Acetylide ions ketones

Certain functional groups may be protected from reduction by conversion to anions that resist reduction. Such anions include the alkoxides of allylic and benzylic alcohols, phenoxide ions, mercaptide ions, acetylide ions, ketone carbanions, and carboxylate ions. Except for the carboxylate, phenoxide, and mercaptide ions, these anions are sufficiently basic to be proton-ated by an alcohol, so they are useful for protective purposes only in the... 

You have already had considerable experience with caibanionic compounds and their- applications in synthetic organic chemistry. The first was acetylide ion in Chapter 9, followed in Chapter 14 by organometallic compounds—Grignaid reagents, for exanple—that act as sources of negatively polarized car bon. In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic, and that this property can be used to advantage as a method for carbon-carbon bond formation. 

Ketones react with acetylide ion (Section 8.7) to give alcohols. For example, the reaction of sodium acetylide with 2-butauone yields 3-methy -l-pentyn-3-ol ... 

Other carbanionic groups, such as acetylide ions, and ions derived from a-methylpyridines have also been used as nucleophiles. A particularly useful nucleophile is the methylsulfinyl carbanion (CH3SOCHJ), the conjugate base of DMSO, since the P-keto sulfoxide produced can easily be reduced to a methyl ketone (p. 549). The methylsulfonyl carbanion (CH3SO2CH2 ), the conjugate base of dimethyl sulfone, behaves similarly, and the product can be similarly reduced. Certain carboxylic esters, acyl halides, and DMF acylate 1,3-dithianes (see 10-10. )2008 Qxj(jatjye hydrolysis with NBS or NCS, a-keto aldehydes or a-... 

Again we need a reagent for an acyl anion synthon (A). Ve find this in the acetylide ion since substituted acetylenes can be hydrated to ketones ... 

Acetylenic alcohols result when acetylides add to ketones and aldehydes (Section 9-7B). Reaction of the acetylide ion with 2-methylpropanal gives one of the groups needed on the triple bond. 

Disconnection of a-hydroxy ketones such as (4) requires acyl anion (5) equivalents. We have already met two of these, the nitro compounds in the last chapter and the acetylenes in Chapter 16. The acetylide ion is satisfaaory here, adduct (6) being hydrated to TM(4) with Hg(II) catalysis. 

In the previous problem, we saw that an acetylide ion can attack a variety of electrophiles. In Chapter 20, we will see that a C=0 bond can also function as an electrophile. Consider the following reaction between an acetylide ion (the nucleophile) and a ketone (the electrophile) ... 

The acetylide ion attacks the ketone, generating an alkoxide ion. After the reaction is complete, a proton source is used to protonate the alkoxide ion. In a synthesis, these two steps must be shown separately, because the acetylide ion will not survive in the presence of H3O+. Using this information, propose a plausible synthesis for allyl alcohol, using acetylene as your only source of carbon atoms ... 

An acetylide ion is another example of a carbon nucleophile that reacts with an aldehyde or a ketone to form a nucleophilic addition product. When the reaction is over, a weak acid (one that will not react with the triple bond, such as the pyridinium ion shown here) is added to the reaction mixture to protonate the alkoxide ion. 

An acetylide ion is used to add a three-carbon fragment to the carbonyl group of the ketone. 

Given the structure or name of an aldehyde or ketone, write an equation for its reaction with the following nucleophiles alcohol, cyanide ion, Grignard reagent or acetylide, hydroxylamine, hydrazine, phenylhydrazine, 2,4-dinitrophenylhydrazine, primary amine, lithium aluminum hydride, and sodium borohydride. 

Treatment of lithium acetylide with a primary alkyl halide (bromide or iodide) or with aldehydes or ketones produces the corresponding monosubstituted acetylenes or propargylic alcohols. Mercuric ion-catalyzed hydration of these furnishes methyl ketones and methyl a-hydroxy ketones, respectively. 

Acetylides are strong bases and are good nucleophiles. So they can add to the carbonyl groups in aldehydes and ketones. The alkoxide ion that is formed during the reaction can be protonated to an alcohol by treating it with aqueous dilute acid. 

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