Glaser coupling homocoupling

The synthesis of conjugated diynes via the Glaser coupling reaction " is the classical method for homocoupling of terminal alkynes. The coupling reaction is catalyzed by CuCl or Cu(OAc)2 in the presence of an oxidant and ammonium chloride or pyridine to yield symmetrically substituted diynes. " The oxidative dimerization appears to proceed via removal of the acetylenic proton, formation of an alkynyl radical, and its dimerization. 

Cu-catalyzed Glaser coupling Preparation of symmetrical conjugated diynes and polyynes by the oxidative homocoupling of terminal alkynes in the presence of copper salts. 186... 

The main driving forces behind the development of new tertiary phosphine palladium complexes for C(sp )—C(sp) couplings have been (i) a reduction or elimination of side reactions, such as Glaser-type homocouplings (ii) the development of environmentally friendly reaction protocols, such as copper-free reactions in benign solvents (iii) the improvement of catalyst stabihty and activity [higher turnover number (TON) and turnover frequency (TOP)] and (iv) a cost reduction by using less-expensive aryl bromides, or even aryl chlorides under mild reaction conditions, for example, at ambient temperature. 

It must be emphasized that current mechanistic understanding of copper-mediated oxidative acetylenic couplings is unsatisfactory. Several studies have shown the strong dependency of the mechanism on the experimental setup, suggesting highly complex coherences and interactions. Nevertheless, the mechanistic idea of Bohlmann et al. described above still provides the most accepted picture for Glaser-type oxidative acetylenic homocouplings. 

Some of the most useful synthetic transformations of terminal alkynes involve intermolecular and intramolecular homo- and cross-coupling reactions between their. sp-carbon centers, leading to butadiyne or polyyne derivatives. The two most widely used and practical systems are (i) oxidative homocoupling reactions, i.e. Glaser and Eglington reactions and (ii) heterocoupling reactions, i.e. Chodkiewicz-Cadiot coupling of a terminal alkyne with a haloalkyne. 

Several minor variations and practical improvements on the Glaser and related acetylenic coupling reactions have been developed. For example, when Cul is substituted for CuCl in the standard Hay conditions (TMEDA, O2, acetone) a greatly improved yield of 12 is obtained, which was ascribed to the more soluble CuI-2TMEDA in acetone. The homocoupling of 2-ethynyl-l-methylpyrrole (13) only proceeds in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which may be due to the decreased acidity of the acetylenic hydrogen. ... 

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