Preparation of Alkynes by Elimination Reactions

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 in which both halogens are on the same carbon, or it may be a vicinal dihalide, one in which the halogens are on adjacent carbons. 

The most frequent applications of these procedures lie in the preparation of terminal alkynes. Because the terminal alkyne product is acidic enough to transfer a proton to amide anion, one equivalent of base in addition to the two equivalents required for double dehydrohalogenation is needed. Adding water or acid after the reaction is complete converts the sodium salt to the corresponding alkyne. 

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

Give the structures of three isomeric dibromides that could be used as starting materials for the preparation of 3,3-dimethyl-l-butyne. 

Because vicinal dihalides are prepared by addition of chlorine or bromine to alkenes (Section 6.14), alkenes, especially terminal alkenes, can serve as starting materials for the preparation of alkynes as shown in the following example  

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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 ... 

Alkynes can be prepared by elimination reactions of 1,2-dihalides, using a strong base. 

Alkenes may be prepared by elimination reactions with a regio-chemistry (Hofmann or SaytzefO that depends on the structure of the substrate and the reaction conditions. Alkenes may also be obtained from carbonyl compounds by the Witlig reaction and by the hydrogenation of alkynes. 

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... 

Alkenyl and alkynyl derivatives have been prepared by elimination reactions. Phosphorus penta-chloride treatment of the 5-acetyl derivative (390) and heating with catalytic amount of aluminum tribromide gave the a-chloro vinyl derivative (391). The initial product is an a,a-dichloro derivative which subsequently loses HCl under the influence of AlBr3. Treatment with strong base leads to further HCl elimination and formation of the 5-alkyne (392). This reaction proceeds well when the... 

Alkynes can be prepared by elimination reactions under conditions similar to those used to form alkenes. Because an alkyne has two n bonds, two molar equivalents of HX must be eliminated from the starting material. One such suitable reactant is a vicinal dihahde, a compound with halogen atoms on adjacent carbon atoms. We recall that such compounds result from the addition of a halogen to the double bond of an alkene (Section 6.5). A geminal dihahde, which has both halogens on the same carbon atom, can also be used to synthesize alkynes. Geminal dichlorides can be made by reaction of phosphorus pentachloride with an aldehyde or ketone. 

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

The cyclic enol ether 255 from the functionalized 3-alkynoI 254 was converted into the furans 256 by the reaction of allyl chloride, and 257 by elimination of MeOH[132], The alkynes 258 and 260, which have two hydroxy groups at suitable positions, are converted into the cyclic acetals 259 and 261. Carcogran and frontalin have been prepared by this reaction[124]. 

Hydrazoyl halides are useful reagents for the synthesis of pyrazolines and pyrazoles (80JHC833). The elimination of HX, usually with triethylamine, is now the preferred method for the generation of the nitrilimine (621) in situ. Although in some cases it is not clear if the mechanism involves a nitrilimine (621) (as for example in the Fusco method in which sodium salts of /3-diketones are used), in other reactions it is the most reasonable possibility. For example, the synthesis of pyrazolobenzoxazine (633) from the hydrazoyl halide (631) probably occurs via the nitrilimine (632). Trifluoromethylpyrazoles (634) have been prepared by the reaction of a hydrazoyl halide and an alkynic compound in the presence of triethylamine (82H(19)179). 

Dipolar cycloaddition reaction of azomethine ylides to alkynes or alkenes followed by oxidation is one of the standard methods for the preparation of pyrroles.54 Recently, this strategy has been used for the preparation of pyrroles with CF3 or Me3Si groups at the (3-positions.55 Addition of azomethine ylides to nitroalkenes followed by elimination of HN02 with base gives pyrroles in 96% yield (Eq. 10.48).56... 

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