Acidic alkynyl proton

The other two compounds display no intramolecular Au- Au interactions, although the molecular packing of [Au(C6F5)(PPh2C=CH)] involves an Au- -H contact from the acidic alkynyl proton (Au- -H distance of 3.07 A, C-H-Au angle of 143°) that links the molecules by translation parallel to the a-axis.33... 

The parent dialkyl ethynylphosphonate was first prepared in 1960 by the addition of stoichiometric ethynylmagnesium bromide to the appropriate dialkyl chlorophosphates in THF. ° The product was isolated in low yield (12-25%), presumably because of side reactions involving the relatively acidic alkynyl proton. The yields can be increased slightly (25-35%) by the use of (toxic) dialkyl fluorophosphates. Despite advances in developing methodologies for the elaboration of dialkyl... 

The acidity of alkynyl protons means that alkynyllithium cyclisations are rather trivial to devise - the starting alkyne can be deprotonated with LDA or LiHMDS and cyclised even onto aldehydes. An example from Kende s laboratory demonstrates the use of an alkynyllithium cyclisation in the synthesis of an enediyne 125 64... 

In the same vein, Schmalz has proposed a facile construction of the colchicine skeleton by a rhodium-catalyzed cyclization/cycloaddition cascade [56]. A TMS group has to be introduced on the alkyne moiety of 66 in order to avoid participation of the relatively acidic alkynyl hydrogen atom in undesired proton transfers. The resulting 6,7,7 of 67a and 67b architecture was assembled in a remarkably diastereoselective manner (14 1) and in satisfactory yield (Scheme 30). 

In general, the reaction can be performed only with organometallics of active metals such as lithium, sodium, and potassium, but Grignard reagents abstract protons from a sufficiently acidic C—H bond, as in R—C=C—H —> R—C=C—MgX. This method is best for the preparation of alkynyl Grignard reagents. ... 

Clearly, a large body of diverse evidence indicates that the acid-catalyzed hydration of alkynyl ethers and thioethers proceeds via a rate-determining protonation through a vinyl cation. However, these vinyl cations are unique in that they have a resonance form where the positive charge resides on the... 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

Significant mechanistic insights into the DABCO-catalysed isomerization of y-hydroxy-o ,/3-alkynyl esters to y-oxo-a,p-trans-alkenyl esters have been reported.33 The reaction mechanism involves cumulene formation, protonation with the conjugate acid of the amine, and protonation of the resulting allenol with water. 

The alternative, reaction of a Grignard reagent with an alkynyl ketone, is not acceptable in this case. The acidic terminal alkyne C—H would transfer a proton to the Grignard reagent. 

Aldehydes and ketones have been alkynylated using indium(III) and Hunig s base (PrjNEt) as catalysts.213 IR and NMR evidence support a dual-activation role for indium it is a Lewis acid for the hard electrophile (carbonyl compound), and has sufficient n -coordination ability for a soft nucleophile such as a terminal alkyne. For the latter substrate, the amine then assists proton abstraction. 

Additionally, acetylene itself is a useful two-carbon building block but is not very convenient to handle as it is an explosive gas. Trimethylsilyl acetylene is a distillable liquid that is a convenient substitute for acetylene in reactions involving the lithium derivative as it has only one acidic proton. The synthesis of this alkynyl ketone is an example. Deprotonation with butyl lithium provides the alkynyl lithium that reacted with the alkyl chloride in the presence of iodide as nucleophilic catalyst (see Chapter 17). Removal of the trimethylsilyl group with potassium carbonate in methanol allowed further reaction on the other end of the alkyne. 

Preparation of Derivatives. Enoate derivatives are prepared from the corresponding chiral alcohol by treatment with acry-loyl chloride in the presence of Triethylamine and catalytic 4-Dimethylaminopyridine or the appropriate carboxylic acid chloride and Silveril) Cyanide. Alkynyl ethers are readily available from the potassium alkoxide by treating with Trichloroethylene, in situ dechlorination with n-Butyllithium, and electrophilic trapping. Trapping the intermediate anion with a proton source or lodomethane followed by Lindlar reduction of the alkynyl ether affords the corresponding vinyl and l-(Z)-propenyl ether, respectively, while reduction of the alkynyl ether with Lithium Aluminum Hydride affords the l-( )-propenyl ether. 

The alkynyl-silicon bond is not cleaved readily by protonic acids, but cleavage can be accomplished by Ag+ under mild conditions . For example, treatment of 237 with aqueous ethanolic silver nitrate followed by liberation of the free alkyne from the silver salt by aqueous KCN gave 238 in 80% yield. 

Attachment of two alkynyl groups to a CHj group confers an unusually high acidity on the protons, and as a consequence 1,4-diynes undergo basc-catalysed rearrangement under very mild conditions. Thus, l-phenyl-l,4-pentadiyne (269) gives l-phenyl-l,3-pentadiyne (271) in good yield upon treatment with ethanolic... 

Because of the acidity of the protons of terminal alkynes, they are easily converted into alkynyl... 

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