Phenylacetylene deprotonation

Zn(OTf)2) and amine base (e.g. triethylamine, Hiinig s base) could be readily monitored by observation of the disappearance of the stretch corresponding to the terminal C-H bond of phenylacetylene. Consistent with a reversible metalation, reappearance of the terminal C-H stretch could be observed on stepwise addition of triflic acid. During treatment of phenylacetylene with either amine base or Zn(OTf)2 alone, no evidence of deprotonation was detected. 

The monosubstituted vinylidene complexes are readily deprotonated with a variety of mild bases (e.g., MeO-, C032 ), and this reaction constitutes the most convenient route to ruthenium acetylide complexes. Experimentally the deprotonation is most easily achieved by passing the vinylidene complex through basic alumina. Addition of a noncomplexing acid (e.g., HPF6) to the acetylide results in the reformation of the vinylidene complex [Eq. (66)]. Reaction of 1 and terminal alkynes such as phenylacetylene in methanol followed by the addition of an excess of... 

The p/fa of carbon acids can depend greatly on the solvent (15.8 for phenylacetylene in diethyl ether, 26.5 in DMSO).22 This effect is explained by the extent of ion-pairing or ion aggregation. The equilibrium for the deprotonation of phenylacetylene is shifted to the right when the solvent is changed from cyclohexylamine (pA a = 20.5) to diethyl ether due to increased stability of the ion pair (for the potassium anion, PhCsC K+) or the ion aggregate (PhCsC K+) . 

A soln. of 2.4 eqs. LiBr in THE added slowly to 1.2 eqs. Cul at —60, after 5 min a soln. of 2.4 eqs. ethylmagnesium bromide in THE added dropwise, after stirring for 1 h 1 eq. phenylacetylene in 20 7 /HMPA added, followed after 5 min by 2.4 eqs. trimethylchlorosilane in HMPA, the mixture slowly warmed to room temp., stirred for 5 h then poured into satd. NH4CI soln. containing a little NaCN (E)-2-phenyl-l-(trimethylsilyl)-l-butene. Y 85%. LiBr suppressed deprotonation. F.e. and electrophilic substitution reactions s. S.-S.P. Chou et al., J. Org. Chem. 54, 868-72 (1989). 

As above stated, the reaction steps in the anionic mechanism take place in reverse order than in the cationic mechanism (Fig. 5.8). Thus, in the anionic mechanism the deprotonation of the alkyne by the external base in complex 2 occurs first, followed by the iodide-for-phosphine substitution. The Gibbs energy profile obtained for the copper-free Sonogashira reaction with phenylacetylene (R = H) through the anionic mechanism is shown in Fig. 5.10. 

The importance of the alkali-metal identity is illustrated once more by the reactivity of the homoleptic bimetallic amido bases MMg(N/Pr2)3 (M = Li, Na) towards phenylacetylene [109]. The lighter alkali-metal congener will deprotonate 2 molar equivalents of substrate resulting in a linear dimeric, tetranuclear complex formulated as [(TMEDA)LiMg(C=CPh)2(N/Pr2)]2, while the sodium derivative will deprotonate a single molar equivalent of alkyne to yield a product of formula [NaMg(C=CPh)(NiPr2)2]2 with an inverse crown motif. 

Electrophilic Trapping of Lithium Species. The deprotonation of arene or heteroarene species followed by quench with electrophiles is a general method for the introduction of functional groups in organic molecules. The arene moiety needs to be electron poor (i.e., thiazoles, imidazoles, pyridine (V-oxides, etc.). Possible electrophiles include sources of halogens, carbonyls, or sulfur. Phenylacetylenes can also be deprotonated with LiO/Bu and quenched with various aldehydes. ... 

A different reaction occurs by reacting 454 with phenylacetylene in acetone, affording the deprotonated complex [Ir2 /t-l,8-(NH)2naphth (C=CPh)(CO)2(PPr 3)2]OTf 458. The latter can be protonated to give 456, but can also react with excess of phenylacetylene to give the neutral diiridium(ll) compound [Ir2 /i-l,8-(NH)2naphth (C=CPh)2(CO)2(PPt 3)2]... 

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