Elimination, of hydrogen bromide from

Joly s method (or modifications) is the best procedure for preparing A " -3-ketones and can be extended to the elimination of hydrogen bromide from a-bromo ketones of all types. Rearrangement is sometimes observed but is not often serious. Selectivity can be improved in some instances by lowering the reaction temperature. The method has been found useful for the preparation of A" -3-ketones from 6-halo-A" -3-ketones ... 

Dehydrochlorination of bis(tnfluoromethylthio)acetyl chloride with calcium oxide gives bis(trifluoromethylthio)ketene [5] (equation 6) Elimination of hydrogen chloride or hydrogen bromide by means of tetrabutylammonium or potassium fluoride from vinylic chlorides or bromides leads to acetylenes or allenes [6 (equation 7) Addition of dicyclohexyl-18-crown-6 ether raises the yields of potassium fluoride-promoted elimination of hydrogen bromide from (Z)-P-bromo-p-ni-trostyrene in acetonitrile from 0 to 53-71 % In dimethyl formamide, yields increase from 28-35% to 58-68%... 

The malonic ester required for synthesis of cyclopal (107) can be obtained by alkylation of diethyl allylmalonate (115) with 1,2-dibromocyclopentane in the presence of excess base. It is probable that the reaction proceeds by elimination of hydrogen bromide from the dihalide as the first step. The resulting allilic halide (116) would be the most reactive electrophile in the reaction mixture and thus would quickly alkylate the anion of the malonate to afford 117. 

The knowledge of the valence tautomerization of benzene oxides to oxepins12 prompted several groups to synthesize oxepins by dehydrohalogenation of 7-oxabicyclo[4.1.0]heptane derivatives. Numerous examples have been described for the base-catalyzed elimination of hydrogen bromide from the 3,4-dibromo-7-oxabicyclo[4.1.0]heptane system. The reaction products are usually obtained as mixtures of oxepin 1 and benzene oxide 2. The 2,7-bis(hydroxy-methyl)oxepin 1 p obtained by this route can be converted to the 2,7-dicarbaldehyde with man-ganese(IV) oxide.23... 

The dehydrohalogenation reaction has been extended to benzannulated oxepins. Elimination of hydrogen bromide from 3-bromo-4-phenyl-2,3-dihydro-l-benzoxepin with 1,5-diazabicyclo-[4.3.0]non-5-ene gives 4-phenyl-1-benzoxepins 15a15 and 15b16 in low yield. 

Elimination of hydrogen bromide from bromohexadienes with dunethyl-benzylamine, 41, 50... 

Scheme 6.32 -Elimination of hydrogen bromide from the 3-bromo-4,5-dihydro-1,5-methano-l H-benzocycloheptenes 134 and 135 by KOtBu and trapping of the intermediates.

The catalysed -elimination of hydrogen bromide from 2-bromoethyl and 1,2-dibromoethyl sulphides, using 9.1.1 or 9.1.2, provides a convenient route to vinyl and alkynyl sulphides, respectively [17, 18], which, as their sulphoxides or sulphones, have considerable utility as dienophiles. Aryl vinyl ethers (>90%) have been obtained by analogous procedures from 2-chloroethyl ethers [19]. 

Elimination of hydrogen bromide from a-bromo-y-butyrolactone with triethylamine, 46,22 Epichlorohydrin, 46, 24 Ether, /-butyl phenyl, id, 89 /j-Ethoxyphenyl isothiocyanate, 46, 21 Ethyl acetoacetate, 46, 82 Ethyl benzoyloxy cyanoacetates, 46, 38 Ethyl y-BROMOBUTYRATE, 46, 42 Ethyl 2-biomocyclopentane acetate, 46, 44... 

The rate of dehydrobromination was found to be three times faster than dehydrochlorination and the reaction followed anti elimination.7 Thus, anti elimination of hydrogen chloride from as.a.s,fran. -l,2,3,4-tetrachlorocyclobutane (16) with potassium hydroxide affords rram-1,3.4-trichlorocyclobutene (17) in 48% yield. However, anti elimination of hydrogen bromide from... 

Elimination, of hydrogen bromide from a dibromide, 22, 50 25, 92 with rearrangement, 24, 33 of hydrogen bromide from a vinyl halide, 25, 93 Emulsin, 22, 53... 

The preferential syn elimination of hydrogen fluoride, in preference to elimination of hydrogen bromide, from 6.15A is particularly surprising [22] (Figure 6.15). 

Elimination of hydrogen bromide from 1,2-dibromo-4-butanol, BrCHjCHBrCHjCHjOH, is accomplished with powdered potassium hydroxide in dry ether. The resulting /3-bromotetrahydrofuran loses another molecule of hydrogen halide when heated with excess powdered base. The over-all yield of dihydrofuran is 62%. This elimination reaction has been extended to the preparation of /3-bromofurans and 2,5-dihydrofurans having two alkyl groups on one of the a-carbon atoms. ... 

Alternatively, the ketone may be converted to the enol acetate and this may be brominated (Scheme 3.85). Elimination of hydrogen bromide from a-bromo ketones leads to a,P-unsaturated ketones. 

The formation of quaternary isoselenazolinium salts is based on the formation of the Se-N bond by intramolecular reaction of an azomethine or amide group with a cyanoseleno or selenomethyl group under acidic conditions (Schemes 29 and 30). Elimination of hydrogen bromide from the appropriate open-chain precursors having amide or bromoseleno groups results in cyclization to isoselenazolidines (Schemes 32 and 33). 

Exploration of the chemistry of cyclopropanimines has been carried out by Quast and coworkers during extensive studies on a-bromoketimines. 1,3-Elimination of hydrogen bromide from 14 by excess potassium t-butoxide in THF yields the imines as colorless oils or low melting solids (equation 6). As shown by their H- and C-NMR spectra, these... 

Base-promoted a-elimination of hydrogen bromide from dibromofluoromethane in the presence of an alkene affords 1-bromo-l-fluorocyclopropanes (see Houben-Weyl, Vol. E19b, pp 1497-1499). 

Addition of bromochlorocarbene (carbenoid) to an alkene results in the formation of 1-bromo-1-chlorocyclopropanes. The preparative value of this process depends on the structure of the carbene source, the type of base used for a-elimination of hydrogen bromide from the haloform, reaction conditions etc. 

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