Terminal alkynes coupling mechanisms

An intriguing variant of this coupling process was reported in 1995. Treatment of acetone with SmD in the presence of 42 resulted in the formation of allene 43 in 81% yield and a 5.7 1 diastereomeric ratio (equation 25)74. Although no mechanism was suggested, it is likely that the reaction involves addition of (CH3)2C=0 to the terminal alkyne,... 

The allenylalkyne 139, the primary product of the coupling of 137 and 138, undergoes further reaction with terminal alkyne. The Pd-catalysed bis-alkynylation of propargylic carbonate 137 occurs to afford enediynes 144. The reaction is understood by the following mechanism. The insertion of one double bond of the allene in 139 to the alkynylpalladium 141, formed from the terminal alkyne 140, generates 142 which... 

Whereas Glaser-type oxidative coupling opens efficient synthetic pathways toward symmetrical diynes, its performance in heterocoupling is poor. The latter may be accomplished by Cadiot-Chodkiewicz coupling of terminal alkynes with 1-haloalkynes (usually 1-bromoalkynes). The reaction is conducted in the presence of an amine and catalytic amounts of a copper(I) salt. Because, in contrast with the Glaser-type reactions described above, it follows a nonoxidative reaction mechanism, oxygen is not necessary - but needs often not to be excluded (Scheme 4) [9]. 

In 2006, Li and Wang reported the palladium-free, silver-catalyzed Sonogashira-type coupling of aryl halides and terminal alkynes. The reaction proceeds in high yield in the presence of catalytic silver iodide, triphenylphosphine, and potassium carbonate. Although the mechanism remains unclear, it is evident that the silver acetylide has a role, as the acetylide is formed on mixing of the reagents (Scheme 1.54).122... 

Ruthenium vinylidene intermediates have also been proposed in the mechanism of the coupling of unactivated alkenes with terminal alkynes to afford 1,3-dienes as a mixture of two isomers, linear and branched derivatives. The linear one was favored [56] (Eq. 42). The same system has allowed the ruthenium-catalyzed alkenylation of pyridine [57]. 

More often such bromo- and iodoalkynes are employed with another synthetic goal in mind, namely, in the Cadiot-Chodkiewicz reaction for the formation of symmetric or asymmetric 1,3-diynes by reaction of the haloalkyne with a terminal alkyne (Figure 13.25). Additional reagents essential for the success of this reaction are one equivalent or more of an amine and a substoichiometric amount of Cul. As with the Cacchi and Stephens-Castro coupling reactions of Section 13.3.4, a Cu-acetylide is the reactive species in the Cadiot-Chodkiewicz coupling. It is formed in step 1 of the mechanism illustrated in Figure 13.25. 

The InCl3-NaBH4-MeCN system shows high regio- and stereoselectivity for the dimerization of terminal alkynes to give enynes 91-93 (Scheme 114). The reaction is considered to proceed via the radical addition and coupling mechanism (Scheme 115).386... 

Mechanism The Pd complex such as Pd(PPh3)4 activates the organic halides by oxidative addition into the carbon-halogen bond. The copper(I) halides react with the terminal alkyne and produce copper acetylide, which acts as an activated species for the coupling reactions. The oxidative addition step is followed by the transmetallation step. The proposed catalytic cycle is shown in Scheme 5.21. 

Scheme 11.44 Mechanism and an example of the cross-coupling of propargylic compounds with terminal alkynes (X = 0C02Me, OAc, Cl).

Bohm, V. P. W., Herrmann, W. A. Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions, 13 a copper-free procedure for the palladium-catalyzed Sonogashira reaction of aryl bromides with terminal alkynes at room temperature. Eur. J. Org. Chem. 2000,3679-3681. 

Terminal alkynes can be coupled directly to aryl and to vinyl halides in the presence of a palladium catalyst and a base. The mechanism of this reaction appears to involve oxidative addition of the sp halide to palladium(O), followed by alkynylation of the intermediate organopalladium halide and reductive elimination of the disubstituted alkyne (equations 17 and... 

The actinide complexes Cp 2AnMe2 (An = Th, U) have been found to effectively catalyze the coupling reaction of terminal alkynes and /-butylisonitrile, B NC. The catalytic conversion of the isonitrile and alkyne to l-aza-1,3-enynes was achieved in toluene or benzene at 90-100 °G, while no reaction was observed in the absence of a catalyst. Scheme 97 illustrates a plausible mechanism for the catalytic coupling of BuNC and terminal alkynes mediated by Cp 2AnMe2.193... 

One of the most well-studied bimetallic catalysts used for the C-C coupling of alkynes are the thiolato-bridged diruthenium complexes 7 (Scheme 5) [22]. In the presence of NH4BF4 these complexes catalyse the head-to-head dimerisation of a number of terminal alkynes to selectively yield Z-enynes [23]. In contrast, related monometallic Ru catalysts typically yield a mixture of E- and Z-isomers, with the E-isomer more commonly favoured [24-26]. Previous work has shown that diruthenium complexes such as 7 are exceptionally robust due to the strong bridging ability of the thiolate ligands, which results in retention of the dinuclear core during reaction [27]. The proposed mechanism for the dimerisation reaction involves a concerted activation process where both Ru centres activate one alkyne each via the catalytic cycle shown in Scheme 5. Initial coordination of the first alkyne yields the vinylidene intermediate 8. The second alkyne then coordinates to... 

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