Ethynyl Carbanions

Alkynes react with haloethenes [38] to yield but-l-en-3-ynes (55-80%), when the reaction is catalysed by Cu(I) and Pd(0) in the presence of a quaternary ammonium salt. The formation of pent-l-en-4-ynes, obtained from the Cu(I)-catalysed reaction of equimolar amounts of alk-l-ynes and allyl halides, has greater applicability and versatility when conducted in the presence of a phase-transfer catalyst [39, 40] although, under strongly basic conditions, 5-arylpent-l-en-4-ynes isomerize. Symmetrical 1,3-diynes are produced by the catalysed dimerization of terminal alkynes in the presence of Pd(0) and a catalytic amount of allyl bromide [41]. No reaction occurs in the absence of the allyl bromide, and an increased amount of the bromide also significantly reduces the yield of the diyne with concomitant formation of an endiyene. The reaction probably involves the initial allylation of the ethnyl carbanion and subsequent displacement of the allyl group by a second ethynyl carbanion on the Pd(0) complex. 

The most convenient method for the preparation of sodium acetylide appears to be by reaction of acetylene with sodium methylsulfinyl carbanion (dimsylsodium). The anion is readily generated by treatment of DMSO with sodium hydride, and the direct introduction of acetylene leads to the reagent. As above, the acetylide may then be employed in the ethynylation reaction. 

Methylsulfinyl carbanion (dimsyl ion) is prepared from 0.10 mole of sodium hydride in 50 ml of dimethyl sulfoxide under a nitrogen atmosphere as described in Chapter 10, Section III. The solution is diluted by the addition of 50 ml of dry THF and a small amount (1-10 mg) of triphenylmethane is added to act as an indicator. (The red color produced by triphenylmethyl carbanion is discharged when the dimsylsodium is consumed.) Acetylene (purified as described in Chapter 14, Section I) is introduced into the system with stirring through a gas inlet tube until the formation of sodium acetylide is complete, as indicated by disappearance of the red color. The gas inlet tube is replaced by a dropping funnel and a solution of 0.10 mole of the substrate in 20 ml of dry THF is added with stirring at room temperature over a period of about 1 hour. In the case of ethynylation of carbonyl compounds (given below), the solution is then cautiously treated with 6 g (0.11 mole) of ammonium chloride. The reaction mixture is then diluted with 500 ml of water, and the aqueous solution is extracted three times with 150-ml portions of ether. The ether solution is dried (sodium sulfate), the ether is removed (rotary evaporator), and the residue is fractionally distilled under reduced pressure to yield the ethynyl alcohol. 

The ethynyl anion provides an example of an sp hybridized carbanion. The high degree of s character in the lone-pair orbital leads to a very large electron affinity for the ethynyl group. Photoelectron spectroscopy indicates an electron binding energy of nearly 70 kcal/mol for the HC=C anion. ... 

Carbanions have been less studied, apart from CH3, 167,168 but included in a more recent set of calculations169 on several carbanions (CH3-, C2H5-, and ethynyl anion) are calculations on C2H3-. For reliable calculations on this type of molecule, diffuse orbitals must be added to the basis set. Several different basis sets were used, but the geometry of the neutral parent molecule was used in some of the calculations. The main aim of this paper was to investigate the electron density and difference densities, electron affinities, and proton affinities. The inversion barrier in the vinyl anion was ca. 142 kJ mol-1 which was in good agreement with that found by Lehn et a/.148 in an earlier calculation. 

Via an amidoalkylation of bis(trimethylsilyl)ethyne (12) with methyl 2-chloro-N-ethoxycarbonylglycinate (57) under the influence of aluminum chloride the corresponding N-(ethoxycarbonyl)-a,a-TMS-ethynyl-glycinate (55) is isolated which is converted by means of lithium-di-isopropylamide (LDA) into a carbanion that reacts with alkylhalide to the substituted glycinate 59. Finally, after alkaline hydrolysis the unprotected a-acetylenic-a-aminoacid (60) (e. g. R = Benzyl a-ethynyl-a-phenyl-alanine is then obtained (Scheme 7). 

Difficulties arise when process (2) is carried out in the presence of mobile protons. This stems from the fact that carbanionic intermediates such as those shown in Scheme 6 are the precursors to the substitution product. Interception of these intermediates by proton traps may prevent the formation of the desired product or at least diminish its yield. Moreover, process (2) proceeds more rapidly in the presence of an aprotic solvent than a protic solvent. This is well illustrated in the synthesis of ethynyl ethers . When methanol is used as the solvent in equation (277), the... 

One final structure in this series is the bis-TMEDA-solvated bis(phenylethynyl)magnesium species characterized as a monomer (111). ° Note the octahedral geometry of the central magnesium with two axial ethynyl ligands. This series of alkynic structures, (106)-(lil). serves to underscore the unpredictability of carbanion crystal structures. The alkynic carbanions have coordination numbers of one, two or three in these complexes. 

In the previous sub-section, the carbon anion was either inherently stable, e.g. the cyanide or ethynyl ions, or arose from the heterolytic cleavage of an organometallic bond, e.g. the Grignard reagent. In this sub-section, we will look at reactions that involve carbanions in which additional stabilisation is provided by a carbonyl group. This is one of the commonest methods of providing such... 

From the mechanistic viewpoint, a remarkable synthesis of perchloro-phenylacetylene consists in the dehydrohalogenation (96) with hydroxide ion of styrenes [94] (R = H, CH3) (p. 328) (M. Ballester and C. Fernandez-Llamazares, unpublished). When R = C1, the reaction occurs only under rather drastic conditions (in glyme/DMSO containing catalytic amounts of 18-crown-6). However, when R = (C6Cl5)2CH, the reaction occurs smoothly at room temperature in aqueous ammonia-THF. Quantitative formation of the red vinyl-substituted carbanion [95] is observed this undergoes moderately slow conversion into the violet ethynyl-substituted ion [97]. Under such reaction conditions, perchlorotoluene is recovered quantitatively. It therefore appears that the high concentration of the carbanion [95] causes the easy chloride-ion elimination to the allenoid intermediate [96], which isomer-izes immediately to the carbanion [97] (97). 

Addition of such carbanions to chloroacetylene results in the introduction of an ethynyl substituent. The reaction consists in addition of PT-generated carbanions to chloroacetylene produced in situ via PT-catalyzed p-elimination of HCl from vinylidene chloride, followed by p-elimination of HCl from the initial adduct. The reaction should be carried out in ethyl ether, which forms a nonexplosive complex with chloroacetylene ... 

Compound 11 is, however, unexpectedly unreactive with Wittig-Horner reagents. Upon heating with the carbanion of ester phosphonates an addition across the allenic bond occurs [14]. In contrast, a slow normal 1,2-addition takes place [14] with the ylide from cyano-methylphosphonate but, unexpectedly, this proceeds with concomitant inversion at the chiral axis as shown in Scheme 3, to give a mixture of 6R or 6S, and (9E)- or (9Z)-isomers 12-15. However, a fast and very clean 1,2-addition occurs with the ethynyl ketone 18 to yield the esters 19 and 20 (Scheme 4). DIB AH reduction of the separated stereoisomers gives the allenic alcohols 21 and 22 in high yield. Mild oxidation to the aldehydes 23 and 24, followed by their condensation with the acetylenic Cio-bis-ylide 25, leads to the stereoisomeric 15,15 -didehydromimulaxanthins 26 and 28, respectively (Schemes 5 and 6). The optically active. 

In a similar way, upon oxidation the lithium stabilized carbanion of an ethynyl compound or the ethynyl compound itself (E 1.4 V/SCE for Fc-C H) leads to a radical that reacts with the surface of carbon as shown in Fignre 3.69. The presence of the Fc group on the surface is ascertained by cyclic voltanunetry and the surface coverage F = 8.1 x 10 mol cm- is equivalent to about 1.8 monolayers [419]. 

Like the pyrrole synthesis, the assembly of 4-methylene-3-oxa-l-azabicyclo[3.1.0]hexanes is likely triggered by the formation of O-vinyl oxime (detected by GLC and NMR). The deprotonation of O-vinyl oxime in a-position relative to the oxime function and the further intermolecular nucleophilic substitution of the vinyloxy group can lead to azirine A (Scheme 1.178). The latter reacts with acetylene (in the form of carbanion) to give acetylenic ethynyl aziri-dine B (the nitrogen analog of the Favorsky reaction), which is added to the third... 

The fonnation of N-ethynyl derivatives testifies that under these conditions, the allenyl carbanion nndergoes prototropic isomerization to N-propargylic (C pj-centered) carbanion. 

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