To alkyne

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]. 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Diaryl disulfides and diselenides add to alkynes to afford the (Z)-l, 2-bis(ar-ylthio)alkenes 193 and (Z)-l,2-bis(arylseleno)alkenes 194. Under CO pressure, carbonylative addition takes place to give thio esters and the selenoketones 195[I07], The selenoketones are converted into the /J-seleno-a, 3-unsaturated aldehydes 196 by Pd-catalyzed hydrogenolysis with HSnBu3[108,109],... 

Addition of a hydroxy group to alkynes to form enol ethers is possible with Pd(II). Enol ether formation and its hydrolysis mean the hydration of alkynes to ketones. The 5-hydroxyalkyne 249 was converted into the cyclic enol ether 250[124], Stereoselective enol ether formation was applied to the synthesis of prostacyclin[131]. Treatment of the 4-alkynol 251 with a stoichiometric amount of PdCl2, followed by hydrogenolysis with formic acid, gives the cyclic enol ether 253. Alkoxypalladation to give 252 is trans addition, because the Z E ratio of the alkene 253 was 33 1. 

Amines undergo aminopalladation to alkynes. The intramolecular addition of amines to alkynes yields cyclic imines. The 3-alkynylamine 273 was cyclized to the 1-pyrroline 274, and the 5-alkynylamine 275 was converted into the 2,3,4,5-tetrahydropyridine 276[137]. Cyclization of o-(l-hexynyl)aniline (277)... 

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

Alkynes react with many of the same electrophilic reagents that add to the carbon-carbon double bond of alkenes Hydrogen halides for example add to alkynes to form alkenyl halides... 

Evidence from a variety of sources however indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations and their involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

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... 

Furthermore kinetic studies reveal that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third order overall and second order in hydrogen halide... 

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

The addition of nitronic esters to alkynes to produce aziridines was postulated to proceed through a 4-isoxazoline as one of the intermediates (Scheme 132). A biradical intermediate (492) was also included in the mechanistic pathway for the reaction (77JA6667). 

Few isothiazoles undergo simple cycloaddition reactions. 4-Nitroisothiazoles add to alkynes (see Section 4.17.7.4). With 5-thiones (84) and dimethyl acetylenedicarboxylate, addition to both sulfur atoms leads to 1,3-dithioles (85) (77SST(4)339, 80H(14)785, 81H(16)156, 81H(16)595). Isothiazol-3-one 1-oxide and the corresponding 1,1-dioxide give normal adducts with cyclopentadiene and anthracene (80MI41700), and saccharin forms simple 1 1 or 1 2 adducts with dimethyl acetylenedicarboxylate (72IJC(B)881). 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

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 hexabutyldistannane used in this reaction is not involved in the propagation sequence but may be involved in initiation or scavenging of potential chain-termination radicals. Intermolecular additions of alkyl radicals to alkynes have also been observed. 

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