Direct cross-coupling with terminal acetylenes

A particularly interesting application of this protocol, in conjunction with the direct cross-coupling with terminal acetylenes is shown in Scheme 5-2. Here, the first coupling occurs between the alkynyltin reagent 50 and the cyclized palladium intermediate 49 and is followed by the second direct coupling with the terminal acetylene 52 at the vinyl bromide moiety to yield unsymmetrical cyclic dienediyne 53 [32]. 

This protocol and the Pd-Cu-catalyzed direct cross-coupling with terminal acetylenes (Section 5.3.5) directly complement each other. The following examples, which cannot be applied to the latter, give normal coupling products only by Stille method [Eqs. 17-20] [28-31]. This method is especially useful for the cross-coupling reactions of organometallic complexes with cyclopentadienyl ligand that are sensitive to amines [Eq. 19] [30]. 

As described above (Section 5.2), the Stephens-Castro reaction of alkynylcopper with aryl and vinyl halides in boiling pyridine is a useful route to aryl and vinyl acetylenes. Direct cross-coupling of organic halides, such as sp halides, with terminal alkynes is a more convenient procedure. Such a reaction is not so easy, but it can be done using a Pd-complex catalyst [41]. Especially facile Pd-catalyzed cross-coupling of aryl and alkenyl halides with terminal alkynes proceeds smoothly under mild conditions in the presence of a cocatalyst of cuprous iodide in amine solvents [Eq. (28)] [42]. This methodology is now used widely for the constiuction of conjugated arylalkyne or enyne systems [43], as described below. It is attractive from a synthetic point of view because mild reaction conditions and simplicity of the procedure are associated with recent developments in modem acetylene chemistry [44]. 

Recently, direct synthesis of terminal acetylenes via Pd-catalyzed cross-coupling of aryl or alkenyl halides with ethynylmetals was developed by Negishi and co-workers (Section ni.2.8.2). ... 

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