Alkynyl coupling

Scheme 10.6. General scheme for alkynyl coupling and 1,3-diyne cleavage reactions via metallacyclo-cumulenes.

Treatment of allene (1,2-propadiene) with 2equiv. of butyllithium leads to an intermediate dilithio species which adds to ketones and aldehydes to afford homopropargylic alcohols in high yield (Table 9.3) [7]. This intermediate also reacts with geranyl chloride to afford the alkynyl coupling product uncontaminated by the allene regioisomer. 

Extensive theoretical studies of the alkynyl coupling reactions have been reported. An early MO study of the relationship between L2M(/r-C2R)2ML2 and L2M(/z-RC4R)ML2 used EH and MNDO techniques and encompassed a range of Main Group elements as well as Ti and Zr ° and showed the transition between symmetrical, asymmetric, and linked C4 ligands. The structural evidence... 

Scheme 25 CO-promoted intramolecular allenylidene-alkynyl coupling...

An intramolecular allenylidene-alkynyl coupling was also observed in the reaction of the mixed alkynyl-allenylidene rhodium(I) complex 73 with carbon monoxide (Scheme 25). In this case, the initially formed thermally unstable allenyl derivative 74 evolved into the metallated cyclobutenone 75 when an excess of CO was present [276]. 

Figure 8.3 Schematic presentation of the mechanism for the aryl-alkynyl coupling reaction...

In addition to aryl alkynyl coupling reactions involving dihaloarenes and acetylenes or dialkynylarenes which lead to poly(arylene acetylene)s, it is worth noting that these polymers have also been obtained by the Pd(0)-catalysed heteropolycondensation of dialkoxy-substituted dibromoarenes with bis(tribu-tylstannyl)acetylenes [122]. 

Iodobenzyl 2-butynyl ethers undergo a Pd-mediated cyclisation with terminal alkynes to give lf/-[2]benzopyrans. The absence of a Cu(I) co-catalyst allows an intramolecular 6-exo-dig cyclisation to compete successfully with an intermolecular Sonogashira aryl-alkynyl coupling <02T9007>. 

Figure 7 Probable reaction path for vinyl-alkynyl coupling on palladium.

A direct and satisfactory procedure for tertiary alkyl-alkynyl coupling has been developed by Negishi and Baba, who used trialkynylaluminums readily obtainable from the corresponding alkynyllithiums and anhydrous AICI3 [92]. For instance, tris(l-hexynyl)aluminum underwent a remarkably clean reaction with 1-adamantyl bromide to produce cross-coupled product 96 in 96 % yield. It is noteworthy that the reaction enables novel geminal alkyl-alkynylation of ketones this reaction should find a considerable application in natural product synthesis (Sch. 60). 

Palladium-catalyzed cross-coupling involving alkynylmetals and related alkynyl nucleophiles alkynyl-aryl, alkynyl-alkenyl, and alkynyl-alkynyl coupling... 

Subsequent studies have shown that alkenylzirconium derivatives generated in situ by hydrozirconation [46] of alkynes also undergo Pd- or Ni-catalyzed cross-coupling. As observed with alkenylalanes, Ni catalysts are generally satisfactory in the cases of alkenyl-aryl coupling [47] (Scheme 1-12), whereas Pd catalysts are more selective and distinctly superior to Ni catalysts in alkenyl-alkenyl and alkenyl-alkynyl coupling [48] (Scheme 1-13). 

Hydrometallation-cross-coupling and carbometallation-cross-coupling tandem protocols and other alkenyl-aryl, alkenyl-alkenyl, and alkenyl-alkynyl coupling reactions double metal catalysis... 

---