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
Jiaqiong Sun, Weiliang Yuan, Rong Tian, Peiyuan Wang, Xuepeng Zhang, Xingwei Li
Abstract
Abstract 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 δ ‐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 η 4 ene‐allyl species with π‐agostic interaction undergoes SN 2 ′‐type external attack by the nitrogen nucleophile, instead of C−N reductive elimination, as the stereodetermining step.