Chiral Brønsted Acid from Chiral Phosphoric Acid Boron Complex and Water: Asymmetric Reduction of Indoles

A new chiral Brønsted acid, generated in situ from a chiral phosphoric acid boron (CPAB) complex and water, was successfully applied to asymmetric indole reduction. This “designer acid catalyst”, which is more acidic than TsOH as suggested by DFT calculations, allows the unprecedented direct asymmet...

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Bibliographic Details
Published in:Angewandte Chemie 2020-02, Vol.132 (8), p.3320-3325
Main Authors: Yang, Kai, Lou, Yixian, Wang, Chenglan, Qi, Liang‐Wen, Fang, Tongchang, Zhang, Feng, Xu, Hetao, Zhou, Lu, Li, Wangyang, Zhang, Guan, Yu, Peiyuan, Song, Qiuling
Format: Article
Language:English
Subjects:
Acids
Aromatic compounds
Asymmetrische Katalyse
Asymmetry
Bor
Boron
Brønsted-Säure
Catalysts
Chemistry
Functional groups
Heterocyclen
Indoles
Mathematical analysis
Phosphoric acid
Protonation
Reduction
Substitutes
Wasser
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Summary:A new chiral Brønsted acid, generated in situ from a chiral phosphoric acid boron (CPAB) complex and water, was successfully applied to asymmetric indole reduction. This “designer acid catalyst”, which is more acidic than TsOH as suggested by DFT calculations, allows the unprecedented direct asymmetric reduction of C2‐aryl‐substituted N‐unprotected indoles and features good to excellent enantioselectivities with broad functional group tolerance. DFT calculations and mechanistic experiments indicates that this reaction undergoes C3‐protonation and hydride‐transfer processes. Besides, bulky C2‐alkyl‐substituted N‐unprotected indoles are also suitable for this system. Es ist komplex: Eine neue chirale Brønsted‐Säure, die in situ aus einem chiralen Phosphorsäure‐Bor(CPAB)‐Komplex und Wasser erzeugt wird, ermöglicht die direkte asymmetrische Reduktion von C2‐Aryl‐substituierten N‐ungeschützten Indolen. Die Reaktion zeichnet sich durch gute bis exzellente Enantioselektivitäten mit breiter Verträglichkeit funktioneller Gruppen aus.
ISSN:0044-8249
1521-3757
DOI:10.1002/ange.201913656