Alkynide ion

This synthesis fails when secondary or tertiary halides are used because the alkynide ion acts as a base rather than as a nucleophile, and the major results is an E2 elimination. 

Very strong bases (such as sodium amide, NaNH2) deprotonate terminal acetylenes to form carbanions called acetylide ions (or alkynide ions). Hydroxide ion and alkoxide ions are not strong enough bases to deprotonate alkynes. Internal alkynes do not have acetylenic protons, so they do not react. 

The alkylation of j p-carbon can, in principle, involve the alkyne (acetylene) either as the nucleophile or the electrophile. In practice by far the most important process involves the alkyne as nucleophile since the acidity of the alkyne proton (pK = 25) allows the ready formation of alkynide ions. These are excellent nucleophiles and they readily undergo acylation and alkylation with appropriate electrophiles. The recent introduction of palladium-catalyzed reactions, usually involving copper(I) salts but also other cations, has greatly increased the use made of arylation and vinylation reactions. In this chapter only the alkylation of the alkynide ion will be discussed acylation, vinylation and arylation reactions are discussed elsewhere. The alkylation of alkynide anions is a reaction of considerable synthetic use and has been extensively reviewed. ... 

The low acidity of 1-alkynes means that strong bases must be used to form the alkynide ions and that water is not a suitable solvent aqueous solutions have a very low concentration of alkynide ions. Some transition metal alkynides can be prepared by precipitation from aqueous solution because their solubilities are very low. Suitable solvents for the preparation of alkynide ions must be less acidic than the alkyne, and preferably allow the alkyne and the alkynide ion to remain in solution. Liquid ammonia, te-trahydrofuran, ether and hydrocarbons have all been used, particularly the first, the alkynide anion being readily formed by metal amides. Alkynides of many types have been prepared from various metals. Besides Groups I and III, copper(I), silver, gold(I), zinc, mercury and, more recently, aluminum alkynides have been synthesized. The alkynides of Groups I and II have been principally used as nucleophiles in alkylation reactions, but there are now many examples of other metal alkynides in this role. Palladium-catalyzed reactions, as remarked above, have become increasingly important for the reactions of alkynides of metals other than Groups I and II, but these have not usually involved alkylation. 

The alkynide ion can undergo alkylation with a variety of alkylating reagents, such as haloalkanes and alkyl sulfates, with the formation of a carbon-carbon bond. The alkynide ion is also strongly basic so that elimination reactions may accompany or subvert the substitution reaction. Group I metal alkynides in liquid ammonia give mainly substitution products with primary haloalkanes but secondary and tertiary haloalkanes give mainly elimination products, as do 2-substituted primary haloalkanes (equation 1). 

Yamaguchi and Hirao found that alkyneboron difluorides, which are prepared from the alkynide ion and boron trifluoride etherate, react with oxiranes to give the 2-alkynols (Scheme 15). The same research group also found that ring opening of oxetane could be accomplished, but in this case boron trifluoride had to be added as a Lewis acid (Scheme 15). 

As was previously remarked, water and its associated base are, respectively, too acidic and insufficiently basic to act as solvent and base for the preparation of alkynide ions. Lissel has shown, however, that with the addition of the crown ether 18-crown-6, alkylation of phenylacetylenes can occur with KOH and an iodoalkane (Scheme 29). 

In their synthesis of erythionolide B (21), Corey and cowoikers used two alkylations of an alkynide ion in the preparation of one half of the molecule. One alkylation involved regiospecific ring opening of a resolved oxirane to provide the desired compound with the correct absolute stereochemistry, while the second used a simple methylation (Scheme 40). 

The anion obtained when the acetylenic hydrogen is removed is known as an alkynide ion or an acetylide ion. As we shall see in Section 7.11, these ions are useful in synthesis ... 

STRATEGY AND ANSWER We make use of two reactions that we have just studied in this chapter syn addition of hydrogen to an alkyne, and alkylation of alkynide ions. 

The reaction proceeds by addition of the alkynide ion, PhC ", to the triple bond of another molecule of phenylacetylene. 

The basic sites abstract a proton from (CHsisSiC H to form the alkynide ion, (CH3)3SiC , which attacks at the silicon atom of another reactant molecule. 

PRACTICE the skill 10.5 In each of the following cases, determine if the base is sufficiently strong to deprotonate the terminal alkyne. That is, determine whether the equilibrium favors formation of the alkynide ion. 

In total, three equivalents of the amide ion are required two equivalents for the two E2 reactions, and one equivalent to deprotonate the terminal alkyne and form the alkynide ion. After the alkynide ion has formed and the reaction is complete, a proton source can be... 

However, under these conditions, this internal alkyne quickly isomerizes to form a terminal alkyne that is subsequently deprotonated to form an alkynide ion ... 

This reaction has powerful synthetic utility, because the resulting alkynide ion can function as a nucleophile when treated with an alkyl halide ... 

This process is only cient with methyl or primary alkyl halides. When secondary or tertiary alkyl halides are used, the alkynide ion functions primarily as a base and elimination products are obtained. This observation is consistent with the pattern we saw in Section 8.14 (substitution vs. elimination). 

The conjugate base of a terminal alkyne, called an alkynide ion, can only be formed with a sufficiently strong base, such as NaNH2. 

Count the carbon atoms in the starting material and in the desired product. There are seven carbon atoms in the starting material, and there are nine carbon atoms in the product. Therefore two carbon atoms must be installed. We have only learned one reaction capable of installing two carbon atoms on an existing carbon skeleton. This process requires the use of an alkynide ion and an alkyl halide ... 

Alternatively, this reaction can be viewed from the perspective of the alkyl halide. That is, the alkyl halide is the starting material, and an alkynide ion is used to achieve the installation of a triple bond onto an existing carbon skeleton ... 

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