| Research Abstract |
(1) Asymmetric Hydrogenation of Ketones: Chiral RuC1_2(binap)(1, 2-diamine) complexes allow rapid, enantioselective hydrogenation of various simple, unfunctionalized ketones with a high substrate/catalyst molar ratio up to 2, 400, 000. As high practicality of this procedure became recognized, we have been encouraged to develop a robust (pre)catalyst that promotes the enantioselective reaction under base-free conditions. During the course of this program, we established that RuH(η^1-BH_4)(binap)(1, 2-diamine) complexes allow for asymmetric hydrogenation of simple ketones in 2-propanol without an additional strong base. Various base-sensitive ketones are convertible to chiral alcohols in a high enantiomeric purity. More recently, we discovered that N-tosylethylenediamine-Ru(II) complexes catalyze asymmetric hydrogenation of ketones giving the corresponding chiral carbinols in high enantiomeric purity. These Ru(II)-complexes, known as excellent catalysts for asymmetric transfer hydrogenat
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ion, can be used for asymmetric hydrogenation as well. This discovery provides a long-thought method for enantioselectively hydrogenating simple ketones under nonbasic and acidic conditions. Further kinetic studies, isotope labeling experiments, and a set of different ID and 2D NMR analyses as well as computational analysis supported a metal-ligand bifunctional mechanism passing through a six-membered pericyclic transition state. We believe the present bifunctional molecular catalyst offers a great opportunity to open up new fundamentals for stereoselective molecular transformation.
(2) Creation of Acid-Base Catalysis Involving Water Functions: Compared with functional group conversions attained by many types of atom-economical oxidation and/or reduction methods, efficient catalysis that mediates carbon-carbon bond formation still remains a considerable challenge. In particular, the development of mild and efficient transformations involving enol and enolate equivalents in the presence of a molecular catalyst has been a topic of continuous interest in organic synthesis. Several drawbacks that come from a high catalyst loading, low atom economy and generation of undesirable by-products need improvement. In order to sustain high reactivity to achieve maximum conversion, future catalysts should be structurally stable with catalytic species that are inert to the air and water. Such catalysts should be also capable of discriminating and binding to a single functionality for complex, multi-functional group substrates. Recently we discovered a novel molecular catalyst that shows high catalytic activity through molecular recognition events. These catalysts have aqua-cluster structures composed of aminoorganoboron and water. The structural properties of these catalysts are very unique: only one water molecule is captured by the combination of two hydrogen bonds and one coordination bond. We proposed that a special arrangement of the water molecule and multiple amines within the catalyst serves as a catalytically active site, thereby mimicking enzyme-like activity by forming cooperative acid-base interactions with organic substrates.
(3) Direct Carbon-hydrogen Bond Arylation: Organic molecures having (hetero)aryl-(hetero)aryl bonds represent privileged structural motifs frequently found in natural products or used in pharmaceuticals as well as functional organic materials. Therefore, the development of efficient methods for biaryl has been a topic of unparalleled importance in all aspects of pure and applied chemistry. Although metal-catalyzed cross-coupling reactions of organometallics and organic halides have enjoyed widespread applications, the direct C-H arylation of heteroarenes and arenes holds significant synthetic potential as it eliminates pre-activation of coupling components. As a part of our program aimed at establishing programmable synthesis of (hetero)aryl-core starburst molecules, we developed a new transition metal catalyst for direct C-H arylation of heteroarenes that manifested high activity paired with broad scope. The fact that our effort has culminated in the finding of direct C-H arylation of simple arenes speaks well for the potential of the present catalysis for further development and applications. Elucidation of reaction mechanism development of more active catalysts, and applications of present direct arylation technology for materials and pharmaceutical chemistry are currently ongoing. Less
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