The University of Tokyo develops new technology using single-walled carbon nanotubes for synthetic organic aquachemistry

A research group led by Research Associate Taku Kitanosono and Project Professor Shu Kobayashi from the Graduate School of Science at the University of Tokyo has developed a new technology for catalytic asymmetric synthesis in water. While conducting chemical reactions in water is extremely important from the perspective of environmentally friendly and safe green chemistry, conventional chiral Lewis acid catalysts are vulnerable to it and quickly lose their functionality, which has been a major challenge. The research group successfully achieved stable immobilization by ensuring that the chiral Lewis acid catalyst they had designed was adsorbed onto single-walled carbon nanotubes (SWNTs) without using covalent bonds. This achievement will significantly contribute to the future manufacturing of pharmaceuticals and high-performance chemical products. The results were published in Chemical Science.

Previous research has employed methods of robustly immobilizing catalysts using covalent bonds. While this approach made it possible to prevent catalyst leaching and maintain high performance, it had the disadvantage of reduced design flexibility due to the complex catalyst preparation process.

In this research, the researchers challenged themselves to immobilize catalysts using a method that does not require covalent bonds. The key was SWNTs. SWNTs have extremely thin tubular structures and possess excellent electrical and structural properties.

The research group thought that the SWNT surface would help stabilize the catalyst. In their experiments, they succeeded in making an immobilized catalyst function simply by adsorbing their uniquely designed scandium complex onto the SWNT surface. They utilized the attractive force of π-π interactions that work between SWNTs and the catalyst. As a result, they found that the electronic state of the catalyst was well-controlled and worked more stably.

This catalyst system demonstrated high performance in important chemical reactions in water. Furthermore, even when the catalyst was used repeatedly up to 10 times, virtually no decline in performance or leaching of catalyst components was observed. This represents high reusability comparable to covalently immobilized catalysts.

When compared with other carbon materials, the superiority of this achievement became clearer. With carbon black (CB) and multi-walled carbon nanotubes (MWNTs), catalyst performance quickly declined, or reactions hardly proceeded at all. These results revealed that SWNTs function not merely as supports that hold catalysts, but as active partners that help the catalysts themselves work.

Furthermore, the group discovered that performance could be further improved by chemically modifying the SWNT surface or changing the molecular shape on the catalyst side. This provided important guidelines for future catalyst development. This research represents an extremely important achievement that has solved the challenging problem of chemical reactions in water through a new approach. By utilizing the special properties of SWNTs, it became possible to use catalysts with high performance and stability without relying on covalent bonds. This technology holds the potential for freer and easier catalyst design. In the future, it is expected that by applying this technology, manufacturing processes for pharmaceuticals and fine chemicals (specialty chemical products) will be transformed into more environmentally friendly and efficient ones.

Journal Information
Publication: Chemical Science
Title: Noncovalent immobilization of chiral Lewis acids on single-walled carbon nanotubes as a tool for synthetic organic aquachemistry
DOI: 10.1039/D5SC05390K