Alkynes by dehydrohalogenation

Alkynes by dehydrohalogenation. Alkynes can be prepared from vie-dibromides by treatment at elevated temperatures with KOH dissolved in DMSO. ... 

Geminal dihalides can also be converted to alkynes by dehydrohalogenation. 

We have encountered haloalkenes—alkenyl halides—as intermediates in both the preparation of alkynes by dehydrohalogenation and also the addition to alkynes of hydrogen halides. Alkenyl halides have become increasingly important as synthetic intermediates in recent years as a result of developments in organometalUc chemistry. These systems do not, however, follow the mechanisms familiar to us from our survey of the haloalkanes (Chapters 6 and 7). This section discusses their reactivity. 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Double dehydrohalogenation (Section 9 7) Reaction in which a geminal dihahde or vicinal dihahde on being treated with a very strong base such as sodium amide is converted to an alkyne by loss of two protons and the two halogen substituents... 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

A complex of homoadamantyne (273) has been prepared by dehalogena-tion of 272 in the presence of tris(triphenylphosphine)platinum(0) [Eq. (43)]." Trapping free homoadamantyne was presumed to be the mechanism of this reaction. However, reduction of neither an initially formed 77-com-plex2,82 nor a C-Br insertion product93 was excluded. Interestingly, the free alkyne, generated by dehydrohalogenation of the vinyl bromide, was successfully trapped with diphenyl-wo-benzofuran. 

Chapter 9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert... 

The trans double bond in the target molecule is a product of reduction of a triple bond with Li in NH3.The alkyne was formed by an alkylation of a terminal alkyne with bromomethane. The terminal alkyne was synthesized from the starting alkene by bromination, followed by dehydrohalogenation. 

A. Alkynes can be made by dehydrohalogenation of vinylic halides in a reaction that is essentially an F.2 process. In studying the stereochemistry of this elimination, it was found that (Z)-2-chloro-2-butcnedioic acid reacts 50 times as fast as the corres()onding E isomer. What conclusion can you draw about the stereochemistry of eliminations in vinylic halides How does this result compare with eliminations of alkyl halides ... 

Cyclobutenones can be prepared by several routes including the reaction of ketenes, generally prepared in situ by dehydrohalogenation of acyl chlorides, with electron-rich alkynes such as alkoxyace-tylenes, alkylthioacetylenes or ynamines. ... 

In dehydrohalogenation reactions, hydrogen and halogen are the atoms eliminated from adjacent carbons. Bases such as potassium hydroxide and sodium amide are the reagents. Both alkenes and alkynes can be synthesized by dehydrohalogenation. 

Dehydrohalogenation [1, 1035-1037, at end]. In a general procedure for the conversion of alkenes into alkynes by bromination and dehydrobromination, Ward and van Dorp recommend sodium amide and liquid ammonia for the dehydrobromination step.103 The preparation of propargyl aldehyde diethyl acetal is an example ,cb... 

Chapter 9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert trans-2,5-dimethyl-3-hexene to 2,5-dimethyl-3-hexyne by adding bromine across the double bond, then doing a double elimination. The infrared and mass spectra of the major product are shown here. 

Owing to their tendency to dimerize to furoxans (1,2,5-oxadiazole 2-oxides), nitrile oxides 5 are usually generated in situ, i.e., in the presence of suitable dipolarophiles such as alkenes, alkynes, etc., from stable precursors such as aldoximes 12 (X = H) or from primary nitroalkanes 13 (Scheme 2) [5,57-67]. Generation of nitrile oxides 5 from aldoximes 12 (X = H) involves either direct oxidation or halogenation of aldoximes 12 (X = H) to hydroximoyl halides 12 (X = Cl or Br) followed by dehydrohalogenation [5,57-67,79,80]. Alternatively, nitrile oxides 5 are conveniently generated via dehydration of primary nitroalkanes 13 [ 17,38,39,65,66,81-95]. This review covers the literature in the last 10-15 years pertaining to the chemistry of isoxazoHnes synthesized from primary nitroalkanes 13. 

Synthesis of Alkynes by Elimination Reaotions 314 [ A MECHANISM FOR THE REACTION ] Dehydrohalogenation of w c-Dibromides to Form Alkynes 315... 

Purpose To demonstrate the preparation of an alkyne by a double dehydrohalogenation. 

But-l-en-3-yne ( vinylacetylene ), F, is made industrially by the CuCl-catalyzed dimerization of ethyne. How would you make it We learned that alkenes can be converted to alkynes by halogenation-double dehydrohalogenation (Section 13-4). This approach suggests 1,3-butadiene as a starting material. We have seen, however, that halogenation of 1,3-butadiene, such as bromination, is not as straightforward as that of a monoalkene 1,2- and 1,4-addition occurs (Section 14-6). Is that a problem Answer No. The 1,2-dihalobutene will eliminate normally, first to 2-bromobutadiene (via deprotonation at the more acidic allylic position) and then to F ... 

Double dehydrohalogenation to form terminal alkynes may also be carried out by heating geminal and vicinal dihalides with potassium tert butoxide m dimethyl sulfoxide... 

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