Propyne, 3- anion

The alkylation reaction is limited to the use of primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane. 

A The alkynyl hydrogen of propyne is acidic and hence the propyne will form an organometallic derivative. The Grignard derivative will react with carbon dioxide to give the carboxylic acid. Cis alkenes are formed by the catalytic hydrogenation of alkynes using a Lindlar catalyst. Hence a propynyl alcohol is a precursor to compound (b). This could be obtained by the addition of a propyne anion to propanone ... 

Active Figure 8.6 MECHANISM A mechanism for the alkylation reaction of acetylide anion with bromomethane to give propyne. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Another modification of the deprotonation/isomerization sequence starts with easily accessible 1-thio-substituted 1-propynes 303. Their deprotonation at the y-position generates allenyl anions that could be trapped regioselectively by different electrophiles R2X (Scheme 8.81) [167-169]. The resulting C-l-substituted allenyl sulfides 304 were obtained in high yields. 

Conversion of a terminal alkyne to its alkynylsilane prevents loss of the relatively acidic terminal hydrogen (pKa of ethyne c. 25) during later synthetic steps. For example, the terminal hydrogen of propyne was masked whilst its propargylic anion was used in a synthesis of Cecropia juvenile hormone, a chemical which plays ail important role in insect development (Figure Si5.2). 

A mechanism for the ilkylation reaction of acetylide anion with bromomethane to give propyne. 

The nucleophilic acetylide ion uses an electron pair to attack the positively, polarized, electrophilic carbon atom of bromomethane. As the new C-C bond forms, Br" departs, taking with it the electron pair from the former C-Br bond and yielding propyne as product. We call such a reaction an alkyla-l tion because a new alkyl group has become attached to the starting alkyne. Alkyne alkylation is not limited to acetylene itself, Any terminal alkyne s can be converted into its corresponding anion and then alkylated by treat-j ment with an alkyl halide, yielding an internal alkyne. For example, con/ version of 1-hexyne into its anion, followed by reaction with 1-bromobutane,] yields 5-decyne ... 

The essential feature of the catalyst systems is that they are formed by the combination of a palladium(II) species with a ligand containing a 2-pyridylphosphine moiety and a proton source containing anions weakly coordinating to palladium [8]. These catalysts are very efficient for the conversion of propyne (for example) as the alkyne and methanol as the nucleophilic co-reagent. 

The catalytically relevant palladium species in propyne carbonylation is thought to be a d square-planar cationic palladium(II) complex of Structure 1 [8], which is symbolized by [L2PdX]X, where L stands for the 2-pyridylphosphine ligand and X for the anion it is thought that complexes of the type 1 are assembled in situ from the three components added in the catalytic experiments (eq. (3)) [9, 10]. 

The specific generation of carbanions in the gas phase by proton abstraction is associated with several problems. In the first place, even simple organic hydrocarbons may have more than one acidic site, and deprotonation may therefore give rise to a mixture of isomers. For example, proton abstraction by OH - or NH2 from propyne 310 generates both the propargyl (311) and the methylacetylide anion 312150 (reaction 121). 

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