Gold-Catalyzed 4C+2C Cycloadditions of Allenedienes, including an Enantioselective Version with New Phosphoramidite-Based Catalysts: Mechanistic Aspects of the Divergence between 4C+3C and 4C+2C Pathways

Gold(I) complexes featuring electron acceptor ligands such as phosphites and phosphoramidites catalyze the 4C+2C intramolecular cycloaddition of allenedienes. The reaction is chemo- and stereoselective, and provides trans-fused bicyclic cycloadducts in good yields. Moreover, using novel chiral phosphoramidite-based gold catalysts it is possible to perform the reaction with excellent enantioselectivity. Experimental and theoretical data dismiss a cationic mechanism involving intermediate II and suggest that the formation of the 4C+2C cycloadducts might arise from a 1,2-alkyl migration (ring contraction) in a cycloheptenyl Au-carbene intermediate (IV), itself arising from a 4C+3C concerted cycloaddition of the allenediene. Therefore, these 4C+2C allenediene cycloadditions and the previously reported 4C+3C counterparts most likely share such cycloaddition step, differing in the final 1,2-migration step.