text: Alkyne activation with π–acidic metals such as Au or Pt is a conventional method in complex organic manifold synthesis, however how this activation exacts reactivity is not fully understood and thus mechanism is largely proposed on the basis of reaction outcome and theoretical calculations. Cationic Au(I) and Pt(II) catalyst are attractive choices as they display strong Lewis acid character and the ability to stabilize cationic intermediates while being bench stable.
  A versatile function of Au(I) catalyzed enyne cycloisomerization is the construction of asymmetric medium–sized rings, which is a challenge in the synthesis of ornamented molecular design.  Convenient access to asymmetric 7– and 8–membered carbocycles is possible using chiral BINAP Au(I) gold catalyst, giving a wide variety of products.
  It is proposed that intramolecular cyclopropanation occurs via a 1,2–shift of the propargyl ester mediated by Au to give a syn–Au vinyl carbenoid species (29). Computational studies show that the syn–intermediate, 29, is formed under kinetic control and it is suggested that it is in equilibrium with the thermodynamically favorable cis–intermediate which may be intercepted by a nucleophile leading to vinyl cyclopropane diene products, however this is beyond the scope of this article.
  Vinylcycloalkenes is another functional class of products accessible through alkyne activation of enynes with π–acidic metals. PtCl has been shown to catalyze the formation of a variety of exotic vinylcycloalkenes from readily accessible starting materials (figure 10).
  Notably, a ring expansion is observed for enynes with cyclic alkene motifs. This is rationalized by a formal insertion of the methylene group of the olefin between the two carbons of the alkyne; a mechanistic reasoning for this ring expansion has also been proposed. The formation of these vinycloalkenes in concert with its ability to undergo a ring expansion was exploited to construct intermediate 36 en route to streptorubin B. A similar transformation is possible using cationic Au(I) complexes, however here one can select for vinycycloalkene products through a mechanism proceeding via an initial 5–exo–dig or bicyclopropanes can be produced via an initial 6–endo–dig. It is suggested through DFT calculations that the 5–exo–dig cyclization is favored for Au(I) complexes as it has a lower activation barrier relative to the 6–endo–dig and indeed numerous examples of vinylcycloalkenes products produced via an initial 5–exo–dig are given (figure 11).
  The reactivity can be reversed by careful selection of reaction conditions, catalyst selection and substrate. The divergent reactivity of these transition metal catalyzed cycloisomerizations further demonstrates their synthetic utility in building unique molecular skeletons.