B(C6F5)3/Chiral Phosphoric Acid Catalyzed Ketimine–Ene Reaction of 2-Aryl-3H-indol-3-ones and α-Methylstyrenes |
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Authors: | Qing-Xia Zhang Yao Li Jie Wang Chen Yang Cheng-Jun Liu Prof Xin Li Prof Jin-Pei Cheng |
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Institution: | State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin, 300071 P. R. China |
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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-3H-indol-3-ones with α-methylstyrenes were achieved by utilizing a B(C6F5)3/chiral phosphoric acid (CPA) catalyst. These ketimine–ene reactions proceed well with low catalyst loading (B(C6F5)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(C6F5)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(C6F5)3/CPA, thereby inducing the improvement of stereoselectivity. |
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Keywords: | 2-Allyl-indolin-3-one Chirale Phosphorsäuren Ketimin-En-Reaktionen Organokatalyse Wasserstoffbrücken |
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