Organic synthesis plays a pivotal role in modern society, offering tools for synthesizing countless kinds of organic molecules such as medicines, agrochemicals, and organic materials. Organic molecules required nowadays has become much more complex than ever before, and methods for synthesis should be much more efficient. Professor Dr. Koichiro Oshima has continued to pursue efficiency of synthetic methods
by presenting new reactions for "highly selective synthesis of what we need." His pioneering work has provided a number of new reactions which show unprecedented reactivity and selectivity, stimulating the fields of synthetic organic chemistry and organometallic chemistry. Although his work covers development of a vast range of synthetic organic reactions, his work is largely based on the concept of "combined use of organometallic reagents and radical species." His work is summarized as follows:
1. Highly Selective Radical Reactions Initiated by Triethylborane
In 1980s, radical reactions were not so popular as ionic reactions in organic synthesis, since radical reactions were considered to be difficult to control. In 1987, he found that triethylborane can initiate radical reactions in the presence of a trace amount of oxygen. The triethylborane-induced system has allowed for performing radical reactions at extremely low temperatures such as -78 °C, and made radical reactions controllable and selective. Now his initiation method is widely used for highly stereoselective, even enantioselective, reactions and the synthesis of complex biologically interesting compounds.
2. Selective Carbon?Carbon Bond Formation Reactions in Aqueous Media
He applied the triethylborane-induced system to radical reactions in aqueous media. Triethylborane-induced radical cyclization reaction of allyl iodoacetate proved to proceed smoothly in water to selectively afford gamma-lactone in high yield. Intriguingly, in organic solvents, none of the lactone is obtained and polymeric products are formed instead. The use of triethylborane has revealed the favorable solvent effect of water in synthetic radical reactions. He extended his interest in reactions in aqueous media to transition metal-catalyzed carbon-carbon bond formations, such as cyclotrimerization of alkynes in an aqueous/organic biphasic system which offers selective synthesis of medium- or large-sized rings.
3. Selective Carbon-Carbon Bond Formation Reactions by Ate Complexes.
He created several ate complexes, and used them for development of selective yet highly efficient reactions, by taking advantage of radical character of the ate complexes. Treatment of 6-halo-1-hexene derivatives with tributylmanganate leads to reductive cyclization reactions. The cyclization, via a radical process, features single electron transfer from the electron-rich organomanganate to the halohexene derivatives. Interestingly, treatment of the same halohexenes with arylmagnesium reagents in the presence of a cobalt complex results in the formation arylative cyclization reactions via a radical process. Cross-coupling reactions of secondary alkyl halides with arylmagnesium reagents proceed in high yield in the presence of a cobalt(II) chloride ? diamine complex. His cobalt-catalyzed reactions culminate in the cross-coupling reactions of tertiary alkyl halides and allyl Grignard reagents. He has also developed cobalt-catalyzed Mizoroki-Heck type reactions of styrenes with alkyl halides by using a single electron transfer process.
4. Highly Selective Silylmetalations with Ate Complexes
In 1983, He found that treatment of terminal alkynes with silylmagnesium reagents under platinum catalysis resulted in syn-silylmetalation of the alkynes to yield (E)-1-silylalkenes. In contrast, copper-catalyzed silylzincations of alkynes with silylzincate complexes proceeded to afford 2-silyl-1-alkenes selectively. These reactions are recognized as the pioneering work on "metalometalation" of carbon-carbon multiple bonds with "Interelement compounds."
Professor Dr. Koichiro Oshima was born in Hyogo, Japan, in 1947. He received his B.S. and Ph.D. degrees from Kyoto University in 1970 and 1975 under the guidance of Professor Hitosi Nozaki. He spent two and a half years as a postdoctoral fellow with Professor Barry Sharpless at MIT from 1975 to 1977, and became an Assistant Professor at Kyoto University in 1977. He was promoted to Lecturer in 1984, Associate Professor in 1986, and Professor in 1993. He has been involved in development of organic reactions via organometallic reagents and received the Award for Young Chemists of the Society of Synthetic Organic Chemistry, Japan in 1983 and the Japan Synthetic Organic Chemistry Award in 2004. His current research interests include the development of new synthetic methods via radical reactions and organometallic reagents.