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

Abstract

AbstractWe 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.