B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>/Chiral Phosphoric Acid Catalyzed Ketimine–Ene Reaction of 2‐Aryl‐3<i>H</i>‐indol‐3‐ones and α‐Methylstyrenes
Qing‐Xia Zhang, Yao Li, Jie Wang, Chen Yang, Cheng‐Jun Liu, Xin Li, Jin‐Pei Cheng
Abstract
Abstract 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.