Ethynyl anions

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. 

Carbon-centred nucleophiles follow the same trend—the table shows that, as size increases from the slender ethynyl anion through primary and secondary organometallics to f-BuMgBr, the axial selectivity drops off correspondingly. 

The ethynyl anion is by far the most stable of the three, and may be formed under quite mild conditions by the addition of a suitable base. To remove the proton from a carbon that is hybridised... 

FIGURE 3.69 Electrografting of ethynyl anions, with the voltammogram of a GC-teC-Fc electrode in CHjClj. (Adapted from Sheridan, M. V. et al. Angew. Chem. Int. Ed. 52,2013 1-5.)... 

Addition of lithiated alkoxy ethynyl anion with chiral Al-sulfinyl imines proceeds with dr >95 5, which can be reversed in the presence of BF3. Excellent diastereoselec-tivity has been reported for zinc-mediated addition of methyl and terminal alkynes to chiral IV-f-butanesulfinyl ketimines (to form 3-amino oxindoles). Zinc-BINOL complexes have been used to achieve enantioselective addition of terminal alkynes to N-(diphenylphosphinoyl)imines (up to 96% cc) and terminal 1,3-diynes to IV-arylimines to trifluoropyruvates (up to 97% yield and 97% ee). ... 

Finally, a simple method, albeit one limited in scope, for 1,4-functionalized compounds is the successive reaction of ethynyl anions with carbonyl compounds (Figure 20.32). The triple bond may be partially or wholly reduced, and the usual transformations are available for the alcohols. 

The synthesis of spiro compounds from ketones and methoxyethynyl propenyl ketone exemplifies some regioselectivities of the Michael addition. The electrophilic triple bond is attacked first, next comes the 1-propenyl group. The conjugated keto group is usually least reactive. The ethynyl starting material has been obtained from the addition of the methoxyethynyl anion to the carbonyl group of crotonaldehyde (G. Stork, 1962 B, 1964A). 

The 17a-ethynyl compound (59) has been prepared in 88% yield from estr-4-ene-3,17-dione (58) and acetylene, at 2-3 atm pressure in tetrahydro-furan in the presence of potassium t-butoxide. Presumably the A-ring enone system is protected as the enolate anion during the course of the reaction. 

Disubstituted isotellurazoles 1 (4-11%) and bis((3-acylvinyl)tellurides 3 (3-10%) were isolated in very low yields from the reaction mixture as the products of nucleophilic addition of telluride anion to the triple bond of the initial ethynyl ketones (83S824). This method cannot be applied to the synthesis of 3//-isotellurazoles. When a-acetylenic aldehydes were used instead of ethynyl ketones, bis((3-cyanovinyl)tellurides 4 obtained in 14-20% yields were the only products (83S824). 

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. 

Cyclisation of o-hydroxyphenyl ethynyl ketones under basic conditions is known to produce benzopyran-4-ones and benzofuranones by 6-endo-dig and 5-exo-dig processes, respectively. However, both cyclisations are reversible in aprotic media thereby generating anions, of which that derived from the pyranone is rapidly and irreversibly protonated and hence selective formation of the chromone results <96T9427>. 

In contrasi to many acetylenes RCsCH, chloroacetylene can be successfully coupled with ketones in liquid ammonia via the lithium compound [80,85], The excellent yield in the reaction with acetone indicates that practically no formation of enolate occurs. Similar good results have been obtained with lithiated ethynyl thioethers, (LiCsCSR), lithiated enyne thioethers, (LiCsCCH=CHSR), lithiated 1,3-diynes (RC=CC=CLi), and lithiated aiylacetylenes (LiCsCAryl)[2], A possible explanation for the small extent of enolization of the ketone is that all these acetylides are less basic due to some stabilization of the anion. 

Alkynyl(phenyl)iodonium salts can be used for the preparation of substituted alkynes by the reaction with carbon nucleophiles. The parent ethynyliodonium tetrafluoroborate 124 reacts with various enolates of /J-dicarbonyl compounds 123 to give the respective alkynylated products 125 in a high yield (Scheme 51) [109]. The anion of nitrocyclohexane can also be ethynylated under these conditions. A similar alkynylation of 2-methyl-1,3-cyclopentanedione by ethynyliodonium salt 124 was applied in the key step of the synthesis of chiral methylene lactones [110]. 

Monosubstitution of acetylene itself to prepare terminal alkynes is not easy. Therefore, trimethylsilylacetylene (134) is used as a protected acetylene. After the coupling, the silyl group is removed by the treatment with fluoride anion. The hexasubstitution of hexabromobenzene (135) with 134 afforded hexaethynylbenzene (136) after desilylation in total yield of 28%. The product was converted to tris(benzocyclobutadieno)benzene (137) using a Co catalyst (see Section 7.2.1). Hexabutadiynylbenzene was prepared similarly [60], As another method, terminal alkynes 139 are prepared in excellent yields by the coupling of commercially available ethynyl Grignard (138) or ethynylzinc bromide with halides, without protection and deprotection [61]. 

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