Dehydrohalogenation from halides

Carbenes from Halides by a-Elimination. The a-elimination of hydrogen halide induced by strong base (Scheme 10.8, Entry 4) is restricted to reactants that do not have (3-hydrogens, because dehydrohalogenation by (3-elimination dominates when it can occur. The classic example of this method of carbene generation is the generation of dichlorocarbene by base-catalyzed decomposition of chloroform.152... 

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). 

The mechanism and stereochemsitry of dehydrohalogenations from vinyl halides have been extensively reviewed - It should however be pointed out that the effects of particular bases and solvents and of temperature cannot always be predicted with confidence as to rate of reaction and product distribution (acetylene, allene, diene). Therefore a variety of combinations of base, solvent and reaction conditions should be tried in order to obtain satisfactory results. 

The kinetics of dehydrohalogenation from the configurational isomers of vinyl halides have been determined for numerous reactions. Thus cw-/7-nitro-P-bromo-styrene in the presence of ethanolic NaOH is converted quantitatively by trans elimination to /7-nitrophenylacetylene within a few minutes (equation 21), whereas the trans isomer hardly reacts at all in that short time. However, the latter affords l,l-diethoxy-2-p-nitrophenylethane in high yield when kept under the above conditions for 20 days (equation 22). The mechanism of the latter reaction could not... 

Alkenes are prepared from alcohols either by direct dehydration or by de-hydrohalogenation of intermediate alkyl halides to avoid rearrangement we often select dehydrohalogenation of halides even though this route involves an extra step. (Or, sometimes better, we use elimination from alkyl sulfonates.)... 

Vinyl halides are typically prepared by three methods (1) addition of HX, X2, or HCX3 to a triple bond, (2) elimination of HX (dehydrohalogenation) from polyhalogenated alkanes, or (3) elimination of X2 fi om polyhalogenated alkanes. Dehydrohalogenation is often accomplished by heating with alkali, while elimination of X2 is carried out by heating with zinc dust in ethanol. 

Dehydrohalogenation (s. a. Ethylene derivatives from halides) CC Hal Dehydrohalogenation-rearrangement 16, 938 Deoxymercuration, stereospecific 16, 970 Dequaternization s. Amines, tert., from ammonium salts, quaternary Derivatives s. a. subentry derivatives... 

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide It IS one of the most useful methods for preparing alkenes by p elimination... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

Dehydrohalogenation (Section 5 14) Reaction in which an alkyl halide on being treated with a base such as sodium ethoxide is converted to an alkene by loss of a proton from one carbon and the halogen from the adjacent carbon... 

The two most common elimination reactions arc dehydroUalogenalion—the loss of HX from an alkyl halide—and dehydration—(he loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocvclohexane yields cyclohexene when treated with KOH in ethanol solution. 

Dehydrohalogenation (Sections 7.1, 11.8) The loss of HX from an alkyl halide. Alkyl halides undergo dehydrohalogenation to yield alkenes on treatment with strong base. 

Monodehydrohalogenation of allylic halides is another classical method for diene synthesis24. This method is complementary to double dehydrohalogenation as both the 1,2-dihalides and allylic halides are readily accessed from alkenes. The commonly employed protocol for diene synthesis, particularly for cyclic 1,3-dienes, is through the allylic monobromination of the alkene with A-bromosuccinimide or related reagents followed by dehydrobromination with hindered bases such as DBN or DBU (equation l)25. 

Nitrile oxides are normally obtained from oximes in two steps halogenation of the aldoxime (5) to give a hydroximoyl halide (4) and subsequent dehydrohalogenation... 

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