Metal acetylide coupling

Corey and Kang have shown that, under suitable conditions, metal acetylides couple regioselectively with allenic bromides to give high yields of 1,4-diynes. This new procedure was used as a step in the first synthesis of ll(/ )-HETE, a natural product related to arachidonic acid (Scheme 82). ... 

In the Favorski reaction [8], etbyne is coupled with a carbonyl compound in the presence of powdered alkali hydroxide suspended in an organic solvent, in which the acetylene has good solubility. Some acetylenic carbinols, derived from ketones, can be obtained in high yields by introducing acetylene at atmospheric pressure. The active intermediate possibly is a metal acetylide formed in low concentration. 

In the total synthesis of harveynone (152), reaction of the iodide 150 bearing labile functionality took place with the tin acetylide 151, and it was claimed that no reaction occurs with the free amine [70]. However, this claim is not always true, and the coupling of the similar iodide 153 was carried out with the corresponding free terminal alkyne in the presence of diisopropylamine [71]. The coupling of the Mg acetylide 155 with vinyl carbamate 154 to give the enyne 156 is catalysed by a Ni complex [72]. It is true that free alkynes give better results than the corresponding metal acetylides in some cases [73]. 

Various metal acetylides are used for smooth coupling with propargylic halides and acetates. 2,3-Alkadien-5-yn-l -ols are obtained by the reaction of 2-(l -alkynyl)oxiranes [28,29], As a synthetic application, the unstable 2,3-octadiene-5,7-diyn-l-ol (136), a fungus metabolite, has been synthesized by the coupling of 4-trimethylsilylbutadiy-nylzinc chloride (134) with 2-ethynyloxirane (135) followed by desilylation [31]. 

As hydrocarbons, terminal acetylenes enjoy a rich reaction chemistry [1]. This is in no small part because of a unique feature of terminal acetylenes that differentiates them from other hydrocarbons - the acidity of the terminal proton (p-K,= 25). It is suggested that the lability of the terminal C-H towards deprotonation results its being bound to an sp-hybridized carbon [2]. This characteristic has been recognized for some time and has led to a diversity of methods for generation of metal acetylides which can participate in coupling reactions. 

Alkynyl complexes, also known as metal acetylides, possess both rigid linear skeleton and tt-conjugation. In the structural aspect, alkynyls make excellent linear bridging units. By means of the reactions of transmetallation (see Transmetalation), poly-Pt-acetylides are readily synthesized (Scheme 24). Copper(I)-mediated oxidative coupling... 

In recent years, other metal acetylides such as Cu, Zn, B, Al, Si, and Sn have also been prepared and employed in various applications. The direct introduction of an acetylenic bond by catalytic coupling reactions has also been practiced [Eqs. (9-13), 3-6]. 

Recently, Navarro and coworkers described that well-defined complexes XLV and XLVI (Fig. 5.6) with commercially available NHC ligands allowed A -coupling reactions to take place at room temperature, in open air, and in much shorter reaction times [106]. The activities of XLV and XLVI were notably affected by the nature of the counteranion X (OAc>Cl>Br), suggesting that metal acetylide formation (169 Scheme 5.74) might be favored with less bulky anions. 

Similar heterometallic cluster formation is induced by treatment of 121 with group 6 metal acetylides, Gp(OG)3M-G=GR (M = Mo, W), which usually afford a mixture of products including, for example, the acetylide-coupling products, 222 and 223. " " When the reaction with FpG=GR is carried out in an NEt3-H20 mixture under irradiation, the 113-ECCU2R complex 224 is formed, where the GHz hydrogen atoms come from H2O as confirmed by a labeling experiment. ... 

Vinyl iodides/triflates have been reported to undergo carbonylative palladium-catalyzed coupling reaction with organostannanes, organoboron derivatives, and metal acetylide. The palladium-catalyzed three-component coupling reaction of vinyl iodides with trimethyl- or tributylvinylstannanes gives unsymmetrical divinyl... 

---