B(C 6 F 5 ) 3 /Chiral Phosphoric Acid Catalyzed Ketimine–Ene Reaction of 2‐Aryl‐3 H ‐indol‐3‐ones and α‐Methylstyrenes

The enantioselective ketimine–ene reaction is one of the most challenging stereocontrolled reaction types in organic synthesis. In this work, catalytic enantioselective ketimine–ene reactions of 2‐aryl‐3 H ‐indol‐3‐ones with α‐methylstyrenes were achieved by utilizing a B(C 6 F 5 ) 3 /chiral phosphoric acid (CPA) catalyst. These ketimine–ene reactions proceed well with low catalyst loading (B(C 6 F 5 ) 3 /CPA=2 mol %/2 mol %) under mild conditions, providing rapid and facile access to a series of functionalized 2‐allyl‐indolin‐3‐ones with very good reactivity (up to 99 % yield) and excellent enantioselectivity (up to 99 % ee ). Theoretical calculations reveal that enhancement of the acidity of the chiral phosphoric acid by B(C 6 F 5 ) 3 significantly reduces the activation free energy barrier. Furthermore, collective favorable hydrogen‐bonding interactions, especially the enhanced N−H⋅⋅⋅O hydrogen‐bonding interaction, differentiates the free energy of the transition states of CPA and B(C 6 F 5 ) 3 /CPA, thereby inducing the improvement of stereoselectivity.