Preparation of alkynes

The preparations of alkynes on insoluble supports include alkylations, vinylations, arylations, and alkynylations of other alkynes. Most of these reactions can be realized with either the alkyne or the alkylating agent linked to the support. 

Pd2(dba)3, P(oTol)3, NEt3, DMF, 100 °C, 20 h (does not proceed with 

Histrionicotoxin is a defensive toxin that protects Dendrobates histrionicus from potential predators. These small poison dart frogs inhabit the moist humid floor of tropical rainforests, and are commonly found in western Ecuador and Colombia. Histrionicotoxin acts by interfering with nerve transmission in mammals, resulting in prolonged muscle contraction. 

Alkynes are prepared by elimination reactions, as discussed in Section 8.10. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne by two 

Because vicinal dihalides are synthesized by adding halogens to alkenes, an alkene can be converted to an alkyne by the two-step process illustrated in Sample Problem 11.2. 

Sample Problem 11.2 Convert alkene A into alkyne B by a stepwise method. 

Convert each compound to 1-hexyne, HC=CCH2CH2CH2CH3. a. Br2CH(CH2)4CH3 b. CH2=CCI(CH2)3CH3 c. CH2=CH(CH2)3CH3 

Allies can be synthesized from metallic acetlylides and allq l halides. 

If a dihaloalkane is heated in alcoholic KOH solution, the two halogen atoms, bonded to the neighbour carbons, are eliminated and the alkane forms a triple bond and becomes an allqme. This reaction is called an elimination reaction. 

Explain whether these elimination reactions would be a good way to prepare these alkenes  

Explain which of these reactions would provide a better synthesis of 2-pentene  

Alkynes can be prepared from dihaloalkanes by elimination of two molecules of HX. This reaction requires very strongly basic conditions so potassium hydroxide at elevated temperatures or the stronger base sodium amide (NaNH2) is commonly employed. Examples are provided by the following equations  

In these reactions, elimination of the first molecule of HX results in the formation of a vinyl halide—an alkene with a halogen bonded to one of the carbons of the double bond. A second, more difficult elimination (this is why the strong base is necessary) pro- 

CHAPTER 10 SYNTHETIC USES OF SUBSTITUTION AND ELIMINATION REACTIONS 

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PREPARATION OF ALKYNES BY ALKYLATION OF ACETYLENE AND TERMINAL ALKYNES... 

Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes... 

Both strategies are applied to the preparation of alkynes In this section we shall see how to prepare alkynes while building longer carbon chains By attaching alkyl groups to acetylene more complex alkynes can be prepared... 

Because vicinal dihalides are prepared by addition of chlorine or bromine to alkenes (Section 6 14) alkenes especially terminal alkenes can serve as starting mate rials for the preparation of alkynes as shown m the following example... 

The properties of organometallic compounds are much different from those of the other classes we have studied to this point Most important many organometallic com pounds are powerful sources of nucleophilic carbon something that makes them espe cially valuable to the synthetic organic chemist For example the preparation of alkynes by the reaction of sodium acetylide with alkyl halides (Section 9 6) depends on the presence of a negatively charged nucleophilic carbon m acetylide ion... 

Examples of alkylation, dealkylation, homologation, isomerization, and transposition are found in Sections 1,17,33, and so on, lying close to a diagonal of the index. These sections correspond to such topics as the preparation of alkynes from alkynes carboxylic acids from carboxylic acids and... 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

The base-catalyzed fragmentation of 4-alkyl-l,2,3-thiadiazoles is a useful method for the preparation of alkyne-1-thiolates <1996T3171>. These alkyne-l-thiolates can then react with carbon disulfide to afford l,3-dithiole-2-thiones. This strategy has been developed to give a synthesis of some novel tetrathiafulvene derivatives <1996T3171>. 

The same methodology was successfnUy applied for the preparation of alkynes 152 and alkenes 153 from a-sulfinyl ketones 150 and S-mesyloxy snlfoxides 151, respectively (equation 52). 

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). 

Preparation of alkynes The reaction of primary alkyl halides and metal acetylides or alkynides (R C=CNa or R C=CMgX) yields alkynes. The reaction is limited to 1° alkyl halides. Higher alkyl halides tend to react via elimination. 

Irradiation of certain 1-p-toluenesulfonate-1,2,3-triazole anions has been employed in the preparation of alkynes.95... 

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