Journal
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
Volume 59, Issue 11, Pages 4550-4556Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/anie.201915226
Keywords
2-allyl-indolin-3-ones; chiral phosphoric acids; hydrogen bonding; ketimine-ene reactions; organocatalysis; theoretical calculation
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Funding
- Program for the National Natural Science Foundation of China [21971120, 21933008]
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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 alpha-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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