Dr. Hou's research has targeted the development of new catalysts, new reactions, and new functional materials that are superior or complementary to the existing ones. His studies have spanned broad areas of chemical synthesis, ranging from the design and synthesis of organometallic complexes with novel structure and reactivity to the activation and transformation of small molecules and strong chemical bonds, selective and efficient synthesis of fine chemicals, and precision polymerization and copolymerization of non-polar and polar olefins. His major achievements are summarized below.
1. Half-Sandwich Rare-Earth Catalysts for Organic Synthesis and Olefin Polymerization
Rare-earth elements possess unique chemical and physical properties that are different from those of main-group and late-transition metals. By taking the advantage of these unique properties, Dr. Hou has designed and synthesized a brand-new series of rare-earth-based catalysts for organic synthesis and olefin polymerization. By combining the strong Lewis acidity and heteroatom affinity of the rare-earth metal ions with the high C=C insertion activity and basicity (C-H activation ability) of the rare-earth metal alkyl species in a half-sandwich-type ligand environment, Dr. Hou has achieved for the first time the catalytic regioselective C-H addition of anisoles, pyridines, amines, and sulfides to a wide range of alkenes and styrenes, thereby constituting a 100% atom-efficient route for C-H alkylation. Based on these findings, he has developed the first C−H polyaddition of dimethoxyarenes to unconjugated dienes and simultaneous chain-growth and step-growth polymerization of methoxystyrene, which offered a unique route for the construction of novel macromolecular architectures that are difficult to make by other means.
By using the unique interaction between rare-earth metal atom and heteroatom, Dr. Hou has synthesized for the first time a series of heteroatom (O, S, Se, N, and P)-functionalized polyolefins with high molecular weights and controllable functional monomer contents through the copolymerization of ethylene and heteroatom-containing a-olefins. By using this heteroatom-assisted copolymerization strategy, he has successfully synthesized a series of brand-new functional polymers that possess unique multi-block microstructures and show excellent self-healing and shape memory properties. This work has opened up a new avenue in the copolymerization of non-polar and polar olefins for the synthesis of functional polymers.
By using chiral cyclopentadienyl ligands, Dr. Hou has synthesized the first series of chiral half-sandwich rare-earth alkyl complexes that show excellent enantioselectivity and activity in various transformations, such as asymmetric C−H addition of pyridines to alkenes, enantioselective intermolecular hydroamination of cyclopropenes with amines, diastereodivergent asymmetric carboamination/annulation of cyclopropenes with aminoalkenes, and enantioselective construction of silicon-stereogenic silanes through the intermolecular asymmetric hydrosilylation of alkenes with dihydrosilanes. These studies have opened up a new frontier in asymmetric catalysis.
2. N-Heterocyclic Carbene (NHC)-Copper Catalysts for Efficient Use of Carbon Dioxide in Organic Synthesis
Carbon dioxide (CO2) is an abundant, low-cost, and inherently renewable C1 resource. The search for selective and efficient catalysts for the use of CO2 as a C1 building block in organic synthesis is of great interest and importance. In this endeavor, Dr. Hou has established the N-heterocyclic carbene (NHC)-coordinated copper complexes as an excellent catalyst platform for the incorporation of CO2 into a variety of organic compounds, which featured excellent functional group tolerance, high regio- and stereoselectivity, and well-defined reaction mechanism. In addition to the efficient synthesis of a variety of synthetically useful carboxylation products, a novel family of lithium boracarbonate and boralactone ion-pair compounds, which could show high potential as a new class of electrolytes for lithium ion batteries, have been selectively obtained by the copper-catalyzed multi-component coupling of carbon dioxide, diborane, and unsaturated compounds such as aldehydes and alkynes in the presence of a lithium alkoxide.
3. Multimetallic Polyhydrides for Activation of Small Molecules and Strong Chemical Bonds
As part of the efforts to explore the synergistic effects of multimetallic complexes in chemical transformations, Dr. Hou has synthesized and examined a series of well-defined multimetallic polyhydride complexes. He has found that titanium hydride clusters bearing sterically demanding cyclopentadienyl or analogous ancillary ligands possess remarkable reactivity toward various inactive molecules, such as the cleavage and hydrogenation of dinitrogen at ambient temperature and pressure, conversion of dinitrogen to organic nitriles under mild conditions, C-C bond cleavage and rearrangement of benzene at room temperature, and hydrodenitrogenation of pyridines and quinolines under mild conditions. The DFT studies have revealed that the cooperation of multiple metal hydrides plays an important role in these novel transformations. This work has not only opened up a new frontier in metal hydride cluster chemistry, but also offered unprecedented insight into the mechanistic aspects of some important industrial processes on solid catalysts such as the Haber-Bosh ammonia synthesis and petroleum refining.
As described above, Dr. Hou's research has gained a worldwide reputation for excellence in organometallic chemistry and catalysis, polymerization catalysis, and reaction chemistry. For these outstanding achievements, Dr, Hou is honored with the Chemical Society of Japan Award.