Chemical reactions are intriguing phenomena in nature and particularly in biology, where molecules change their structures. Chemical reactions are initiated by activating molecules by heating, irradiation, or contact with chemical reagents, which is followed by structural changes in an energetically downhill manner. Studies of chemical reactions in basic sciences promote our understanding of nature and biology, and those in the applied sciences provide methods to produce useful compounds. Professor M. Yamaguchi has significantly contributed to the development, understanding, and application of organic chemical reactions, which involve the rearrangement of covalent and noncovalent chemical bonds. He developed a number of synthetic chemical reactions with novel activation modes and also explored the dynamic function of helical materials based on the bottom-up synthesis of helical molecules, molecular aggregates, and molecular assemblies.
1. Development of Synthetic Reactions Based on Novel Molecular Activation Methods
Development of synthetic methods for organosulfur and organophosphorus compounds is important for the development of drugs and functional materials, and is an important goal in organic chemistry. Professor Yamaguchi developed transition-metal-catalyzed methods employing novel activation modes of heteroatom reagents, although such catalysis had been considered not feasible, because of the strong affinity of heteroatoms for transition metals. Studies of the catalyzed reactions led to the development of novel concepts such as the similar reactivity of rhodium/palladium catalysts, corelations of the reactivity of substrates and products for efficient synthesis, and equilibrium shifts to increase product yield. A novel scientific field has been explored through the fusion of organoheteroatom chemistry and transition metal chemistry. Development of asymmetric reactions using proline salt catalysis is another achievement of Professor Yamaguchi, which involves the activation of enones and enals by the organocatalyst. This is recognized as pioneering work conducted in the 1990s, for it was followed by a burst of activity in the study of organocatalytic methods in the 2000s. The lithium acetylide/BF3 reagent system he developed is widely used in organic synthesis.
2. Dynamic Functions of Synthetic Helical Molecules and Molecular Systems
A helix is an ordered structure ubiquitous in nature with either right-handed or left-handed chirality. Helical organic materials broadly appear in biology as exemplified by DNA at the molecular and molecular aggregate levels, the tubulin protein at the molecular assembly level, and the vines of morning-glory or orchid flowers at the bulk level, which exhibit important biological functions. The chemical study of helical materials, however, has been difficult, because of the lack of methods to synthesize such materials systematically. Professor Yamaguchi has developed a method to prepare large quantities of a helical organic compound, helicene, and synthesized various helical organic materials including small molecular helicene derivatives, oligomeric compounds, molecular aggregates, and molecular assemblies in the bottom-up manner.
He synthesized various helicene derivatives, and developed novel functions, which include chiral recognition in complexation with double-stranded DNA, the right/right and left/left rule in noncovalent bond formation, helical asymmetric catalysts, optical resolution using helicene-grafted silica nanoparticles, and diode-switching electronic devices. Helicene oligomers were synthesized by connecting helicene with linkers from monomers up to tenmers, where more than 100 oligomers were systematically prepared. Discontinuous phenomena appeared, where the property of the oligomers largely changed beyond a certain number of helicene units. It was found that cyclic acetylene trimers form a dimeric aggregate without forming higher aggregates, and linear oligomers form homo- and hetero-double-helices, which reversibly aggregate and disaggregate upon cooling and heating, respectively. This pioneering work on double-helix forming oligomers has attracted much attention in recent years. Various double-helix forming compounds containing amide, sulfonamide, and aminomethylene linkers have been developed, which are characterized by their dynamic functions. This work yielded a general method to develop double-helix forming molecules. Multidomain oligomers, in which different double-helix forming oligomers were connected, were synthesized. Each domain behaved independently; therefore a novel function emerged owing to the combination of the different functions exhibited by each domain: For example, ααββ tetrameric aggregates, which occur in protein aggregation in biology, were formed.
Achievements were made in the area of nonequilibrium thermodynamic properties of double-helix forming molecules, which were then applied to a novel molecular switching function. On the basis of the finding of thermal hysteresis in dilute solutions at the molecular level, it was determined that a metastable disaggregated random-coil state is formed by simple heating and cooling, and that self-catalysis is involved during the formation of a double-helix from a random-coil. In addition, the transformation exhibited various notable nonequilbrium thermodynamic phenomena at the molecular level, including temperature threshold phenomena, sensing of temperature increase/decrease, sensing of concentration increase, counting the numbers 1 and 2, and equilibrium crossing.
Also noteworthy is the development of the bottom-up synthesis of helical organic materials from the molecular level to molecular aggregates and the assembly level and the development of their dynamic functions. The materials systems include solid surfaces, nanoparticles surfaces, self-assembly gels, vesicles, and liquid crystals, in which reversible double-helix formation is integrated into the bulk dynamic functions. These studies suggest that bulk dynamic functions could be programmed into the molecular structures of helicene oligomers.
These studies have provided various dynamic functions of helical materials involving noncovalent bond formation, and unraveled the concepts and principles, that govern such phenomena. It should be emphasized that a chiral world of helical molecules is as rich as the chiral world of molecules with central chirality. It is not unreasonable to state that Professor Yamaguchi is a founder of the helical chiral world of organic molecules.
To summarize, Professor Yamaguchi has studied a broad range of chemical reactions both with covalent and noncovalent bond formations using organic synthetic methodology, and has made important contributions to the development of synthetic methodology, molecular activation modes, and dynamic function, as well as the production of useful organic compounds. His contribution is highly valued in the advancement of organic chemistry and related scientific fields, and he certainly merits the Chemical Society of Japan Award.