Coupling of Two Alkynes

Takahashi and co-workers have designed a [4 + 3]-cycloaddition based on their previously reported [4 + 4]- and [4 + 5]-reactions (Sections 10.13.2.4 and 10.13.2.7) involving zirconacyclopentadienes as four-carbon components. The zirconacyclopentadienes are prepared by the coupling of two alkynes with Cp2ZrR2 (where R = Et, Bu, or... 

Enynes.1 Pd(OAc)2 when complexed with this phosphine (1 1) can effect coupling of two alkynes to provide dimeric 1,3-enynes. 

Alternatively, rhodium catalysts have been revealed to be effective for the coupling of two alkynes with an isocyanide to afford the iminocyclopentadienes 62 and 62 in high yield (Tab. 11.9) [36bj. The coordinating solvent dibutyl ether, in combination with por-tionwise addition of the isocyanides, is key to the success of this transformation. 

The stereoselective synthesis of 1,4-disubstituted-l,3-dienes proceeds by head-to-head oxidative coupling of two alkynes with formation of an isolable metallacyclic biscarbene ruthenium complex [23], as shown in Scheme 6. Several key experiments involving labeled reagents and stoichiometric reactions and theoretical studies support the formation of a mixed Fischer-Schrock-type biscarbene complex which undergoes protonation at one carbene carbon atom whereas the other becomes accessible to nucleophilic addition of the carboxylate anion (Scheme 6) [23]. 

Similarly, coupling of two alkynes can form the diene metallacycles. 

Depending on the nature of the Cp ligands, the metal, the stoichiometry, and the reaction conditions, very different complexes have been obtained by treatment of (25-28) with 1,3-butadiynes. For instance, the formation of (29) and (30) results of cleavage of the C-C bond of the alkynes. The coupling of two alkyne units in the coordination sphere of the metal gives (31). ... 

Oxidative coupling of two alkynes to give the metallole, followed by CO insertion and reductive elimination. 

With suitably placed hydroxy and ester funcdonal groups, tandem lactonization can ensue (Scheme 11.91). The intermolecular coupling of two alkynes with this system is also possible (Scheme 11.92), provided that only one alkyne is terminal, and the acceptor alkyne is electron poor, otherwise extensive self-coupling of the alkynes may result. " ... 

The excellent coordination properties of alkynes with ruthenium catalysts led to their use as partners for the coupling with a large variety of unsaturated molecules. The first examples of dimerization of terminal alkynes involved acetylide or vinylidene intermediates. By contrast, with (CsR5)Ru catalysts, a completely different stoichiometric head-to-head coupling of alkynes has been discovered affording ruthenacyclopentatrienes, which are cyclic biscarbenes produced by the oxidative coupling of two alkynes [1-3] (Scheme 1). 

Scheme 1 Oxidative coupling of two alkynes with (C5R5)Ru catalysts...

The other congener in the class of heterocumulenes, ketene, is perhaps the least studied and most challenging coupling partner. Unlike the studies outlined above, no data on nickel-mediated oxidative coupling between ketenes and alkynes currently exists. Similarly, no data exist on ketene insertion into a five-membered nickelacycle arising from homooxidative coupling of two alkynes. However, a handful of... 

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