Catalysts are crucial for production of many thousands of products and materials required by modern society. They are also used for the reduction of water and air pollution, thus decreasing the waste of natural resources and energy. Recent advances in sustainable science and technology demand more powerful and practical catalysts, which promote highly efficient molecular transformations. Prof. Takao Ikariya has been engaged in exploring such a green and sustainable transformations of by conceptually new bifunctional molecular catalysis, named concerto catalysis in both conventional reaction phases and supercritical fluids.
His major contributions to the progress in the chemistry of molecular catalysis include the following three points:
1. In early 1980's he discovered a very practical and powerful chiral BINAP-Ru catalyst, Ru2Cl4(binap)2N(C2H5)3 that effected efficiently asymmetric hydrogenation of alkenes and ketones and was successfully applicable to commercial synthetic process for synthesis of an intermediate of carbapenemes. The structure of the chiral Ru complex was later revised to be the ethylamine coordinated Ru complex, Ru2Cl4(binap)2NH2(C2H5) by X-ray crystallographic analysis, which revealed that the Ru/NH bifunctional unit was crucial for determining the catalyst performance. The emergence of his powerful BINAP-Ru catalyst has stimulated the intense development in the research on chiral Ru catalyst and also triggered the extraordinary development of Noyori's BINAP-Ru chemistry.
2. Prof. Ikariya has developed conceptually new bifunctional molecular catalysts, Ru[N-sulfonylated dpen](η6-arene) for asymmetric transfer hydrogenation of ketones and imines. This prototype of bifunctional Ru catalyst is characterized by the unique M/NH acid-base unit responsible for the metal-ligand cooperating catalysis and its excellent catalytic performance in terms of reactivity and selectivity. Since then, he has polished up the original Ru catalysts by fine-tuning of their structures based on a deep understanding the mechanism of the catalysis, leading to very practical and powerful tools for organic synthesis. His newly developed bifunctional catalysts can efficiently promote not only transfer hydrogenation of carbonyl compounds but also hydrogenation of polarized functionalities, imides, epoxides, esters, and amides, which is a greener alternative to the metal hydride reductions. He found that the bifunctional catalysts activate even carbon dioxde under supercritical conditions to promote its catalytic hydrogenation into formic acid derivatives. In addition, the bifunctional catalysts are also applicable to C-C and C-N bond formation and to aerobic oxidation of alcohols.
He successfully expanded the unique concept of the bifunctional mononuclear molecular catalyst to dinuclear catalyst systems which have a sulfonylimido-bridge ligands. The bifunctional dinuclear catalyst, [(Cp*Rh)2(μ-NTs)2], can promote catalytic aerobic oxidation of H2 or alcohols. The redox interconversion between the diimido and the diamido complexes is crucial for determining the catalytic performance. Thus, the metal-cooperating ligand can activate the substrates in a concerted manner to facilitate the molecular transformation. Now it is realized that the bifunctional molecular catalyst based on the M/NH cooperation effect is a general and practical tool in organic synthesis.
3. He is one of the pioneering researchers who opened up a new chemistry of molecular catalysis in supercritical fluids having unique physical properties. In 1995, Ikariya, together with Noyori and Jessop discovered extremely rapid hydrogenation of CO2 in supercritical CO2 containing molecular Ru catalysts into useful formic acid derivatives, in which the use of the supercritical conditions accelerates significantly the hydrogenation reaction. The finding has had a great impact on academic research and the development of unprecedented molecular catalysis. He also developed many practical catalytic reactions in supercritical fluids, demonstrating that the supercritical fluid has the beneficial effects of both liquid- and gas-phase chemistry, and is a very promising reaction medium for homogeneous and heterogeneous and even enzymatic catalysis. He investigated dynamic behavior of CO2 molecules and molecular interaction between carbon dioxide and catalyst molecules under the supercritical conditions by using high pressure and variable temperature NMR spectroscopy. In particular, based on the supercritical NMR study of the reaction of CO2 and amines to carbamic acids, he developed numerous reactions effective CO2 fixation into valuable functionalized compounds. Supercritical CO2 can be now used as a reaction medium and can potentially replace conventional organic solvents to serve as an environmentally benign reaction solvent.
The powerful concerto catalysis with bifunctional molecular catalysts established by Ikariya is now utilized in academic and industrial research to develop valuable chemicals and materials. His achievement will provide a great leap towards the goal of more efficient, sustainable and green production processes.