Elimination reactions dehalogenation

OTHER 1,2-ELIMINATION REACTIONS Dehalogenation of Vicinal Dihalides... 

Considerable attention has been directed to dehalogenation mediated by corrinoids and porphyrins in the presence of a chemical reductant (references in Gantzer and Wackett 1991 Glod et al. 1997 Workman et al. 1997). Illustrations are provided by the dechlorination and elimination reactions carried out by titanium(III) citrate and hydroxocobala-min (Bosma et al. 1988 Glod et al. 1997). The involvement of corrinoids and porphyrins is consistent with the occurrence of analogous mechanisms for biological reactions that... 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Some elimination reactions which yield ynamines involve the dehalogenation or dehydrohalogenation of substituted amines or amides as shown in Eq. (21). 

The elimination reactions involving dehalogenation, dehydrohalogenation, and dehydration are often laborious compared to the more recent techniques involving dehalogenation of gem-dihalocyclopropanes [10a, b]. However, the availability of the starting materials is the deciding factor. 

Tris(phenylthio)antimony, 337 Dechlorination (see Reduction reactions) Deconjugation (see Isomerization) Defluorination (see Reduction reactions) Dehalogenation (see Elimination reactions, Reduction reactions) Dehydration (see also Elimination reactions)... 

Organocopper reagents, 207 Organovanadium reagents, 219 By desilylation of vinylsilanes Tetrabutylammonium fluoride, 286 By desulfurization of vinyl sulfides and related sulfur compounds Sodium dithionite, 281 Elimination reactions By dehalogenation Dichlorobis(cyclopentadienyl)-titanium, 102... 

The main reaction paths for the electroreductive cleavage of the carbon-halogen bond lead to total or partial dehalogenated products with the formation of C-H and/or single, double and triple C-C bonds according to the prevailing coupling or elimination reactions. 

An acetylenic bond may, however, be introduced also by dehalogenation of vicinal dihaloalkenes, occasionally by cleavage of nitrogenous compounds, or from enol esters by elimination reactions.178... 

Reactions. - Elimination and Dehalogenation. Mild conditions for eliminating primary alkyl halides to form 1-alkenes have been reported in which a solution of the halide and DBU in THF is added to a solution prepared by the reaction of an equivalent of dichlorobis(triphenylphosphinato)nickel and two equivalents each of triphenylphosphine and butyl-lithium.1 Polyethylene glycol promoted basic dehydrohalogenation of 1,1-dibromoalkenes has been... 

In dehalogenation and decarboxylative elimination reactions, the double bond is formed in a defined position. 

Extensive discussion of these and many other types of elimination reactions can be found in several reviews.We will focus here on 1,2-eliminations, especially dehydrohalogenation, dehydration, dehalogenation, deamination reactions, and pyrolytic eliminations. 

In each of the examples of 1,2-elimination reactions discussed above, one of the groups eliminated was a proton. Another iype of 1,2-elimination reaction is the dehalogenation of vicinal dihalides. Iodide ion can be used as the dehalogenating agent in both protic and aprotic solvents (equation 10.40). 

Elimination reactions can be regarded as the reverse of addition reactions. One substrate is converted into at least two molecules, with dehydrogenation, dehydration, dehalogenation, and dehydrohalogenation reactions being of highest technical... 

Elimination reactions have been induced under phase transfer conditions by a variety of techniques [1, 2, 4, 5, 25—28]. Predominant among these is the dehydrohalogena-tion of alkyl halides or sulfonates [1, 2, 5] and the dehalogenation of vicinal dihalides [25]. In the former case, some elimination is observed as a side reaction of the intended Sn 2 halogen exchange reaction. The direct displacement of halide by halide (the... 

Vicinal dehalogenation has been reported under phase transfer conditions [25]. In this case, a series of vicinal dibromides was treated with an aqueous solution of sodium iodide and sodium thiosulfate in the presence of a quaternary ammonium catalyst. 1,2-Dibromooctane was thereby converted to 1-octene in 92% yield (see Eq. 9.8) [25]. It was found that the elimination reaction could be effected by sodium thiosulfate itself, but that this method was slower than when iodide ion was present. Only a catalytic amount of iodide ion was required in the reaction, it being continuously regenerated by reaction with thiosulfate. 

Alkenes may be made from saturated compounds by various -elimination reactions. A vicinal dihalide may be dehalogenated by sodium iodide or activated zinc to give a double bond specifically between the carbons that... 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Although the foregoing reactions involve dehalogenation by reduction or elimination, nucleophilic displacement of chloride may also be important. This has been examined with dihalomethanes using HS at concentrations that might be encountered in environments where active anaerobic sulfate reduction is taking place. The rates of reaction with HS exceeded those for hydrolysis and at pH values above 7 in systems that are in equilibrium with elementary sulfur, the rates with polysulfide exceeded those with HS. The principal product from dihalomethanes was the polythio-methylene HS (CH2-S) H (Roberts et al. 1992). 

As a consequence of facile homolytic cleavages, sulfonyl halides (I > Br > Cl F unsuitable) are able to add to unsaturated C—C systems. To prevent (or reduce) competing polymerizations, the additions of sulfonyl chlorides have been recommended to be carried out in the presence of copper(I/II) salts (Asscher-Vofsi reaction ). Comprehensive surveys have been published on the resulting j8-halogeno sulfones (or their vinyloguous compounds) as well as on their dehalogenation products (vinyl sulfones, 1-sulfonyl-l, 3-dienes, etc.). Table 5 reviews a series of sulfonyl halide additions and facile hydrogen halide eliminations. 

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