Alkynes, reaction with strong bases

A terminal alkyne is an alkyne that has a hydrogen substituents (Fig. H). This hydrogen substituent is acidic and can be removed with strong base (e.g. sodium amide NaNH2) to produce an alkynide (Fig. I). This is an acid-base reaction. 

Alkynes also undergo a reaction that has no analogy in alkene chemistry. Because a C - H bond of an alkyne is more acidic than a C - H bond in an alkene or an alkane, alkynes are readily deprotonated with strong base. The resulting nucleophiles react with electrophiles to form new carbon-carbon o bonds, so that complex molecules can be prepared from simple starting materials. The study of alkynes thus affords an opportunity to learn more about organic synthesis. 

Butyne is isomerized to 1-butyne by treatment with strong base, (a) Write a stepwise mechanism for this process, (b) Explain why a more stable internal alkyne can be isomerized to a less stable terminal alkyne under these reaction conditions. 

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... 

Many of the methods that have been reported for coupling alkyl electrophiles with terminal alkynes require a strong base and therefore have limited functional-group compatibility [35]. In contrast, the Sonogashira reaction, which is widely used to cross-couple aryl and vinyl electrophiles with terminal alkynes, employs a mild base (e.g., an amine). Expanding the scope of Sonogashira couplings to include alkyl electrophiles would be a welcome advance in alkyne chemistry. 

Some chiral polyhydroxyalkynes have been prepared by treating 2-alkoxy-l-chlorosugar derivatives with strong bases (UNH2/NH3 or LDA) the conversion of (89), derived from D-xylose, into alkyne (90) is typical.80 Periodate cleavage of 3,4-O-isopropylidene-D-mannitol, followed by Wadsworth-Emmons reaction, gave diene diester (91). 1... 

Alkynes can also be prepared from other alkynes. The reaction of terminal alkynyl anions with alkylating agents, snch as primary haloalkanes, oxacyclopropanes, aldehydes, or ketones, results in carbon-carbon bond formation. As we know (Section 13-2), such anions are readily prepared from terminal alkynes by deprotonation with strong bases (mostly alkyllithium reagents, sodinm amide in liqnid ammonia, or (jrignard reagents). Alkylation... 

Unlike haloalkanes, alkenyl halides are relatively unreactive toward nucleophiles. Although we have seen that, with strong bases, alkenyl halides undergo elimination reactions to give alkynes, they do not react with weak bases and relatively nonbasic nucleophiles, such as iodide. Similarly, SnI reactions do not normally take place, because the intermediate alkenyl cations are species of high energy. 

In the remainder of this chapter, particular reactions are selected for examination of their synthetic potential. Acetylide ions are useful for linking carhon chains, particularly where a double bond is desired with stereoselectivity. Acetylene and 1-alkynes may be deprotonated with strong bases such as LDA and then treated with alkyl halides or carbonyl compounds. Preformed lithium acetylide complexed with ethylenediamine is available as a dry powder. Several alkynes derived from acetylide and carbon dioxide or formaldehyde are available, including propargyl alcohol (HC CCHjOH), propargyl bromide (HC CCH Br), and methyl propio-late (HC=CC02CH3). 

Double dehydrohalogenation (Section 9 7) Reaction in which a geminal dihahde or vicinal dihahde on being treated with a very strong base such as sodium amide is converted to an alkyne by loss of two protons and the two halogen substituents... 

One way to generate carbanions is to combine an acidic molecule with one equivalent of a very strong base, such as n-butyl lithium (n-BuLi). For example, reaction of the alkyne shown below with n-BuLi leads to a carbanion of formula CsH, 02 , which then undergoes an Sn2 reaction with n-propyl bromide (n-PrBr),... 

Based on the extraordinary selectivity in hydrosilylation reactions when an alkyne competes with other groups for a silicon-bonded active hydrogen, further derivatisation can be carried out. The hydrosilylation of 2-methyl-3-butynol, which works very well with polymeric siloxanes, gives hydroxyal-kenylsilicon compounds - a l-silylalkenyl/2-silylalkenyl mixture from cis-addition across the triple bond. Elimination of water from the tert. alcohol produced, catalyzed by traces of a strong acid, results in isoprenylic siloxanes in more than 90 % overall yield (Eq. 8). 

The 1 1 complexes of alkynylgold compounds (L)AuC=CR with strong donors L can also be conveniently prepared from the corresponding gold halides (L)AuX (X = C1, Br, I), an alkyne HC=CR, and a base. For many representative examples, the reactions with an alkali alkanolate in an alcohol58 or with triethylamine in dichloro-methane are the methods of choice (Equation (13)).59... 

A characteristic and synthetically important reaction of ethyne and 1-alkynes is salt ( acetylide ) formation with very strong bases. In such reactions the alkynes behave as acids in the sense that they give up protons to suitably strong bases ... 

Some organometallic compounds are prepared best by the reaction of a strong base or an alkyl metal derivative with an acidic hydrocarbon, such as an alkyne ... 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

Fig. I. Reaction of a terminal alkyne with a strong base.

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