Catalytic‐Type Excited‐State N−H Proton‐Transfer Reaction in 7‐Aminoquinoline and Its Derivatives

7‐Aminoquinoline (7AQ) and various amino derivatives thereof (‐NHR) have been strategically designed and synthesized to study their excited‐state proton‐transfer (ESPT) properties. Due to the large separation between the proton donor ‐NHR and the acceptor ‐N‐ site, ESPT in 7AQ derivatives, if availa...

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Bibliographic Details
Published in:Chemistry : a European journal 2019-11, Vol.25 (65), p.14972-14982
Main Authors: Chang, Kai‐Hsin, Liu, Ying‐Hsuan, Liu, Jiun‐Chi, Peng, Yu‐Chiang, Yang, Yu‐Hsuan, Li, Zhi‐Bin, Jheng, Ren‐Hua, Chao, Chi‐Min, Liu, Kuan‐Miao, Chou, Pi‐Tai
Format: Article
Language:English
Subjects:
7-aminoquinoline
Acidity
Aminoquinolines
Basicity
Canonical forms
Catalysis
Chemistry
Configurations
Derivatives
excited-state proton transfer
heterocycles
Methanol
protic solvent
Protons
Quinoline
tautomerism
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Summary:7‐Aminoquinoline (7AQ) and various amino derivatives thereof (‐NHR) have been strategically designed and synthesized to study their excited‐state proton‐transfer (ESPT) properties. Due to the large separation between the proton donor ‐NHR and the acceptor ‐N‐ site, ESPT in 7AQ derivatives, if available, should proceed under protic solvent catalysis. ESPT is found to be influenced by the acidity of ‐NHR and the basicity of the proton‐acceptor ‐N‐ in the quinoline moiety. The latter is varied by the resonance effect at the quinoline ‐N‐ site induced by the ‐NHR substituent. For those 7AQ derivatives undergoing ESPT, increased quinoline basicity results in a faster rate of ESPT, implying that proton donation from methanol to the quinoline moiety may serve as a key step in the process. Our studies also indicate the existence of an equilibrium between cis and trans arrangements of ‐NHR in terms of its hydrogen‐bond (H‐bond) configuration with methanol, whereby only the cis‐H‐bonded form undergoes methanol‐assisted ESPT. With one exception, the interconversion between cis and trans configurations is much faster than the rate of ESPT, yielding amino‐type (normal form) and imine‐type (proton‐transfer tautomer) emissions with distinct relaxation dynamics. Solvent‐assisted proton transfer: 7‐Aminoquinoline derivatives undergo excited‐state proton transfer (ESPT) catalyzed by methanol. An increase in quinoline basicity results in a faster rate of ESPT, implying that proton donation from methanol to the quinoline moiety may be a key step in the process.
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.201904027