Nanotechnologies / Materials
Organic synthesis, Pharmaceutical chemistry

Fixation of CO2 as a C1 source into organic compounds
catalyzed by lanthanide metal complexes
Department of Materials Engineering Science, Graduate School of Engineering Science
Associate Professor Hayato Tsurugi
https://researchmap.jp/h_tsu?lang=en

Abstract

Nanotechnologies / Materials

Methylation reaction of amine derivatives has been developed as
an organic reaction utilizing carbon dioxide as a carbon source. In
this reaction, a catalytic amount of lanthanum complexes and boron compounds, hydrosilanes as reducing agents, and secondaryamines as substrates are used under atmospheric pressure of CO2
to selectively afford N-methylated tertiary amines. Computational
study on the reaction mechanism clarifies that the ion-pair formation consisting of the lanthanum complex and the hydridoborateanion is active for the initial hydrosilylation reaction of carbon dioxide, and the subsequent reaction with the secondary amine anddeoxygenative reduction gives the final products.

atmosphere, thereby making it possible to directly utilize carbon
dioxide in the atmosphere for organic synthesis in near future.

Figure 1 Methylation of sec-amines▍ Background & Results
Carbon dioxide (CO2) is a thermodynamically stable molecule,and its partial reduction leads to reactive CO2-derived molecules
such as formic acid and formaldehyde. However, because of thehigh stability of CO2, its reduction requires energy, and over-reduction of the initial CO2-reduced compounds provides chemicallyinert methane as the final product; it is thus important to quickly
use the initially formed reduced molecules for organic synthesis.
In this study, we found that by using a lanthanum complex witha large ionic radius as a catalyst, triarylboron and hydrosilane, alanthanum complex with a hydridoborate anion is generated andthe hydrosilylation of CO2 proceeds under mild conditions. Furthermore, it was found that when a secondary amine is added to thereaction mixture, the initially reduced products from hydrosilylationof CO2 reacts with secondary amines to give amides, and the deoxygenative reduction of the amides undergoes under the same reaction conditions, leading to the methylation reaction of secondaryamines. During the catalyst screening, it was found that the ionicradius of the central metal affects the reactivity; in fact, the catalyticactivity of rare earth metals other than lanthanum decreases. The
reaction proceeded efficiently, especially when aniline derivatives
were used as substrates, and dialkylaniline derivatives with various
functional groups were obtained as the final product. In order toclarify the details of this reaction, each reaction steps were investigated using theoretical calculations. The transition state consistsof carbon dioxide bound to the lanthanum center with adjacenthydridoborate anion. The transition state energy required for thisreaction is greatly influenced by the Lewis acidity of the triarylboronused as a co-catalyst. It was also found that the non-covalent interaction between the cationic lanthanum complex and the anionic
hydrido borate is important for lowering the transition state energyof the reaction.

Figure 2 Molecular structure of catalytically active species

Figure 3 Mechanism of CO2 hydrogenation▍ Significance of the research and Future perspective
Reactions using CO2, a greenhouse gas, as a carbon source inorganic synthesis are important research targets from the viewpoint of carbon cycle without relying on fossil fuels. In particular,
the development of catalysts for the direct incorporation of CO2 asa C1 source into organic compounds is one of the scientific technologies that will contribute for lowering carbon dioxide concentration. We are conducting research on improving possible catalysts
that can selectively utilize only carbon dioxide under a mixed gas
P a t e n t
Tr e a t i s e

Shinohara, Koichi; Tsurugi, Hayato; Mashima, Kazushi. N-methylation of aniline derivatives with CO2 and phenylsilane catalyzed by lanthanum
hydridotriarylborate complexes bearing a nitrogen tridentate ligand. ACS Catalysis 2022, 12 (14), 8220-8228. doi: 10.1021/acscatal.2c01658

URL
K e y w o r d CO2 fixation, homogeneous catalyst, synthetic organic chemistry