Vinylidenes in Gold Catalysis

Relative to Au(III), in general, and In(III), in this case, Au(I) is more inclined to react through vinylidene intermediates due, in part, to its linear geometry, tvhich offers little steric impediment to incoming nucleophiles. 

Mild conditions discovered for the cyclization of propargyl pyridine 145 were applied to other substituted and elaborated heterocydes, giving indolizidine-type products in good-to-excellent yields (Table 9.17). 

Intramolecular nucleophilic attack on the a-position of the vinylidene complex 148 affords a zwitterionic species (149). Next, a formal 1,3-H-shift must occur before release of the observed product (146). Based on the results of an isotope labeling study, this process is believed to occur via two discrete 1,2-H-migration events (149— 150— 147). Interestingly, no back-migration of heteroatom substituents is observed [50]. 

In2000, Hashmi and coworkers reported that certain alkynyl furans (151) undergo rapid cycloisomerization to give bicyclic phenols (152) in the presence of AUCI3 at room temperature (Equation 9.16) [51]. A number of late-transition metal catalysts promote this transformation [52]. Echavarren and coworkers have studied the Pt-catalyzed variant [53], which is believed to proceed via a mechanism involving Pt-cydopropylcarbene intermediates [54]. 

Entry Substrate Product Croup Time (h) Yield (%) 

---