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 Rh(III)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 Rh(III)alkenyl intermediate undergoes delta-hydrogen elimination of the allylic C-H via a six-membered ring transition state to produce a Rh(III)enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare Rh-III eta(4)ene-allyl species with pi-agostic interaction undergoes SN2 '-type external attack by the nitrogen nucleophile, instead of C-N reductive elimination, as the stereodetermining step.