopropane derivatives only in 3 cases. Depending on the nucleophilicity of the alkene and the haloform, different proportions of CBr,, CBrI, and CI, adducts were obtained. The most nucleophilic acceptor trapped those carbenes preferentially that were formed by elimination of iodide.

Syntheses ofgem-Dihalocyclopropanes and Their Use in Organic Synthesis

“…Dibromoiodomethane and bromodiiodomethane, when treated with a concentrated NaOH solution in a PTC system in the presence of alkenes, form mixtures of all three possible dihalocyclopropanes. 326 Thus, there is no convenient method for the preparation of gem-bromoiodocyclopropanes via the addition of bromoiodocarbene to alkenes. Knochel et al described the syntheses of configurationally stable, ester-functionalized cyclopropylmagnesium reagents, which react stereoselectively with electrophiles, including iodine or 1,2-dibromotetrachloroethane, 327 to produce gem-bromoiodocyclopropane derivatives (eq 57).…”

Concurrent Cyclopropanation by Carbenes and Carbanions

“…Hine also demonstrated that CCl 2 could be captured by other halide ions to yield trihalomethide carbanions, and thence “mixed” haloforms 1b and the phase transfer catalytic generation of CCl 2 , CBr 2 , CBrCl, and CBrI . In the latter cases, rapid dihalocarbene/halide/trihalomethide equilibrations are clearly implicated. , …”

“…Trihalomethides are well-established intermediates in other dihalocarbene generative methods, including the reaction of NaI with PhHgCCl 2 Br and the phase transfer catalytic generation of CCl 2 , CBr 2 , CBrCl, and CBrI . In the latter cases, rapid dihalocarbene/halide/trihalomethide equilibrations are clearly implicated. , …”

Facile Halogen Exchange Reactions: Chloroform with Bromoform and Carbon Tetrachloride with Carbon Tetrabromide

“…Chloroform Exchange with Bromoform. The halogen exchange in haloforms, such as that between chloroform and bromoform, under basic conditions is well known. − This redistribution reaction has served preparative purposes . The mechanism of exchange has been interpreted differently by various workers.…”