Deprotonation of terminal alkynes

The reactivity of the ultrasonically formed butyllithiums was also examined. Thus, sec-BuLi was used to deprotonate a phosphonium salt and the resulting ylide used in the standard Wittig reaction. Similarly, ortho-lithiation of anisole could be effected in a one-pot process (Scheme 22). However, deprotonation of terminal alkynes or 1,3-dithianes requires use of lithium containing at least 2% sodium, in contrast to the previous examples, which were all carried out using low-sodium content lithium wire (<0.02%). There is no satisfactory explanation for this observation as yet. 

There are a number of procedures for coupling of terminal alkynes with halides and sulfonates, a reaction that is known as the Sonogashira reaction.161 A combination of Pd(PPh3)4 and Cu(I) effects coupling of terminal alkynes with vinyl or aryl halides.162 The reaction can be carried out directly with the alkyne, using amines for deprotonation. The alkyne is presumably converted to the copper acetylide, and the halide reacts with Pd(0) by oxidative addition. Transfer of the acetylide group to Pd results in reductive elimination and formation of the observed product. 

In 1999, Carreira identified Zn(II) as a metal that, like Ag(I) and Cu(I), is capable of effecting the metalation of terminal acetylenes under mild conditions. Thus, treatment of terminal alkynes with Zn(OTf)2 and NEt3 at room temperature led to the formation of zinc alkynylides (Eq. 4). The zinc salt and the amine base work in synergy to weaken the acetylenic proton, with the acetylene undergoing complexation to the Zn(II) center and the base effecting subsequent deprotonation (Fig. 1) [11]. 

Addition of internal alkynes to (t)5-C5H5)(PR3)2RuCI does not lead to the formation of the corresponding disubstituted vinylidene (68). The failure of this reaction could reflect the relative difficulty of a 1,2-alkyl shift for internal alkynes as compared to the 1,2-proton shift for the successful rearrangement of terminal alkynes (Scheme 9). Alternatively, if the deprotonation-reprotonation route is important in the rearrangement of terminal alkynes (vide supra), then clearly internal alkynes would not undergo a similar isomerization. 

The electron-rich acetylide anion produced from deprotonation of 1-alkynes with Na in NH3 (liq.) is reluctant to accept an electron, allowing the selective reduction of an internal triple bond in the presence of a terminal one. Reduction to the corresponding 1-alkenes can be achieved in the presence of ammonium sulfate. ... 

A more general method for arylation of terminal alkynes as well as electron-deficient alkynes is the Negishi Pd-catalyzed cross-coupling of aryl halides with alkynyl-zinc reagents. When using functionally substituted alkynylzincs, the deprotonation of 1-alkynes must be done with LDA instead of alkylithiums. 

Technically, XMgC=CCH3 is a Grignard reagent because it is an organometallic compound of magnesium. However, it is not made in the usual fashion it is made by deprotonating the terminal alkyne as shown. 

Since sp-hybridized carbons are relatively acidic (pKa=25), strong bases can be used to deprotonate a terminal alkyne. If a Grignard reagent is used as the base, then the resulting product is drawn as a Grignard rather than an anion (this essentially sacrifices a more reactive, cheaper Grignard in the production of a new, more stable one— the desired alkynyl Grignard). 

PRACTICE the skill 10.5 In each of the following cases, determine if the base is sufficiently strong to deprotonate the terminal alkyne. That is, determine whether the equilibrium favors formation of the alkynide ion. 

In total, three equivalents of the amide ion are required two equivalents for the two E2 reactions, and one equivalent to deprotonate the terminal alkyne and form the alkynide ion. After the alkynide ion has formed and the reaction is complete, a proton source can be... 

Identify which of the following bases can be used to deprotonate a terminal alkyne ... 

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