Recently, the international academic journal Angewandte Chemie International Edition published a paper entitled "Oxidase-Type C−H/C−H Coupling Using an Isoquinoline-Derived Organic Photocatalyst" online. This new research result was achieved under the joint efforts of theSchool of Chemistry and Materials Science, Hangzhou Institute for Advanced Study (HIAS), UCAS, École Polytechnique Fédérale de Lausanne (EPFL) and the University of Basel. (https://doi.org/10.1002/anie.202202649) In this research, an isoquinoline-derived organic photosensitizer has been developed and used in the oxidative coupling of the C(Sp3)-C(Sp2) bond. Oxygen in air, as the only oxidant, features mild conditions and good selectivity. Based on the in-depth mechanism-centered research, the authors of the paper believe that the self-quenching of the photosensitizer plays a key role in the reaction process.
The vigorous development of photocatalysis in organic synthesis has given rise to increasing requirements for photocatalysts. Organic photosensitizers are thus widely used in organic synthetic chemistry due to their advantages of low prices, easily adjusted structures and sound stability. The authors started the research by designing the organic photosensitizerA based on the diaryl ketone structure (Figure 1). The benzene ring was replaced with an isoquinoline one to red-shift the photosensitizer's light absorption band, so as to realize the photocatalytic reaction under visible light conditions. When the photosensitizer was used in the oxidative coupling of the C(Sp3)-C(Sp2) bond, the authors found through reaction intermediate observation and fluorescence quenching analysis that the reaction did not directlyabstract a hydrogen atom from an alkane to form an alkyl radical through the expected triplet intermediate B.Further mechanism research shows that the newly introduced isoquinoline part plays a unique role in the reaction. A photosensitizer self-quenching process to generate the hydrogen atom transfer (HAT) catalyst has been proposed, which is to generate the isoquinoline radical cationD by oxidizing the ground-state photosensitizer with the excited-state photosensitizerB. As the reaction substrate also contains an isoquinoline or similar heterocyclic structure, the radical cationD' may also be available to participate in the reaction. The formation of such intermediates has been confirmed via transient fluorescence spectroscopy.The isoquinoline radical cation D (D')can abstract hydrogen radicals from non-activated alkanes to produce the alkyl radicals required for the reaction. This has been demonstrated using atmospheric pressure photoionization mass spectrometry.
Figure 1: Mechanism for Isoquinoline-derived Organic Photosensitizer to Catalyze Oxidative Coupling of C(Sp3)-C(Sp2) Bond
The isoquinoline-derived photocatalyst has been fully characterized, including the determination of ultraviolet-visible absorption spectrum, redox potential and excited-state lifetime. The catalytic system is suitable for both the hydrogen transfer of activated and non-activated C(Sp3)-H bonds and the oxidative coupling reaction between the C-H bonds and heterocycles of aldehyde substrates. These substrates are widely applicable and oxygen in the air, as the only oxidant, saves the trouble of using other hazardous oxides. The in-depth study of the reaction mechanism, especially the proposal and confirmation of the photosensitizer self-quenching process, also provides a reference for the design and development of organic photosensitizers.
Dr. Zhang Lei, the first author of the paper, graduated from the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences in July 2016. Later, he did postdoctoral research at EPFL and officially joined the School of Chemistry and Materials Science, HIAS, UCAS in June 2021. Professor Xile Hu from EPFL and Professor Oliver S. Wenger from the University of Basel are co-corresponding authors.
Source | School of Chemistry and Materials Science
Typesetter | Tong Sihui
Executive Editor | Wang Xia
Scan the QR code and follow the official account of "HIAS-UCAS"!
HIAS-UCAS
WeChat ID: UCAS-HIAS
Official website丨http://hias.ucas.ac.cn/
Email: hangzhou@ucas.ac.cn
Published in Zhejiang
Scan with WeChat
Follow us
