Gold-catalyzed [4C+3C] intramolecular cycloaddition of allenedienes: Synthetic potential and mechanistic implications

An experiment was conducted to discover that Au complex generated in situ from [(IPr)AuCl] and AgSbF6 catalyzes the [4C+3C] intramolecular cycloaddition of a variety of allenedienes at room temperature. Allenediene was treated with 10 mol% of AuCl3 or AuCl, the desired [4C+3C] cycloadduct was obtained in good yield. Screening of other Au catalysts revealed that the cycloaddition was particularly rapid and efficient with the Au complex, which features a N-heterocyclic carbene (IPr) ligand. NMR monitoring of the cycloaddition of allenediene using 10 mol% of several catalysts revealed that while exhibiting a significantly lower energy barrier for the 1,2-H shift step. The reaction of cyclopentylallene derivative in the presence of catalyst, in addition to providing the expected tricycle that results from a 1,2 alkyl shift in the intermediate carbene species, led to a minor cycloadduct identified as the bicycle.