text: Enyne cycloisomerization, an alkyne variant of the Alder-ene reaction (figure 5), is an intramolecular rearrangement of 1,n–enynes to give the corresponding cyclic isomer.
  Although the rearrangement may occur under thermal conditions, the scope of the thermal rearrangement is limited due to the requirement of high temperatures, thus transition metals such as Au, Pd, Pt, Rh and Ir are often employed as catalysts. As synthesis quarrels to build complex structural motifs in the presence of inductive, stereoelectronic and steric demands this rearrangement has recently been developed as a robust method for constructing carbo– and heterocyclic scaffolds with excellent chemo–, regio– and diastereoselective outcomes.
  There is not a single mechanism that can be used to describe enyne cycloisomerizations as the mechanism depends on reaction conditions and catalyst selection. Intermediates of the metal catalyzed cycloisomerization in which the metal coordinates the alkyne or alkene activating either or both are possible and are shown in figure 6.
  Activation of the alkyne by complexation with the metal leading to an η–metal intermediate such as 18 opens up the alkyne to nucleophilic attack and engenders carbocation intermediates. This Pull–push reactivity is important for understanding reactions mediated by π–acids. Complexation of the alkyne to the metal fragment depletes electron density in the bond (‘pull’), in concert with the ability of the metal to back donate (“push”) arouses the observed consecutive electrophilic and nucleophilic character to the vicinal carbon atoms of the alkyne (figure 7).
  Metallacycle intermediates  (19) are the result of the simultaneous complexation and activation of both partners. Hydrometallation of the alkyne giving a vinyl metal species that may in turn carbometalate the olefin is also possible (20). An example of a 1,6–enyne cycloisomerization proceeding through an η–activated metal intermediate is given in figure 8, which is common for enyne cycloisomerizations mediated by Pt or Au due to their π–acidic nature.  Notably, in this example chirality transfer takes place in which the absolute stereochemistry of the enyne (26) controls stereochemistry of the product (27).