Dihalides from dehydrohalogenation

Dehydrohalogenations can be performed under mild conditions using solid potassium t-butoxide in petroleum ether in the presence of catalytic amounts of 18-crown-6. These conditions are particularly effective for converting 1,2-dihalides (from terminal alkenes) and 1,1-dihalides (from aldehydes) into terminal acetylenes, and gem-dihalides from symmetrical ketones into internal acetylenes. 

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. 

Alkynes are prepared from alkyl dihalides via elimination of atoms or groups from adjacent carbons. Dehydrohalogenation of vicinal- or gemiwaZ-dihahdes is a particularly useful method for the preparation of alkynes (see Section 5.4.5). 

Telluropheno[2,3-b]quinolines were obtained by dehydrohalogenation of 2,3-dihydro-telluropheno[2,3-A]quinoline 1,1-dihalides with l,5-diazabicyclo[5.4.0]undec-7-ene (DBU) in yields ranging from 12 to 35%. The dihalides were prepared from the 2,3-dihydro tclluropheno[2,3-6]quinolines (p. 374) and /V-bromosuccinimide or iodine in carbon tetrachloride. 

In some cases, we can generate a carbon-carbon triple bond by eliminating two molecules of HX from a dihalide. Dehydrohalogenation of a geminal or vicinal dihalide gives a vinyl halide. Under strongly basic conditions, a second dehydrohalogenation may occur to form an alkyne. 

Sec. 8.2) from dihalides by base-induced double dehydrohalogenation... 

A triple bond may be formed by dehydrohalogenation of dihalides and olefinic halides of the general types RCX= CHj, RCH= =CHX, RCH= =CXR, RCHXCHjX, RCHXCHXR, RCHjCHX, and RCXjCHjR. The choice of a base depends somewhat on the position desired for the triple bond in the product. Sodium amide tends to rearrange the triple bond toward the end of the chain," and potassium hydroxide favors reverse isomerization toward the center of the chain,Although neither rearrangement is dependable from a synthetic standpoint, it is best to choose the base favoring the desired product. 

Dehydrohalogenation of vicinal dihalides is particularly useful since the dihalides themselves are readily obtained from the corresponding alkenes by addition of halogen. This amounts to conversion—by several steps—of a double bond into a triple bond. 

Dehydrohalogenation of Alkyl Halides Dehalogenation of 12-Dihalides Dehydration of Alcohols Alkenes from Ethers... 

The necessary vicinal dihalides are themselves readily available by addition of Bi2 or CI2 to alkenes. Huts, the overall halogenation/dehydrohalogenation sequence makes it possible to go from an alkene to an alkyne. For example, diphenylethylene is converted into diphenylacetylene by reaction with Bt2 and subsequent base treatment. 

This alkyne can be produced from the corresponding alkyl dihalide by dehydrogalogenation. The dehydrohalogen-ation is carried out in two steps with KOH (ale) that produces a vinyl halide and NaNH2, which is a stronger base, that gives the alkyne. To obtain the dihalide. 

Show which of the following compounds could be synthesized in good yield by a double dehydrohalogenation from a dihalide. In each case,... 

Initial dehydrohalogenation of the w r-dihalide produces a mixture of two bromoalkenes that are not isolated but that undergo a second dehydrohalogenation. The terminal alkyne that results from this step is deprotonated (because of its acidity) by the third mole of sodium amide (see Section 7.9). To complete the process, addition of ammonium chloride converts the sodium alkynide to the desired product, 1-butyne. 

A more direct approach to acetylenes has been reported which utilizes phase-transfer generated tetrabutylammonium hydroxide to dehydrohalogenate a-halo-olefins or bis-dehydrohalogenate vicinal dihalides. An example of the latter is the reaction of 1,2-dibromo-l-phenylethane in pentane with 50% aqueous sodium hydroxide in the presence of tetrabutylammonium bisulfate. The product, phenylacetylene, is produced in 87% yield in less than an hour by this method (see Eq. 9.11) [27]. The advantage of this approach is apparent when one considers that in the traditional method, jS-bromostyrene (from the dehydrohalogenation and decarboxylation of cinnamic acid dibromide) is heated with KOH at over 200° and the phenylacetylene distills as formed in 67% yield [28]. 

The reactions of halogenomethylpyridines Table 6.13) with nucleophilic reagents are normal and require no comment is. The familiar reaction of dehydrohalogenation to produce acetylene derivatives is of considerable value in its application to side-chain halogen derivatives of the pyridine series. Its value arises from the possibility of carrying out the sequence of reactions, 2(or 4)-picoline 2(or 4)-styrylpyridine (p. 334) -> 2 (or 4)-styrylpyridine dihalide (p. 349) 2-(or 4)-tolazole. The dehydrohalogena-... 

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