Homocoupling reactions terminal alkynes

Ranu and Banerjee developed a [bmim][OH] TSIL for oxidative homocoupling of terminal alkynes to 1,4-disubstituted 1,3-diynes in atmospheric conditions using Cu(ii) without using either palladium catalyst, amines, oxidants or organic solvents. Significant advantages stated by the authors include fast kinetics, high yields and mild reaction conditions. 

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. 

Pd-catalyzed homocoupling of terminal alkynes proceeds in the presence of a catalytic amount of Cu(l) salts and amines. During this process a terminal alkyne reacts with copper(I) salt in the presence of an amine to give an alkynyl copper compound that reacts with the Pd catalyst. Therefore, the homocoupling of terminal alkynes falls into the category of homocoupling of organometals and follows the same reaction mechanism. 

To date, most research on NHCP transition-metal catalysis has been devoted to cross-couplings [13] or related reactions such as hydroarylation of alkenes [18], direct arylation of alkynes [17], or oxidative homocoupling of terminal alkynes [19]. All NHCP systems used in these studies feature one or two... 

In tMs section, Pd-catalyzed homocoupling of terminal alkynes, cross-coupling of terminal alkynes with internal alkynes, and cross-coupling of terminal alkynes with allenes will be discussed. All three types of reactions involve (i) activation of the C—bond of a terminal alkyne, (ii) alkynylpalladation of another molecule of alkyne or allene, and (iii) reductive elimination or protonation to produce a conjugated enyne. For alkynylpallada-tions of allenes followed by trapping with nucleophiles, see Sect IV.7. 

The alkynyl iodide 359 undergoes cross-coupling with a terminal alkyne to give the 1,3-diyne 360[264]. No homocoupling product is formed. This reaction offers a good synthetic method for unsymmetrical 1,3-diynes. 

For the development of the oxidative homocoupling reaction, in 1955 Chodkiewicz and Cadiot explored a Cu(I)-catalyzed heterocoupling reaction of terminal alkynes with 1-bromoalkyne in the... 

Most transition metal-catalyzed cross-coupling reactions also yield small quantities of the product of homocoupling of the nucleophilic reactant[16, 114,115]. In particular terminal alkynes [116, 117] or metalated terminal alkynes [118] readily dimerize to the corresponding 1,3-butadiynes (Scheme8.14). 

A problem that may be encountered when using the original procedure is that some functionally substituted acetylenes do not survive the acidic reaction conditions. Furthermore, one or both undesired homocoupled diynes may be formed, and these are often difficult to separate from the desired product. A number of methods are available that minimize or circumvent these shortcomings. For example, coupling of terminal alkynes with in situ preformed copper alkynylides accommodates the cleavage-prone Me3Si moiety. " ... 

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. 

JS2 OXIDATIVE HOMOCOUPLING REACTIONS OF TERMINAL ALKYNES... 

However, the use of basic anions is not the sole approach followed to obtaining basic ILs. Liquid salts bearing this additional property can be prepared also by incorporation of a basic center into the cation. This approach generally affords more thermally stable ILs than those based on basic anions, which frequently present relatively low decomposition temperatures. Basic ionic liquids bearing aliphatic or aromatic amines on the side chain(s) have been synthesized and, recently, some of these have been used as both the solvent and base for Heck, copper free Sonogashira, and for homocoupling reactions of terminal alkynes (Figure 4.2). 

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