New electrochemically activated molecular catalyst developed by Yokohama National University

A research group consisting of Associate Professor Naoki Shida and Professor Mahito Atobe from Yokohama National University and Associate Professor Daisuke Yokogawa and Project Researcher Kayo Suda from the University of Tokyo, in collaboration with a research group from Hokkaido University, has developed a new molecular catalyst that electrochemically activates halogen bonding interactions between halogen atoms and Lewis bases, enabling proton-coupled electron transfer (PCET). Through this catalyst, the group achieved intramolecular C-N bond formation reactions of N-protected aminobiphenyls in an electrochemical and catalytic manner and also succeeded in quantitatively evaluating its performance. The interaction of this catalyst is activated by electrochemical oxidation, and the catalyst molecule forms complexes with substrates and bases. Through experiments and theoretical calculations, the researchers revealed that this complex enables PCET, significantly improving both the reaction rate and chemoselectivity. This research provides new design guidelines for electrochemical catalysts, for which unified design principles have not yet been established. The findings were published in the Journal of the American Chemical Society.

Molecular conversion technology using electrochemistry has attracted attention recently, but electrolysis reactions that directly convert molecules on electrodes face challenges such as requiring more energy than necessary (overpotential) and electrode degradation. Catalyst molecules called mediators have been seen as a potential means to solve these challenges. Mediator molecules contribute to solving these challenges by mediating electron transfer between the electrode and the molecules to be reacted and can greatly improve the selectivity and efficiency of chemical reactions. However, there are no established guidelines for what kind of molecules should be designed as mediators.

The research group succeeded in developing new mediator molecules equipped with electrochemically switchable interactions. In particular, applying electrochemically activated halogen bonding interactions to organic reactions is unprecedented and will contribute to the development of diverse catalyst molecules and the advancement of electrochemistry. In this reaction, the mediator molecule first undergoes one-electron oxidization on the electrode and is converted to an electron-deficient state, thereby activating the interaction. Aminobiphenyls interact with the activated mediator molecules to form halogen bonding complexes. Through the formation of the complex, enabling PCET and driving the reaction forward, and ultimately bonds are formed between the intramolecular aromatic ring and the carbon-nitrogen.

Furthermore, by adopting an anthracene skeleton for the mediator molecule, the researchers stabilized the normally unstable one-electron oxidized state, thereby achieving catalytic carbon-nitrogen bond formation reactions. Based on this strategy, they were able to synthesize carbazoles in high yields from aminobiphenyls with four different protecting groups using mediator molecules with various halogens.

They also analyzed the reaction mechanism in detail by combining theoretical calculations and electrochemical measurements. As a result, it became clear that the interactions do indeed exist, as evidenced by the stabilization of the halogen bonding complexes. In addition, they found that by forming complexes, very energetically favorable PCET occurs using weak bases.

The "mediator design capable of electrochemically switching interactions" established in this research will contribute to realizing more energy-efficient and sustainable molecular conversion by reducing overpotential in electrochemical processes and suppressing electrode degradation. Moving forward, the researchers plan to advance molecular design with precise control of halogen bonding strength and electronic states to further improve reaction rate and selectivity and expand the range of applicable substrates. They are also considering applications to the formation reactions of other important bonds, such as carbon-carbon bond formation, rather than just carbon-nitrogen bonds. Furthermore, they aim to generalize the reaction mechanism by combining theoretical calculations and electrochemical measurements, systematize guidelines for mediator design, and apply them to synthesis processes for pharmaceuticals and functional materials.

Journal Information
Publication: Journal of the American Chemical Society
Title: Redox-Switchable Halogen Bonding in Haloanthracene Mediators Enables Efficient Electrocatalytic C-N Coupling
DOI: 10.1021/jacs.5c18175