Alkynes formation

Chlorobenzenes activated by coordination of Cr(CO)3 react with terminal alkynes[253). The 1-bromo-1,2-alkadiene 346 reacts with a terminal alkyne to afford the alka-l,2-dien-4-yne 347[254], Enol tritlates are used for the coupling with terminal alkynes. Formation of 348 in the syntheses of ginkgolide[255] and of vitamin D are examples[256] Aryl and alkenyl fluorides are inert. Only bromide or iodide is attacked when the fluoroiodoalkene 349 or fluoroiodoar-ene is subjected to the Pd-catalyzed coupling with alkynes[257-259]. 

No intennolecular reaction of malonate or /3-keto esters with halides has been reported, but the intramolecular reaction of /3-diketones such as 790 and malonates proceeds smoothly[652,653]. Even the simple ketone 791 can be arylated or alkenylated intramolecularly. In this reaction, slow addition of a base is important to prevent alkyne formation from the vinyl iodide by elim-ination[654]. 

Particularly interesting are the results obtained with the phosphonium ylides including an acyl rest derived from aminoacid if the N-H bond reactivity is blocked by an amide protection, the alkyne formation takes place [25,27], but if the N-H bond is not deactivated, an intramolecular cyclization occurs to give a new stabilized ylide [27,28]. 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Resin-bound 4/7-1,3-oxazines 115 were synthetized by the stepwise condensation of an amide resin 489, an aldehyde, and an alkyne. Formation of the oxazine ring took place in the presence of the catalyst BF3-Et20 via a hetero-Diels-Alder cycloaddition of the alkyne and the acyliminium 491 arising from the condensation of the amide and the aldehyde (Scheme 92). The quantitative efficacy of the process was determined by elemental analysis of a model system bearing a bromine atom on the aldehyde moiety (R =C6H4Br(p)), which indicated a 78% conversion for the heterocyclization <2001CEJ2318, 2004JC0846>. 

In some cases, such a cyclization is accompanied by alkyne formation due to 1,2-migration of a substituent in the intermediate alkylidenecarbene [46,48,49]. 

Sn2 mechanism (Scheme 47) [138,139]. Concerted,in-plane nucleophilic substitutions of this type are facilitated by the exceptional nucleofugality of iodoben-zene, and appear to be operative with weakly basic nucleophiles. Similar reactions of ( )-/ phenylvinyl(phenyl)iodonium tetrafluoroborate are slower, less stereoselective, and more prone to competitive alkyne formation. 

When silylene transfer was attempted to alkynes substituted with halides or sulfones, however, silacyclopropene formation was not observed.101 Instead, acetylenic silanes 113 were observed (Scheme 7.32). Treatment of silacyclopropene 112c (or 112d) with acetophenone and substoichiometric amounts of Cul also induced alkyne formation. 

Ag-catalyzed silyl-substituted alkyne formation mechanism... 

On the basis of crossover experiments, Woerpel and Clark attributed the formation of alkynes to an intramolecular rearrangement mechanism (Scheme 7.33).101 For silver-promoted alkyne formation, insertion of the acetylene into the Ag-Si bond produced 116. Then (3 elimination of bromide occurred more rapidly than silacy-clopropene formation to generate silyl-substituted alkyne 117. Substitution of L with bromide then afforded the observed product (113a). 

Once again (cf equation 205), the behavior of tetrabutylammonium fluoride (TBAF) is exceptional. Thus, the treatment of (E)-1 -decenyl(phenyl)iodonium tetrafluoroborate with ten equivalents of TBAF in acetonitrile leads exclusively to 1-decyne (equation 212)146. Furthermore, the reactions of the a- and /1-deuterio isotopomers of the decenyliodonium salt with one equivalent of TBAF in dichloromethane are clearly consistent with alkyne formation via the a-elimination-alkylidenecarbene pathway (equations 213 and 214)146. 

Although alkyne formation from / ,/ -disubstituted vinyliodonium salts via the a-elimination-alkylidenecarbene pathway is expected, the generation of 1-decyne from ( )-l-decenyl(phenyl)iodonium fluoroborate (equation 230) might also occur via syn -elimination. That a-elimination is dominant, even in this case, has been demonstrated by treatment of the a- and / -deuterio-l-decenyl isotopomers with triethylamine (equations 233 and 234)128. 

Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

Various cyclopropenone derivatives react with peracids to yield the corresponding alkynes as primary products. However, the alkynes are themselves oxidized under the reaction conditions and the reaction is impractical as an alkyne synthesis. Two possible modes of alkyne formation are depicted in equation 97" ". Reaction of diphenylcyclopropenone with singlet oxygen or potassium permanganate gives" " desoxybenzoin. 

Reaction of terminal alkynes formation of l,2-alkadien-4-ynes. . . ... 

Reactions of Terminal Alkynes Formation of l,2-Alkadien-4-ynes... 

Addition of thiols to alkenes/alkynes. Formation of 2,3-dihydro-1,4-oxathiins by cycloaddition and alkenyl suffides by simple addition is catalyzed by Mn(0Ac)3-2H20. 

Elimination reactions can also be used to prepare alkynes. Dihalides can be converted into alkynes by treatment with base, provided that the two halogen atoms are on the same, or adjacent, carbon atoms. In general, alkyne formation requires more drastic reaction conditions than alkene formation, and a strong base, such as sodium amide, is usually employed (Reactions 5.4 and 5.5). The product of Reaction 5.5, phenylethyne, is shown in Figure 5.1 as a ball and stick model. 

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