Alkynes, addition reactions

In this last section of the chapter we shall leave elimination reactions to look at addition reactions. Alkynes react with some reducing agents stereoselectively to give either the Z double bond or the E double bond. Some of these reactions were described briefly in Chapter 24. 

Not only intrinsic atom-economic nature but also easier availability of the carbon substrate are the important sustainable advantages of the addition reaction. Alkynes are usually commercially available, while corresponding vinyl halides may require separate synthesis. 

Alkynes undergo stoichiometric oxidative reactions with Pd(II). A useful reaction is oxidative carboiiyiation. Two types of the oxidative carbonyla-tion of alkynes are known. The first is a synthesis of the alkynic carbox-ylates 524 by oxidative carbonylation of terminal alkynes using PdCN and CuCh in the presence of a base[469], Dropwise addition of alkynes is recommended as a preparative-scale procedure of this reation in order to minimize the oxidative dimerization of alkynes as a competitive reaction[470]. Also efficient carbonylation of terminal alkynes using PdCU, CuCI and LiCi under CO-O2 (1 I) was reported[471]. The reaction has been applied to the synthesis of the carbapenem intermediate 525[472], The steroidal acetylenic ester 526 formed by this reaction undergoes the hydroarylalion of the triple bond (see Chapter 4, Section 1) with aryl iodide and formic acid to give the lactone 527(473],... 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Table 9 2 summarizes the methods for preparing alkynes Section 9 8 Like alkenes alkynes undergo addition reactions... 

Methylarsine, trifluoromethylarsine, and bis(trifluoromethyl)arsine [371-74-4] C2HAsF, are gases at room temperature all other primary and secondary arsines are liquids or solids. These compounds are extremely sensitive to oxygen, and ia some cases are spontaneously inflammable ia air (45). They readily undergo addition reactions with alkenes (51), alkynes (52), aldehydes (qv) (53), ketones (qv) (54), isocyanates (55), and a2o compounds (56). They also react with diborane (43) and a variety of other Lewis acids. Alkyl haUdes react with primary and secondary arsiaes to yield quaternary arsenic compounds (57). 

Furans, thiophenes and pyrroles have all been obtained by addition of alkynic dienophiles to a variety of other five-membered heterocycles, as illustrated in Scheme 104. As the alkynic moiety provides carbons 3 and 4 of the resulting heterocycle, this synthetic approach provides an attractive way of introducing carbonyl containing substituents at these positions, especially as many of the heterocyclic substrates are readily generated. Such reactions do... 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

Alkynes react when heated with trifluoroacetic acid to give addition products. Mixtures of syn and anti addition products are obtained. Similar addition reactions occur with trifluoromethanesulfonic acid. These reactions are analogous to acid-catalyzed hydration and proceed through a vinyl cation intermediate. 

Alkynes undergo addition reactions with halogens. The reaction has been thoroughly examined from a mechanistic point of view. In the presence of excess halogen, tetrahaloalkanes are formed, but mechanistic studies can be carried out with a limited... 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

Alkynes react with mercuric acetate in acetic acid to give addition products. In the case of 3-hexyne, the product has -stereochemistry, but the Z-isomer is isolated from diphenylacetylene. The kinetics of the addition reaction are first-order in both alkyne and... 

The most common synthetic application of mercury-catalyzed addition to alkynes is the conversion of alkynes to ketones. This reaction is carried out under aqueous acidic conditions, where the addition intermediate undergoes protonation to regenerate Hg. ... 

The classification of hydrocar bons as aliphatic or ar omatic took place in the 1860s when it was aheady apparent that there was something special about benzene, toluene, and their- derivatives. Their molecular- for-mulas (benzene is CgHg, toluene is CyKj ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and alkynes, however, benzene proved to be inert. Benzene does react with Br-2 in the presence of iron(III) bromide as a catalyst, but even then addition isn t observed. Substitution occurs instead ... 

Unsaturated hydrocarbons undergo a variety of reactions. Experimentally, alkenes and alkynes undergo addition reactions, whereas aromatic molecules, such as benzene, undergo substitution reactions instead. Why ... 

Thomson wVY Click Organic Interactive to use a web-based palette to predict products for alkyne addition reactions. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Addition of alkynes to a-alkoxy aldehydes is most favorably performed with the corresponding zinc reagents (Table 12)46. As with Grignard reagents, the chelation-controlled addition of zinc alkynes proceeds with higher diastereoselectivity when diethyl ether rather than tetrahydrofuran is used as reaction solvent. 

The exact behavior and mechanism of electrophilic additions to alkynes is clearly strongly dependent upon the reaction conditions. In a highly polar and strongly acidic but weakly nucleophilic solvent such as trifluoroacetic acid, addition via a vinyl cation intermediate is favored whereas in less polar, more nucleophilic solvents such as acetic acid, a different mechanism prevails. 

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