Dehydrohalogenations acetylenes

Amyl alcohol was selected as a solvent with a convenient boiling point for the dehydrohalogenation. Acetylenic triple bonds are attacked by water in the presence of strong alkali or strong acid. 

Dehydrohalogenation. Acetylenes can be prepared by dehydrogenation of compounds of the type RCH=CXR or RCHX—CHXR by ion-pair extraction (6, 565). Yields are generally superior to those obtained by classical methods. ... 

In early work, vinyl chloride had been heated with stoichiometric amounts of alkaU alkoxides in excess alcohol as solvent, giving vinyl ethers as products (210). Supposedly this involved a Williamson ether synthesis, where alkaU alkoxide and organic haUde gave an ether and alkaU haUde. However, it was observed that small amounts of acetylene were formed by dehydrohalogenation of vinyl chloride, and that this acetylene was consumed as the reaction proceeded. Hence acetylene was substituted for vinyl chloride and only catalytic amounts of alkaU were used. Vinylation proceeded readily with high yields (211). 

Subsequent dehydrohalogenation afforded exclusively the desired (Z)-olefin of the PGI2 methyl ester. Conversion to the sodium salt was achieved by treatment with sodium hydroxide. The sodium salt is crystalline and, when protected from atmospheric moisture and carbon dioxide, is indefinitely stable. A variation of this synthesis started with a C-5 acetylenic PGF derivative and used a mercury salt cataly2ed cyclization reaction (219). Although natural PGI has not been identified, the syntheses of both (6R)- and (65)-PGl2, [62777-90-6] and [62770-60-7], respectively, have been described, as has that of PGI3 (104,216). 

In 1980 Sonogashira reported a convenient synthesis of ethynylarenes - the Pd-catalyzed cross-coupfing of bromo- or iodoarenes with trimethylsilylacetylene followed by protiodesilylation in basic solution [15]. Prior to this discovery, formation of terminal acetylenes required manipulation of a preformed, two-carbon side chain via methods that include halogenation/dehydrohalogenation of vinyl- and acetylarenes, dehalogenation of /1,/1-dihaloalkenes, and the Vils-meier procedure [ 14]. With the ready availability of trialkylsilylacetylenes, the two-step Sonogashira sequence has become the cornerstone reaction for the construction of virtually all ethynylated arenes used in PAM and PDM synthesis (vide infra). 

Sodium amide has been extensively used in the synthesis of a large number of acetylenic compounds by dehydrohalogenation. An example is ... 

Several acetylenic derivatives are prepared by dehydrohalogenation of vicinal dibromo compounds, which are obtained by adding bromine to olefinic compounds. 

The base-promoted dehydrohalogenation of vinyl or allylic halides affords poor yields of allenes because of the competing side reaction leading to acetylenes [21,22], The presence of tertiary hydrogens a to the halogen also helps to... 

Dicarboxanilide-acetylene Di(N-phenyl-carboxamide) acetylene or Bis(carboxanilide)- cetylene, CgHjHNOC.CsC.CONHCeHj mw 264.27, N 10.60% pinkish-white crysts (from MeOH-water), mp 194-96° was prepd from acetylenedicacboxylic acid by a series of reactions to give bromofumaranilide which was dehydrohalogenated by a dil soln of methanolic KOH. 

SAMPLE SOLUTION (a) Reasoning backward, we recognize 1,1-dichloroethane as the product of addition of two molecules of hydrogen chloride to acetylene. Thus, the synthesis requires converting ethylene to acetylene as a key feature. As described in Section 9.7, this may be accomplished by conversion of ethylene to a vicinal dihalide, followed by double dehydrohalogenation. A suitable synthesis based on this analysis is as shown ... 

The amide ions are powerful bases and may be used (i) to dehydrohalogenate halo-compounds to alkenes and alkynes, and (ii) to generate reactive anions from terminal acetylenes, and compounds having reactive a-hydrogens (e.g. carbonyl compounds, nitriles, 2-alkylpyridines, etc.) these anions may then be used in a variety of synthetic procedures, e.g. alkylations, reactions with carbonyl components, etc. A further use of the metal amides in liquid ammonia is the formation of other important bases such as sodium triphenylmethide (from sodamide and triphenylmethane). 

With volatile products which may arise in reactions which do not lead to the initial formation of metal salts (i.e. dehydrohalogenations, alkylations leading to terminal acetylenes) the following procedure is recommended.35... 

The designated starting material, 1,1-dichloroethane, is a geminal dihalide and can be used to prepare acetylene by a double dehydrohalogenation. 

Monovinylacetylene has been prepared by the decomposition of a diquatemary ammonium base,3 by the dimerization of acetylene,4 and by the dehydrohalogenation of dihalobutenes in liquid ammonia.5 The last method produces a metal salt. The procedure described has been published.6... 

Three examples of the many reactions that can be advantageously carried out in high-temperature molten salts have been selected to illustrate this molten-salt technique. When 1,1-dichloroethane is passed through a ZnCl2/KCl melt at 330 °C, chloroethene is formed in 97 cmol/mol yield by dehydrohalogenation. Likewise, the addition of hydrogen chloride to acetylene proceeds with 89 cmol/mol yield in the same melt. A combination of these two steps allows a ready synthesis of the technically important chloroethene from acetylene and 1,1-dichloroethane according to Eq. (3-1) [25, 31]. 

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