Alkynes alkynyl anions

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

LDA is a strong base. Two E2 eliminations give an alkyne, which is deprotonated by the excess LDA to give an alkynyl anion. This species then reacts with Mel by an Sjsj-2 process. 

R = H, Ph) have been obtained from the anionic [Ru(N4Meg)] and ethyne or HC CPh, respectively [51], In this case, formation of an intermediate r -alkyne complex is unlikely, the probable mechanism being deprotonation of the alkyne and coordination of the alkynyl anion followed by proton transfer. 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles... 

The converse of this idea is central to a useful bit of chemoselectivity that can be obtained in the reactions of dianions. 1-Propynol can be deprotonated twice by strong bases—first, at the hydroxyl group to make an alkoxide anion (the pIQ of the OH group is about 16) and, secondly, at the alkyne (pKa of the order of 25) to make a reacts with electrophiles it always reacts at the alkynyl anion and not at the alkoxide. 

Making alkenes by addition to alkynes offers two distinct advantages. Firstly, although the reaction is not connective in the sense that the Wittig and Julia reactions are, the starting materials can often be made straightforwardly by alkylation of alkynyl anions. Secondly, the same alkyne can be used to make either E- or Z-alkene—an advantage shared with the Peterson reaction but here the... 

A filled sp orbital is lower in energy than filled sp or sp orbitals since it is closer to the positively charged nucleus. This imparts sufficiently greater acidity to acetylene and 1-alkynes (pA a 24-26) so that bases such as alkyllithiums, lithium dialkylamides, sodium amide in liquid ammonia, and ethylmagnesium bromide may be used to generate the alkynyl anions (see Section 8.2). 

The alkynyl anion can act as a nucleophile. Reaction with primary alkyl halides will lead to alkylation (by an SN2 mechanism) and the introduction of an alkyl group on the terminal carbon atom of the alkyne. 

Preparation of Alkynes by Alkylation of Alkynyl Anions The other major alkyne preparation is based on the easy accessibility of nucleophilic carbanions from terminal alkynes (Section 13-2). So, a wide variety of internal alkynes may be made from any terminal alkyne, via the sehenie... 

How will this aldehyde react with nucleophiles such as lithiated alkynes Consider a Felkin— Anh transition state again, we know that the substituted nitrogen atom, being electronegative and bulky, will lie perpendicular to the carbonyl group in the most reactive conformation. Looking at the two alternatives shown below, it s easy to see that the one on the right allows unhindered attack, and in the synthesis an alkynyl anion was used to make the product shown. 

This property is useful, because strong bases such as sodium amide in liquid ammonia, alkylUthiums, and Grignard reagents can deprotonate terminal alkynes to the corresponding alkynyl anions. These species react as bases and nucleophiles, much like other carbanions (Section 13-5). 

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

Preparation of Alkynes from Alkynyl Anions CHAPTER 13... 

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