Additions to alkynes

Alkynes undergo 1,3-dipolar addition reactions with organic azides to form triazolesCommonly, hydrazoic acid additions to alkynes also give triazoles and this aspect of their chemistry has been 

Ethoxyacetylene possesses a strong electron-releasing substituent a- to the triple bond and as a result hydrazoic acid is able to react by an Ad mechanism (equation 85). The intermediate monozizide 

Electron-withdraw al by the carbonyl groups cannot be the only factor determining the nature of the product (188) since a triazole is 

The ruthenium-catalyzed addition of diazo compounds to alkynes has led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoalkane and one of alkyne [106] (Eqs. 82,83). The stereoselective formation of these conjugated dienes results from the selective creation of two C=C double bonds rather than leading to the cyclopropene derivative. This is expected to be due to the possibility for the C5Me5RuCl moiety to accomodate two cis carbene ligands. 

This reaction applied to enynes allowed the one-step selective synthesis of alkenylbicyclo[3.1.0] hexane derivatives by addition of one equivalent of diazoalkane to enyne. This novel reaction involves the stereoselective formation of 

This reaction manifold has been heavily exploited in the assembly of indoles and benzofurans [2]. For example, Taylor and McKillop reported in 1985 that ortho-alkynyl substituted anilines 1 can undergo cyclization in the presence of PdCl2 catalysts to 

R = aryl, alkyl R = aryl, alkyl R = aryl, alkyl, CH20Bn R = H, Ph R = Me, COOMe, H R = H, Me NuH = alcohol, phenol, dIketone 

Michael addition of the nudeophile to initiate the palladium-catalyzed cydization similar to that noted above, followed by allyl coupling. 

With substituents on the alkyne that can stabilize a non-bridging carbocation, mixed stereochemistry is found in the products. The reaction of 1-phenylpropyne with Br2 in acetic acid gives the products shown in Eq. 10.23. The reaction obeys a kinetic expression such as Eq. 10.21 ([alkene] replaced by [alkyne]), and a p value of-5.17 for para substituents on the phenyl ring was found. The data support the intermediacy of a vinyl cation (see margin) that can combine with both bromide or solvent to form either E or Z products. 

Interestingly, the kinetic term in the reaction of 1-phenylpropyne with Br2 that has a Bra dependence is only observed when bromide anion is added. Furthermore, the reaction that corresponds to this term gives solely the anti addition product. This supports a concerted mechanism with nucleophilic attack by bromide simultaneous with electrophilic addition of bromine (Eq. 10.24). 

Most alkene reactions also work as reactions of one or both of the two n bonds of an alkyne. 

R —H dark, cold no peroxides / R H vinyl halide dark, cold no peroxides / R H geminal dihalide 

Fill in the blank in the title of Synthetic Transformation 11.1 with the words Markovnikov or 

Label the vinyl halides in Model 1 cis, trans, or neither, as appropriate. (A vinyl group is any group directly attached to a C=C double bond, so a vinyl bromide has a Br attached to a C=C.) 

What evidence is there in Model 1 that Synth. Transf. 11.2 proceeds via a cyclic intermediate such as a bromonium ion  

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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]. 

For further discussion of this topic see the article The Electrophilic Addition to Alkynes in the November 1993 edition of theVourna/ of Chemical Education (p 873) Additional common tary appeared in the Novem ber 1996 issue... 

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 ... 

The three basic mechanisms that have been considered to be involved in electrophilic additions to alkynes are shown below. The first involves a discrete vinyl cation. In general, it can lead to either of the two stereoisomeric addition products. The second mechanism is a termolecular process which would be expected to lead to stereospecific anti addition. The... 

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 cyanobenzylpiperidine 94 acts as an effective benzoyl anion equivalent for addition to alkynes. As shown, the initial adduct 95 is readily cleaved with copper sulfate to give the ketone product 96 (93LA375). 

Sulfonyl bromides and iodides react similarly217-218-225 copper-salt catalysis in these cases facilitates the additions but is not absolutely necessary however, it influences the stereochemistry of the additions. Addition of sulfonyl iodides226 as well as the uncatalyzed thermal addition of sulfonyl bromides227 to alkynes leads to an exclusive trans-addition, whereas CuBr2 catalysis in the latter case causes the formation of cis-addition products to some extent (11 16%) correspondingly, copper-salt catalysis in sulfonyl chloride additions to alkynes leads to the formation of a mixture of Z,E-isomers228-229 (equation 40). 

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. 

When 1,3-dienes remain in an anti orientation, inhibiting a [l,3]-shift of the first formed vinylphosphirane, a second phosphinidene addition can occur to give bisphosphiranes 35 [80]. Bis- and trisphosphirenes (36-38) result from addition to alkyne substituted phosphirenes (39) [81,82]. 

As in the P(III) chemistry above, both late metal (Pd) and lanthanide catalysts have been used for P(V)-H additions to alkynes, alkenes, aldehydes, and imines. In addition, titanium, aluminum, and zinc catalysts have been employed. Typical P(V) substrates include dialkyl phosphites P(0R)2(0)H and phosphine oxides PR2(0)H. 

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