Exploiting zwitterions in organic synthesis and catalysis Takashi Ooi Nagoya University, Japan
Zwitterions, which have both positive and negative charges in a single molecule, are easy to generate and handle, serving as versatile species in organic chemistry. Compared to simple ionic species, zwitterions are often isolable owing to the stabilization by the inductive and resonance effects. Another important aspect is that various molecular transformations are possible by utilizing the nucleophilic and basic properties of the anion moiety and the electrophilicity of the cation moiety. In conjunction with our research program directed toward molecular design of organic ions and eliciting their functions as catalysts capable of precisely controlling counterions, 1 we have been interested in the characteristic features of zwitterions, particularly in view of extending our approach to the development and control of radical-mediated reactions. Concretely, we have been engaging in the catalyst and reaction development by harnessing the reactivity of electron-deficient radical cations generated from the stable zwitterions via single-electron transfer (SET). For instance, we have devised a bench-stable, zwitterionic 1,2,3-triazolium amidate and achieved catalytic hydrogen-atom transfer (HAT) reactions based on the generation of the amidyl radical via intermolecular SET with a photoredox catalyst. 2 Furthermore, the discovery that the conjugate acid of the triazolium amidate, 1,2,3-triazolium amide, behaves as a one-electron acceptor and oxidatively regenerates photoredox catalysts with dehydrogenation has led to the development of acceptorless dehydrogenative cross-coupling (ADC) reactions. 3 However, the effective concentration of amidyl radicals generated in the intermolecular SET was extremely low, and hence the intrinsic reactivity of the triazolium amidate is yet to be fully realized. To solve this problem, we have pursued the possibility of imparting two functions to a single organic molecule: photoinduced one-electron redox ability and capability as a HAT catalyst. In this lecture, I would like to share a stream of research, in which a simple idea was exploited to develop a new zwitterion and elicit its function as a catalyst to promote HAT reaction from inactive C(sp3)-H bonds. As the other recent advancement in extending our approach, reaction development by the combination of common zwitterions with SET will also be discussed.
References 1. (a) Uraguchi, D.; Ueoka, F.; Tanaka, N.; Kizu, T.; Takahashi, W.; Ooi, T. Angew. Chem. Int. Ed. 2020, 59, 11456.
(b) Ohmatsu, K.; Morita, Y.; Kiyokawa, M.; Ooi, T. J. Am. Chem. Soc. 2021, 143, 11218. 2. Ohmatsu, K.; Suzuki, R.; Furukawa, Y.; Sato, M.; Ooi, T. ACS Catal. 2020, 10, 2627. 3. Minami, K.; Ohmatsu, K.; Ooi, T. ACS Catal. 2022, 12, 1971.
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© The Author(s), 2023
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