Alkyne acetylide anions from

Formation of acetylide anions from terminal alkynes (11 6B)... 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Alkyne carbon atoms are sp-hybridized, and the triple bond consists of one sp-sp a bond and two p-p tt bonds. There are relatively few general methods of alkyne synthesis. Two good ones are the alkylation of an acetylide anion with a primary-alkyl halide and the twofold elimination of HX from a vicinal dihalide. 

Acetylide anion (Section 8.7) The anion formed by removal of a proton from a terminal alkyne. 

Finally, the peculiar formation of iodoalkynes from iodine and acetylenes with relatively low (< 25) pK values in liquid ammonia should be mentioned [121]. The most likely intermediates occurring are acetylide "anions" formed in very low concentrations from the acetylene and the base ammonia. The conversions proceed very slowly and iodinadon of the lithiated alkynes in the same solvent is undoubtedly a far superior method. 

Reactions that form carbon-carbon bonds are extremely important in synthesis because they enable larger compounds, containing more carbons, to be constructed from smaller compounds. This requires the reaction of a carbon nucleophile with a carbon electrophile. The most important carbon nucleophiles that we have encountered so far are cyanide ion and acetylide anions (see Section 10.8). If we remember that acetylide anions can be reduced to c/.v-alkenes (see Section 11.12), then all of the addition products of this chapter are accessible from simple alkynes. 

Sec. 8.8) from terminal alkynes by alkylation of acetylide anions... 

Terminal alkynes are weakly acidic. The alkyne hydrogen can he removed by s strong base 9uch ae Na NH.. to yield nn a<%tylide anipn An acetylide anion ads as a nucleophile and can displace a halide ion from a primary alkyl halide in a n alkylation reaction. Acetylide anions are more stable iJian either alkyl anions or vinylic anions because their m ative charge is in a hybrid orbital with 50% s character, allowing the charge to be doser to the nucleus. 

The electron-rich acetylide anion produced from deprotonation of 1-alkynes with Na in NH3 (liq.) is reluctant to accept an electron, allowing the selective reduction of an internal triple bond in the presence of a terminal one. Reduction to the corresponding 1-alkenes can be achieved in the presence of ammonium sulfate. ... 

Another simple, yet important, reaction is the addition of an acetylide anion, C2H, to a carbonyl group. The anion may be formed by the reaction between a strong base, such as NaNH2, and an alkyne. Write down the complete reaction sequence from the starting materials to the final product. 

Alkynyl complexes contain metal-carbon bonds in which the metal is bound to the sp-hybridized carbon at the terminus of a metal-carbon triple bond. The materials properties of these complexes have been investigated extensively. The properties of these complexes include luminescence, optical nonlinearity, electrical conductivity, and liquid crystallinity. These properties derive largely from the extensive overlap of the metal orbitals with the ir-orbitals on the alkynyl ligand. The M-C bonds in alkynyl complexes appear to be considerably stronger than those in methyl, phenyl, or vinyl complexes. Alkynyl complexes are sometimes prepared from acetylide anions generated from terminal alkynes and lithium bases (e.g., method A in Equation 3.42), but the acidity of alkynyl C-H bonds, particularly after coordination of the alkyne to the transition metal, makes it possible to form alkynyl complexes from alkynes and relatively weak bases (e.g., method B in Equation 3.42). Alkynyl copper complexes are easily prepared and often used to make alkynylnickel, -palladium, or -platinum complexes by transmetallation (Equation 3.43). This reaction is a step in the preparation of Ni, Pd, or Pt alkynyl complexes from an alkyne, base, and a catalytic amoimt of Cul (Equation 3.44). This protocol for... 

Because of the ready availability of acetylene and the ease with which it is converted to a nucleophile, alkylation of acetylide anions is the most convenient laboratory method used for the synthesis of other alkynes. The process can be repeated, and a terminal alkyne in turn can be converted to an internal alkyne. An important feature of this reaction is that a new carbon-carbon skeleton can be made, allowing for the construction of larger carbon skeletons from smaller ones. In the following scheme, the carbon skeleton of 3-heptyne is constructed from acetylene and two lower-molecular-weight haloalkanes. 

In the synthesis of propargylic alcohols, we saw the reaction of an alkynyl nucleophile (either the anion RC=CNa or the Grignard RC CMgBr, both prepared from the alkyne RC CH) with a carbonyl electrophile to give an alcohol product. Such acetylide-type nucleophiles will undergo Sn2 reactions with alkyl halides to give more substituted alkyne products. With this two-step sequence (deprotonation followed by alkylation), acetylene can be converted to a terminal alkyne, and a terminal alkyne can be converted to an internal alkyne. Because acetylide anions are strong bases, the alkyl halide used must be methyl or 1° otherwise, the E2 elimination is favored over the Sn2 substitution mechanism. 

Internal alkynes can be made from terminal alkynes by converting the terminal alkyne to an acetylide anion and then treating the anion with a primary alkyl halide. Propose a mechanism for the alkylation. (See Section 8.15.)... 

Most other alkynes are prepared from acetylene by taking advantage of the acidity of the C — H bond. In the presence of a very strong base, such as sodium amide (NaNH2), the amide anion removes the proton from acetylene to form ammonia and the salt sodium acetylide. The acetylide can then react with an alkyl halide, such as CHsBr ... 

The nucleophilic acetylide ion uses an electron pair to attack the positively, polarized, electrophilic carbon atom of bromomethane. As the new C-C bond forms, Br" departs, taking with it the electron pair from the former C-Br bond and yielding propyne as product. We call such a reaction an alkyla-l tion because a new alkyl group has become attached to the starting alkyne. Alkyne alkylation is not limited to acetylene itself, Any terminal alkyne s can be converted into its corresponding anion and then alkylated by treat-j ment with an alkyl halide, yielding an internal alkyne. For example, con/ version of 1-hexyne into its anion, followed by reaction with 1-bromobutane,] yields 5-decyne ... 

As noted above, alkyne anions are very useful in Sn2 reactions with alkyl halides, and in acyl addition reactions to a carbonyl.46 Alkyl halides and sulfonate esters (tosylates and mesylates primarily) serve as electrophilic substrates for acetylides. A simple example is taken from Kaiser s synthesis of niphatoxin B, in which propargyl alcohol (36) is treated with butyllithium and then the OTHP derivative of 8-bromo-1-octanol to give a 47% yield of 37.48... 

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