From Alkynyl Anions

Preparation of Alkynes from Alkynyl Anions CHAPTER 13... 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

R = H, Ph) have been obtained from the anionic [Ru(N4Meg)] and ethyne or HC CPh, respectively [51], In this case, formation of an intermediate r -alkyne complex is unlikely, the probable mechanism being deprotonation of the alkyne and coordination of the alkynyl anion followed by proton transfer. 

It was also reported that treatment of -alkynyl iodides 17 and 18, having a triple bond activated by conjugation either with an aromatic ring or a double bond, with zinc dust in THF resulted in the formation of the cyclic products 19 and 20 respectively (equation 8)20. However, their formation was ascribed to a zinc-induced radical cyclization process due to the failure to detect any open-chain organozinc species prior to cyclization as well as unsuccessful attempts to efficiently functionalize any alkenylzinc species that would have been normally expected from an anionic pathway20. 

Preparation of Aikynes by Alkylation of Alkynyl Anions A synthesis of internal aikynes starting from terminal aikynes. 

Preparation of Alkynes by Alkylation of Alkynyl Anions The other major alkyne preparation is based on the easy accessibility of nucleophilic carbanions from terminal alkynes (Section 13-2). So, a wide variety of internal alkynes may be made from any terminal alkyne, via the sehenie... 

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... 

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

Alkynes can also be prepared from other alkynes. The reaction of terminal alkynyl anions with alkylating agents, snch as primary haloalkanes, oxacyclopropanes, aldehydes, or ketones, results in carbon-carbon bond formation. As we know (Section 13-2), such anions are readily prepared from terminal alkynes by deprotonation with strong bases (mostly alkyllithium reagents, sodinm amide in liqnid ammonia, or (jrignard reagents). Alkylation... 

Many other types of reactions may be catalyzed by ammonium or phosphonium salt PT catalysts. Figure 23.15 shows some examples. The first is an elimination, and the second the alkylation at the a-position of a nitrile. The third example involves the reaction of an alkynyl anion with a ketone without the catalyst, the yield is only 7 %. Both this and the previous example will appear surprising—the acid from which the proton is removed has, in both cases, quite a high pK,... 

Differences between cyanide and acetylide complexes arise from the instability of the acetylide ligand to hydrolysis. The alkynyl anions are strong bases, and anhydrous solvents such as liquid ammonia must be used in the preparation of many of their complexes. Metal thiocyanates,... 

The number of possible structural devices increases as functionality gets further from the anionic center in functionalized organolithium compounds. So, d-func-tionalized organolithium compounds can be classified according to the hybridization of the carbon atom bonded to the lithium in alkyllithium compounds (XXI and XXII), allylic and benzylic derivatives (XXIII-XXV) and alkenyl (XXVI-XXVIII) and alkynyl systems (XXK). 

Acetylenedimagnesium bromide, 66, 84, 137 Acyl-alkyl diradical disproportionations, 299 Acyl-alkyl diradical recombination, 296 Alkaline hydrogen peroxide, 10, 12, 20 Alkylation of formyl ketones, 93 Alkylation via enolate anions, 86 17a-Alkynyl steroids from 17-ketones, 67 2a-Al]yl-17jS-hydroxy-5a-androstan-3 -one, 9 5 Allylic acetoxylation, 242 Allylmagnesium bromide, 64 17 -Aminoandrost-5-en-3 -ol, 145 17 a-Aminomethy 1-5 a-androstane-3, 1718-diol, 387... 

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