Rhodium(III)‐Catalyzed Asymmetric [4+1] and [5+1] Annulation of Arenes and 1,3‐Enynes: A Distinct Mechanism of Allyl Formation and Allyl Functionalization

We report chiral RhIII cyclopentadienyl‐catalyzed enantioselective synthesis of lactams and isochromenes through oxidative [4+1] and [5+1] annulation, respectively, between arenes and 1,3‐enynes. The reaction proceeds through a C−H activation, alkenyl‐to‐allyl rearrangement, and a nucleophilic cyclization cascade. The mechanisms of the [4+1] annulations were elucidated by a combination of experimental and computational methods. DFT studies indicated that, following the C−H activation and alkyne insertion, a RhIII alkenyl intermediate undergoes δ‐hydrogen elimination of the allylic C−H via a six‐membered ring transition state to produce a RhIII enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare RhIII η4 ene‐allyl species with π‐agostic interaction undergoes SN2′‐type external attack by the nitrogen nucleophile, instead of C−N reductive elimination, as the stereodetermining step. RhIII‐catalyzed [4+1] and [5+1] annulation of N‐methoxy benzamides and 1,3‐cyclohexanediones with 1,3‐enynes has been realized for the enantioselective synthesis of lactams and isochromenes, respectively. DFT studies suggest an unusual pathway of alkenyl‐to‐allyl rearrangment and SN2′‐type allylic substitution for the [4+1] annulation system.