Transfer dehydrohalogenation

The catalytic activity of polyethylene glycol (PEG) phosphonium salts has been evaluated, in phase-transfer dehydrohalogenation reactions, as slightly better than that of the corresponding PEG ammonium compounds886 (reaction 271). By comparison... 

The complexity of the peroxide curing system arises from a range of possible side reactions such as /3-cleavage of the oxy radical, addition reaction, polymer scission, radical transfer, dehydrohalogenation, oxygenation, and acid catalyzed decomposition of the peroxide." ... 

The most frequent applications of these procedures he in the preparation of terminal alkynes Because the terminal alkyne product is acidic enough to transfer a proton to amide anion one equivalent of base m addition to the two equivalents required for dou ble dehydrohalogenation is needed Adding water or acid after the reaction is complete converts the sodium salt to the corresponding alkyne... 

ALKYNES VIA PHASE TRANSFER-CATALYZED DEHYDROHALOGENATION PROPIOLALDEHYDE DIETHYL ACETAL... 

The dehydrohalogenation of 1- or 2-haloalkanes, in particular of l-bromo-2-phenylethane, has been studied in considerable detail [1-9]. Less active haloalkanes react only in the presence of specific quaternary ammonium salts and frequently require stoichiometric amounts of the catalyst, particularly when Triton B is used [ 1, 2]. Elimination follows zero order kinetics [7] and can take place in the absence of base, for example, styrene, equivalent in concentration to that of the added catalyst, is obtained when 1-bromo-2-phenylethane is heated at 100°C with tetra-n-butyl-ammonium bromide [8], The reaction is reversible and 1-bromo-l-phenylethane is detected at 145°C [8]. From this evidence it is postulated that the elimination follows a reverse transfer mechanism (see Chapter 1) [5]. The liquidrliquid two-phase p-elimination from 1-bromo-2-phenylethanes is low yielding and extremely slow, compared with the PEG-catalysed reaction [4]. In contrast, solid potassium hydroxide and tetra-n-butylammonium bromide in f-butanol effects a 73% conversion in 24 hours or, in the absence of a solvent, over 4 hours [3] extended reaction times lead to polymerization of the resulting styrene. 

Isotope effects and element effects associated with hydron-transfer steps during methoxide promoted dehydrohalogenation reactions of jo-CF3C6H4C HClCH2X (X=Br, Cl, or F) have also been discussed, with regard to distinction between E2 and multi-step pathways. The Arrhenius behaviour of hydrogen isotope effects was used to calculate the amounts of internal hydrogen return associated with the two-step mechanism. 

The dehydrohalogenation of monohalogenated cyclopropanes under phase transfer conditions has been of greater preparative importance Monoalkylated bromo-cyclopropanes have been converted to alkylidene cyclopropanes, and 3,3-disub-stituted cyclopropylhalides provide the corresponding cyclopropenes in good yield [158]. 

A variety of sulfur reagents have been used for the dehydrohalogenation of vic-dihaloalkanes to alkenes. Aqueous sodium trithiocarbonate in the presence of a phase transfer catalyst was reported to give high yields under very mild conditions [217]. A radical mechanism involving a one-electron transfer was proposed as the first step in this reductive elimination. 

The products most commonly observed in the reactions of bare metal cations (M+) with alkyl halides, summarized in Scheme 11, can be described as arising from halide transfer (a), halogen transfer (b) dehydrohalogenation (c) and alkene elimination (d). 

Dehydrohalogenation. Aryl vinyl ethers are prepared conveniently by dehy-drohalogenation of aryl 2-haloethyl ethers with aqueous sodium hydroxide with tetra-n-butylammonium hydrogen sulfate as phase-transfer catalyst (equation I). 

DEHYDROHALOGENATION Crown ethers. Phase-transfer catalysts. Quinoline. 

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