Alkyne anions from alkynes

Halogenoalkynes are also formed from alkynes and aqueous solutions of hypohalites. In these reactions, alkynylide anions are transient intermediates. The conversions give good results only when the acetylene has a low pK (e.g. aryl-feCH and ROCCsCH) or good solubility in water (e.g..HC-CCXCHj OH) [122], The hypohalite method of Strauss [123] has most applications in the synthesis of bromoacetylenes. 

The amide ions are powerful bases and may be used (i) to dehydrohalogenate halo-compounds to alkenes and alkynes, and (ii) to generate reactive anions from terminal acetylenes, and compounds having reactive a-hydrogens (e.g. carbonyl compounds, nitriles, 2-alkylpyridines, etc.) these anions may then be used in a variety of synthetic procedures, e.g. alkylations, reactions with carbonyl components, etc. A further use of the metal amides in liquid ammonia is the formation of other important bases such as sodium triphenylmethide (from sodamide and triphenylmethane). 

After addition of the alkyne anion to the alkylborane, iodination facilitates alkyl group migration from boron to carbon in a transfer that resembles the one seen in the synthesis of (Z)-alkenes described in Section B4.1. Elimination to give the product alkyne occurs under the iodination reaction conditions (Figure B4.4). 

The first alkyne cyclisations, from 377, 379 and 381, predate the early alkene cyclisations by a couple of years these three date from 1966173 and 1967,174 and illustrate the favourability of both exo and endo-dig cyclisation. All three generate benzylic vinyllithiums (378, 380 and 382), and both aryl (377, 379) and alkyl halides (381) are successful starting materials. Similar organomagnesium cyclisations were described at about the same time.175 However, it is not clear in these reactions how much of the product is due to participation of radicals in the mechanism - alkylbromides undergo halogen-metal exchange with alkyllithiums via radical intermediates (chapter 3).176 If it really is an anionic cyclisation, cyclisation to 378 is remarkable in being endo. Endo-dig anionic cyclisations are discussed below. 

When sodium hydroxide was used as nucleophile, salt 3c was rapidly converted to alkyne 63. This alkyne now features only a single stereocenter, and this is evident from its spectra. Its structure was initially assigned incorrectly, but the alkyne structure (63) has been rigorously established by X-ray crystallography. It is curious that azide and hydroxide should attack on the periphery of the TTF ring system, while the bulkier malonate anion attacks internal carbons. It is also intriguing that none of the observed modes of attack give rise to the sulfur ylid 64, which should be more stable than 61. 

The substitution reactions of alkyl halides by carbon nucleophiles derived from alkynes and enolate anions provide major methods for the... 

Formation of acetylide anions from terminal alkynes (11 6B)... 

As will be indicated later, 7-rings can be made from alkynes. However, the carb-anion process of (69) is a rare example in which competition between 6- and 7-ring closure is present—only the 6-ring forms . 

Anions from the stronger carbon acids are accessible and give expected products (see Table 22, p. 897). Invariably, the alkyne carries an activating substituent. Cyclic products form when possible (the additions of the azallyl anions of equation (184)... 

The benzoate ion is the product under the reaction conditions but the alkyne anion collects a oton from a water molecule, regenerating the second hydroxide ion, which therefore is a base... 

Recent evidence for the formation of hemiketal intermediates iqmn acylation of alkynides has been obtained from glucopyranolactones. Treatment of tetrabenzyl (55) with the anion from l-benzyloxy-3-bu-tyne gave a quantitative yield of hemiketal (56) which showed IR absorptions for the OH and alkyne portions of the molecule (X = 3350 cm" and 2250 cnr, respectively). This compound was stereospecifi-cally reduced to the C-glycoside (57) with triethylsilane/BFs-etherate (overall yield 72% Scheme 18). None of the other stereoisomer or ring-opened product was obtained. ... 

As noted above, alkyne anions are very useful in Sn2 reactions with alkyl halides, and in acyl addition reactions to a carbonyl.46 Alkyl halides and sulfonate esters (tosylates and mesylates primarily) serve as electrophilic substrates for acetylides. A simple example is taken from Kaiser s synthesis of niphatoxin B, in which propargyl alcohol (36) is treated with butyllithium and then the OTHP derivative of 8-bromo-1-octanol to give a 47% yield of 37.48... 

The first proton to be removed by base will be from the alcohol and this will need a reasonably strong base such as NaH. Removal of the alkyne proton requires a much stronger base such as BuLi. You might represent the product as an alkyne anion or a covalently bonded alkyllithium. 

Triazole Derivatives. Triazole derivatives are known to possess tumor necrosis factor-a (TNF-a) production inhibitor activity. The synthesis of triazole derivatives can be achieved from alkynes or diynes by a tandem cascade reaction involving 1,3-dipolar cycloaddition, anionic cyclization and sigmatropic rearrangement on reaction with sodium azide. Some of the benzoyl triazole derivatives were considered to be potent local anaesthetics and are comparable with Lidocaine. The triazoles can also be prepared from benzoyl acetylenes,triazoloquina-zoline derivatives, 2-trifluoromethyl chromones, aliphatic alkynes, 2-nitroazobenzenes, ring opening of [ 1,2,4]triazolo [5,1-c] [2,4]benzothiazepin-10 (5//)-one, alkenyl esters and dendrimers. A number of these reactions are outlined in eqs 44 8. 

Alkynyl complexes contain metal-carbon bonds in which the metal is bound to the sp-hybridized carbon at the terminus of a metal-carbon triple bond. The materials properties of these complexes have been investigated extensively. The properties of these complexes include luminescence, optical nonlinearity, electrical conductivity, and liquid crystallinity. These properties derive largely from the extensive overlap of the metal orbitals with the ir-orbitals on the alkynyl ligand. The M-C bonds in alkynyl complexes appear to be considerably stronger than those in methyl, phenyl, or vinyl complexes. Alkynyl complexes are sometimes prepared from acetylide anions generated from terminal alkynes and lithium bases (e.g., method A in Equation 3.42), but the acidity of alkynyl C-H bonds, particularly after coordination of the alkyne to the transition metal, makes it possible to form alkynyl complexes from alkynes and relatively weak bases (e.g., method B in Equation 3.42). Alkynyl copper complexes are easily prepared and often used to make alkynylnickel, -palladium, or -platinum complexes by transmetallation (Equation 3.43). This reaction is a step in the preparation of Ni, Pd, or Pt alkynyl complexes from an alkyne, base, and a catalytic amoimt of Cul (Equation 3.44). This protocol for... 

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