Gold vinylidenes

Thanks to a recent renaissance in gold catalysis, new gold-mediated transformations are being discovered with ever-increasing frequency. Some of these discoveries have been attributed to the action of gold vinylidenes. Fiirstner and coworkers uncovered one such example while screening catalysts for intramolecular alkyne hydroarylation (Scheme 9.23) [46]. 

Bertrand and coworkers invoke a retroiminoene-type mechanism for the formation of gold vinylidenes in an interesting crosscouphng of enamines and terminal alkynes to give allenes [60]. This reaction constitutes both a new reaction manifold for metal vinylidenes and a new retron for stereochemically defined allenes. 

Scheme 17 The benzofulvene products more directly reflect the gold vinylidene intermediate. The em-diaurated species as an instant dual activation precatalyst shows three times the activity of other gold catalysts...

Scheme 18 The dibenzopentalaie products also are good indicators for a gold vinylidene intermediate...

More recently, Au(I)-catalyzed ring-closure reactions of 1,2-diethynylbenzenes 158-160 bearing a terminal acetylene unit afford benzofulvene and dibenzopen-talene derivatives 161-163, as illustrated in Scheme 6.37 [79]. Gold vinylidene complexes are postulated to generate the respective reaction intermediates. These reactions are of importance for the construction of asymmetric polycyclic systems. 

On the other hand, it was recently presented by Ye et al. [35] that Au complex is another choice for this transformation (Scheme 21). Interestingly, this reaction required addition of a stoichiometric amount of strong acid such as MsOH and proceeded under mild conditions (typically, at room temperature). On the basis of mechanistic study, the gold-vinylidene pathway is less likely. That is, this reaction would proceed via activation of alkyne part by coordination with Au center (formation of A) and the following 5-endo-dig cyclization. 

In 2004, Mamane et al. reported the cycloaromatization of biphenyl bearing an alkyne moiety at the ortho position, 65, into phenanthrene derivatives 66 via gold-vinylidene... 

Using a similar dual gold catalysis of diynes model, Hashmi, Hansmann, and coworkers explored a dual gold-catalyzed cyclization of 3,4-diethynylthiophenes to generate pentaleno[c]thiophenes through gold-vinylidenes and C-H bond activation. In these cases, 5-e (io-dig cyclization pathway was observed. This protocol can be used to construct thiophene/furan or thiophene/pyrrole 101 system skeleton from 3,4-diethynylthiophene derivatives 100 (Scheme 12.46) [49]. 

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